| By Author | [ A B C D E F G H I J K L M N O P Q R S T U V W X Y Z | Other Symbols ] |
| By Title | [ A B C D E F G H I J K L M N O P Q R S T U V W X Y Z | Other Symbols ] |
| By Language |
|
Download this book: [ ASCII ] Look for this book on Amazon Tweet |
Title: Volcanoes: What They are and What They Teach
Author: Judd, John W. (John Wesley)
Language: English
As this book started as an ASCII text book there are no pictures available.
*** Start of this LibraryBlog Digital Book "Volcanoes: What They are and What They Teach" ***
This book is indexed by ISYS Web Indexing system to allow the reader find any word or number within the document.
WHAT THEY TEACH ***
Transcriber Note
Text emphasis denoted as _Italics_.
THE
International Scientific Series
VOL. XXXV.
_Frontispiece._
[Illustration]
Sections of Igneous Rocks, illustrating the passage from the glassy to
the crystalline structure.
1. Vitreous Rock. 2. Semi-Vitreous Rock. 3. Vitreous Rock with
Sphærulites. 4. Rock with Crypto-crystalline Base. 5. Rock with
Micro-crystalline Base. 6. Rock of Granite Structure built up
entirely of Crystals.
[_See pp._ 63-68.
VOLCANOES
WHAT THEY ARE and WHAT THEY TEACH
BY
JOHN W. JUDD, F.R.S.
PROFESSOR OF GEOLOGY IN THE ROYAL SCHOOL OF MINES
_WITH 96 ILLUSTRATIONS_
SIXTH EDITION
LONDON
KEGAN PAUL, TRENCH, TRÜBNER & CO. Ltd.
PATERNOSTER HOUSE, CHARING CROSS ROAD
1903
(_The rights of translation and of reproduction are reserved._)
PREFACE.
In preparing this work, I have aimed at carrying out a design suggested
to me by the late Mr. Poulett Scrope, the accomplishment of which has
been unfortunately delayed, longer than I could have wished, by many
pressing duties.
Mr. Scrope's well-known works, 'Volcanoes' and 'The Geology and Extinct
Volcanoes of Central France'--which passed through several editions
in this country, and have been translated into the principal European
languages--embody the results of much careful observation and acute
reasoning upon the questions which the author made the study of his
life. In the first of these works the phenomena of volcanic activity
are described, and its causes discussed; in the second it is shown that
much insight concerning these problems may be obtained by a study of
the ruined and denuded relics of the volcanoes of former geological
periods. The appearance of these works, in the years 1825 and 1827
respectively, did much to prepare the minds of the earlier cultivators
of science for the reception of those doctrines of geological
uniformity and continuity, which were shortly afterwards so ably
advocated by Lyell in his 'Principles of Geology.'
Since the date of the appearance of the last editions of Scrope's
works, inquiry and speculation concerning the nature and origin of
volcanoes have been alike active, and many of the problems which were
discussed by him, now present themselves under aspects entirely new and
different from those in which he was accustomed to regard them. No one
was ever more ready to welcome original views or to submit to having
long-cherished principles exposed to the ordeal of free criticism than
was Scrope; and few men retained to so advanced an age the power of
subjecting novel theories to the test of a rigorous comparison with
ascertained facts.
But this eminent geologist was not content with the devotion of his
own time and energies to the advancement of his favourite science, for
as increasing age and growing infirmities rendered travel and personal
research impossible, he found a new source of pleasure in seeking
out the younger workers in those fields of inquiry which he had so
long and successfully cultivated, and in furthering their efforts by
his judicious advice and kindly aid. Among the chosen disciples of
this distinguished man, who will ever be regarded as one of the chief
pioneers of geological thought, I had the good fortune to be numbered,
and when he committed to me the task of preparing a popular exposition
of the present condition of our knowledge on volcanoes, I felt that I
had been greatly honoured.
In order to keep the work within the prescribed limits, and to avoid
unnecessary repetitions, I have confined myself to the examination of
such selected examples of volcanoes as could be shown to be really
typical of all the various classes which exist upon the globe; and
I have endeavoured from the study of these to deduce those general
laws which appear to govern volcanic action. But it has, at the
same time, been my aim to approach the question from a somewhat new
standpoint, and to give an account of those investigations which have
in recent times thrown so much fresh light upon the whole problem.
In this way I have been led to dwell at some length upon subjects
which might not at first sight appear to be germane to the question
under discussion;--such as the characters of lavas revealed to us
by microscopic examination; the nature and movements of the liquids
enclosed in the crystals of igneous rocks; the relations of minerals
occurring in some volcanic products to those found in meteorites; the
nature and origin of the remarkable iron-masses found at Ovifak in
Greenland; and the indications which have been discovered of analogies
between the composition and dynamics of our earth and those of other
members of the family of worlds to which it belongs. While not evading
the discussion of theoretical questions, I have endeavoured to keep
such discussions in strict subordination to that presentation of the
results attained by observation and experiment, which constitutes the
principal object of the work.
The woodcuts which illustrate the volume are in some cases prepared
from photographs, and I am indebted to Mr. Cooper for the skill with
which he has carried out my wishes concerning their reproduction.
Others among the engravings are copies of sketches which I made in
Italy, Hungary, Bohemia, and other volcanic districts. The whole of the
wood-blocks employed by Mr. Poulett Scrope in his work on Volcanoes
were placed at my disposal before his death, and such of them as
were useful for my purpose I have freely employed. To Captain S. P.
Oliver, R.A., I am obliged for a beautiful drawing made in the Island
of Bourbon, and to Mr. Norman Lockyer and his publishers, Messrs.
Macmillan & Co., for the use of several wood-blocks illustrating
sun-spots and solar prominences.
J. W. J.
London: _May 1881_.
CONTENTS.
CHAPTER I.
PAGE
INTRODUCTORY: NATURE OF THE ENQUIRY
1
CHAPTER II.
THE NATURE OF VOLCANIC ACTION
7
CHAPTER III.
THE PRODUCTS OF VOLCANIC ACTION
39
CHAPTER IV.
THE DISTRIBUTION OF THE MATERIALS EJECTED FROM VOLCANIC VENTS
67
CHAPTER V.
THE INTERNAL STRUCTURE OF VOLCANIC MOUNTAINS
112
CHAPTER VI.
THE VARIOUS STRUCTURES BUILT UP AROUND VOLCANIC VENTS
161
CHAPTER VII.
THE SUCCESSION OF OPERATIONS TAKING PLACE AT VOLCANIC CENTRES
186
CHAPTER VIII.
THE DISTRIBUTION OF VOLCANOES UPON THE SURFACE OF THE GLOBE
224
CHAPTER IX.
VOLCANIC ACTION AT DIFFERENT PERIODS OF THE EARTH'S HISTORY
247
CHAPTER X.
THE PART PLAYED BY VOLCANOES IN THE ECONOMY OF NATURE
281
CHAPTER XI.
WHAT VOLCANOES TEACH US CONCERNING THE NATURE
OF THE EARTH'S INTERIOR
307
CHAPTER XII.
THE ATTEMPTS WHICH HAVE BEEN MADE TO EXPLAIN
THE CAUSES OF VOLCANIC ACTION
331
INDEX
371
ILLUSTRATIONS.
Sections of igneous rocks illustrating the passage from the
glassy to the crystalline structure
_Frontispiece_
Fig. Page
1. Stromboli, viewed from the north-west, April 1874 _to face p._ 10
2. Map of the Island of Stromboli 11
3. Section through the Island of Stromboli from north-west to
south-east 13
4. The crater of Stromboli as viewed from the side of the
Sciarra during an eruption on the morning of April 24,
1874. 14
5. Vesuvius in eruption, as seen from Naples, April 26, 1872.
(_From a photograph_) _to face p._ 24
6. View of Vulcano, with Vulcanello in the foreground--taken
from the south end of the Island of Lipari 43
7. Minute cavities, containing liquids, in the crystals of rocks.
(_After Zirkel_) _to face p._ 60
8. Minute liquid-cavity in a crystal, with a moving bubble.
(_After Hartley_) 63
9. Cavity in crystal, containing carbonic-acid gas at a
temperature of 86° F., and passing from the liquid to
the gaseous condition. (_After Hartley_) 64
10. Monte Nuovo (440 ft high) on the shores of the Bay of Naples.
(_After Scrope_) 76
11. Map of the district around Naples, showing Monte Nuovo and the
surrounding volcanoes of older date 78
12. Outlines of the summit of Vesuvius during the eruption of
1767. (_After Sir W. Hamilton_) _to face p._ 80
13. Crater of Vesuvius formed during the eruption of 1822
(_After Scrope_) 82
14. Crater of Vesuvius in 1756, from a drawing made on the spot.
(_After Sir W. Hamilton_) 84
15. The summit of Vesuvius in 1767, from an original drawing.
(_After Sir W, Hamilton_) 85
16. Summit of Vesuvius in 1843 86
17. Outlines of Vesuvius, showing its form at different periods
of its history 87
18. Cascade of lava tumbling over a cliff in the Island of
Bourbon. (_After Capt. S. P. Oliver, R.A._) 93
19. Lava-stream (obsidian) in the Island of Vulcano, showing
the imperfect liquidity of the mass 95
20. Interior of a rhyolitic lava-stream in the Island of Lipari,
showing broad, sigmoidal folds, produced by the slow
movements of the mass 96
21. Interior of a rhyolitic lava-stream in the Island of Lipari,
showing the complicated crumplings and puckerings,
produced by the slow movements of the mass 96
22. Vesuvian lava-stream of 1858, exhibiting the peculiar
'ropy' surfaces of slowly-moving currents.
(_From a photograph_) _to face p._ 98
23. Vesuvian lava-stream of 1872, exhibiting the rough cindery
surfaces characteristic of rapidly flowing currents.
(_From a photograph_) _to face p._ 96
24. Concentric folds on mass of cooled lava. (_After Heaphy_) 100
25. Mass of cooled lava formed over a spiracle on the slopes
of Hawaii. (_After Dana_) 100
26. Group of small cones thrown up on the Vesuvian lava-current
of 1855. (_After Schmidt_) 101
27. Natural section of a lava-stream in the Island of Vulcano,
showing the compact central portion and the scoriaceous
upper and under surfaces 104
28. Section of a lava-stream exposed on the side of the river
Ardèche, in the south-west of France. (_After Scrope_) 106
29. Portion of a basaltic column from the Giant's Causeway,
exhibiting both the ball-and-socket and the
tenon-and-mortise structure 107
30. Vein of green pitchstone at Chiaja di Luna, in the Island
of Ponza, breaking up into regular columns and into
spherical masses with a concentric series of joints.
(_After Scrope_) 108
31. Illustration of the 'perlitic structure' in glassy rocks 109
32. Transverse section of a lava-stream 111
33. The Kammerbühl, or Kammerberg, Bohemia (as seen from
the south-west) 113
34. Section of the Kammerbühl in Bohemia 114
35. Natural section of a volcanic cone in the Island of Vulcano 116
36. Section in the side of the Kammerbühl, Bohemia 118
37. Experimental illustration of the mode of formation of
volcanic cones, composed of fragmental materials 120
38. Natural section of a tuff-cone, forming the Cape of Misenum,
and exhibiting the peculiar internal arrangement,
characteristic of volcanoes composed of fragmentary
materials. (_After Scrope_) 121
39. Section of a small scoria-cone formed within the crater of
Vesuvius in the year 1835, illustrating the filling up of
the central vent of the cone by subsequent ejections.
(_After Abich_) 122
40. Volcanic cones composed of scoriæ, and breached on one
side by the outflow of lava-currents. (_After Scrope_) 128
41. Campo Bianco, in the Island of Lipari. A pumice-cone
breached by the outflow of an obsidian lava-stream
_to face p._ 124
42. Volcanic cones in Auvergne, which have suffered to some
extent from atmospheric denudation. (_After Scrope_) 124
43. Experimental illustration of the mode of formation of
volcanic cones composed of viscid lavas. (_After Reyer_) 126
44. The Grand Puy of Sarcoui, composed of trachyte, rising
between two breached scoria-cones (Auvergne). (_After
Scrope_) 126
45. Volcanic cone (Mamelon) composed of very viscid lava
(Island of Bourbon). (_After Bory de St. Vincent_) 127
46. Another Mamelon in the Island of Bourbon, with a crater
at its summit. (_After Bory de St. Vincent_) 127
47. Cliff-section in the Island of Madeira, showing how a
composite volcano is built up of lava-streams, beds of
scoriæ, and dykes. (_After Lyell_) 125
48. Section seen at the cascade, Bains du Mont Dore. (_After
Scrope_) 130
49. Section in the Island of Ventotienne, showing a great
stream of andesitic lava overlying stratified tuffs.
(_After Scrope_) 130
50. Cliff on the south side of the Island of San Stephano 131
51. The headland of Monte della Guardia, in the Island of Ponza 131
52. Western side of the same headland, as seen from the north
side of Luna Bay 132
53. Sea-cliff at Il Capo, the north-east point of Salina,
showing stratified agglomerates traversed by numerous
dykes, the whole being unconformably overlaid by
stratified, aqueous deposits 137
54. Section observed in the Val del Bove, Etna, showing a
basaltic dyke, from the upper part of which a
lava-current has flowed 138
55. Basaltic dykes projecting from masses of stratified scoriæ
in the sides of the Val del Bove, Etna 134
56. Sheets of igneous rock (basalt) intruded between beds of
sandstone, clay, and limestone (Island of Skye) 137
57. Plan of the dissected volcano of Mull in the Inner
Hebrides _to face p._ 142
58. Section of the volcano of Mull along the line A B " 142
59. Summit of the volcano of Monte Sant' Angelo, in Lipari,
exhibiting a crater with walls worn down by denudation 158
60. Outlines of lava-cones 160
61. Diagram illustrating the formation of parasitic cones along
lines of fissure formed on the flanks of a great volcanic
mountain 162
62. Outline of Etna, as seen from Catania 162
63. Outline of Etna, as seen from the Val del Bronte 163
64. Plan of the volcano forming the Island of Ischia 163
65. A primary parasitic cone, with a secondary one at its
base--Ischia 164
66. Scoria-cone near Auckland, New Zealand, with a lava-current
flowing from it. (_After Heaphy_) 165
67. Section of rocks below the ancient triassic volcano of
Predazzo in the Tyrol 165
68. Cotopaxi, as seen from a distance of ninety miles. (_After
Humboldt_) 168
69. Citlaltepetl, or the Pic d'Orizaba, in Mexico, as seen from
the Forest of Xalapa. (_After Humboldt_) 169
70. Lac Paven, in the Auvergne. (_After Scrope_) 171
71. The crater-lake called Lago del Bagno, in Ischia, converted
into a harbour 172
72. Lake of Gustavila, in Mexico. (_After Humboldt_) 172
73. Peak of Teneriffe, surrounded by great crater-rings. (_After
Piazzi-Smyth_) 175
74. The volcano of Bourbon, rising in the midst of a crater-ring
four miles in diameter. (_After Bory de St. Vincent_) 176
75. The volcano of Bourbon, as seen from another point of
view, with three concentric crater-rings encircling its
base. (_After Bory de St. Vincent_) 176
76. Vesuvius as seen from Sorrento, half encircled by the
crater-ring of Somma 177
77. Outlines of various volcanoes illustrating the different
relations of the craters to cones _to face p._ 178
78. Island thrown up In the Mediterranean Sea in July and
August, 1831. (_After the Prince de Joinville_) 179
79. Sinter-cones surrounding the orifices of geysers 183
80. Diagram illustrating the mode of formation of travertine-
and sinter-terraces on the sides of a hill of tuff 185
81. Map of the volcanic group of the Lipari Islands, illustrating
the position of the lines of fissure upon which
the volcanoes have been built up 192
82. The Puy de Pariou, in the Auvergne, illustrating the shifting
of eruption along a line of fissures 193
83. Ideal section of the Puy de Pariou 194
84. Fissure formed on the flanks of Etna during the emotion
of 1865. (_After Silvestri_) 194
85. Plan of the Island of Vulcano, based on the map of the
Italian Government 196
86. Vulcanello, with its three craters 197
87. Section of basalt from Ovifak, Greenland, with particles of
metallic iron diffused through its mass 319
88. Diagram illustrating the relations between the terrestrial
and the extra-terrestrial rocks _to face p._ 322
89. A group of sun-spots. (_After Secchi_) 362
90. A sun-spot, showing the great masses of incandescent
vapour rising or falling within it. (_After Secchi_) 363
91. The edge of a sun-spot, showing a portion of the prominent
masses of incandescent gas (A) which detached itself
at B and floated into the midst of the cavity.
(_After Norman Lockyer_) 363
92. Drawing of a solar prominence made by Mr. Norman
Lockyer, March 14, 1869, at 11 h. 5 m. A.M. 364
93. The same object, as seen at 11 h. 15 m. on the same day.
(_After Norman Lockyer_) 365
94. Drawings of a solar prominence at four different periods
on September 7, 1871. (_After Young_) 366
95. A group of Lunar craters (Maurolycus, Barocius, &c.), the
largest being more than sixty miles in diameter 368
VOLCANOES.
CHAPTER I.
INTRODUCTORY: NATURE OF THE INQUIRY.
'What is a volcano?' This is a familiar question, often addressed to us
in our youth, which 'Catechisms of Universal Knowledge,' and similar
school manuals, have taught us to reply to in some such terms as the
following: 'A volcano is a burning mountain, from the summit of which
issue smoke and flames.' Such a statement as this, it is probable, does
not unfairly represent the ideas which are, even at the present day,
popularly entertained upon the subject.
But in this, as in so many other cases, our first step towards the
acquirement of scientific or exact knowledge, must be the unlearning of
what we have before been led to regard as true. The description which
we have quoted is not merely incomplete and inadequate as a whole,
but each individual proposition of which it is made up is grossly
inaccurate, and, what is worse, perversely misleading. In the first
place, the action which takes place at volcanoes is not 'burning,' or
combustion, and bears, indeed, no relation whatever to that well-known
process. Nor are volcanoes necessarily 'mountains' at all; essentially,
they are just the reverse--namely, holes in the earth's crust, or
outer portion, by means of which a communication is kept up between
the surface and the interior of our globe. When mountains do exist at
centres of volcanic activity, they are simply the heaps of materials
thrown out of these holes, and must therefore be regarded not as the
causes but as the consequences of the volcanic action. Neither does
this action always take place at the 'summits' of volcanic mountains,
when such exist, for eruptions occur quite as frequently on their sides
or at their base. That, too, which popular fancy regards as 'smoke'
is really condensing steam or watery vapour, and the supposed raging
'flames' are nothing more than the glowing light of a mass of molten
material reflected from these vapour clouds.
It is not difficult to understand how these false notions on the
subject of volcanic action have come to be so generally prevalent.
In the earlier stages of its development, the human mind is much
more congenially employed in drinking in that which is marvellous
than in searching for that which is true. It must be admitted, too,
that the grand and striking phenomena displayed by volcanoes are
especially calculated to inspire terror and to excite superstition,
and such feelings most operate in preventing those close and accurate
observations which alone can form the basis of scientific reasoning.
[Sidenote: IDEAS OF THE ANCIENTS.]
The ancients were acquainted only with the four or five active
volcanoes in the Mediterranean area; the term 'volcano' being the
name of one of these (Vulcano, or Volcano, in the Lipari Islands),
which has come to be applied to all similar phenomena. It is only in
comparatively modern times that it has become a known £act that many
hundreds of volcanoes exist upon the globe, and are scattered over
almost every part of its surface. Classical mythology appropriated
Vulcano as the forge of Hephæstus, and his Roman representative Vulcan,
while Etna was regarded as formed by the mountains under which a
vengeful deity had buried the rebellious Typhon; it may be imagined,
therefore, that any endeavour to more closely investigate the phenomena
displayed at these localities would be regarded, not simply as an act
of temerity, but as one of actual impiety. In mediæval times similar
feelings would operate with not less force in the same direction, for
the popular belief identified the subterranean fires with a place of
everlasting torment; Vulcano was regarded as the place of punishment
of the Arian Emperor Theodosius, while Etna was assigned to poor Anne
Boleyn, the perverter of faith in the person of its stoutest defender.
That such feelings of superstitious terror in connection with volcanoes
are, even at the present day, far from being extinct, will be attested
by every traveller who, in carrying on investigations about volcanic
centres, has had to avail himself of the assistance of guides and
attendants from among the common people.
Among the great writers of antiquity we find several who had so far
emancipated their minds from the popular superstitions as to be able
to enunciate just and rational views upon the subject of volcanoes.
Until quite recent times, however, their teaching was quite forgotten
or neglected, and the modern science of Vulcanology may be said to have
entirely grown up within the last one hundred years.
The great pioneer in this important branch of research was the
illustrious Italian naturalist Spallanzani, who, in the year 1788,
visited the several volcanoes of his native land, and published an
account of the numerous valuable and original observations which he
had made upon them. About the same time the French geologist Dolomieu
showed how much light might be thrown on the nature of volcanic action
by a study of the various materials which are ejected from volcanic
vents; while our own countryman. Sir William Hamilton, was engaged in
a systematic study of the changes in form of volcanic mountains, and
of the causes which determine their growth. At a somewhat later date
the three German naturalists. Von Buch, Humboldt, and Abich, greatly
extended our knowledge of volcanoes by their travels in different
portions of the globe.
[Sidenote: CHARACTER OF MODERN RESEARCHES.]
The first attempt, however, to frame a satisfactory theory of volcanic
action, and to show the part which volcanoes have played in the past
history of our globe, together with their place in its present economy,
was made in 1825, by Poulett Scrope, whose great work, 'Considerations
on Volcanoes,' may be regarded as the earliest systematic treatise on
Vulcanology. Since the publication of this work, many new lines of
inquiry have been opened up in connection with the subject, and fresh
methods of research have been devised and applied to it. More exact
observations of travellers over wider areas have greatly multiplied
the facts upon which we may reason and speculate, and many erroneous
hypotheses which had grown up in connection with the subject have been
removed by patient and critical inquiry.
We propose in the following pages to give an outline of the present
state of knowledge upon the subject, and to indicate the bearings
of those conclusions which have already been arrived at, upon the
great questions of the history of our globe and the relations which
it bears to the other portions of the universe. In attempting this
task we cannot do better than take up the several lines of inquiry
in the order in which they have been seized upon and worked out by
the original investigators; for never, perhaps, is the development
of thought in the individual mind so natural in its methods, and so
permanent in its effects, as when it obeys those laws which determined
its growth in the collective mind of the race. In our minds, as in our
bodies, development in the individual is an epitome, or microcosmic
reproduction, of evolution in the species.
CHAPTER II.
THE NATURE OF VOLCANIC ACTION.
The dose investigation of what goes on within a volcanic vent may
appear at first sight to be a task beset with so many difficulties and
dangers that we may be tempted to abandon it as altogether hopeless. At
the first recorded eruption of Vesuvius the elder Pliny lost his life
in an attempt to approach the mountain and examine the action which
was taking place there; and during the last great outburst of the same
volcano a band of Neapolitan students, whose curiosity was greater than
their prudence, shared the same fate.
But in both these cases the inquirers paid the penalty of having
adopted a wrong method. If we wish to examine the mode of working of a
complicated steam-engine, it will be of little avail for us to watch
the machinery when the full blast of steam is turned on, and the rapid
movements of levers, pinions, and slides baffle all attempts to follow
them, and render hopeless every effort to trace their connection with
one another. But if some friendly hand turn off the greater part of
the steam-supply, then, as the rods move slowly backwards and forwards,
as the wheels make their measured revolutions, and the valves axe
seen gradually opening and shutting, we may have an opportunity of
determining the relations of the several parts of the machine to one
another, and of arriving at just conclusions concerning the plan on
which it is constructed. Nor can we doubt that the parts of the machine
bear the same relation to one another, and that their movements take
place in precisely the same order, when the supply of steam is large as
when it is small.
Now, as we shall show in the sequel, a volcano is a kind of great
natural steam-engine, and our best method of investigating its action
is to watch it when a part of the steam-supply is cut off. It is
true that we cannot at will control the source of supply of steam to
a volcano, as we can in a steam-engine, but as some volcanoes have
usually only a small steam-supply, and nearly all volcanoes vary
greatly in the intensity of their action at different periods, we can,
by a careful selection of the object or the time of our study, gain all
those advantages which would be obtained by regulating its action for
ourselves.
Spallanzani appears to have been the first to perceive the important
fact, that the nature of volcanic action remains the same, however
its intensity may vary. Taking advantage of the circumstance that
there exists in the Mediterranean Sea a volcano--Stromboli--which
for at least 2,000 years has been in a constant and regular, but not
in a violent or dangerous, state of activity, he visited the spot,
and made the series of careful observations which laid the foundation
of our knowledge of the 'physiology of volcanoes.' Since the time
of Spallanzani, many other investigators have visited the crater
of Stromboli, and they have been able to confirm and extend the
observations of the great Italian naturalist, as to the character of
the action which is constantly taking place within it. We cannot better
illustrate the nature of volcanic action than by describing what has
been witnessed by numerous observers within the crater of Stromboli,
where it is possible to watch the series of operations going on by the
hour together, and to do so without having our judgment warped either
by an excited imagination or the sense of danger.
[Sidenote: APPEARANCE OF STROMBOLI FROM A DISTANCE.]
In the sketch, fig. 1, which was made on April 20, 1874, I have shown
the appearance which this interesting volcano usually presents, when
viewed from a distance. The island is of rudely circular outline, and
conical form, and rises to the height of 3,090 feet above the level of
the Mediterranean. From a point on the side of the mountain, masses
of vapour are seen to issue, and these unite to form a cloud over
the mountain, the outline of this vapour-cloud varying continually
according to the hygrometric state of the atmosphere, and the direction
and force of the wind. At the time when this sketch was made, the
vapour-cloud was spread in a great horizontal stratum overshadowing
the whole island, but it was clearly seen to be made up of a number
of globular masses, each of which, as we shall hereafter see, is the
product of a distinct outburst of the volcanic forces.
Viewed at night-time, Stromboli presents a far more striking and
singular spectacle. The mountain, with its vapour canopy, is visible
over an area having a radius of more than 100 miles. When watched from
the deck of a vessel anywhere within this area, a glow of red light is
seen to make its appearance from time to time above the summit of the
mountain; this glow of light may be observed to increase gradually in
intensity, and then as gradually to die away. After a short interval
the same appearances are repeated, and this goes on till the increasing
light of the dawn causes the phenomenon to be no longer visible. The
resemblance presented by Stromboli to a 'flashing light' on a most
gigantic scale is very striking, and the mountain has long been known
as 'the lighthouse of the Mediterranean.'
It must be pointed out, however, that in two very important particulars
the appearances presented by Stromboli differ markedly from those
rhythmical gleams exhibited by the 'flashing-lights' of our coasts.
In the first place, the intervals between successive flashes are very
unequal, varying from less than one minute to twenty minutes, or even
more; and in the second place, the duration and intensity of the red
glow above the mountain are subject to like variation, being sometimes
a momentary scarcely visible gleam, and at others a vivid burst of
light which illuminates the sky to a considerable distance round.
[Illustration: Fig. 1.--Stromboli, viewed from the North-west, April
1874.]
[Illustration: Fig. 2.--Map of tub Island of Stromboli. (Scale about
two inches to a mile.)]
[Sidenote: GENERAL FEATURES OF THE MOUNTAIN.]
Let us now draw near and examine this wonderful phenomenon of a
mountain which seemingly ever burns with fire, and yet is not consumed.
The general form of the Island of Stromboli will be gathered from an
inspection of the plan, fig. 2, which is copied from a map published by
the Italian Government. When we land upon the island, we find that it
is entirely built up of such materials as we know to be ejected from
volcanoes; indeed, it resembles on a gigantic scale the surroundings
of an iron furnace, with its heaps of cinders and masses of slag. The
irregularity in the form of the island is at once seen to be due to the
action of the wind, the rain, and the waves of the surrounding sea,
which have removed the loose, cindery materials at some points, and
left the hard, slaggy masses standing up prominently at others.
This great heap of cindery and slaggy materials rises, as we have said,
to a height of more than 3,000 feet above the sea-level, but even this
measurement does not give a just idea of its vast bulk. Soundings in
the sea surrounding the island show that the bottom gradually shelves
around the shores to the depth of nearly 600 fathoms, so that Stromboli
is a great conical mass of cinders and slaggy materials, having a
height of over 6,000 feet, and a base whose diameter exceeds four miles.
The general form and proportions of this mass will be better understood
by an examination of the section, fig. 3, which is also constructed
from the materials furnished by the map of the island issued by the
Italian Government. The same section, and the map, fig. 2, will serve
to make clear the position and relations of the point on the mountain
at which the volcanic activity takes place. At a spot on the north-west
slope of the mountain, about 1,000 feet below its summit, and 2,000
feet above the level of the sea, there exists a circular depression,
the present active 'crater' of the volcano; and leading down from this
to the sea there is a flat slope making an angle of about 35° with the
horizon, and known as the 'Sciarra.' The Sciarra is bounded by steep
cliffs, as shown in the sketch fig. 1, and the plan fig. 2.
[Illustration: Fig. 3.--Section through the Island of Stromboli from
n.w. to s.e.
_a._ Highest summit of the mountain, _c._ Cratère del Fossa, _b._ Point
overlooking the crater, _d._ Steep slope known as the Sciarra del
Fuoco. _e._ Continuation of the same slope beneath the level of the
sea. _f._ Steep cliffs of the Punta dell' Omo.]
[Sidenote: FORM AND FUNCTION OF THE CRATER.]
If we climb up to this scene of volcanic activity, we shall be able to
watch narrowly the operations which are going on there. On the morning
of the 24th of April, 1874, I paid a visit to this interesting spot in
order to get a near view of what was taking place. On reaching a point
upon the side of the Sciarra, from which the crater was in full view
before me, I witnessed, and made a sketch of, an outburst which then
took place, and this sketch has been reproduced in fig. 4. Before the
outburst, numerous light curling wreaths of vapour were seen ascending
from fissures on the sides and bottom of the crater. Suddenly, and
without the slightest warning, a sound was heard like that produced
when a locomotive blows off its steam at a railway-station; a great
volume of watery vapour was at the same time thrown violently into
the atmosphere, and with it there were hurled upwards a number of
dark fragments, which rose to the height of 400 or 500 feet above the
crater, describing curves in their course, and then falling back upon
the mountain. Most of these fragments tumbled into the crater with a
loud, rattling noise, but some of them fell outside the crater, and a
few rolled down the steep slope of the Sciarra into the sea. Some of
these falling fragments were found to be still hot and glowing, and in
a semi-molten condition, so that they readily received the impression
of a coin thrust into them.
[Illustration: Fig. 4.--The Crates of Stromboli as viewed from the side
of the Sciarra during an eruption on the morning of April 24, 1874.]
[Sidenote: APERTURES AT THE BOTTOM OF THE CRATER.]
But on the upper side of the crater, at the point marked 6, on the
section fig. 3, there exists a spot from which we can look down upon
the bottom of the crater, and view the operations taking place there.
This is the place where Spallanzani and other later investigators have
carried on their observations, and, when the wind is blowing from
the spectator towards the crater, he may sit for hours watching the
wonderful scene displayed before him. The black slaggy bottom of the
crater is seen to be traversed by many fissures or cracks, from most
of which curling jets of vapour issue quietly, and gradually mingle
with and disappear in the atmosphere. But besides these smaller cracks
at the bottom of the crater, several larger openings are seen, which
vary in number and position at different periods; sometimes only one of
these apertures is visible, at others as many as six or seven, and the
phenomena presented at these larger apertures are especially worthy of
careful investigation.
These larger apertures, if we study the nature of the action taking
place at them, may be divided into three classes. From those of the
first class, steam is emitted with loud, snorting puffs, like those
produced by a locomotive-engine, but far less regular and rhythmical
in their succession. In the second class of apertures masses of molten
material are seen welling out, and, if the position of the aperture
be favourable, flowing outside the crater; from this liquid molten
mass steam is seen to escape, sometimes in considerable quantities.
The openings of the third class present still more interesting
appearances. Within the walls of the aperture a viscid or semi-liquid
substance is seen slowly heaving up and down. As we watch the seething
mass the agitation within it is observed to increase gradually, and at
last a gigantic bubble is formed which violently bursts, when a great
rush of steam takes place, carrying fragments of the scum-like surface
of the liquid high into the atmosphere.
If we visit the crater by night, the appearances presented are found to
be still more striking and suggestive. The smaller cracks and larger
openings glow with a ruddy light. The liquid matter is seen to be red-
or even white-hot, while the scum or crust which forms upon it is of a
dull red colour. Every time a bubble bursts and the crust is broken up
by the escape of steam, a fresh, glowing surface of the incandescent
material is exposed. If at these moments we look up at the vapour-cloud
covering the mountain, we shall at once understand the cause of the
singular appearances presented by Stromboli when viewed from a distance
at night, for the great masses of vapour are seen to be lit up with a
vivid, ruddy glow, like that produced when an engine-driver opens the
door of the furnace and illuminates the stream of vapour issuing from
the funnel of his locomotive.
Let us now endeavour to analyse the phenomena so admirably displayed
before us in the crater of Stromboli. The three essential conditions
on which the production of these phenomena seems to depend are the
following: first, the existence of certain apertures or cracks
communicating between the interior and the surface of the earth;
secondly, the presence of matter in a highly heated condition beneath
the surface; and thirdly, the existence of great quantities of water
imprisoned in the subterranean regions--which water, escaping as steam,
gives rise to all those active phenomena we have been describing.
[Sidenote: CAUSE OF THE GLOWING LIGHT.]
We have said, at the outset, that there exists no analogy whatever
between the action which takes place in volcanoes and the operation of
burning or combustion. Occasionally, it is true, certain inflammable
substances are formed by the action going on within the volcano, and
these inflammable substances, taking fire, produce real flames. Such
flames are, however, in almost all cases only feebly luminous, and do
not give rise to any conspicuous appearances. What is usually taken for
flame during volcanic eruptions is simply, as we have already pointed
out, the glowing red-hot surface of a mass of molten rock, reflected
from a vapour-cloud hanging over it. The red glow observed over a
volcano in eruption is indeed precisely similar in its nature and
origin to that which is seen above London during a night of heavy fog,
and which is produced by the reflection of the gas-lights of the city
from the innumerable particles of water-vapour diffused through the
atmosphere. Fires, of course, occur when the molten and incandescent
materials poured out from a volcano come in contact with inflammable
substances, such as forests and houses, but in these cases the
combustion is quite a secondary phenomenon.
There is another popular delusion concerning volcanic action, which it
may be necessary to refer to and to combat. From the well-known fact
that sulphur or brimstone is found abundantly in volcanic regions, the
popular belief has arisen that this highly inflammable substance has
something to do with the production of the eruptions of volcanoes. In
school-books which were, until comparatively recent years, in constant
use in this country, the statement may be found that by burying certain
quantities of sulphur, iron-pyrites, and charcoal in a hole in the
ground, we may form a miniature volcano, and produce all the essential
phenomena of a volcanic eruption. No greater mistake could possibly be
made. The chemical reactions which take place when sulphur and other
substances are made to act upon each other differ entirely from the
phenomena of volcanic action. The sulphur which is found in volcanic
regions is the result and not the cause of volcanic action. Among the
most common substances emitted from volcanic vents along with the steam
are the two gases, sulphurous acid and sulphuretted hydrogen. When
these two gases come into contact with one another, chemical action
takes place, and the elements contained in them--oxygen, hydrogen,
and sulphur--are free to group themselves together in an entirely new
fashion; the consequence of this is that water and sulphuric acid (oil
of vitriol) are formed, and a certain quantity of sulphur is set free.
The water escapes into the atmosphere, the sulphuric acid combines with
lime, iron, or other substances contained in the surrounding rocks,
and the sulphur builds up crystals in any cavities which may happen to
exist in these rocks.
[Sidenote: VOLCANIC ACTION RESEMBLES BOILING.]
If, however, careful and exact observations, like those carried on
at Stromboli, compel us to reject the popular notions concerning
the supposed resemblance between volcanic action and the combustion
of sulphur or other substances, they nevertheless suggest analogies
with certain other simple and well-known operations. And in pursuing
these analogies, we are led to the recognition of some admirable
illustrations both of the attendant phenomena and of the true cause of
volcanic outbursts.
No one can look down on the mass of seething material in violent
agitation within the fissures at the bottom of the crater of Stromboli,
without being forcibly reminded of the appearances presented by liquids
in a state of boiling or ebullition. The glowing material seems to be
agitated by two kinds of movements, the one whirling or rotatory, the
other vertical or up-and-down in its direction. The fluid mass in this
way appears to be gradually impelled upwards, till it approaches the
lips of the aperture, when vast bubbles are formed upon its surface,
and to the sudden bursting of these the phenomena of the eruption are
due.
Now if we take a tall narrow vessel and fill it with porridge or
some similar substance of imperfect fluidity, we shall be able, by
placing it over a fire, to imitate very closely indeed the appearances
presented in the crater of Stromboli. As the temperature of the mass
rises, steam is generated within it, and in the efforts of this steam
to escape, the substance is set in violent movement. These movements of
the mass are partly rotatory and partly vertical in their direction; as
fresh steam is generated in the mass its surface is gradually raised,
while an escape of the steam is immediately followed by a fall of the
surface. Thus an up-and-down movement of the liquid is maintained, but
as the generation of steam goes on faster than it can escape through
the viscid mass, there is a constant tendency in the latter to rise
towards the mouth of the vessel. At last, as we know, if heat continues
to be applied to the vessel, the fluid contents will be forced up to
its edge and a catastrophe will occur; the steam being suddenly and
violently liberated from the bubbles formed on the surface of the mass,
and a considerable quantity of the material forcibly expelled from
the vessel. The suddenness and violence of this catastrophe is easily
accounted for, if we bear in mind that the escaping steam acts after
the manner of a compressed spring which is suddenly released. Steam
is first formed at the bottom of the vessel which is in contact with
the fire; but here it is under the pressure of the whole mass of the
liquid, and moreover, the viscidity of the substance tends to retard
the union of the steam bubbles and their rise to the surface of the
mass. But when the pressure is relieved by the bursting of bubbles at
the surface, the whole of the generated steam tends to escape suddenly.
[Sidenote: ESCAPE OF STEAM-BUBBLES FROM LAVA.]
Now within the crater of Stromboli we have precisely the necessary
conditions for the display of the same series of operations. In the
apertures at the bottom there exists a quantity of imperfectly fluid
materials at a higher temperature, containing water entangled in its
mass. As this water passes into the state of steam it tends to escape,
and in so doing puts the whole mass into violent movement. When the
steam rises to the surface, bubbles are formed, and the formation of
these bubbles is promoted by the circumstance that the liquid mass,
where exposed to the atmosphere, becomes chilled, and thereby rendered
less perfectly fluid. By the bursting of these bubbles the pressure is
partially relieved, and a violent escape of the pent-up steam takes
place through the whole mass. Equilibrium being thus restored, there
follows a longer or shorter interval of quiescence, during which steam
is being generated and collected within the mass, and the series of
operations which we have described then recommences.
There is one other consideration which must be borne in mind in
connection with this subject. It is well known that if water be
subjected to sufficiently great pressure it may be raised to a very
high temperature and still retain its liquid condition. When this
pressure is removed, however, the whole mass passes at once into the
condition of steam or water-gas; and the gas thus formed at high
temperatures has a proportionably high tension. In a Papin's digester
water confined in a strong vessel is raised to temperatures far above
its ordinary boiling-point, and from any opening in such a vessel the
steam escapes with prodigious violence. Now, at considerable depths
beneath the earth's surface, and under the pressure of many hundreds
or thousands of feet of solid rock, water still retaining its liquid
condition may become intensely heated. When the pressure is relieved by
the formation of a crack or fissure in the superincumbent mass of rock,
the escape of the superheated steam will be of very violent character,
and may be attended with the most striking and destructive results. In
the existence of high temperatures beneath the earth's surface, and the
presence in the same regions of imprisoned water capable of passing
into the highly elastic gas which we call steam, we have a cause fully
competent to produce all the phenomena which we have described as
occurring at Stromboli.
It may at first sight appear that the grand and terrible displays
of violence witnessed during a great volcanic eruption differ
fundamentally in their character and their origin from those feeble
outbursts which we are able to examine closely and analyse rigorously
at Stromboli. But that such is not the case a few simple considerations
will soon convince us.
[Sidenote: STROMBOLI COMPARED WITH VESUVIUS.]
Although Stromboli usually displays the subdued and moderate activity
which we have been describing, yet the intensity of the action going
on within it is subject to considerable variation. Occasionally the
violence of the outbursts is greatly increased--the roaring of the
steam-jets may be heard for many miles around, considerable streams of
incandescent liquefied rock flow down the Sciarra into the sea, and the
explosions in the crater are far more frequent and energetic, cinders
and fragments of rock being scattered all over the island and the
surrounding seas.
On the other hand, volcanoes like Vesuvius, which are sometimes the
scene of eruptions on the very grandest scale, at others subside into a
temporary state of moderate activity quite similar in character to that
which is the normal condition of Stromboli. Thus, shortly before the
great eruption of Vesuvius in April 1872, a small cone was formed near
the edge of the crater, and during some months observers could watch,
in ease and safety, a series of small explosions taking place, quite
similar in their character and attendant phenomena to those which we
have described as occurring at Stromboli. French geologists are in the
habit of defining the condition of activity in a volcano by speaking
of the more quiet and, regular state as the 'Strombolian stage,' and
the more violent and paroxysmal as the 'Vesuvian stage'; but the two
conditions are, as we have seen, presented by the same volcano at
different periods, and pass into one another by the most insensible
gradations.
We must now proceed to compare the grand and terrible appearances
presented during a great eruption with those more feeble displays which
we have been describing, to show that in all their essential features
these different kinds of outbursts are identical with one another, and
must be referred to the action of similar causes.
The volcanic eruption which has been most carefully studied in recent
times is that which we have already referred to as occurring at
Vesuvius, in the month of April 1872. With the exception, perhaps, of
that which took place in October 1822, this eruption was the grandest
which has broken out at Vesuvius during the present century. Owing to
the circumstance of its proximity to the great city of Naples, Vesuvius
has always been the most carefully watched of all volcanoes, and in
recent years the erection of an observatory, provided with instruments
for recording the smallest subterranean tremors affecting the mountain,
has facilitated the carrying on of those continuous and minute
observations which are so necessary for exact scientific inquiry.
[Illustration: Fig 5. Vesuvius in Eruption, as seen from Naples, April
26, 1872. (_From a photograph_)]
[Sidenote: VESUVIUS ERUPTION OF 1872.]
On the occasion of this outburst, the aid of instantaneous photography
was first made available for obtaining a permanent record of the
appearances displayed at volcanic eruptions. In fig. 5 we have one of
these photographs, which was taken at 5 o'clock P.M. on April 26, 1872,
transferred to a wood-block and engraved. In examining it we feel sure
that we are not being misled by any exaggeration or error on the part
of the artist. Vesuvius rises to the height of nearly 4,000 feet above
the level of the sea, and an inspection of the photograph proves that
the vapours and rock-fragments were thrown to the enormous height of
20,000 feet, or nearly four miles, into the atmosphere.
The main features of this terrifying outburst were as follows. For
more than a twelvemonth before, the activity of the forces at work
within the mountain appeared to be gradually increasing, and the great
eruption commenced on April 24, attained its climax on the 26th, and
began to die out on the following day. During the eruption the bottom
of the crater was entirely broken up, and the sides of the mountain
were rent by fissures in all directions. So numerous were these
fissures and cracks that liquid matter appeared to be oozing from every
part of its surface, and, as Professor Palmieri, who witnessed the
outburst from the observatory, expressed it, 'Vesuvius sweated fire.'
One of the fissures was of enormous size, extending from the summit to
far beyond the base of the cone; the scar left by this gigantic rent
being plainly visible at the present day.
From the great opening or crater at the summit, and from some of the
fissures on the sides of the mountain, enormous volumes of steam rushed
out with a prodigious roaring sound, the noise being so terrific that
the inhabitants of Naples, five miles off, fled from their houses
and spent the night in the open streets. Although this roaring sound
appeared at a distance to be continuous, yet those upon the mountain
could perceive that it was produced by detonations or explosions
rapidly following one another. Each of these explosions was accompanied
by the formation of a great globe of white vapour, which, rising into
the atmosphere, swelled the bulk of the vast cloud overhanging the
mountain. An inspection of the photographs (see fig. 5) shows that the
great vapour-cloud over Vesuvius was made up of the globular masses
ejected at successive explosions. Each of these explosive upward
rushes of steam carried along with it a considerable quantity of solid
fragments, and these fell in great numbers all over the surface of
the mountain, breaking the windows of the observatory, and making it
dangerous to be out of doors.
We have said that lava, or molten rock, appeared to be issuing from
the very numerous cracks formed all over the flanks of the mountain.
But at three points this molten rock issued in such quantities as to
form great, fiery floods, which rushed down the sides of the mountain,
and flowed to a considerable distance beyond its base. The largest
of these lava-floods overwhelmed and destroyed the two villages of
Massa di Somma and San Sebastiano, besides many country houses in the
neighbourhood.
[Sidenote: STEAM EMITTED FROM LAVA-CURRENT.]
A very marked and interesting feature exhibited by these three
lava-floods was the quantity of watery vapour which they gave off
during their flow. All along their course, enormous volumes of steam
were evolved from them, as will be seen by an inspection of the
photograph. Indeed, such was the abundance and tension of the steam
thus escaping from the surfaces of the lava-currents that it forced
the congealing rock up into great bubbles and blisters, and gave rise
to the formation of innumerable miniature volcanoes, varying in size
from a beehive to a cottage, some of which remained in a state of
independent activity for a considerable time.
So far, what we have described as taking place at Vesuvius, in April
1872, has been only the repetition on a £Eur grander scale of the
three kinds of action which we have shown to be constantly taking
place at Stromboli; namely, the formation of cracks or fissures in the
earth's surface, the escape of steam with explosive violence from these
openings, often propelling rock-fragments into the atmosphere, and the
outwelling, under the influence of this compressed steam, of masses of
molten materials.
There were some other appearances presented at the great outburst at
Vesuvius, which do not seem at first sight to find any analogies in
the manifestations of the more feeble action continually going on at
Stromboli.
Before and during the great outbreak of April 1872, Vesuvius itself
and the whole country round were visited with earthquake-shocks, or
tremblings of the ground. The sensitive instruments in the Vesuvian
Observatory showed the mountain daring the eruption to be in a constant
state of tremor. These earthquakes are not, as is commonly supposed,
actual upheavings of the earth's surface, but are vibrations propagated
through the solid materials of which the earth is built up. We cannot
stamp our feet upon the ground without giving rise to such vibrations,
though our senses may not be sufficiently acute to perceive them.
The explosive escape of steam from a crack is a cause sufficiently
powerful to produce a shock which is propagated and may be felt for
a considerable distance round. Even on Stromboli an observer at the
edge of the crater may notice that each explosive outburst of steam
is accompanied by a perceptible tremor of the ground, and in the case
of Vesuvius the violent shocks produced by the escape of far larger
volumes of steam give rise to proportionately stronger vibrations. The
nature and origin of those far more terrible and destructive shocks
which sometimes accompany, and more frequently precede, great volcanic
eruptions, we shall consider in the sequel.
[Sidenote: CAUSE OF LIGHTNING DURING ERUPTIONS.]
Another striking phenomenon which was exhibited in the great eruption
of Vesuvius in 1872 was the vivid display of lightning accompanied
by thunder. The uprushing current of steam and rock-fragments forms
a vertical column, but as the steam condenses it spreads out into a
great horizontal cloud which is seen to be made up of the great globes
of vapour emitted at successive explosions. When there is little or
no wind the vertical column with a horizontal cloud above it bears a
striking resemblance to the stone-pine trees which form so conspicuous
a feature in every Neapolitan landscape. Around this column of vapour
the most vivid lightning constantly plays and adds not a little to the
grand and awful character of the spectacle of a volcanic eruption,
especially when it is viewed by night.
In the eruption of 1872 a strong wind blowing from the north-west
destroyed the usual regular appearance of this 'pine-tree appendage' to
the mountain, which is so well known to, and dreaded by the inhabitants
of Naples; the cloud, as will be seen from the photograph (fig. 5,
_facing_ p. 24), was blown on one side, and most of the falling
fragments took the same direction.
It is well known that when high-pressure steam is allowed to escape
through an orifice, electricity is abundantly generated by the
friction, and Sir William Armstrong's hydro-electric machine is
constructed on this principle. Every volcano in violent eruption is
a very efficient hydro-electric machine, and the uprushing column
is in a condition of intense electrical excitation. This result is
probably aided by the friction of the solid particles as they are
propelled upwards and fall back into the crater. The restoration of
the condition of electrical stability between this column and the
surrounding atmosphere is attended with the production of frequent
lightning-flashes and thunder-claps, the found of the latter being
usually, however, drowned in the still louder roar of the uprushing
steam-column.
The discharge of Buch large quantities of steam into the atmosphere
soon causes the latter to be saturated with watery vapour, and there
follows an excessive rainfall; long-continued rain and floods were an
accompaniment of the great Vesuvian outbreak of 1872, as they have
been of almost all great volcanic eruptions. The Italians, indeed,
dread the floods which follow an eruption more than the fiery streams
of lava which accompany it--for they have found the mud-streams (_lave
di fango_), formed by rain-water sweeping along the loose volcanic
materials, to be more widely destructive in their effects than the
currents of molten rock (_lave di fuoco_).
Besides the phenomena which we have now described as accompanying a
great volcanic outburst, many others have undoubtedly been recorded
by apparently trustworthy authorities. But, in dealing with the
descriptions of these grand and terrible events, we must always be on
our guard against accepting as literal facts, the statements made by
witnesses, often writing at some distance from the scene of action, and
almost always under the influence of violent excitement and terror.
The desire to administer to the universal love of the marvellous, and
the tendency to exaggeration, will usually account for many of the
wonderful statements contained in such records; and, even where the
witness is accurately relating events which he thinks passed before his
eyes, we must remember that it is probable he may have had neither the
opportunity nor the capacity for exact observation.
The more carefully we sift the accounts which have been preserved of
great volcanic outbursts, the more are we struck by the fact that the
appearances described can be resolved into a few simple operations, the
true character of which has been distorted or disguised by the want of
accurate observation on the part of the witnesses.
[Sidenote: SIMILARITY OF FEEBLE AND VIOLENT ERUPTIONS.]
We are thus led to the conclusion that the grand and terrible
appearances displayed at Vesuvius and other volcanoes in a state of
violent eruption do not differ in any essential respect from the
phenomena which we have witnessed accompanying the miniature outbursts
of Stromboli. And we are convinced, by the same considerations, that
the forces which give rise to the feeble displays in the latter case
would produce, if acting with greater intensity and violence, all the
magnificent spectacles presented in the former.
In Vesuvius and Stromboli alike, the active cause of all the phenomena
exhibited is found to be the escape of steam from the midst of
masses of incandescent liquefied rock. The violence, and therefore
the grandeur and destructive effects of an eruption, depend upon the
abundance and tension of this escaping steam.
There is one respect in which volcanic phenomena are especially
calculated to excite the fear and wonder of beholders--namely, in the
sudden and apparently spontaneous character of their manifestations.
Eclipses were regarded as equally portentous with volcanic eruptions
till astronomers learned not only to explain the causes which gave
rise to them, but even to predict to the second the times of their
occurrence. If we were able in like manner to warn the inhabitants of
volcanic regions of the approach of a grand eruption, the fear and
superstition with which these events are now regarded would doubtless
be in great part dispelled. The power of prediction is alike the
crucial test and the crowning triumph of a scientific theory.
But, although natural philosophers are able to assign the causes to
which the grand operations of volcanoes are due, and also to explain
all the varied appearances which accompany them, they have not as yet
so far mastered the laws which govern volcanic action as to be able to
predict the periods of their manifestation.
That these operations, like all others going on upon the globe, are
governed by great natural laws we cannot for a moment doubt. And that,
in all probability, more careful and exact observation and reasoning
will at some future time lead us to the recognition of these laws,
every student of nature is sanguine. But at the present time, it must
be confessed, we are very far indeed from being able to afford that
crowning proof of the truth of our theories of volcanic action which is
implied in the power of predicting the period and degree of intensity
of their manifestations.
[Sidenote: ERUPTIONS AND THE INTERVALS BETWEEN THEM.]
There are, however, some observations which lead us to hope that the
time may not be far distant when we shall have so £Eur obtained a
knowledge of the conditions on which volcanic action depends as to be
able to form some judgment as to its manifestations in the future at
any particular locality. But we must recollect that these conditions
axe very numerous and complicated, and that some of them may lie almost
entirely outside our sphere of observation; hence hasty attempts in
this direction, such as have recently been made, are to be deprecated
by every true lover of science.
Concerning the eruptions that have taken place at those volcanic
centres which have been known from a remote antiquity, we have records
from which we can determine the intervals separating these outbursts
and their relative violence. A critical examination of these records
leads to the following conclusions:--
(1.) A long period of quiescence is generally followed by an eruption
which is either of long duration or of great violence.
(2.) A long-continued, or very violent eruption is usually followed by
a prolonged period of repose.
(3.) Feeble and short eruptions usually succeed one another at brief
intervals.
(4.) As a general rule, the violence of a great eruption is inversely
proportional to its duration.
It will be seen that these general conclusions are in perfect harmony
with the theory that volcanic outbursts are due to the accumulation
of steam at volcanic centres, and that the tension of this imprisoned
gas eventually overcomes the repressing forces which tend to prevent
its manifestation. Before astronomers had learnt to determine all
the conditions on which the production of eclipses depends, they had
found that these phenomena succeed one another at regular intervals.
The discovery of such astronomical cycles was a great advance in our
knowledge of the heavenly bodies, and in the same way the determination
of these general relations between the intensity and duration of
volcanic outbursts and the intervals of time which separate them may
be regarded as the first step towards the discovery of the laws which
govern volcanic activity.
In the actual determination of the conditions upon which the occurrence
of volcanic eruptions depends, it must be confessed, however, that
very little has as yet been done. This is in part due to the fact that
some at least of these conditions lie beyond the limits of direct
observation. But it must also be admitted, on the other hand, that
little has been as yet accomplished towards the careful and systematic
observation of those phenomena which may, and probably do, exert an
influence in bringing about volcanic outbursts.
[Sidenote: INFLUENCE OF ATMOSPHERIC CONDITIONS.]
In the Lipari Islands there has prevailed a belief, from the very
earliest period of history, that the feeble eruptions of Stromboli
are in some way dependent upon the condition of the atmosphere.
These islands were known to the ancients as the Æolian Isles, from
the fact that they were once ruled over by a king of the name of
Æolus. It seems not improbable that Æolus was gifted with natural
powers of observation and reasoning far in advance of those of his
contemporaries. A careful study of the vapour-cloud which covers
Stromboli would certainly afford him information concerning the
hygrometric condition of the atmosphere; the form and position
assumed by this vapour-cloud would be a no less perfect index of the
direction and force of the wind; and, if the popular belief be well
founded, the frequence and violence of the explosions taking place
from the crater would indicate the barometric pressure. From these
data an acute observer would be able to issue 'storm-warnings' and
weather-prognostics of considerable value. In the vulgar mind, the
idea of the prediction of natural events is closely bound up with that
of their production; and the shrewd weather-prophet of Lipari was
after his death raised to the rank of a god, and invested with the
sovereignty of the winds.
Whether the popular idea that the outbursts of Stromboli are regulated
by atmospheric conditions has any foundation is still open to grave
doubt. It seems to be certain, however, that during autumn and winter
the more violent paroxysms of the volcano occur, and that in summer
the action which takes place is far more regular and equable. It would
be of the greatest benefit to science if an observatory were erected
beside the crater of Stromboli, where a careful record might be kept of
all atmospheric changes, and of the synchronous manifestations of the
volcanic forces.
A little consideration will show that it is a by no means unreasonable
supposition that variations in atmospheric pressure may exercise a very
important influence in bringing about volcanic outbursts. Changes in
the barometer to the extent of two inches within a very short period
are not uncommon occurrences. A very simple calculation will show that
the fall of the mercury in the barometer to the extent of two inches
indicates the removal of a weight of two millions of tons from each
square mile of the earth's surface where this change takes place. Now,
if we suppose, as we have good ground for doing, that under volcanic
areas vast quantities of superheated water are only prevented from
flashing into steam by the superincumbent pressure, a relief of this
pressure to the extent of two millions of tons on every square mile
could scarcely fail to produce very marked effects. The way in which
explosions in fiery coal-mines generally follow closely upon sudden
falls in the atmospheric pressure is now well known; and coal-mine
explosions and volcanic outbursts have this in common, that both
result from the sudden and violent liberation of subterranean gases.
There are not a few apparently well-authenticated accounts of volcanic
and earthquake phenomena following closely on peculiar atmospheric
conditions, and the whole question of the relation of the volcanic
forces to atmospheric pressure, as Spallanzani himself so long ago
pointed out, is deserving of a most careful and rigorous investigation.
[Sidenote: SUPPOSED TIDAL EFFECTS.]
There is one other consideration which has frequently been urged as
worthy of especial attention, in dealing with the question of the
exciting causes of volcanic outbursts. If volcanoes were, as was at one
time almost universally supposed, in direct communication with a great
central mass of liquefied materials, or even if any large reservoirs
of such liquids existed beneath volcanic districts, as others have
imagined, then the different mobility of the solid and liquid portions
of the earth's mass would give rise to tidal effects similar to those
occurring in the surface waters of the globe. Under such circumstances,
volcanic outbursts, like the tides, would be determined by the relative
positions of the sun and moon to our globe. It is certain, however,
that no very direct relation has yet been established between the
lunar periods and those of volcanic outbursts, though recent close
observations upon the crater of Vesuvius, by Professor Palmieri, do
seem to lend support to the view that such relations may exist.
At the present time, therefore, it must be admitted that vulcanologists
have only just commenced those series of exact and continuous
observations which are necessary to determine the conditions that
regulate the appearance of volcanic phenomena. The study of the laws
of volcanic action is yet in its infancy. But the establishment of
observatories on Vesuvius and Etna 18 fall of promise for the future,
and when we consider the advances which have been made, during the last
one hundred years, in our knowledge of the true nature of volcanic
action, we need not despair that the extension of the same methods
of inquiry will lead to equally important results concerning the
conditions which determine and the laws which govern it.
In the meanwhile, it is no small gain to have established the fact that
volcanic phenomena, divested of all those wonderful attributes with
which superstition and the love of the marvellous have surrounded them,
are operations of nature obeying definite laws, which laws we may hope
by careful observation and accurate reasoning to determine; and that
the varied appearances, presented alike in the grandest and feeblest
outbursts, can all be referred to one simple cause--namely, the escape,
from the midst of masses of molten materials, of imprisoned steam or
water-gas.
CHAPTER III.
THE PRODUCTS OF VOLCANIC ACTION.
While Spallanzani was engaged in investigating the nature of the action
going on at Stromboli and other Italian volcanoes, his contemporary
Dolomieu was laying the foundation of another important branch of
vulcanology by studying the characters of the different materials of
which volcanoes are built up. Since the publication of Dolomieu's
admirable works on the rocks of the Lipari and Ponza Islands, science
has advanced with prodigious strides. The chemist has taught us how
to split up a rock into its constituent elements and to determine the
proportions of these to one another with mathematical precision; the
mineralogist has done much in the investigation of the characters and
mode of origin of the crystalline minerals which occur in these rocks;
and the microscopist has shown how the minute internal structure of
these rocks may be made clearly manifest. We shall proceed to give a
sketch of the present state of knowledge obtained by these different
kinds of investigations, concerning the materials which are ejected
from volcanic vents.
The most abundant of the substances which are ejected from volcanoes
is steam or water-gas, which, as we have seen, issues in prodigious
quantities during every eruption. But with the steam a great number
of other volatile materials frequently make their appearance. The
chief among these are the add gases known as hydrochloric acid,
sulphurous acid, sulphuretted hydrogen, carbonic add, and boracic acid;
and with these acid gases there issue hydrogen, nitrogen, ammonia,
the volatile metals arsenic, antimony, and mercury, and some other
substances. In considering the nature of the products which issue from
volcanic fissures, it must be remembered that many substances which
under ordinary circumstances do not exhibit marked volatility are
nevertheless easily carried away in fine particles when a current of
steam is passed over them. As we shall have to point out in the sequel,
different volatile substances have a tendency to make their appearance
at volcanic vents according as the intensity of the action going on
within it varies.
The volatile substances issuing from volcanic fissures at high
temperatures react upon one another, and many new compounds are thus
formed. We have already seen how, by the action of sulphurous acid
and sulphuretted hydrogen on each other, the sulphur so common in
volcanic districts has been separated and deposited. The hydrochloric
acid acts very energetically on the rocks around the vents, uniting
with the iron in them to form the yellow ferric-chloride. The rocks
all round a volcanic vent are not unfrequently found coated with this
yellow substance, which is almost always mistaken by casual observers
for sulphur. In many volcanoes the constant passage through the rocks
of the various acid gases has caused nearly the whole of the iron,
lime, and alkaline materials of the rocks to be converted into soluble
compounds known as sulphates, chlorides, carbonates, and borates; and,
on the removal of these by the rain, there remains a white, powdery
substance, resembling chalk in outward appearance, but composed of
almost pure silica. There are certain cases in which travellers have
visited volcanic islands where chemical action of this kind has gone on
to such an extent, that they have been led to describe the islands as
composed entirely of chalk.
[Sidenote: GASES EMITTED FROM VOLCANOES.]
Some of the substances issuing from volcanic vents, such as hydrogen
and sulphuretted hydrogen, are inflammable, and when they issue at a
high temperature, these gases burst into flame the moment that they
come into contact with the air. Hence, when volcanic fissures axe
watched at night, faint lambent flames are frequently seen playing over
them, and sometimes these flames are brilliantly coloured, through the
presence of small quantities of certain metallic oxides. Such volcanic
flames, however, are scarcely ever strongly luminous and, as we have
already seen, the red, glowing light which is observed over volcanic
mountains in eruption is due to quite another cause. The study by the
aid of the spectroscope of the flames which issue from volcanic vents
promises to throw much new light on the rarer materials ejected by
volcanoes. Spectroscopic observations of this kind have already been
commenced by Janssen, at Stromboli and Santorin.
Some of the volatile substances issuing from volcanic vents, are at
once deposited when they come in contact with the cool atmosphere,
others form new compounds with one another and the constituents of the
atmosphere, while others again attack the materials of the surrounding
rocks and form fresh chemical compounds with some of their ingredients.
Thus, there are continually accumulating on the sides and lips of
volcanic fissures deposits of sulphates, chlorides, and borates of
the alkalies and alkaline earths, with sal-ammoniac, sulphur, and the
oxides and sulphides of certain metals. The lips of the fissures from
which steam and acid gases issue in volcanoes are constantly seen to
be coated with yellow and reddish-brown incrustations, consisting of
mixtures, in varying proportions, of these different materials, and
these sometimes assume the form of stalactites and pendent masses.
[Sidenote: DEPOSITS AROUND VOLCANIC VENTS.]
Some of these products of volcanic action are of considerable
commercial value. At Vulcano regular chemical works have been
established in the crater of the volcano, by an enterprising Scotch
firm, a great number of workmen being engaged in collecting the
materials which are deposited around the fissures, and are renewed
by the volcanic action almost as soon as they are removed. In fig.
6, I have given a sketch of this singular spot, taken from the high
ground of the neighbouring Island of Lipari. From the village at
the foot of the volcano, where the workmen live, a zig-zag road has
been constructed leading up the side, and down into the crater of
the volcano. On this road, workmen and mules, laden with the various
volcanic materials, may be seen constantly passing up and down.
[Illustration: Fig. 6.--View of Vulcano, with Vulcanello in the
foreground taken from the south end of the Island of Lipari.]
Vulcano appears to have been frequently in a state of violent eruption
during the past 2,000 years--the last great outburst having taken place
in 1786. In 1873 the activity in the crater of Vulcano suddenly became
more pronounced in character, and the workmen hastened to escape from
the dangerous spot, but, before they could do so, several of them
were severely injured by the explosions. After this outburst, which
did not prove to be of very violent character, the quantity of gases
issuing from the fissures in the crater was for a time much greater
than before, and the productiveness of these great natural chemical
works was proportionately increased: but eventually the action died out
almost entirely. The chief products of Vulcano which are of commercial
value, are sal-ammoniac, sulphur, and boracic acid. At one time it was
even contemplated that great leaden chambers should be erected over the
principal fissures at the bottom of the crater of Vulcano, in which
chambers the volatile materials might be condensed and collected. The
change in the condition of the volcano has unfortunately prevented the
carrying out of this bold project.
Besides the volatile substances which issue from volcanic vents,
mingling with the atmosphere or condensing upon their sides, there are
also many solid materials ejected, and these may accumulate around the
orifices, till they build up mountains of vast dimensions, like Etna,
Teneriffe, and Chimborazo. Some of these solid materials are evidently
fragments of the rock-masses, through which the volcanic fissure has
been rent; these fragments have been carried upwards by the force of
the steam-blast and scattered over the sides of the volcano. But the
principal portion of the solid materials ejected from volcanic orifices
consists of matter which has been extruded from sources far beneath the
surface, in a highly-heated and fluid or semi-fluid condition.
[Sidenote: EJECTED ROCK-FRAGMENTS.]
The fragments torn from the sides of volcanic fissures consist of the
rocks through which the eruptive forces may happen to have opened their
way; pieces of sandstone, limestone, slate, granite, &c., are thus
frequently found in considerable numbers among materials which build up
volcanic mountains. Thus, some of the volcanic cones in the Eifel are
very largely made up of fragments of slate, which have been torn from
the sides of the vents by the uprushing currents of steam. At Vesuvius
masses of limestone are frequently ejected, and may be picked up all
over the slopes of the mountains. These limestone-fragments frequently
contain fossils, and Professor Guiscardi, of Naples, has been able to
collect several hundred species of shells, transported thus by volcanic
action from the rock-masses which form the foundation of the volcano
of Vesuvius. The action of water at a high temperature, and under
such enormous pressure as must exist beneath volcanic mountains, has
often produced changes in the rocks of which fragments are ejected
from volcanic vents. The so-called 'lava' ornaments, which are so
extensively sold at Naples, are not made from the materials to which
geologists apply that name, but from the fragments of altered limestone
that have been torn from the rocks beneath the mountain, and scattered
by the eruptive forces all over its sides. The chemical action of the
superheated and highly-compressed steam on the rocks beneath volcanoes
frequently results in the formation of beautifully crystallised
minerals. Such crystallised minerals abound in the rock-fragments
scattered over the sides of Vesuvius and other volcanoes, both active
and extinct. They have been formed in the great chemical laboratories
which exist beneath the volcano, and have been brought to the surface
by the action of the steam-jets issuing from its fissures.
Of still greater interest are those materials which issue from volcanic
orifices in an incandescent, and often in a molten, condition, and
which are evidently derived from sources far below the earth's surface.
It is to these materials that the name of 'lavas' is properly applied.
Lavas present a general resemblance to the slags and clinkers which
are formed in our furnaces and brick-kilns, and consist, like them, of
various stony substances which have been more or less perfectly fused.
When we come to study the chemical composition and the microscopical
structure of lavas, however, we shall find that there are many respects
in which they differ entirely from these artificial products.
Let us first consider the facts which are taught us concerning the
nature and origin of lavas, by a chemical analysis of them.
[Sidenote: CHEMICAL COMPOSITION OF LAVAS.]
Of the sixty-five or seventy chemical elements, only a very small
number occur at all commonly in lavas. Eight elements, indeed, make
up the great mass of all lavas--these are oxygen, silicon, aluminium,
magnesium, calcium, iron, sodium, and potassium. But even these
eight elements are present in very unequal proportions. Oxygen makes
up nearly one-half the weight of all lavas. Almost all the other
elements found in lavas exist in combination with oxygen, so that
lavas consist entirely of what chemists call 'oxides.' This is a most
remarkable circumstance, which, as we shall presently see, is of great
significance. The metalloid silicon makes up about one-fourth of the
weight of most lavas, and the metal aluminium about one-tenth. The
other five elements vary greatly in their relative proportions in
different lavas.
In all lavas the substance which forms the greatest part of the mass
is the compound of oxygen and silicon, known as silica or silicic
acid. In its pure form, this substance is familiar to us as quartz, or
rock-crystal and flint. Silica is present in all lavas in proportions
which vary from one-half to four-fifths of the whole mass. Now, this
substance, silica, has the property of forming more complex compounds
by uniting with the other oxides present in lavas--namely, the oxides
of aluminium, magnesium, calcium, iron, potassium, and sodium. Silica
is called by chemists an _acid_, the other oxides in lavas are termed
_bases_, and the compounds of silica with the bases are known as
_silicates_. Hence we see that lavas are composed of a number of
different silicates--the silicates of aluminium, magnesium, calcium,
iron, potassium, and sodium.
The above statements will perhaps be made clearer by the accompanying
table from which it will be seen that lavas are compounds in varying
proportions of six kinds of salts--namely, the silicates of alumina,
magnesia, lime, iron, potash, and soda.
Composition of Lavas.
Elements Binary Compounds Salts
Acid Bases
{ Silicon Silica--+
{ |
O { Aluminum +--Alumina Silicate of Alumina
x { |
y { Magnesium +--Magnesia " " Magnesia
g { |
e { Calcium +--Lime " " Lime
n { |
{ Iron +--Iron " " Iron
{ |
{ Potassium +--Potash " " Potash
{ |
{ Sodium +--Soda " " Soda
Now, in some lavas the acid constituent, or silica, is present in much
larger proportions than in others. Those lavas with a large proportion
of silica are called 'acid lavas,' those with a lower percentage of
silica, and therefore a higher proportion of the bases, are known as
the 'basic lavas.' It is convenient to employ the term 'intermediate
lavas' for those in which the proportion of silica is lower than in the
acid lavas, and the proportion of the bases is lower than in the basic
lavas.
The acid lavas contain from 66 to 80 per cent, of silica; they are poor
in lime, magnesia, and oxide of iron, but rich in potash and soda. The
basic lavas contain from 45 to 55 per cent, of silica; they are rich in
magnesia, lime, and oxide of iron, but poor in soda and potash. In the
intermediate lavas the proportion of silica varies from 55 to 66 per
cent.
As the basic-lavas contain a larger proportion of oxide of iron and
other heavy oxides than the acid-lavas, the former have usually a
higher specific gravity than the latter; it is, indeed, possible in
most cases to distinguish between these different varieties by simply
weighing them in water and in air.
[Sidenote: DIFFERENT KINDS OF LAVA.]
The basic lavas are usually of much darker colour than the add
lavas--the terms acid lavas, intermediate lavas, and basic lavas
correspond indeed pretty closely with the names trachytes, greystones
and basalt, which were given to the varieties of lavas by the older
writers on volcanoes, at a time when their chemical constitution had
not been accurately studied. Fresh lavas of acid composition are
usually nearly white in colour, intermediate lavas are of various
tints of grey, and basic lavas nearly black. It must be remembered,
however, that colour is one of the least persistent, and therefore
one of the least valuable, characters by means of which rocks can
be discriminated, and also that by exposure to the influence of the
atmospheric moisture the iron present in all lavas is affected, and
the lavas belonging to all classes, when weathered, assume reddish and
reddish-brown tints.
Geologists have devised a great number of names for the various kinds
of lava which have been found occurring round volcanic vents in
different parts of the world, and the study of these varieties is full
of interest. For our present purpose, however, it will be sufficient
to state that they nearly all fall into five great groups, known as
the Rhyolites, the Trachytes, the Andesites, the Phonolites, and the
Basalts. The Rhyolites are acid lavas, the Basalts are basic lavas,
and the Trachytes, Andesites, and Phonolites, different kinds of
intermediate lavas, distinguished by the particular minerals which they
contain.
Before we part from this subject of the classification of lavas
according to their chemical composition, it will be well to point
out that there exists a small group of lavas which stand quite by
themselves, and cannot be referred to either of the classes we have
indicated. They contain a smaller proportion of silica, and a much
larger proportion of magnesia and oxide of iron than the other lavas,
and may be made to constitute a small sub-group, to which we may apply
the term of 'ultra-basic lavas.' Although much less widely distributed
than the other varieties, they are, in some respects, as we shall
presently have to point out, of far greater interest to the geologist
than all the other kinds of lavas.
[Sidenote: MINUTE STRUCTURE OF LAVAS.]
We will now proceed to consider the facts which are brought to light
concerning the nature of lavas, when they are studied by the aid
of the microscope. Although most lavas appear at first sight to be
opaque substances, yet it is easy to prepare slices of them which are
sufficiently thin to transmit light. In such thin transparent slices
we are able to make out, by the aid of the microscope, certain very
interesting details of structure, which afford new and important
evidence bearing on the mode of origin of these rocks.
Host lavas are capable of being melted by the heat of our furnaces;
but the different kinds of lava vary greatly in the degree of their
fusibility. The basic lavas, or those with the smallest proportion of
silica, are usually much more easily fusible than those which contain a
high percentage of silica, the add lavas.
Now, it is a very noteworthy circumstance, that when a lava is
artificially fused it assumes on cooling very different physical
characters to those which were presented by the original rock.
If we examine the freshly-broken surface of a piece of lava, we
shall, in most cases, find that it contains a great number of those
regular-shaped bodies which we call crystals; in some cases these
crystals are so small as to be scarcely visible to the naked eye, in
others they may be an inch or more in length. Most lavas are thus seen
to be largely made up of crystals of different minerals. The minerals
which are usually contained in lavas are quartz, the various kinds of
felspar, augite, hornblende, the different kinds of mica, olivine, and
magnetite.
But when a piece of lava is melted in a furnace, all these crystalline
minerals disappear, and the resulting product is the homogeneous
substance which we call glass. If, as many suppose, lavas acquire the
fluidity which they possess when issuing from volcanic vents as the
result of simple fusion it is strange that artificially fused lavas do
not agree more closely in character with the natural products.
A careful examination of different kinds of lavas, however, will
show that they vary very greatly in character among themselves. Some
lavas are as perfectly glassy in structure as those which have been
artificially fused, while others contain great numbers of crystals,
which may sometimes be of very large size.
If we prepare thin transparent slices of these different kinds of
lavas, and examine them by the aid of the microscope, we shall find
that lavas are made up of two kinds of materials, a base or groundmass
of a glassy character, and distinct crystals of different minerals,
which are irregularly distributed through this glassy base, like the
raisins, currants, and pieces of candied peel in a cake. In some
cases the glassy base makes up the whole mass of the rock; in others,
smaller or larger numbers of crystals are seen to be scattered through
a glassy base; while in others again the crystals are so numerous that
the presence of an intervening glassy base or groundmass can only be
detected by the aid of the microscope.
[Sidenote: STUDY OF LAVAS WITH THE MICROSCOPE.]
If thin slices of the glassy materials of lavas be examined with high
magnifying powers, new and interesting facts are revealed. Through
the midst of the clear glassy substance cloudy patches are seen to be
diffused; and, if we examine them with a still higher power, these
cloudy patches resolve themselves into innumerable particles, some
transparent and others opaque, having very definite outlines. At the
same time fresh cloudy patches are brought into view, which can only
be resolved by yet higher powers of the microscope. In examining these
natural glasses by the aid of the microscope, we are forcibly reminded
of what occurs when the 'Milky Way' and some other parts of the heavens
are studied with a telescope. As the power of the instrument is
increased the nebulous patches are resolved into distinct stars, but
fresh nebulous masses come into view, which are in turn resolved into
stars, when higher powers of the instrument are employed.
In the Frontispiece, No. 1 illustrates the appearance presented by
these volcanic glasses when examined with a high power of a microscope.
Through a glassy base is seen a number of diffused nebulous patches,
which are in places resolved into definite particles.
These minute particles of definite form, which the microscope has
revealed in the midst of the glassy portions of lava, have received the
name of microliths, or crystallites. The study of the characters and
mode of arrangement of these microliths or crystallites has in recent
years thrown much new light on the interesting problems presented by
lavas.
In some glassy lavas the microliths or crystallites, instead of being
indiscriminately diffused through the mass of the base or groundmass,
are found to be collected together into groups of very definite form.
In No. 2 of the Frontispiece we have a section of a glassy rock in
which the crystallites have united together, so as to build up groups
presenting the most striking resemblance to fronds of ferns. Around
these groups spaces of dear glass have been left by the gathering up
of the crystallites, which in other parts of the mass are seen to be
equally diffused through it. In this formation of groups of microliths
we cannot but recognise the action of those crystalline forces, which
on frosty mornings cover our windows with a mimic vegetation composed
of icy particles.
In other cases, again, the crystallites scattered through the glassy
portions of lavas unite in radial groups about certain centres, and
thus build up globular masses to which the name of 'sphærulites' has
been given. No. 3 in the Frontispiece illustrates the formation of
these sphærulites.
Now, a careful study of the microliths or crystallites has proved that
they are the minute elements of which those wonderfully beautiful
objects which we call crystals are built up. In some cases we can
see that the crystallites are becoming united together in positions
determined by mathematical laws, and the group is gradually assuming
the outward form and internal structure of a crystal. In other cases
crystals may be found which are undergoing a disintegrating action,
and are then seen to be made up of minute elements similar to the
crystallites or microliths of glassy rocks.
[Sidenote: CRYSTALLITES AND CRYSTALS.]
The conclusion is confirmed by the fact that if we take an artificially
fused lava and allow it to cool slowly, it will be found that the
glassy mass into which it has resolved itself contains numerous
crystallites. If the cooling process be still further prolonged, these
crystallites will be found to have united themselves into definite
groups, and sometimes distinct crystals are formed in the mass; under
these circumstances the rock frequently loses its glassy appearance and
assumes a stony character.
In connection with this subject, it may be mentioned that some years
ago a very ingenious invention was submitted to trial in the Works
of the Messrs. Chance, of Birmingham. It had been suggested that if
certain lavas of easy fusibility were melted and poured into moulds,
we might thus obtain elaborately ornamented stone-work, composed of
the hardest material, without the labour of the mason. The molten rock
when quickly cooled was found to assume the form of a black glass, but
when very slowly cooled passed into a stony material. Unfortunately,
it was found that this material did not withstand the weather like
ordinary building stones, and, in consequence, the manufacture had to
be abandoned.
Now, the study of the products of volcanoes has led geologists to
recognise the true relations between glassy and crystalline rocks.
In the amorphous mixture of various silicates which compose a glass,
chemical affinity causes the separation of certain portions of
definite composition, and these form the microliths or elements of
which different crystalline minerals are built up. Under the influence
of the crystalline forces, there is a great shaking or agitation in
the mass, and the microliths of similar kind come together and become
united, like the fragments in Ezekiel's valley of dry bones.
Although we cannot see this process taking place under our eyes,
in a mass of lava, yet we may study specimens in which the action
has been arrested in its different stages. In order to understand
the development of an acorn into an oak-tree, it is not necessary
to watch the whole series of changes in a particular case. A visit
to an oak-thicket, in which illustrations of every stage of the
transformation may be found, will afford us equally certain information
on the subject.
In the same way by the examination of such a series of rock-sections
as that represented in the Frontispiece, we may understand how, in the
midst of a mass of mixed silicates constituting a natural glass, the
separation of microliths takes place; these unite into groups which are
the skeletons of crystals, and finally, by the filling up of the empty
spaces in these skeletons, complete crystals are built up. The series
of operations may, however, be interrupted at any stage, and this stage
we may have the chance of studying.
[Sidenote: GLASSY AND CRYSTALLINE LAVAS.]
We are able, as we shall show in a future chapter, to examine many
rock-masses that have evidently formed the reservoirs from which
volcanoes have been supplied, and others that fill up the ducts which
constituted the means of communication between these subterranean
reservoirs, and the surface of the earth. Now in these subterranean
regions the lavas have been placed under conditions especially
favourable for the action of the crystalline forces--they must have
cooled with extreme slowness, and they must have been under an enormous
pressure, produced in part by the weight of the superincumbent rocks,
and in part by the expansive force of the imprisoned steam. We are not,
therefore, surprised to find that in these subterranean regions, the
lavas, while retaining the same chemical composition, have assumed a
much more perfectly crystalline condition. In some cases, indeed, the
whole rock has become a mass of crystals without any base or groundmass
at all.
An examination of the Frontispiece will illustrate this perfect
gradation from the glassy to the crystalline condition of lavas. No.
1 represents a glass through which microliths or crystallites of
different dimensions and character are diffused. In Nos. 2 and 3,
these crystallites have united to form regular groups. In No. 4, which
may be taken as typical of the features presented by most lavas, we
have a glassy groundmass containing microliths (a 'crypto-crystalline
base'), through which distinct crystals are distributed. Nos. 5 and 6
illustrate the characters presented by lavas which have consolidated
at considerable depths beneath the surface; in the former we have
a mans of small crystals (a 'micro-crystalline base') with larger
crystals scattered through it; while the latter is entirely made up of
large crystals without any trace of a base or groundmass.
Now, as all lavas are found sometimes assuming the glassy condition at
the surface, so when seen in the masses which have consolidated with
extreme slowness, and under great pressure, in subterranean regions,
the same materials are found in the condition of a rock which is built
up entirely of crystals. Chemists have found that artificial mixtures
of silicates in which soda and potash are present in considerable
quantities, have a great tendency to assume the glassy condition on
cooling from a state of fusion, and glass manufacturers are always
careful to use considerable proportions of the alkalis as ingredients,
in making glass. It is found, in like manner, that those lavas which
contain the largest portion of the silicates of soda and potash (the
'acid lavas') most frequently assume the condition of a natural glass.
Geologists have given distinct names to the glassy and the perfectly
crystalline conditions of the different kinds of lavas, the glassy
varieties being found in masses which have cooled rapidly near the
surface, and the crystalline varieties in masses which have cooled
slowly at great depths. The names of these two conditions of the five
great classes into which we have divided lavas are as follows:--
[Sidenote: HIGHLY CRYSTALLINE IGNEOUS ROCKS.]
_Crystalline Forms._ _Lavas._ _Glassy Forms._
Granite Rhyolite }
Syenite Trachyte } Obsidian.
Diorite Andesite }
Miascite Phonolite }
Gabbro Basalt Tachylyte.
As vitreous rocks have little in their general appearance to
distinguish them from one another, the glassy forms of the first
four classes of lava have not hitherto received distinct names, but
have been confounded together under the name of obsidian. If we
determine the specific gravities of rocks having the same composition
but different structures, we shall find that they become heavier in
proportion as the crystalline structure is developed in them. Thus
gabbro is heavier, but tachylyte is lighter than basalt, bulk for bulk,
though all have the same chemical composition.
Nor are the crystals contained in lavas less worthy of careful study,
by the aid of the microscope, than the more or less glassy groundmass
in which they are embedded. Mr. Sorby has shown that the crystals found
in lavas, exhibit many interesting points of difference from those
which separate out in the midst of a mass of the same rock, when it has
been artificially melted and slowly cooled. There are other facts which
also point to the conclusion that, while the glassy groundmass of lavas
may have been formed by cooling from a state of fusion, the larger and
well-formed crystals in these lavas must have been formed under other
and very different conditions.
The larger crystals in lavas exhibit evidence of having been slowly
built up in the midst of a glassy mass, containing crystallites and
small crystals. We can frequently detect evidence of the interruptions
which have occurred in the growth of these crystals in the concentric
zones of different colour or texture which they exhibit; and portions
of the glassy base or groundmass are often found to have been caught up
and enclosed in these crystals during their growth.
But when we find, as in the porphyritic pitchstones, a glassy base
containing only minute crystallites, through which large and perfectly
formed crystals are distributed, we can scarcely doubt that the minute
crystallites and the larger crystals have separated from the base under
very different conditions. This is indicated by the bet that we detect
in these cases no connecting links between the embryo microliths and
the perfect crystals; and a confirmation of the conclusion is seen in
the circumstance that many of the crystals are found to have suffered
injury as if from transport, their edges and angles being rounded and
abraded, and portions being occasionally broken off from them.
Hence we are led to conclude that the larger crystals in lavas were
probably separated from the amorphous mass in the subterranean
reservoirs beneath the volcano, and were carried up to the surface in
the midst of the liquefied glassy material which forms the groundmass
of lavas. When we come to examine these crystals more closely, we find
that certain very curious phenomena are exhibited by them which lend
powerful support to this conclusion.
[Illustration: Fig. 7.--Minute Cavities, containing Liquids, in the
Crystals of Rocks.]
[Sidenote: LIQUID CAVITIES IN CRYSTALS.]
It is found convenient by geologists to designate those rocks which
have consolidated in deep-seated portions of the earth's crust
as Platonic Rocks, confining the name of Volcanic rocks to those
consolidating At the surface; but Plutonic and Volcanic Rocks shade
into one another by the most insensible gradations.
When the crystals embedded in granitic rocks, and in some lavas, are
examined with the higher powers of the microscope, they are frequently
seen to contain great numbers of excessively minute cavities. Each of
these cavities resembles a small spirit-level, having a quantity of
liquid and a bubble of gas within it. In fig. 7 we have given a series
of drawings of these cavities in crystals as seen under a high power
of the microscope. In No. 1 a group of such cavities is represented,
one of which is full of liquid, while two others are quite empty; the
remaining cavities all contain a liquid with a moving bubble of gas.
In No. 2 two larger cavities are shown, containing a liquid and a
bubble of gas; and it will be seen from these how varied in form these
cavities sometimes are. In Nos. 3, 4 and 6 the liquid in the cavities
contains, besides the bubbles, several, minute crystals; and in No. 6
we have a cavity containing two liquids and a bubble.
In the largest of such cavities the bubble is seen to change its
place so as always to lie at the upper side of the cavity, when the
position of the latter is altered, just as in a spirit-level. But in
the smallest cavities the bubbles appear to be endowed with a power of
spontaneous movement; like imprisoned creatures trying to escape, these
bubbles are seen continually oscillating from side to side and from end
to end of the cavities which enclose them. In fig. 8 a minute cavity
containing a liquid and bubble is shown, the path pursued by the latter
in its wonderful gyrations being indicated by the dark line. These
cavities are exceedingly minute, and so numerous that in some crystals
there must be millions of them present; indeed, in certain cases, as
we increase the magnifying power of our microscopes, new and smaller
cavities continually become visible. It has been estimated that in some
instances the number of these minute liquid-cavities in the crystals of
rocks amounts to from one thousand millions to ten thousand millions in
a cubic inch of space.
[Illustration: Fig. 8--Minute Liquid-cavity in a Crystal, with a moving
Bubble. (The path of the bubble is indicated by the dark line.)]
[Sidenote: NATURE OF LIQUIDS IN CAVITIES.]
What is the nature of the liquids which are thus imprisoned in these
cavities contained in the crystals of lavas and granites? Careful
experiments have given a conclusive answer to this question. In many
cases the liquid is water, usually containing considerable quantities
of saline matter dissolved in it. Sometimes the saline matters are
present in such abundance that they cannot all pass into solution, but
crystallise out, as in fig. 7--Nos. 3, 4, 5--where cubic crystals of
the chlorides of sodium and potassium are seen floating in the liquid;
in other cases the liquid is a hydrocarbon like the mineral oil which
is present in great abundance in deep-seated rocks in many parts of the
globe. But in some other cases the liquid contained in the cavities
of crystals is found to be one which could scarcely be anticipated
to occur under such circumstances--the gas known as carbonic add,
which under extreme pressure can be reduced to a liquid condition. In
cavities containing liquefied carbonic acid, if the rock be warmed up
to 86° or 90° Fahrenheit the bubble suddenly vanishes, sometimes with
an appearance like ebullition or boiling, as represented in fig. 9.
Now the temperature which we have indicated is the 'critical point' of
carbonic acid, and above that temperature it cannot exist in a liquid
condition, however great may be the pressure to which it is subjected.
The liquid has been converted into a gas which completely fills the
cavity. The carbonic acid in the cavities of crystals has frequently
been isolated and its nature placed beyond doubt by spectroscopic and
ordinary chemical tests.
The presence of these liquids in the cavities of crystals clearly
proves that the latter must have been formed under enormous pressure--a
pressure sufficiently great to reduce, not only steam, but also
volatile hydrocarbons and even gaseous carbonic acid, to the bulk of a
liquid.
[Illustration: Fig. 9.--Cavity in Crystal containing Carbonic-Acid Gas
at a temperature of 86° F., and passing from the liquid to the gaseous
condition.]
Such conditions of enormous pressure we may infer to exist in the
deep-seated reservoirs beneath volcanoes, where, besides the weight
of the superincumbent rock-masses, we have the compressing force of
great quantities of elastic vapour held in confinement. The crystals of
which granitic rocks are entirely built up exhibit clear evidence of
having been all formed under these conditions of enormous pressure. The
glassy base or groundmass of lavas, on the other hand, presents all the
characters of materials that have cooled from a state of fusion. Most
lavas consist in part of crystals, exhibiting fluid-cavities like those
present in granite, and in part of a base, which has evidently been
formed by the cooling of a fused mass. We are therefore justified in
concluding that the crystals have been formed in subterranean recesses,
and that the groundmass or base has consolidated at the surface. The
bearing of these conclusions upon some of the great problems presented
by volcanoes we shall have occasion to point out in the sequel.
[Sidenote: CAUSE OF MOVEMENT OF BUBBLES.]
One of the most interesting inquiries suggested by the study of
the liquid-cavities in volcanic rocks is that of the cause of the
apparently spontaneous movement of the bubbles which we have described
as taking place in some of the smaller of them. The ingenious
experiments of Mr. Noel Hartley have suggested to Professor Stokes
an explanation which is probably the true one. It appears that these
minute globes of vapour are in such a state of unstable equilibrium as
to be affected by the smallest changes of temperature, and that the
variations in the heat of the atmosphere, due to currents of air and
the movement of warm or cold bodies through it, are sufficient to cause
the oscillation of these sensitively poised bubbles.
The short account which we have been able to give in the foregoing
pages of the researches that have been carried on concerning the nature
of the materials ejected from volcanoes will serve to show that these
investigations have already made known many facts of great interest,
and that the farther pursuit of them is full of the highest promise.
To the scientific worker no subject is too vast for his research, no
object so minute as to be unworthy of his most patient study. In some
of our future inquiries concerning the nature of volcanic action, we
shall be led to an investigation of the phenomena displayed in the sun,
moon, comets and other great bodies of the universe; but another road
to truths of the same grandeur and importance is found, as we have
seen, in an examination of the mode of development of crystallites, and
a study of the materials contained in the microscopic cavities of the
minutest crystals.
CHAPTER IV.
THE DISTRIBUTION OF THE MATERIALS EJECTED FROM VOLCANIC VENTS.
The escape of great quantities of steam and other gases from the midst
of a mass of fluid or semi-fluid lava gives rise to the formation of
vast quantities of froth or foam upon its surface. This froth or foam,
which is formed upon the surface of lava by the escape of gaseous
matters from within it, is made up of portions of the lava distended
into vesicles, in the same way that bubbles are formed on the surface
of water. It bears precisely the same relation to the liquid mass of
lava that the white crest of foam upon an advancing wave does to the
sea-water, from the bubbles of which it is formed.
This froth upon the surface of lavas varies greatly in character
according to the nature of the material from which it is formed. In
the majority of cases the lavas consist, as we have seen, of a mass
of crystals floating in a liquid magma, and the distension of such a
mass by the escape of steam from its midst gives rise to the formation
of the rough cindery-looking material to which the name of 'scoria'
is applied. But when the lava contains no ready-formed crystals, but
consists entirely of a glassy substance in a more or less perfect state
of fusion, the liberation of steam gives rise to the formation of the
beautiful material known as 'pumice.' Pumice consists of a mass of
minute glass bubbles; these bubbles have not usually, however, retained
their globular form, but have been elongated in one direction through
the movement of the mass while it was still in a plastic state.
The steam frequently escapes from lava with such violence that the
froth or scum on its surface is broken up and scattered in all
directions, as the foam crests of waves are dispersed by the wind
during a storm. In this way fragments of scoria or pumice are often
thrown to the height of many hundreds or thousands of feet into the
atmosphere, as we have seen is the case at Stromboli and Vesuvius.
Indeed, during violent eruptions, a continuous upward discharge of
these fragments is maintained, the ragged cindery masses hurtling one
another in the atmosphere, as they are shot perpendicularly upwards
to an enormous height and fall back into the vent; or they may rise
obliquely and describe curves so as to descend outside the orifice from
which they were ejected.
[Sidenote: FINENESS OF VOLCANIC DUST.]
During their upward discharge and downward fall, the cindery fragments
are by attrition continually reduced to smaller dimensions. The noise
made by these fragments, as they strike against one another in
the air during their rise and fall, is one of the most noteworthy
accompaniments of volcanic eruptions. It has been noticed that in many
cases there is a constant diminution in the size of the fragments
ejected during a volcanic outburst, this being doubtless due to the
friction of the masses as they are ejected and re-ejected from the
vent. Thus it is related by Mr. Poulett Scrope, who watched the
Vesuvian eruption of 1822, which lasted for nearly a month, that
during the earlier stages of the outburst fragments of enormous size
were thrown out of the crater, but by constant re-ejection these were
gradually reduced in size, till at last only the most impalpable dust
issued from the vent. This dust filled the atmosphere, producing in
the city of Naples 'a darkness that might be felt,' and so excessively
finely divided was it, that it penetrated into all drawers, boxes, and
the most closely fastened receptacles, filling them completely. Mr.
Whymper relates that, while standing on the summit of Chimborazo, he
witnessed an eruption of Cotopaxi, which is distant more than fifty
miles from the former mountain. The fine volcanic dust fell in great
quantities around him, and he estimated that no less than two millions
of tons must have been ejected during this slight outburst. Professor
Bonney has examined this volcanic dust from Cotopaxi, and calculates
that it would take from 4,000 to 25,000 particles to make up a grain in
weight.
Various names have been given by geologists to the fragments ejected
from volcanic vents, which, as we have seen, differ greatly in their
dimensions and other characters. Sometimes masses of more or less
fluid lava are flung bodily to a great height in the atmosphere.
During their rise and fall these masses are caused to rotate, and in
consequence assume a globular or spheroidal form. The water imprisoned
in these masses, during their passage through the atmosphere, tends to
expand into steam, and they become more or less completely distended
with bubbles. Such masses, which sometimes assume very regular and
striking forms, are known as 'volcanic bombs.' Many volcanic bombs have
a solid nucleus of refractory materials. The large, rough, angular,
cindery-looking fragments are termed 'scoriæ.' When reduced to the
dimensions of a marble or pea they are usually called by the Italian
name of 'lapilli.' The still finer materials are known as volcanic sand
and dust.
There are, however, two names which are frequently applied to these
fragmentary materials ejected from volcanoes, which are perhaps
liable to give rise to misconception. These are the terms 'cinders'
and 'ashes.' It must be remembered that the scoriæ or cindery-looking
masses are not, like the cinders of our fires, the product of the
partial combustion of a material containing inflammable gases, but are,
like the clinkers of furnaces and brick-kilns, portions of partially
vitrified and fused rock distended by gases. So, too, volcanic ashes
only resemble the ashes of our grates in being very finely divided;
they are not, like the latter, the incombustible residue of a mass
which has been burnt.
[Sidenote: VOLCANIC BOMBS AND PELE'S HAIR.]
The glassy lavas, when distended by escaping gases, give rise to the
formation of pumice, the white colour of which, as in the case of the
foam of a wave, is due to the reflection of a portion of the light in
its frequent passage from one medium to another--in this case from
air to glass, and from glass to air. The volcanic bombs formed from
glassy lavas are often of especially beautiful and regular forms.
Sometimes the passage of steam through a mass of molten glass produces
large quantities of a material resembling spun glass. Small particles
or shots of the glass are carried into the air and leave behind them
thin, glassy filaments like a tail. At the volcano of Kilauea in Hawaii
this filamentous volcanic glass is abundantly produced, and is known
as 'Pele's Hair'--Pele being the name of the goddess of the mountain.
Birds' nests are sometimes found composed of this beautiful material.
In recent years an artificial substance similar to this Pele's hair
has been extensively manufactured by passing jets of steam through the
molten slag of iron-furnaces; it resembles cotton-wool, but is made up
of fine threads of glass, and is employed for the packing of boilers
and other purposes.
The very finely-divided volcanic dust is often borne to enormous
distances from the volcano out of which it has been ejected. The force
of the steam-current carrying the fragments into the atmosphere is
often so great that they rise to the height of several miles above
the mountain. Here they may actually pass into the upper currents of
the atmosphere and be borne away to the distance of many hundreds
or thousands of miles. Hence it is not an unusual circumstance for
vessels at sea to encounter at great distances from land falling
showers of this finely divided, volcanic dust. We sometimes meet with
this far-travelled, volcanic dust under very unexpected circumstances.
Thus, in the spring of 1875 I had occasion to visit Prof. Vom Rath of
Bonn, who showed me a quantity of fine volcanic dust which had during
the past winter fallen in considerable quantities in certain parts
of Norway. This dust, upon microscopic examination, proved to be so
similar to what was known to be frequently ejected from the Icelandic
volcanoes that a strong presumption was raised that volcanic outbursts
had been going on in that island. On returning to England I found that
the first steamer of the season had just reached Leith from Iceland,
bringing the intelligence that very violent eruptions had taken place
during the preceding months.
[Sidenote: DISPERSION OF PUMICE AND VOLCANIC DUST.]
This finely-divided volcanic dust is thus carried by the winds and
spread over every part of the ocean. Everyone is familiar with the
fact that pumice floats upon water; this it does, not because it is a
material specifically lighter than water, but because cavities filled
with air make up a great part of its bulk. If we pulverise pumice, we
find the powder sinks readily in water, but the rock in its natural
condition floats for the same reason that an iron ship does--because
of the air-chambers which it encloses. When this pumice is ejected from
a volcano and falls into a river or the ocean, it floats for a long
time, till decomposition causes the breaking down of the thin glassy
partitions between the air chambers, and causes the admission of water
into the latter, by which means the whole mass gets water-logged. Near
the Liparis and other volcanic islands the sea is sometimes covered
with fragments of pumice to such an extent that it is difficult for a
boat to make progress through it, and the same substance is frequently
found floating in the open ocean and is cast up on every shore.
During the year 1878 masses of floating pumice were reported as
existing in the vicinity of the Solomon Isles, and covering the surface
of the sea to such extent that it took ships three days to force their
way through them. Sometimes these masses of pumice accumulate in such
quantities along coasts that it is difficult to determine the position
of the shore within a mile or two, as we may land and walk about on
the great floating raft of pumice. Now, recent deep-sea soundings,
carried on in the 'Challenger' and other vessels, have shown that the
bottom of the deepest portion of the ocean, far away from the land, is
covered with these volcanic materials which have been carried through
the air or floated on the surface of the ocean. To these deeper parts
of the ocean no sediments carried down by the rivers are borne, and the
remains of calcareous organisms are, in these abysses, soon dissolved;
under such conditions, therefore, almost the only material accumulating
on the sea bottom is the ubiquitous wind- and wave-borne volcanic
products. These particles of volcanic dust and fragments of pumice by
their disintegration give rise to a clayey material, and the oxidation
of the magnetite, which all lavas contain, communicates to the mass a
reddish tint. This appears to be the true origin of those masses of
'red-clay' which, according to recent researches, are found to cover
all the deeper parts of the ocean, but which probably attain to no
great thickness.
But while some portion of the materials ejected from volcanoes may
thus be carried by winds and waves, so as to be dispersed over every
part of the land and the ocean-bed, another, and in most cases by far
the largest, portion of these ejections falls around the volcanic vent
itself. It is by the constant accumulation of these ejected materials
that such great mountain masses as Etna, Teneriffe, Fusiyama, and
Chimborazo have been gradually built up around centres of volcanic
action.
There are cases in which the formation of volcanic mountains on a small
scale has actually been observed by trustworthy witnesses. There are
other cases in which volcanic mountains of larger size can be shown
to have increased in height and bulk by the fall upon their sides and
summits of fragmentary materials ejected from the volcanic vent. In all
cases the examination of these mountain-masses leads to the conclusion
that they are entirely built up of just such materials as we constantly
see thrown out of volcanoes during eruption.
[Sidenote: FORMATION OF VOLCANIC MOUNTAINS.]
Thus we are led to the conclusion that all volcanic mountains are
nothing but heaps of materials ejected from fissures in the earth's
crust, the smaller ones having been formed during a single volcanic
outburst, the larger ones being the result of repeated eruptions from
the same orifice which may, in some cases, have continued in action for
tens or hundreds of thousands of years.
No observer has done such useful work in connection with the study of
the mode of formation of volcanic mountains as our countryman, Sir
William Hamilton, who was ambassador at Naples from 1764 to 1800, and
made the best possible use of his opportunities for examining the
numerous volcanoes in Southern Italy.
A little to the west of the town of Puzzuoli on the Bay of Naples there
stands a conical hill rising to the height of 440 feet above the level
of the Mediterranean, and covering an area more than half a mile in
diameter. Now we have the most conclusive evidence that in ancient
times no such hill existed on this site, which was partly occupied by
the Lucrine Lake, and the fact is recognised in the name which the hill
bears, that of Monte Nuovo, or the 'New Mountain.' See fig. 10.
Sir William Hamilton rendered admirable service to science by
collecting all the contemporary records relating to this interesting
case, and he was able to prove, by the testimony of several
intelligent and trustworthy witnesses, that during the week following
the 29th of September, 1538, this hill had gradually been formed of
materials ejected from a volcanic vent which had opened upon this site.
[Illustration: Fig. 10. Monte Nuovo (440 ft. high) on the shores of the
Bay of Naples.]
[Sidenote: HISTORY OF THE FORMATION OF MONTE NUOVO.]
The records collected by Hamilton with others which have been
discovered since his death prove most conclusively the following
facts. During more than two years, the country round was affected
by earthquakes, which gradually increased in intensity and attained
their climax in the month of September 1538; on the 27th and 28th of
that month these earthquake shocks are said to have been felt almost
continuously day and night. About 8 o'clock on the morning of the 29th,
a depression of the ground was noticed on the site of the future hill,
and from this depression, water, which was at first cold and afterwards
tepid, began to issue. Four hours afterwards the ground was seen to
swell up and open, forming a gaping fissure, within which incandescent
matter was visible. From this fissure numerous masses of stone, some of
them 'as large as an ox,' with vast quantities of pumice and mud, were
thrown: up to a great height, and these falling upon the sides of the
vent formed a great mound. This violent ejection of materials continued
for two days and nights, and on the third day a very considerable hill
was seen to have been built up by the falling fragments, and this hill
was climbed by some of the eye-witnesses of the eruption. The next day
the ejections were resumed, and many persons who had ventured on the
hill were injured, and several killed by the falling stones. The later
ejections were however of less violence than the earlier ones, and seem
to have died out on the seventh or eighth day after the beginning of
the outburst. The great mass of this considerable hill would appear,
according to the accounts which have been preserved, to have been built
up by the materials which were ejected during two days and nights.
Monte Nuovo is a hill of truncated conical form, which rises to the
height of 440 feet above the waters of the Mediterranean, and is now
covered with thickets of stone-pine. The hill is entirely made up
of volcanic scoriæ, lapilli, and dust, and the sloping sides have
evidently been produced by these fragmentary materials sliding over one
another till they attained the angle of rest; just as happens with the
earth and stones tipped from railway-waggons during the construction of
an embankment. In the centre of this conical hill is a vast circular
depression, with steeply sloping sides, which is of such depth that its
bottom is but little above the sea-level. This cup-shaped depression is
the 'crater' of the volcano, and it has evidently been formed by the
explosive action which has thrown out the materials immediately above
the vent, and caused them to be accumulated around it.
[Illustration: Fig. 11.--Map of the district around Naples, showing
Monte Nuovo and the surrounding volcanoes of older date.]
The district lying to the west of Naples, in which the Monte Nuovo is
situated, contains a great number of hills, all of which present a most
striking similarity to that volcano. All these hills are truncated
cones, with larger or smaller circular depressions at their summits,
and they axe entirely composed of volcanic scoriæ, lapilli, and dust.
Some of these hills are of considerably larger dimensions than the
Monte Nuovo, while others are of smaller size, as shown in the annexed
map, fig. 11. No stranger visiting the district, without previous
information upon the subject, would ever suspect the fact that, while
all the other hills of the district have existed from time immemorial,
and are constantly mentioned in the works of Greek and Roman writers,
this particular hill of Monte Nuovo came into existence less than 350
years ago.
[Sidenote: OLDER VOLCANOES OF THE CAMPI PHLEGRÆI.]
The evidently fused condition of the materials of which these hills are
built up is a dear sign of the volcanic action which has taken place
in it; and this feet was so fully recognised by the ancients that they
called the district the Campi Phlegræi, or 'the Burning Fields,' and
regarded one of the circular depressions in it as the entrance to Hades.
It is impossible for anyone to examine this district without being
convinced that all the numerous cones and craters which cover it
have been formed by the same agency as that by which Monte Nuovo was
produced. We have shown that there is the most satisfactory historical
evidence as to what that agency was.
Now volcanic cones with craters in their centres occur in great numbers
in many parts of the earth's surface. In some districts, like the
Auvergne, the Catacecaumene in Asia Minor, and certain parts of New
Zealand, these volcanic cones occur by hundreds and thousands. In some
instances, these volcanic cones have been formed in historic times,
but in the great majority of cases we can only infer their mode of
origin from their similarity to others of which the formation has been
witnessed.
Most of the smaller volcanic hills, with their craters, have been
thrown up during a single eruption from a volcanic fissure; but, as
Hamilton conclusively proved, the grandest volcanic mountains must
have been produced by frequent repetitions of similar operations upon
the same site. For not only are these great volcanic piles found to be
entirely composed of materials which have evidently been ejected from
volcanic vents, but, when carefully watched, such mountains are found
undergoing continual changes in form, by the addition of materials
thrown out from the vent, and falling upon their sides.
This fact will be well illustrated by a comparison of the series of
drawings of the summit of Vesuvius which were made by Sir William
Hamilton in 1767, and which we have copied in fig. 12. During the
earlier months of that year the summit of the mountain was seen to be
of truncated form, a great crater having been originated by the violent
outbursts of the preceding year. This condition of the mountain-top
is represented in the first figure of the series. The drawing made by
Hamilton, on July 8, shows that not only was the outer rim of the great
crater being modified in form by the fall of materials upon it, but
that in the centre of the crater a small cone was being gradually built
up by the quiet ejections which were taking place.
[Illustration: Fig. 12.--Outlines of the Summit of Vesuvius during the
Eruption of 1767.]
[Sidenote: CHANGES IN FORM OF VESUVIUS.]
If we compare the drawings made at successive dates, we shall find that
the constant showers of falling materials were not only raising the
edge of the great crater but were at the same time increasing the size
of the small cone inside the crater. By the end of October the small
cone had grown to such an extent that its sides were confluent with
those of the principal cone, which had thus entirely lost its truncated
form and been raised to a much greater height. The comparison of these
drawings will be facilitated by the dotted lines, which represent
the outline of the top of the mountain at the preceding observation;
so that the space between the dotted and the continuous line in each
drawing shows the extent to which the bulk of the cone had increased in
the interval between two observations.
But, although the general tendency of the action going on at volcanic
mountains is to increase their height and bulk by the materials falling
upon their summits and aides, it must be remembered that this action
does not take place by any means continuously and regularly. Not only
are there periods of rest in the activity of the volcano, during which
the rain and winds may accomplish a great deal in the way of crumbling
down the loose materials of which volcanic mountains are largely built
up, but sudden and violent eruptions may in a very short time undo the
slow work of years by blowing away the whole summit of the mountain at
once. Thus, before the great eruption of 1822, the cone of Vesuvius, by
the almost constant ejection of ashes during several years, had been
raised to the height of more than 4,000 feet above the level of the
sea; but by the terrible outburst which then took place the cone was
reduced in height by 400 feet, and a vast crater, which had a diameter
of nearly a mile, and a depth of nearly 1,000 feet (see fig. 13), was
formed at the top of the mountain. The enormous quantity of material
thus removed was either distributed over the flanks of the mountain,
or, when reduced to a finely comminuted condition, was carried by the
wind to the distance of many miles, darkening the air, and coating the
surface of the ground with a thick covering of dust.
[Illustration: Fig. 13.--Crater of Vesuvius formed during the eruption
in 1822. (It was nearly 1 mile in diameter and 1,000 ft. deep.)]
[Sidenote: EARLY HISTORY OF VESUVIUS.]
The volcano of Vesuvius, although of somewhat insignificant dimensions
when compared with the grander volcanic mountains of the globe,
possesses great interest for the student of Vulcanology, inasmuch as
being situated in the midst of a thickly populated district and in
close proximity to the city of Naples, it has attracted much attention
during past times, and there is no other volcano concerning which we
have so complete a series of historical records. The present cone of
Vesuvius, which rises within the great encircling crater-ring of Somma,
has a height of about 1,000 feet. But there is undoubted evidence that
this cone, to the top of which a railway has recently been constructed
for the convenience of tourists, has been entirely built up during the
last 1,800 years, and, what is more, that during this period it has
been many times almost wholly destroyed and reconstructed.
Nothing is more certain than the bet that the Vesuvius upon which the
ancient Romans and the Greek settlers of Southern Italy looked, was a
mountain differing entirely in its form and appearance from that with
which we are familiar. The Vesuvius known to the ancients was a great
truncated cone, having a diameter at its base of eight or nine miles,
and a height of about 4,000 feet. The summit of this mountain was
formed by a circular depressed plain, nearly three miles in diameter,
within which the gladiator Spartacus, with his followers, were
besieged by a Roman army. There is no evidence that at this time the
volcanic character of the mountain was generally recognised, and its
slopes are described by the ancient geographers as being clothed with
fertile fields and vineyards, while the hollow at the top was a waste
overgrown with wild vines.
[Illustration: Fig. 14.--Crater of Vesuvius in 1756. (From a drawing
made on the spot)]
But in the year 79 a terrible and unexpected eruption occurred, by
which a vast, crateral hollow was formed in the midst of Vesuvius, and
all the southern side of the great rim surrounding this crater was
broken down. Under the materials ejected during this eruption, the
cities of Pompeii, Herculaneum, and Stabiæ were overwhelmed and buried.
Numerous descriptions and drawings enable us to understand how in the
midst of the vast crater formed in the year 79 the modern cone has
gradually been built up. Fresh eruptions are continually increasing the
bulk, or raising the height of the Vesuvian cone.
The accompanying drawings made by Sir William Hamilton enable us to
understand the nature of the changes which have been continually
taking place at the summit of Vesuvius. The drawing fig. 14 shows the
appearance presented by the crater in the year 1756.
[Illustration: Fig. 15.--The Summit of Vesuvius in 1767. (From an
original drawing.)]
[Sidenote: VESUVIUS IN MODERN TIMES.]
At this time we see that inside the crater a series of cones had been
built up one within the other from which lava issued, filling the
bottom of the crater and finding its way through a breach in its walls,
down the side of the cone. It is evident that the ejected materials
falling on the sides of the innermost cone would tend to enlarge the
latter till its sides became confluent with the cone surrounding it,
and if this action went on long enough, the crater would be entirely
filled up and a perfect cone with only a small aperture at the top
would be produced. But from time to time, grand and paroxysmal
outbursts have occurred at Vesuvius, which have truncated the cone,
and sometimes formed great, cup-shaped cavities, reaching almost to its
base, like that shown in fig. 13.
In 1767 the crater of Vesuvius, as shown in fig. 15, contained a single
small cone in a state of constant spasmodic outburst, like that of
Stromboli.
[Illustration: Fig. 16.--Summit of Vesuvius in 1848.]
In 1843, we find that the crater of Vesuvius contained three such small
cones arranged in a line along its bottom as depicted in fig. 16.
These drawings of the summit of Vesuvius give a fair notion of the
changes which have been continually going on there during the whole of
the historical period. Ever and anon a grand outburst, like that of
1822, has produced a vast and deep crater such as is represented in
fig. 13, and then a long continuance of quiet and regular ejections
has built up within the crater small cones like those shown in figs.
14, 15 and 16, till at last the great crater has been completely filled
up, and the cone reconstructed.
[Illustration: Fig. 17.--Outlines of Vesuvius, showing its Form at
different periods of its history.]
[Sidenote: CHANGES IN OUTLINE OF VESUVIUS.]
In the series of outlines in fig. 17, we have endeavoured to illustrate
the succession of changes which has taken place in Vesuvius during
historical times. In the year 79 one side of the crater-wall of the
vast mountain-mass was blown away. Subsequent ejections built up the
present cone of Vesuvius within the great encircling crater-wall of
Somma, and the form of this cone and the crater at its summit have
been undergoing continual changes during the successive eruptions of
eighteen centuries.
What its _future_ history may be we can only conjecture from analogy.
It may be that a long continuance of eruptions of moderate energy may
gradually raise the central cone till its sides are confluent with
those of the original mountain; or it may be that some violent paroxysm
will entirely destroy the modern cone, reducing the mountain to the
condition in which it was after the great outburst of 79. On the other
hand, if the volcanic forces under Vesuvius are gradually becoming
extinct (but of this we have certainly no evidence at present), the
mountain may gradually sink into a state of quiescence, retaining its
existing form.
The series of changes in the shape of Vesuvius, which are proved by
documentary evidence to have been going on during the last 2,000 years,
probably find their parallel in all active volcanoes. In all of these,
as we shall hereafter show, the activity of the vents undergoes great
vicissitudes. Periods of continuous moderate activity alternate with
short and violent paroxysmal outbursts and intervals of complete rest,
which may in some cases last for hundreds or even thousands of years.
During the periods of continuous moderate activity, the crater of the
volcano is slowly filled up by the growth of smaller cones within it;
and the height of the mountain is raised. By the terrible paroxysmal
outbursts the mountain is often completely gutted and its summit blown
away; but the materials thus removed from the top and centre of the
mass are for the most part spread over its aides, so that its bulk
and the area of its base are thereby increased. During the intervals
of rest, the sides of the mountain which are so largely composed of
loose and pulverulent materials are washed downwards by rains and
driven about by winds. Thus all volcanoes in a state of activity are
continually growing in size every ejection, except in the case of those
where the materials are in the finest state of subdivision, adding to
their bulk; the area of their bases being increased during paroxysmal
outbursts, and their height during long-continued moderate eruptions.
[Sidenote: DEVIATIONS FROM CONICAL FORM.]
We have pointed out that the conical form of volcanic mountains is
due to the slipping of the falling materials over one another till
they attain the angle at which they can rest. There are, however, some
deviations from this regular conical form of volcanoes which it may be
well to refer to.
The quantity of rain which falls during volcanic eruptions is often
enormous, owing to the condensation of the great volumes of steam
emitted from the vent. Consequently the falling lapilli and dust often
descend upon the mountain, not in a dry state but in the condition
of a muddy paste. Many volcanic mountains have evidently been built
up by the flow of successive masses of such muddy paste over their
surfaces. Some volcanic materials when mixed with water have the
property of rapidly 'setting' like concrete. The ancient Romans and
modern Italians, well acquainted with this property of certain kinds of
volcanic dust and lapilli, have in all ages employed this 'puzzolana,'
as it is called, as mortar for building. The volcanic muds have often
set in their natural positions, so as to form a rock, which, though
light and porous, is of tolerably firm consistency. To this kind of
rock, of which Naples and many other cities are built, the name of
'tuff' or 'tufa' is applied. A similar material is known in Northern
Germany as 'trass.'
The cause of the 'setting' of puzzolana and tufa is that rain-water
containing a small proportion of carbonic acid acts on the lime in the
volcanic fragments, and these become cemented together by the carbonate
of lime and the free silica, which are thus produced in the mass.
When a strong wind is blowing during a volcanic outburst, the materials
may be driven to one side of the vent, and accumulate there more
rapidly than on the other. Thus lop-sided cones are formed, such
as may frequently be observed in some volcanic districts. In areas
where constant currents of air, like the trade-winds, prevail, all
the scoria-cones of the district may thus be found to be unequally
developed on opposite sides, being lowest on those from which the
prevalent winds blow, and highest on the sides towards which these
winds blow.
[Sidenote: ANGLE OF SLOPE IN VOLCANIC CONES.]
The examination of any careful drawing, or better still of the
photograph, of a volcanic cone, will prove that the profile of such
cones is not formed by straight lines, but by curves often of a
delicate and beautiful character. The delineations of the sacred
volcano of Fusiyama, which are so constantly found in the productions
of Japanese artists, must have familiarised everyone with the elegant
curved lines exhibited by the profiles of volcanoes. The upper slope
of the mountain is comparatively steep, often exhibiting angles of
30° to 35°, but this steepness of slope gradually diminishes, till it
eventually merges in the surrounding plains. The cause of this elegant
form assumed by most volcanic mountains is probably two-fold. In the
first place we have to remember that the materials falling upon the
flanks of the mountain differ in size and shape, and some will rest
on a steeper slope than others. Thus, while some of the materials
remain on the upper part of the mountains, others are rolling outwards
and downwards. Hence we find that those cones which are composed of
uniform materials have straight sides. But in some cases, we shall
see hereafter, there has certainly been a central subsidence of the
mountain mass, and it is this subsidence which has probably given rise
to the curvature of its flanks.
We have hitherto considered only the methods by which the froth or
foam, which accumulates on the surface of fluid lava, is dispersed.
But in many cases not only is this scum of the lava ejected from the
volcanic vent by the escaping steam, but the fluid lava itself is
extruded forcibly, and often in enormous quantities.
The lava in a volcanic vent is always in a highly heated, usually
incandescent, condition. Seen by night, its freshly exposed surface
is glowing red, sometimes apparently white-hot. But by exposure to
the atmosphere the surface is rapidly chilled, appearing dull red by
night, and black by day. Many persons are surprised to find that a
flowing stream of lava presents the appearance of a great mass of rough
cinders, rolling along with a rattling sound, owing to the striking of
the clinker-like fragments against each other. When viewed by night,
the gleaming, red light between these rough, cindery masses betrays the
presence of incandescent materials below the chilled surface of the
lava-stream.
No fact in connection with lavas is more striking than the varying
degrees of liquidity presented by them in different cases. While some
lava-streams seem to resemble rivers, the material flowing rapidly
along, filling every channel in its course, and deluging the whole
country around, others would be more fitly compared to glaciers,
creeping along at so slow a rate that the fact of their movement can
only be demonstrated by the most careful observation. Even when falling
over a precipice such lavas, owing to their imperfect liquidity, form
heavy, pendent masses like a 'guttering' candle, as is shown by fig.
18, which is taken from a drawing kindly furnished to me by Capt. S. P.
Oliver, R.A. The causes of these differences in the rate of motion of
lava-streams we must proceed to consider.
[Illustration: Fig. 18.--Cascade of Lava tumbling over a cliff in the
Island of Bourbon.]
[Sidenote: TEMPERATURE OF LAVA-STREAMS.]
There can be no doubt that the temperature of lavas varies greatly in
different cases. This is shown by the fact that while some lavas are in
a state of complete fusion, similar to that of the slags of furnaces,
and like the latter, such lavas on cooling form a glassy mass, others
consist of a liquid magma in which a larger or smaller number of
crystals are found floating. In these latter cases the temperature of
the magma must be below the fusing-point of the minerals which exist
in a crystalline condition in its midst. It has indeed been suggested
that the whole of the crystals in lavas are formed during the cooling
down of a completely fused mass; but no one can imagine that the
enclosed crystals of quartz, felspar, leucite, olivine, &c., have
been so formed, such crystals being sometimes more than an inch in
diameter. The microscopic examination of lavas usually enables us to
discriminate between those complete crystals which have been formed at
great depths and carried up to the surface, and the minute crystalline
particles and microliths which have been developed in the glassy mass
during cooling. Crystals of the former class, indeed, exhibit abundant
evidence, in their liquid cavities and other peculiarities, that they
have not been formed by simple cooling from a state of fusion, but
under the combined action of heat, the presence of water and various
gases, and intense pressure.
As we have already seen, the different lavas vary greatly in their
degrees of fusibility. The basic lavas, containing a low percentage
of silica, are much more fusible than the acid lavas, which contain
a high percentage of silica. When the basic lavas are reduced to a
complete state of fusion their liquidity is sometimes very perfect, as
is the case at Kilauea in Hawaii, where the lava is thrown up into jets
and fountains, falling in minute drops, and being drawn out into fine
glassy threads. On the other hand, the less fusible acid lavas appear
to be usually only reduced to the viscous or pasty condition, which
artificial glasses assume long before their complete fusion. Of this
fact I have found many proofs in the Lipari Islands, where such glassy,
acid lavas abound. In fig. 6 (page 43) a lava-stream is represented on
the side of the cone of Vulcano.
[Sidenote: IMPERFECTLY FLUID LAVAS.]
This lava is an obsidian--that is to say, it is of the add type and
completely glassy--but its liquidity must have been very imperfect,
seeing that the stream has come to a standstill before reaching the
bottom of a steep slope of about 35°. In fig. 19 there is given a side
view of the same stream of obsidian, from which it will be seen that
it has flowed slowly down a steep slope and heaped itself up at the
bottom, as its fluidity was not complete enough to enable it to move on
a slighter incline. An examination of the interior of such imperfectly
fluid lavas affords fresh proofs of the slow and tortuous movements
of the mass. Everywhere we find that the bands of crystallites and
sphærulites are, by the movement of the mass, folded and crumpled and
puckered in the most remarkable manner, as is illustrated in figs. 20
and 21. Similar appearances occur again and again among the vitreous
and semi-vitreous acid lavas of Hungary.
[Illustration: Fig. 19.--Lava-stream (obsidian) in the Island of
Vulcano showing the imperfect liquidity of the mass.]
[Illustration: Fig. 20.--Interior of a Rhyolitic Lava-stream in the
Island of Lipari, showing broad sigmoidal folds produced by the slow
movements of the mass.]
[Illustration: Fig. 21.--Interior of a Rhyolitic Lava-stream in the
Island of Lipari, showing the complicated crumplings and puckerings
produced by the slow movements of the mass.]
[Sidenote: RATE OF MOVEMENT OF VESUVIAN LAVAS.]
But, although the temperature of lava-streams and the fusibility of
their materials may in some cases account for their condition of
either perfect liquidity or viscidity, it is clear that in other
instances there must be some other cause for this difference. Thus
it has been found that at Vesuvius the lavas erupted in modern times
have all a striking similarity to one another in chemical composition,
in the minerals which they contain, and in their structure. They are
all basic lavas, which when examined by the microscope are seen to
consist of a more or less glassy magma, in the midst of which numerous
crystals of augite, leucite, olivine, magnetite, and other minerals
are scattered. Yet nothing can be more strikingly different than the
behaviour of the lavas poured out from Vesuvius at various periods. In
some cases the lava appears to be in such a perfectly liquid condition
that, issuing from the crater, it has been described as rushing down
the slope of the cone like a stream of water, and such exceedingly
liquid lavas have in some cases flowed to the distance of several
miles from the base of the mountain in a very short time. But other
Vesuvian lavas have been in such a viscid condition that their rate
of movement has been so extremely slow as to be almost imperceptible.
Such lava-streams have continued in movement during many years, but the
progress has been so slow (often only a few inches in a day) that it
could only be proved by means of careful measurements.
If we examine some of these Vesuvian lavas which have exhibited such
striking differences in their rate of flow, we shall find that they
present equally marked differences in the character of their surfaces.
The lava-current of 1858 was a remarkable example of a slow-flowing
stream, and its surface, as will be seen in fig. 22, which is taken
from a photograph, has a very marked and peculiar character. A
tenacious crust seems to have formed on the surface, and by the further
motion of the mass this crust or scum has been wrinkled and folded
in a very remarkable manner. Sometimes this folded and twisted crust
presents a striking resemblance to coils of rope. Precisely similar
appearances may be observed on the surface of many artificial slags
when they flow from furnaces, and are seen to be due to the same
cause, namely, the wrinkling up of the chilled surface-crust by the
movement of the liquid mass below. Lavas which present this appearance
are frequently called 'ropy lavas'; an admirable example of them is
afforded in the lava-cascade of the Island of Bourbon represented in
fig. 18 (page 93).
But lavas in which the rate of flow has been very rapid, exhibit
quite a different kind of surface to that of the ropy lavas. The
Vesuvian lava-stream of 1872 was remarkable for the rapidity of its
flow, and its surface presents a remarkable contrast to that of the
slow-moving lava of 1858. The surface of the lava-current of 1872 is
covered with rough cindery masses, often of enormous dimensions, and
it is exceedingly difficult to traverse it, as the ragged projecting
fragments tear the boots and lacerate the skin. The appearance
presented by this lava-stream is illustrated by fig. 23, which is also
taken from a photograph.
[Illustration: Fig. 22.--Vesuvian Lava-stream of 1858, exhibiting the
peculiar 'Ropy' Surfaces of Slowly Moving Currents.
(_From a Photograph._)]
[Illustration: Fig. 23.--Vesuvian Lava-stream of 1872, exhibiting the
Rough Cindery Surfaces characteristic of Rapidly Flowing Currents.
(_From a Photograph._)]
[Sidenote: VESUVIAN LAVA-STREAM OF 1872.]
Now it is found that those lava-streams which move slowly and present
ropy surfaces give off but little steam during their flow, while those
lava-streams which flow more rapidly and present a rough and cindery
appearance give off vast quantities of steam. The extraordinary amount
of vapour given off from the lava-streams which flowed from Vesuvius
in 1872 is illustrated in the photograph copied in fig. 5 (facing page
24), in which the three lava-currents are each seen to be surmounted
by enormous vapour-clouds rising to the height of several thousands
of feet above them, and mingling with the column that issued from
the central vent. By the escape of this enormous quantity of steam
the surface of the lava was thrown into rugged cindery projections,
and in some places little cones were formed upon it, which threw out
small scoriæ and dust. The quantity of vapour was, in fact, so great,
that little parasitical volcanoes were formed on the surface of the
lava-stream. Some of these miniature volcanoes were of such small
dimensions that they were carried away on boards to be employed as
illustrations in the lecture-rooms of the University of Naples.
The arrangement of the materials forced out from fissures on the
surfaces of lava-streams by the disengaged vapours and gases depends
on the degree of fluidity of the lava, and the force of the escaping
steam-jets. In very viscous lavas the materials may issue quietly,
forming great concentric masses like coils of rope; such were described
by Mr. Heaphy as occurring in New Zealand (see fig. 24).
[Illustration: Fig. 24.--Concentric Folds on mass of cooled Lava.]
In other cases the lava, if somewhat more liquid, may in issuing
quietly without great outbursts of steam, accumulate in great
bottle-shaped masses, which have been compared to 'petrified
fountains.' Cases of this kind have been described by Professor Dana as
occurring on the slopes of Hawaii (see fig. 25).
[Illustration: Fig. 25.--Mass of cooled Lava formed over a spiracle on
the slopes of Hawaii.]
[Sidenote: MINIATURE CONES ON LAVA-STREAMS.]
When the steam escapes with explosive violence from a spiracle
('bocca') on the surface of a lava-stream, minute cinder cones,
like those described as being formed in 1872, are the result. Fig.
26 represents a group of miniature cones thrown up on the Vesuvian
lava-stream of 1855: it is taken from a drawing by Schmidt.
[Illustration: Fig. 26.--Group of small Cones thrown up on the Vesuvian
Lava-current of 1856.]
Some of these appear like burst blisters or bubbles, while others
are built up of scoriaceous masses which have been ejected from the
aperture and have become united while in a semi-fluid condition. Other
examples of these spiracles or bocche on the surfaces of lava-currents
may be seen in the figs. 22 and 23, which are copied from photographs.
The facts we have described all point to the conclusion that the
presence of large quantities of water imprisoned in a mass of lava
contributes greatly to its mobility. And this conclusion is supported
by so many other considerations that it is now very generally accepted
by geologists. The condition of this imprisoned water in lavas is one
which demands further investigation at the hands of physicists. It has
been suggested, with some show of reason, that the water may exist
in the midst of the red-hot lava as minute particles in the curious
'spheroidal condition' of Boutigny, and that these flash into steam as
the lava flows along.
Lava, when extruded from a volcanic crater in a more or less completely
fluid state, flows down the side of the cone, and then finds its way
along any channel or valley that may lie in its course, obeying in its
movements all the laws of fluid bodies. The lava-currents thus formed
are sometimes of enormous dimensions, and may flood the whole country
for many miles around the vent.
Lava-streams have been described, which have flowed for a distance of
from fifty to a hundred miles from their source, and which have had
a breadth varying from ten to twenty miles. Some lava-streams have a
thickness of 500 feet, or even more. These measures will give some
idea of the enormous quantities of material brought from the earth's
interior by volcanic action and distributed over its surface. The mass
of lava which flowed out during an eruption off Reykjanes in Iceland,
in the year 1783, has been calculated to be equal in bulk to Mont Blanc.
There are many parts of the earth's surface, such as the Western Isles
of Scotland and the North-east of Ireland, the Deccan of India, and
large tracts in the Rocky Mountains, where successive lava-sheets have
been piled upon one another to the height of several thousands of feet,
and cover areas of many hundreds or even thousands of square miles.
[Sidenote: FEATURES OF LAVA-STREAMS.]
The more fusible basic lavas are as a general rule more liquid in
character than any others, and it is these very liquid lavas that are
usually found forming plateaux built up of successive lava-streams. The
less liquid lavas, like those of Hungary and Bohemia, are not usually
found flowing to such distances from the vent, but form dome-shaped
mountain-masses.
Lava-streams usually exhibit in their upper and under surfaces a
scoriaceous texture due to the escape of steam from the upper surface,
portions of the cindery masses so formed falling off from the end of
the stream, and being rolled over by the stream so as to form its
base. The thickness of this scoriaceous upper and lower part of a
lava-stream varies according to the quantity of steam imprisoned in
it; but all thick lava-streams have a compact central portion which
is composed of hard, solid rock. Very good examples of the internal
structure of lava-streams may sometimes be examined in the sea-cliffs
of volcanic islands. In fig. 27 we have given a copy of a drawing made
while sailing round the shores of Vulcano. The scoriaceous portions of
lava-streams are sometimes employed, as at Volvic in the Auvergne, as
a building material, or as at Neidermendig in the Eifel and in Hungary
for mill-stones; the compact portions are employed for building and
paving, and for road metal. The rock of some of the modern lava-streams
of Vesuvius is largely quarried for paving the streets of Naples.
This solid portion of the lava-streams in slowly cooling down from its
highly-heated condition undergoes contraction, and in consequence is
rent asunder by a number of cracks. Sometimes these cracks assume a
wonderfully regular arrangement, and the rock may be broken up into
very symmetrical masses.
[Illustration: Fig. 27.--Natural section of a Lava-stream in the Island
of Vulcano, showing the compact central portion and the scoriaceous
upper and under surfaces.]
[Sidenote: COLUMNAR STRUCTURE OF LAVAS.]
If we imagine a great sheet of heated material, like a lava-stream,
slowly cooling down, it is evident that the contraction which must
take place in it will tend to produce fissures breaking up the mass
into prisms. A little consideration will convince us what the form of
these prisms must be. There are only three regular figures into which
a surface can be divided, namely, equilateral triangles, squares, and
regular hexagons; the first being produced by the intersection of
sets of six lines radiating at angles of 60° from certain centres;
the second by the intersection of sets of _four_ lines radiating from
centres at angles of 90°; and the third from sets of _three_ lines
radiating from centres at an angle of 120°. It is evident that a less
amount of contractile force will be required to produce the sets of
_three_ cracks rather than those of four or six cracks; or, in other
words, the contractile force in a mass will be competent to produce the
cracks which give rise to hexagons rather than those which form squares
or triangles. This is no doubt the reason why the prisms formed by the
cooling of lava, as well as those produced during the drying of starch
or clay, are hexagonal in form.
The hexagonal prisms or columns formed by contraction during the
consolidation of lavas vary greatly in size, according to the rate of
cooling, the nature of the materials, and the conditions affecting the
mass. Sometimes such columns may be found having a diameter of eight
or ten feet and a length of five hundred feet, as in the Shiant Isles
lying to the north of the Island of Skye; in other cases, as in certain
volcanic glasses, minute columns, an inch or two in length and scarcely
thicker than a needle, are formed; and examples of almost every
intermediate grade between these two extremes may sometimes be found.
The largest columns are those which are formed in very slowly cooling
masses.
The columnar structure is exhibited by all kinds of lava, and indeed
in other rock-masses which have been heated by contact with igneous
masses and gradually cooled. The rocks which display the structure in
greatest perfection, however, are the basalts.
Mr. Scrope first called attention to the fact that the upper and lower
portions of lava-streams sometimes cool in very different ways, and
hence produce columns of dissimilar character. The lower portion of the
mass parts with its heat very slowly, by conduction to the underlying
rocks, while the upper portions radiate heat more irregularly into
the surrounding atmosphere. Hence we often find the lower portions of
thick lava-streams to be formed of stout, vertical columns of great
regularity; while the upper part is made up of smaller and less regular
columns, as shown in fig. 28.
[Illustration: Fig. 28.--Section of a Lava-stream exposed on the side
of the river Ardèche, in the south-west of France.]
The remarkable grotto known as Fingal's Cave in the Island of Staffa
has been formed in the midst of a lava-stream such as we have been
describing; the thick vertical columns, which rise from beneath the
level of the sea, are divided by joints and have been broken away by
the action of the sea; in this way a great cavern has been produced,
the sides of which are formed by vertical columns, while the roof is
made up of smaller and interlacing ones. The whole structure bears some
resemblance to a Gothic cathedral; the sea finding access to its floor
of broken columns, and permitting the entrance of a boat during fine
weather. Similar, though perhaps less striking, structures are found in
many other parts of the globe wherever basaltic and other lava-streams
exhibit the remarkable columnar structure as the result of their slow
cooling. Portions of basaltic columns are often employed for posts
by the road-sides, as in Central Germany and Bohemia, or for paving
stones, as in Pompeii and at the Monte Albano near Rome.
[Illustration: Fig. 29.--Portion of a Basaltic Column from the Giant's
Causeway, exhibiting both the ball-and-socket and the tenon-and-mortise
structures.]
[Sidenote: OTHER JOINT-STRUCTURES IN LAVAS.]
Occasionally basaltic lava-streams exhibit other curious structures in
addition to the columnar. Thus some basaltic columns are found divided
into regular joints by equidistant, curved surfaces, the joints thus
fitting into one another by a kind of ball-and-socket arrangement.
Sometimes we find processes projecting from the angles of the curved
joint-surfaces, which cause the blocks to fit together as with a tenon
and mortise. This kind of structure is admirably displayed at the
Giant's Causeway, Co. Antrim, in the North of Ireland. A portion of a
basaltic column from this locality is represented in fig. 29.
[Illustration: Fig. 30.--Vein of green Pitchstone, at Chiaja di Luna
in the Island of Ponza, breaking up into regular columns, and into
spherical masses with a concentric series of joints.]
While the ordinary columnar structures are very common in basalts,
the ball-and-socket and tenon-and-mortise structures are exceedingly
rare. The question of the mode of origin of these remarkable structures
has given rise to much discussion, and the opinions of geologists and
physicists are by no means unanimous upon the subject.
Sometimes we find masses of lava traversed by curved joints, and
occasionally we find curious combinations of curved and plane joints,
giving rise to appearances scarcely less remarkable than those
presented by the columns of the Giant's Causeway. Some of the more
striking examples of this kind have been described and explained by
Professor Bonney.
[Illustration: Fig. 31.--Illustration of the 'Perlitic structure' in
glassy Rocks.
a. Perlltic structure, as seen in a lava from Hungary.
b. The same structure, artificially produced in Canada Balsam
during cooling.
]
[Sidenote: PERLITIC-STRUCTURE IN LAVAS.]
In the Ponza Islands there occurs a remarkable example of a columnar
pitchstone, which is also traversed by a member of curved concentric
joints, causing the rock to break up into pieces like the coats of an
onion. This remarkable rock-mass is represented in fig. 30.
A very similar structure is often seen in certain glassy lavas, when
they are examined in thin sections under the microscope. Such glassy
lavas exhibit the peculiar lustre of mother-of-pearly doubtless in
consequence of the interference of light along the cracks. Lavas
exhibiting this character are known to geologists as 'perlites.' The
perlitic structure has been produced artificially by Mr. Grenville Cole
in Canada Balsam, and by MM. Fonqué and Michel Lévy, in chemically
deposited silica. See fig. 31.
A thick lava-stream must take an enormous period to cool down--probably
many hundreds or even thousands of years. It is possible to walk over
lava-streams in which at a few inches below the surface the rock is
still red-hot, so that a piece of stick is lighted if thrust into a
crack. Lava is a very bad conductor of heat, and loose scoriæ and
dust are still worse conductors. During the eruption of Vesuvius in
1872, masses of snow which were covered with a thick layer of scoriæ,
and afterwards by a stream of lava, were found three years afterwards
consolidated into ice, but not melted. The city of Catania is
constantly supplied with ice from masses of snow which have been buried
under the ejections of Etna.
During the cooling down of lavas, the escape of steam and various
gases gives rise to the deposition of many beautiful crystalline
substances in the cavities and on the surfaces of the lava. Deposits
of sulphur, specular-iron, tridymite, and many other substances are
often thus produced, and the colour and appearance of the rock-masses
are sometimes completely disguised by these surface incrustations, or
by the decomposition of the materials of the lava by the action of the
add gases, and vapours upon it.
[Sidenote: SINKING OF SURFACES OF LAVA-STREAMS.]
Very frequently the surface of a lava-stream becomes solid, while the
deeper portions retain their fluid condition; under such circumstances
the central portions may flow away, leaving a great hollow chamber or
cavern. In consequence of this action, we not unfrequently find the
upper surface of a lava-current exhibiting a depression, due to the
falling in of the solidified upper portions when the liquid lava has
flowed away and left it unsupported, as in fig. 32.
[Illustration: Fig. 32.--Transverse section of a Lava stream. (The
dotted line indicates the original surface.)]
CHAPTER V
THE INTERNAL STRUCTURE OF VOLCANIC MOUNTAINS.
Near the high-road which passes between the towns of Eger and
Franzenbad in Bohemia, there rises a small hill known as the Kammerbühl
(see fig. 33), which has attracted to itself an amount of interest
and attention quite out of proportion to its magnitude or importance.
During the latter part of the last century and the earlier years of
the present one, the fiercest controversies were waged between the
partisans of rival schools of cosmogony over this insignificant hill;
some maintaining that it originated in the combustion of a bed of coal,
others that its materials were entirely formed by some kind of 'aqueous
precipitation,' and others again that the hill was the relic of a small
volcanic cone.
Among those who took a very active part in this controversy was
the poet Goethe, who stoutly maintained the volcanic origin of the
Kammerbühl, styling it 'a pocket edition of a volcano.' To Goethe
belongs the merit of having suggested a Very simple method by which the
controversies concerning this hill might be set at rest: he proposed
that a series of excavations should be undertaken around the hill, and
a tunnel driven right under its centre.
[Illustration: Fig. 33.--The Kammerbühl of Kammerberg, Bohemia.
(As seen from the south-west)]
[Sidenote: THE KAMMERBÜHL.]
The poet's friend, Count Caspar von Sternberg, determined to put this
project into execution. This series of excavations, which was completed
in 1837, has for ever set at rest all doubts as to the volcanic origin
of the Kammerbühl. A plug of basalt was found filling the centre of
the mass, and connected with a small lava-stream flowing down the side
of the hill; while the bulk of the hill was shown to be composed of
volcanic scoriæ and lapilli. The section fig. 34 will illustrate the
structure of the hill as revealed by these interesting excavations.
[Illustration: Fig. 34.--Section of the Kammerbühl, in Bohemia.
_a a._ Metamorphic rocks. _b._ Basaltic scoriæ. _c._ Solid plug
of basalt rising through the centre of the volcanic pile, _d d._
Lava-stream composed of the same rock. _e e._ Alluvial matter
surrounding the old volcano.
(The dotted lines indicate the probable former outline of the volcano.)]
[Sidenote: VOLCANOES DISSECTED BY DENUDATION.]
It can of course very seldom happen that actual mining operations, like
those undertaken in the case of the Kammerbühl, will be resorted to
in order to determine the structure of volcanic mountains. Geologists
have usually to avail themselves of less direct, but by no means less
certain, methods than that of making artificial excavations in order
to investigate the earth's crust. Fortunately it happens that what we
cannot accomplish ourselves, nature does for us. The action which we
call 'denudation' serves as a scalpel to dissect volcanic mountains
for us, and to expose their inner recesses to our view. Many portions
of the earth's surface are complete museums crowded with volcanic
'subjects,' exhibiting every stage of the process of dissection. In
some, rains and winds have stripped off the loose covering of cinders
and dust, and exposed the harder and more solid parts--the skeleton of
the mountain. In others, the work of destruction has proceeded still
further, and slowly wearing rivers or the waves of the sea may have cut
perfect, vertical sections of the mountain-mass. Sometimes the removal
of the materials of the volcanic mountain has gone on to such an extent
that its base and ground-plan are fully exposed. It only requires the
necessary skill in piecing together our observations on these dissected
volcanoes, in order to arrive at just views concerning the 'comparative
anatomy' of volcanoes. As the knowledge of the structure of animals
remained in the most rudimentary condition until the practice of
dissection was commenced, so our knowledge of volcanoes was likewise
exceedingly imperfect till geologists availed themselves of the
opportunities afforded to them of studying naturally dissected volcanic
mountains.
In some cases we may find that the sea has encroached on the base
of a volcanic hill, till one half of it has been washed away, and
the structure of the mass to its very centre is exposed to our view.
Thus in fig. 6 (page 43), it will be seen that there lies in front of
Vulcano a peninsula called Vulcanello, consisting of three volcanic
cones, united at their base, with the lava-streams which have flowed
from them. One half of the cone on the left-hand side of the picture
has been completely washed away by the sea, and a perfect section
of the internal structure of the cone is exposed. The appearances
presented in this section are shown in the sketch, fig. 35. Some
portions of the face of this section are concealed by the heaps of
fragments which have fallen from it, but enough is visible to convince
us that three kinds of structures go to make up the cone. In the first
place, we have the loose scoriæ and lapilli, which in falling through
the air have arranged themselves in tolerably regular layers upon the
sides of the cone.
[Illustration: Fig. 35.--Natural section or a Volcanic Cone in the
Island of Vulcano.
_a._ Crater. _b b._ Lava-streams. _c._ Dykes which have clearly formed
the ducts, through which the lava has risen to the crater. _d d._
Stratified volcanic scoriæ. _e._ Talus of fallen materials.]
In the second place, we have lava-streams which have been ejected from
the crater or from fissures on the flanks of the cone, and flowed down
its sides. And thirdly, we find masses of lava filling up cracks in
the cone; these latter are called 'dykes.' Of these three kinds of
structures most volcanic mountains are built up, but in different
cases the part played by these several elements may be very unequal.
Sometimes volcanoes consist entirely of fragmentary materials, at
others they are made up of lavas only, while in the majority of
cases they have been formed by alternations of fragmentary and fluid
ejections, the whole being bound together by dykes, which are masses of
lava injected into the cracks formed from time to time in the sides of
the growing cone.
If we direct our attention in the first place to the fragmentary
ejections, we shall find that they affect a very marked and peculiar
arrangement, which is best exhibited in those volcanic cones composed
entirely of such materials.
[Sidenote: INTERNAL STRUCTURE OF VOLCANIC CONES.]
Everyone who examines volcanoes for the first time will probably be
struck by the regular stratification of materials of which they are
composed. Thus the tuffs covering the city of Pompeii are found to
consist of numerous thin layers of lapilli and volcanic dust, perfectly
distinct from one another, and assuming even the arrangement which we
usually regard as characteristic of materials that have been deposited
from a state of suspension in water. The fragmentary materials in
falling through the air are sorted, the finer particles being carried
farther from the vent than the larger and heavier ones. The force
of different volcanic outbursts also varies greatly, and sometimes
materials of different character are thrown out during successive
ejections. These facts will be illustrated by fig. 36, which is a
drawing of a section exposed in a quarry opened in the side of the
Kammerbühl. In this section we see that the falling scoriæ have been
arranged in rudely parallel beds, but the regular deposition of these
has been interrupted by the ejection of masses of burnt slate torn from
the side of the vent, probably during some more than usually violent
paroxysm of the volcano. In those volcanoes which are built up of tuffs
and materials which have fallen in the condition of a muddy paste, the
perfect stratification of the mass is often very striking indeed, and
large cones are found built up of thin uniformly-spread layers of more
or less finely-divided materials, disposed in parallel succession.
Such finely-stratified tuff-cones abound in the district of the Campi
Phlegræi.
[Illustration: Fig. 36.--Section in the side of the Kammerbühl, Bohemia.
_a a._ stratified basaltic scoriæ. _b b._ Bands made up of fragments
of burnt slate. _c._ Stratified basaltic scoriæ. _d d._ Pseudo-dykes
occupying lines of fault.]
[Sidenote: ARRANGEMENT OF FRAGMENTAL MATERIALS.]
If, in consequence of any subterranean movements, fissures are produced
in the sides of the cones formed of fragmentary materials, these often
become gradually filled with loose fragments from the sides of the
fissure, and in this manner 'pseudo-dykes' are formed. An example of
such pseudo-dykes is represented in fig. 36, where the beds composing
the volcanic cone of the Kammerbühl are seen to have been broken across
or faulted, and the fissures produced in the mass have been gradually
filled with loose fragments.
It is not difficult to imitate, on a small scale, the conditions which
exist at those volcanic vents from which only fragmentary materials
are ejected. If we take a board having a hole in its centre, into
which a pipe is inserted conveying a strong air-blast, we shall, by
introducing some light material like bran or sawdust into this pipe
cause an ejection of fragments, which will, when the board is placed
horizontally, fall around the orifice of the pipe and accumulate there
in a conical heap (fig. 37). It will be found necessary, as was shown
by Mr. Woodward, who performed the experiment before the Physical
Society, to adopt some contrivance, such as a screw, for forcing the
material into the air-pipe. If we alternately introduce materials of
different colours, like mahogany- and deal-sawdust into the pipe, these
materials will be arranged in layers which can be easily recognised,
and the mode of accumulation of the mass will be evident. By means of
a sheet of tin or cardboard we may divide this miniature volcanic cone
vertically into two portions, and if we sweep one of these away the
internal structure of the other half will be clearly displayed before
our eyes.
In this way we shall find that the conical heap of sawdust with the
hole in its centre has a very peculiar and definite arrangement of its
materials. It is made up of a number of layers each of which slopes in
opposite directions, towards the centre of ejection and away from that
centre. These layers are thickest along the line of the circle where
the change in slope takes place, and they thin away in the direction of
the two opposite slopes.
[Illustration: Fig. 37.--Experimental illustration of the mode of
Formation of Volcanic Cones composed of fragmental materials.]
[Sidenote: CAUSE OF THIS ARRANGEMENT.]
The cause of this peculiar arrangement of the materials is evident.
The sawdust thrown up by the air blast descends in a shower and tends
to accumulate in a circular heap around the orifice, the area of this
circular heap being determined by the force of the blast. Within this
circular area, however, the quantity of falling fragments is not
everywhere the same; along a circle surrounding the vent at a certain
distance, the maximum number of falling fragments will be found to
descend, and here the thickest deposit will take place. As this goes
on, a circular ridge will be formed, with slopes towards and away
from the centre of injection. As the ridge increases in height, the
materials will tend to roll down either one slope or the other, and
gradually a structure of the form shown in the figure will be piled up.
The materials sliding down the outer slope will tend to increase the
area of the base of the cone, while those which find their way down the
inner slope will fall into the vent to be again ejected.
[Illustration: Fig. 38.--Natural section of a Tuff-cone forming the
Cape of Misenum, and exhibiting the peculiar internal Arrangement
characteristic of volcanoes composed of fragmentary materials.]
Volcanic cones composed of scoriæ, dust, &c. are found to have exactly
the same internal structure as is exhibited by the miniature cone of
sawdust. The more or less regular layers of which they are made up dip
in opposite directions, away from and towards the vent, and thin out
in the direction of their dip (see fig. 38). In small cones the crater
or central cavity is of considerable size in proportion to the whole
mass, but as the cone grows upwards and outwards, the dimensions of
the crater remain the same, while the area of the base and the height
of the cone are continually increasing. This is the normal structure
of volcanic cones formed of fragmentary materials, though, as we shall
hereafter show, many irregularities are often produced by local and
temporary causes.
[Illustration: Fig. 39.--Section of a small Scoria-cone formed within
the crater of Vesuvius in the year 1885, illustrating the filling-up of
the central tent of the cone by subsequent ejections.]
In some cases the central vent of a volcanic scoria-cone may be
filled up by subsequent ejections. A beautiful example of this kind
was observed by Abich, in the case of a small cone formed within the
crater of Vesuvius in 1835, and is represented in fig. 39.
[Illustration: Fig. 40.--Volcanic Cones composed of Scoriæ, and
breached on one side by the outflow of lava-currents.]
[Sidenote: BREACHED CONES.]
Many cones formed in the first instance of scoriæ, tuff, and pumice may
give rise to streams of lava, before the vent which they surround sinks
into a state of quiescence. In these cases, the liquid lava in the vent
gives off such quantities of steam that masses of froth or scoriæ are
formed, which are ejected and accumulate around the orifice. When the
force of the explosive action is exhausted, the lava rises bodily in
the crater, which it more or less completely fills. But, eventually,
the weaker side of the crater-wall yields beneath the pressure of the
liquid mass, and this part of the crater and cone is swept away before
the advancing lava-stream. Examples of such 'breached cones' abound in
Auvergne and many other volcanic districts (see fig. 40). A beautiful
example of a cone formed of pumice, which has been breached by the
outflow of a lava-stream of obsidian, occurs in the Lipari Islands, at
the Rocche Rosse. It is this locality which supplies the whole world
with pumice (see fig. 41).
[Illustration: Fig. 41.--Campo Bianco, in the Island of Lipari. A
Pumice-cone breached by the Outflow of an Obsidian Lava-current.]
It is often surprising to find how volcanic cones composed of loose
materials, such as tuffs, scoriæ, or pumice, retain their distinctive
forms, and even the sharpness of their outlines, during enormous
periods of time. Thus, in the scoria-cones which abound in the
Auvergne, and were, in all probability, formed before the historical
period, the sharp edges of the craters appear to have suffered scarcely
any erosion, and the cones are as perfect in their outlines as though
formed but yesterday. It is probable that the facility with which these
cindery heaps are penetrated by the rain which falls upon them is the
cause why they are not more frequently washed away.
[Illustration: Fig. 42.--Volcanic Cones in Auvergne which have suffered
to some extent from atmospheric denudation.]
Sometimes, however, scoria-cones are found reduced by atmospheric
waste to mere heaps of cinders, in which the position of the crater is
indicated only by a slight depression, as in fig. 42.
[Sidenote: CONES COMPOSED OF LAVA.]
When but little explosive action takes place at the volcanic vent, and
only fluid lava is ejected, mountains are formed differing very greatly
in character from the cones composed of fragmentary materials.
If the lavas be of very perfect liquidity, like those erupted in the
Sandwich Islands, they flow outwards around the vent to enormous
distances. By the accumulation of materials during successive
outbursts, a conical mass is built up which has but a slight elevation
in proportion to the area of its base. Thus in Hawaii we find great
volcanic cones, composed of very fluid lavas, which have a height of
nearly 14,000 feet with a diameter of base of seventy miles. In these
Hawaiian mountains the slope of the sides rarely exceeds 6° to 8°.
But if, on the other hand, the lavas be of much more viscid
consistency, the character of the volcanic cones which are produced by
their extrusion will be very different. The outwelling material will
tend to accumulate and heap itself up around the vent. By successive
ejections the first-formed shell is forced upwards and outwards, and
a steep-sided protuberant mass is formed, exhibiting in its interior
a marked concentric arrangement. Dr. Ed. Reyer, of Grätz, has devised
a very ingenious method for reproducing on a miniature scale the
characteristic features of these eruptions of viscid lavas. He takes a
quantity of plaster of Paris reduced to a pasty consistence, which he
forces through a hole in a board. The plaster accumulates in a great
rounded boss about the orifice through which it has been forced. If the
plaster have some colouring matter introduced into it, the mass, on
being cut across, will exhibit in the disposition of its colour-bands
the kind of action which has gone on during its extrusion, fig. 43.
[Illustration: Fig. 43.--Experimental illustration of the Mode of
Formation of volcanic cones composed of viscid lavas.]
[Illustration: Fig. 44.--The Grand Puy of Sarcoui, composed of
trachyte, rising between two breached scoria-cones (Auvergne).]
There are many volcanic cones which exhibit clear evidence of having
thus been formed by the extrusion of a viscid mass of lava through
a volcanic fissure. Among such we may mention the domitic Puys of
Auvergne, fig. 44, many andesitic volcanoes in Hungary, the phonolite
hills of Bohemia, and the so-called 'mamelons' of the Island of
Bourbon. See figs. 45 and 46. When the interior of these masses is
exposed by natural or artificial sections, they are all found to
exhibit the onion-like structure which occurs in the plaster models.
[Sidenote: INTERNAL STRUCTURE OF LAVA-CONES.]
[Illustration: Fig. 45.--Volcanic Cone (Mamelon) composed of very
viscid lava. (Island of Bourbon.)]
[Illustration: Fig. 46.--Another Mamelon in the Island of Bourbon, with
a crater at its summit.]
But while some volcanoes are composed entirely of the fragmentary
ejections and others are wholly formed by successive outflows of lava,
the majority of volcanoes, especially those of larger dimensions, are
built up of alternations of these different kinds of materials.
[Illustration: Fig. 47.--Cliff-section in the Island of Madeira,
showing how a composite volcano is built up of lava-streams, beds of
scoriæ, and dykes.]
[Sidenote: NATURAL SECTIONS OF CONES.]
The structure of these composite cones may be understood by an
inspection of the accompanying fig. 47, which shows the appearances
presented in a cliff on the coast of the Island of Madeira. We see
that the mass is made up of numerous layers of volcanic scoriæ,
alternating with sheets of lava. The latter, which are represented
in transverse section in the drawing, are seen to thin out on either
side, and to vary greatly in breadth. Besides the alternating masses
of scoriæ and the lava-sheets, there are seen in the section, bands
of a bright-red colour, which are represented in the drawing by black
lines. These are layers of soil, or volcanic dust, which, by the
passage of a lava-stream over their surface, have been burnt so as
to acquire a brick-red colour. These bands of red material, to which
the name of 'laterite' has been frequently applied, very commonly
occur in sections of composite volcanic cones. Crossing the whole of
the horizontally-disposed masses in the section, we find a number of
'dykes,' which are evidently great cracks filled with lava from below.
Some of these run vertically through the cliffs, others obliquely. In
some cases the lava, rising to fill a dyke, has flowed as a lava-stream
at the surface. Last of all, we must call attention to the fact that
the section exhibits evidence of great movements having taken place
subsequently to the accumulation of the whole of the materials. A
great crack has been produced, on one side of which the whole mass has
subsided bodily, giving rise to the phenomenon which geologists call a
'fault.'
In the section, fig. 27, p. 104, copied from a drawing of a sea-cliff
in the Island of Vulcano, a transverse section of a lava-stream is
represented on a somewhat larger scale. The upper and under surface of
the lava-stream is seen to have a scoriaceous structure, but the thick
central mass is compact, and divided by regular joint-planes. This
section also illustrates the fact that, before the lava-stream flowed
down the sides of the mountain, a valley had been cut by meteoric
agencies on the flanks of the volcano, the dykes which traverse the
lower beds of tuff being abruptly truncated.
In mountain ravines, upon the slopes of ancient volcanoes, and in the
cliffs of volcanic islands, we are often able to study the way in which
these great mountain masses are built up of alternating lava-currents,
beds of volcanic agglomerate, scoriæ, tuff and dust, and intersecting
dykes. In fig. 48, the features above described are illustrated by a
section in the sides of the great volcano of Mont Dore.
[Illustration: Fig. 48.--Section seen at the cascade. Bains du Mont
Dore.]
[Illustration: Fig. 49.--Section in the Island of Ventotienne, showing
a great stream of andesitic lava overlying stratified tuffs.]
[Sidenote: SECTIONS IN THE PONZA ISLANDS.]
In figs. 49, 50, 51, and 52, we have given drawings of portions of the
sea-cliffs in several of the Ponza Islands, a small volcanic group off
the Italian coast.
[Illustration: Fig. 50.--Cliff on the south side of the Island of San
Stephano.
_a._ Trachyte lava-stream, with a scoriaceous upper surface overlaid by
stratified tuffs, _b_.]
[Illustration: Fig. 51.--The headland of Monte della Guardia, in the
Island of Ponza.
_a._ Columnar trachyte. _b._ Stratified tuffs. _c._ Pumiceous
agglomerates. _d._ Dyke of rhyolite.]
[Illustration: Fig. 52.--Western side of the same headland, as seen
from the north side of Luna Bay.
_a._ Trachyte lava. _b._ Stratified tuffs. _c._ Dykes of rhyolite, with
their edges passing into pitchstone. _d._ Pumiceous agglomerate.]
[Illustration: Fig. 53.--Sea-cliff at Il Capo, the north-east point of
Salina showing stratified agglomerates traversed by numerous dykes, the
whole being unconformably overlaid by stratified aqueous deposits.]
Fig, 53 represents a cliff-section in the island of Salina, one of the
Liparis, exhibiting evidence that a series of volcanic agglomerates
traversed by dykes of Andesite have been denuded and covered by a
recent stratified deposit.
[Sidenote: PART PLAYED BY DYKES IN CONE-BUILDING.]
In the formation of these great composite cones, a minor but by no
means insignificant part is played by the dykes, or lava-filled
fissures, which are seen traversing the mass in all directions. That
dyke-fissures often reach the surface of a volcanic cone, and that
the material which injects them then issues as a lava-stream, is
illustrated by fig. 54. The formation of these cracks in a volcanic
cone, and their injection by liquid lava, must of course distend
the mountainous mass and increase its volume. If we visit the great
crater-walls of Somma in Vesuvius, and of the Val del Bove in Etna,
we shall find that the dykes are so numerous that they make up a
considerable portion of the mass. When the loose scoriæ and tuffs are
removed by denudation, these hard dykes often stand up prominently like
great walls, as represented in fig. 55. Even in such cases as these,
however, it is doubtful whether the bulk of all the dykes put together
exceeds one-tenth of that of the lavas and fragmentary materials.
[Illustration: Fig. 54.--Section observed in the Val del Bove, Etna,
showing a basaltic dyke, from the upper part of which a lava-current
has flowed.]
[Illustration: Fig. 55.--Basaltic Dykes projecting from masses of
stratified scoriæ in the sides of the Val del Bove, Etna.]
Hence we are led by an examination of the internal structure of
volcanic mountains to conclude that scoriæ- and tuff-cones, and cones
formed of very liquid lavas, increase by an _exogenous_ mode of growth,
all new materials being added to them from without; in the cones formed
of very viscid lavas, on the other hand, the growth is _endogenous_,
taking place by successive accretions within it. The composite cones
owe their origin to both the _exogenous_ and the _endogenous_ modes of
growth, but in a much greater degree to the former than the latter. The
layers of scoriæ, tuff, and dust, and the successive lava-streams are
added to the mass from without, and the lava forming the dykes from
within it.
[Sidenote: THEORY OF ELEVATION CRATERS.]
There are doubtless cases in which, when a tuff-cone is formed, a mass
of very viscid lavas is extruded into its interior, and the mass is
distended like a gigantic bubble. But inasmuch as the very viscid lavas
do not appear to give rise to scoriæ to anything like the same extent
as the more liquid kinds, such 'cupolas,' as they have been called
by some German geologists, are probably not very numerous, and may
be regarded as constituting the exception rather than the rule. The
idea which was formerly entertained by some geologists that all great
volcanic mountains were formed of masses originally deposited in a
horizontal position, and subsequently blown up into a conical form, has
been effectually disposed of by the observations of Lyell and Scrope.
The condition of the great fluid masses which underlie volcanic vents
is another point on which much light has been thrown by the study
of naturally-dissected volcanoes. In some cases, as was shown by
Hochstetter during his admirable researches among the New Zealand
volcanoes, the rising lavas form a great chamber for themselves in the
midst of a volcanic cinder-cone, taking the place of loose materials
which are re-ejected from the vent, or have been re-fused and absorbed
into the mass of lava itself. From this central reservoir of lava,
eruptions are kept up for some time, but when the volcano sinks
into a state of quiescence the lava slowly consolidates. In such
slowly solidified masses of lava, very beautiful groups of radiating
columns are often exhibited Northern Germany abounds with examples
of such basaltic masses, which have once formed the centres of great
cinder-cones; but in consequence of the removal of the loose materials
and the surrounding strata by denudation, these central reservoirs of
the volcanoes have been left standing above the surface, and exhibit
the peculiar arrangements of the columns formed in them during the
process of cooling.
[Sidenote: INTRUSIVE LAVA-SHEETS.]
But in the majority of the more solidly-built composite volcanoes no
such liquid reservoir can be formed within the volcanic cone itself.
Under these circumstances, the lavas, especially those of more liquid
character, tend to force passages for themselves among the rocks
through which they are extruded. Wherever a weak point exists, there
such lavas will find their way, and as the planes of stratification
in sedimentary rocks constitute such weak places, we constantly find
sheets of lava thus inserted between beds of aqueous origin. The areas
over which these intrusive sheets of rock sometimes extend may be very
great, but the more fusible, basic lavas (basalt, &c.) usually form
much more widely-spreading sheets than the less fusible, acid lavas.
In some cases these great intrusive sheets are found extending to a
distance of twenty or thirty miles from the centre at which they were
ejected, and they often follow the bedding of the strata with which
they are intercalated in so regular a manner, that it is difficult for
an observer to believe at first sight that they can have been formed in
the way which we have described. A closer examination will generally
reveal the fact that while these intrusive lava-sheets retain their
parallelism with the strata among which they have been intruded, over
considerable areas, yet they sometimes break across, or send offshoots
into them, as shown in fig. 56. In all cases, too, the rocks lying
above and below such sheets will be found to be more or less baked and
altered, and this affords a very convincing evidence of the intrusion
of the igneous mass between the strata so altered.
[Illustration: Fig. 56.--Sheets of Igneous Rock (Basalt) intruded
between beds of sandstone, clay, and limestone. (Island of Skye.)]
That in the case of most great volcanic mountains, or systems of
mountains, vast reservoirs of liquid lava must exist in the earth's
crust far below the surface, there can be little room for doubt.
Whether such fluid masses are in direct or indirect communication with
a great central reservoir, even supposing such to exist, is a totally
different question. In many cases the outburst of volcanoes in more or
less close proximity has been observed to take place simultaneously,
while in others the commencement of the eruption of one volcano has
coincided with the lapse into quiescence of another in its vicinity.
On the other hand, the remarkable case of the volcanoes of Hawaii
seems to indicate that two vents in close proximity may be supplied
from perfectly distinct reservoirs of lava. The active craters of
Mauna Loa and Kilauea are situated at the heights of 14,000 and 4,000
feet respectively above the sea level; yet the former is sometimes
in a state of violent activity, with which the latter shows no signs
of sympathy whatever. We shall, in a future chapter, adduce evidence
that the liquid lavas in underground reservoirs may undergo various
stages of change in the enormous periods of time during which habitual
volcanic vents are supplied from them.
We have already shown that the character assumed by a mass of fused
material in cooling varies greatly according as the cooling takes place
rapidly at the surface or slowly under enormous pressure. In the former
case a glassy base is formed containing a greater or smaller number
of crystallites or embryo crystals, in the latter the whole rock is
converted into a mass of fully-developed crystals.
[Sidenote: CONSOLIDATION OF LAVAS AT GREAT DEPTHS.]
The lavas which are poured out at the surface consist, as we have
seen, of a glassy magma in which a greater or smaller number of
crystals are found which have been borne up from below. The great
dykes and intrusive sheets consist for the most part of a mass of
small or imperfectly developed crystals in which a number of large and
perfectly formed crystals are embedded. Such rocks are said to have a
'porphyritic' structure. The rocks formed by the consolidation of the
liquid masses in the underground reservoirs are found to be perfectly
crystallised, the crystals impressing one another on every side and
making up the whole mass to the exclusion of any paste or magma between
them. The crystals in those rocks which have consolidated at these
vast depths exhibit evidence, in their enclosed watery solutions and
liquefied carbonic acid, of the enormous pressures under which they
must have been consolidated. The lavas, the more or less porphyritic
rocks of the dykes and sheets, and the perfectly crystalline (granitic)
rocks of the underground reservoirs pass into one another, however, by
the most insensible gradations.
We sometimes find examples of volcanoes which, by the action of
denuding forces, have had their very foundations exposed to our view.
Such examples occur in the Western Isles of Scotland, in the Euganean
Hills near Padua in Northern Italy, and in many other parts of the
earth's surface. In these cases we are able to trace the ground-plan of
the volcanic pile, and to study the materials which have consolidated
deep beneath the surface in the very heart of the mountain.
In studying these 'basal wrecks' of old volcanoes it is always
necessary to bear in mind that the appearance and general characters of
a volcanic rock may be completely disguised by chemical changes going
on within it. It is through want of attention to this fact that so many
mistakes were made by the Wernerian school of geologists who declared
that they could find no analogy between the basaltic rocks of the globe
and the products of active volcanoes, and were hence led to refer the
origin of the former to some kind of 'aqueous precipitation.'
Many of the hard and crystalline marbles which are employed as
ornamental stones were originally loose masses of shells and corals,
as we easily perceive when we examine the polished faces. But these
incoherent heaps of organic _débris_ have been converted into a compact
and solid rock in consequence of the mass being penetrated by water
containing carbonate of lime in solution. Crystals of this substance
were deposited in every cavity and interstice of the mass, and thus
the accumulation of separate organisms was gradually transformed to a
material of great solidity and hardness.
[Sidenote: FORMATION OF AMYGDALOIDS.]
In precisely the same way loose heaps of scoriæ, lapilli, or pumice
may, by the passage through them of water containing various substances
in solution, have their vesicles filled with crystals, and thus be
converted into the hardest and most solid of rock-masses. Similarly
the scoriaceous portions of lava-streams have their vesicles filled
with crystalline substances deposited from a state of solution, and
are thus converted into a solid mass which may at first sight appear
to offer but little resemblance to the vesicular materials of recent
lava-streams. To these vesicular rocks which have their cavities filled
with crystalline substances geologists apply the name of amygdaloids
(_L. amygdalus_, an almond). The cavities in lava-rocks are usually
more or less elongated, owing to the movement of the mass while in
a still plastic state, and the crystalline materials filling these
cavities take the almond-like shape; hence the name.
When the amygdaloids and altered fragmentary ejections of volcanoes
are studied microscopically, their true character is at once made
manifest. The exposure of faces of these altered volcanic rocks to the
weathering influences of the atmosphere, in many cases also causes
their true nature to be revealed, the crystalline materials filling
the interstices and vesicles of the mass are dissolved away by the
rain-water containing carbonic acid, and the rock regains its original
cavernous structure and appearance. But this repeated passage of water
through volcanic rock-masses may result in the removal of so large a
portion of their materials that the remainder crumbles down into the
condition of a clay or mud.
In the basal wrecks of volcanoes, of which we have spoken, we usually
find only small and fragmentary remains of the great accumulations
of loose and scoriaceous materials which originally constituted the
bulk of the mountain mass. In the centre of the ground-plan of such
a denuded volcano we find great masses of highly crystalline or
granitic rock, which evidently occupy vast fissures broken through
the sedimentary or other rocks upon which the volcanic pile has been
reared. These highly crystalline rocks exhibit, as we have shown,
clear evidence of having been consolidated from a state of fusion
with extreme slowness and under enormous pressure, but their ultimate
chemical composition is identical with that of the lavas which have
been ejected from the volcano.
When, as frequently happens, the volcano, after pouring out one kind
of lava for a certain period, has changed the nature of its ejections,
and given rise to materials of different composition, we find clear
evidence of the fact in studying the basal wreck or ground-plan of
the volcano. A great intrusive crystalline mass, of the same chemical
composition as the first-extruded lava, is found to be rent asunder and
penetrated by a similarly crystalline mass having the composition of
the lavas of the second period. Thus, in the volcanoes of the Western
Isles of Scotland, which are reduced by the action of denudation to
this condition of basal wrecks, we find that rhyolites, trachytes, and
andesites were ejected during the earlier periods of their history, and
basalts during the later periods.
[Illustration: Fig. 57.--Plan of the Dissected Volcano of Mull, in the
Inner Hebrides.]
[Illustration:
_a_ Rocks on which the Volcano has been built up.
_b_ Great intrusive masses of acid and intermediate rocks.
_c_ Lara currents of basalt which have flowed from _d_.
_d_ Intrusive masses of gabbros & dolerite.
_e_ Lava currents which have flowed from _b_.
_f_ Volcanic tuffs and agglomerates.
Fig. 58.--Section of the Volcano along the line _A B_.]
[Sidenote: ANCIENT VOLCANO OF MULL.]
We perceive on studying the ground-plan of these volcanoes that
great masses of granite, syenite, and diorite--the crystalline
representatives of the first-extruded lavas--are penetrated by
intrusions of gabbro--the granitic form of the later-ejected lavas.
These features are admirably illustrated by the ruined volcano now
constituting the Island of Mull, one of the Inner Hebrides, a plan
of which is given in fig. 57, and a section in fig. 58. This volcano
probably had a diameter at its base of nearly thirty miles, and a
height of from 10,000 to 12,000 feet, but is now reduced to a group of
hills few of which exceed 3,000 feet in height.
From these great intrusive masses of highly crystalline rocks there
proceed in every direction great spurs or dykes, which are evidently
the radiating fissures formed during the outwelling of igneous
materials from below, injected by these fluid substances. The rock
forming these dykes is often less perfectly crystalline than that which
constitutes the centre of the mass, and we may indeed detect among the
materials of these dykes examples of every variety of structure, from
the perfectly crystalline granite to the more or less glassy substance
of lavas. Besides the vertical or oblique dykes we also find horizontal
sheets, which, passing from these central masses, have penetrated
between the surrounding strata, often, as we have seen, to enormous
distances.
For the sake of simplicity, we have spoken of these ground-plans, or
basal wrecks of volcanoes, as constituting a flat plain; as a matter of
fact, however, the unequal hardness of the materials composing volcanic
mountains causes them to assume, under the influence of denuding
agencies, a very rugged and uneven surface. The hard crystalline
materials filling the central vent stand up as great mountain groups;
each large dyke, by the removal of the surrounding softer materials, is
left as a huge wall-like mass, while the remnants of lava-streams are
seen constituting a number of isolated plateaux.
The great Island of Skye is the basal wreck of another volcano which
was also in eruption during Tertiary times; probably, many millions
of years ago. This immense volcano had originally a diameter at its
base of about thirty miles, and a height of 12,000 to 15,000 feet,
and must have been comparable to Etna or Teneriffe in its dimensions.
At the present time, there is nothing left of this vast pile but the
highly crystalline granites and gabbros filling up the great fissures
through which the eruption of igneous materials took place. These, worn
by denudation into rounded dome-like masses and wild rugged peaks,
constitute the Red Mountains and Coolin Hills of Skye, which rise to
the height of more than 3,000 feet above the sea-level. From these
great, central masses of crystalline rocks, innumerable radiating dykes
may be found rising through the surrounding rock-masses, with isolated
patches of the scoriæ and lapilli ejected from the volcano, which have
here and there escaped removal by denudation. Along what were the
outskirts of this great mountain-mass are found flat-topped hills,
built up of lava-streams, only small portions of which have escaped
removal by denudation.
[Sidenote: RESERVOIRS BENEATH VOLCANOES.]
But this wearing away of the structure of a volcanic cone by the
denuding forces may proceed even one stage farther, and we may then
have revealed for our inspection and study the mass of originally fluid
materials, from which one or more volcanoes have been fed, cooled and
consolidated in their original reservoir. There are many examples of
masses of granitic or highly crystalline rocks, having precisely the
same composition as the different varieties of lavas, which are found
lying in the midst of the sedimentary rocks, and sending off into
these rocks veins and dykes of the same composition with themselves.
No one who has carefully studied the appearances presented by volcanic
mountains in different stages of dissection, by the action of denuding
forces, can avoid recognising these great granitic masses as the cooled
reservoirs from which volcanoes have in all probability been supplied
during earlier periods of the earth's history.
The eruption of these great masses of incandescent rock, impregnated
with water and acid gases, through strata of limestone, sandstone,
clay, coal, &c., may be expected to produce striking and wonderful
chemical changes in the latter. Nor are we disappointed in these
anticipations. Whenever we examine the sedimentary materials around
volcanic vents, we find that, in contact with the once-fused materials,
they everywhere exhibit remarkable evidences of the chemical action
to which they have been subjected. The limestones are converted into
statuary marble, the sandstones pass into quartzite, the days assume
the hardness and lustre of porcelain, while the coals have lost their
volatile ingredients and assumed a form like coke or graphite. And
these changes are found to extend in many cases to the distance of many
hundreds of yards from the planes of junction between the igneous and
the sedimentary materials.
Among the most interesting effects resulting from the extrusion of
masses of incandescent rock, charged with water and various gases,
through beds of limestone, clay, sandstone, &c., we may mention the
production of those beautiful crystalline minerals which adorn our
museums and are so highly prized as gems. By far the larger part of
these beautiful minerals have been formed, directly or indirectly, by
volcanic agencies.
These gems and beautiful minerals are, for the most part, substances
of every-day occurrence, which entirely owe their beauty to the
crystalline forms they have assumed. The diamond is crystallised
carbon, the ruby and sapphire are crystallised alumina, the amethyst
and a host of other gems are crystallised silica; and in almost all
cases the materials of gems are common and widely diffused, it is only
in their finely crystalline condition that they are rare and therefore
valuable.
[Sidenote: FORMATION OF VOLCANIC MINERALS.]
Crystals are formed during the slow deposition of a substance, either
by the evaporation of a liquid in which it is dissolved, by its
volatilisation, or its cooling from a state of fusion. In many cases it
can be shown that the formation of large and regular crystals is aided
if the work goes on with extreme slowness and under great pressure.
By sealing up various substances in tubes containing water which can
be kept at a high temperature, minute crystals of many well-known
minerals have been artificially formed by chemists. Part of the water
converted into steam has formed a powerful spring, which, reacting
upon the remainder of the liquid in the tube, has subjected it to
enormous pressure, and under these conditions of extreme pressure and
temperature, chemical actions take place of which we have no experience
under ordinary circumstances. The experiments of Mr. Hannay seem to
prove that when carbon is separated from certain organic substances
at a high temperature and under great pressure, it may crystallise in
the form of the diamond. And the recent discovery of diamonds in the
midst of materials filling old volcanic vents in South Africa seems
to show that this was in many cases the mode in which the gem was
originated. Even under the conditions which prevail at the earth's
surface, however, minute and unnoticed chemical actions taking place
during long periods of time, produce most remarkable results. This has
been well illustrated by M. Daubrée, who has shown that in the midst of
masses of concrete which the Romans built up around the hot springs of
Plombières and other localities, many crystalline minerals have been
formed, in the course of 2,000 years, by the action of the waters upon
the ingredients of the concrete.
But most of the crystals of minerals which have been thus artificially
formed are of minute, indeed often of microscopic, dimensions. In the
underground reservoirs beneath volcanoes, however, we have all the
necessary conditions for the formation of crystals of minerals on a
far grander scale. High temperatures, pressures far greater than any
we can command at the earth's surface, the action of superheated steam
and many acid gases on the various constituents of both igneous and
sedimentary rocks, and, above all, time of almost unlimited duration;
these constitute such a set of conditions as may fairly be expected
to result in the formation of crystals, similar to those artificially
produced but of far greater size and beauty.
If we visit those parts of the earth's surface where great masses of
fused volcanic rock have slowly cooled down in contact with sedimentary
materials, we shall not be disappointed in our expectations. Diamonds,
rubies, sapphires, emeralds, topazes, garnets, and a host of equally
beautiful, if less highly prized, crystalline substances, are found in
such situations, lying in the subterranean chemical laboratories in
which they have been formed, but now, by the action of denuding forces,
revealed to our view.
In some cases it is not necessary to penetrate to these subterranean
laboratories in order to find these beautiful gems and other
crystallised minerals; for the steam jets which issue from volcanic
fissures carry up fragments of rock torn from the side of the vent,
and in the cavities and fissures of such ejected masses beautiful
crystallised products are often found. Such rock-fragments containing
minerals finely crystallised are found abundantly on the flanks of
Vesuvius and other active volcanoes, and among the materials of the
Laacher See and other extinct volcanoes.
[Sidenote: FORMATION OF MINERAL-VEINS.]
But it is not only the finely crystallised minerals and gems which
we owe to volcanic action. The various metallic minerals have nearly
all been brought from deep-seated portions of the earth's crust and
deposited upon the sides of rock-fissures by the agency of the same
volcanic forces. It is these forces which have, in the first instance,
opened the cracks through the solid rock masses; and, in the second
place, have brought the metallic sulphides, oxides, and salts--either
in fusion, in solution, or in a vaporised condition--from the
deep-seated masses within the earth, causing them to crystallise upon
the sides of the fissures, and thus form those metallic lodes and veins
which are within reach of our mining operations.
There is still one other important class of minerals which owe the
existence, though indirectly, to volcanic agencies. The cavities of
igneous rocks, especially the vesicles formed by the escape of steam,
constitute, when filled with water, laboratories in which complicated
chemical reactions take place. The materials of the lava are gradually
dissolved and re-crystallised in new combinations. By this means the
most beautiful examples of such minerals as the agates, the onyxes,
the rock-crystals, the Iceland-spars, and the numerous beautiful
crystals classed together as 'Zeolites' have been formed. No one can
visit a large collection of crystalline minerals without being struck
with the large number of beautiful substances which have thus been
formed as secondary products from volcanic materials.
CHAPTER VI.
THE VARIOUS STRUCTURES BUILT UP AROUND VOLCANIC VENTS
From what has been said in the preceding chapters it will be seen that
while some of the materials ejected from volcanic vents are, by the
movements of the air and ocean, distributed over every part of the
face of the globe, another, and by far the larger, part of the matter
so ejected, accumulates in the immediate vicinity of the vent itself.
By this accumulation of erupted materials, various structures are
built up around the orifices from which the ejections take place, and
the size and character of these structures vary greatly in different
cases, according to the quantity and nature of the ejected materials,
and the intensity of the eruptive forces by which they were thrown from
the orifice. We shall proceed in the present chapter to notice the
chief varieties in the forms and characters of the heaps of materials
accumulated round volcanic vents.
These heaps of materials vary in size from masses no bigger than a
mole-heap up to mountains like Etna, Teneriffe, and Chimborazo. The
size of volcanic mountains is principally determined by the conditions
of the eruptive action at the vent around which they are formed. If
this action exhausts itself in a single effort, very considerable
volcanic cones, like the Monte Nuovo with many similar hills in its
vicinity, and the Puys of Auvergne, may be formed; but if repeated
eruptions take place at longer or shorter intervals from the same vent,
there appears to be scarcely any limit to the size of the structures
which may, under such conditions, be formed. It is by this repeated
action from the same volcanic vent going on for thousands or even
millions of years, that the grandest volcanic mountains of the globe
have been built up. Such volcanoes have sometimes a diameter at their
base of from 30 to 100 miles, and an elevation of from 10,000 to 25,000
feet.
The _form_ of volcanic mountains is determined in part by the nature
of the materials ejected, and in part by the character of the eruptive
action.
From what has been said in the preceding chapter, it will be gathered
that the volcanoes built up by ejections of fragmentary materials
differ in many striking particulars from those formed by the outwelling
of lavas from volcanic vents. In a less degree, the volcanoes composed
of the same kind of volcanic materials also vary among themselves.
[Sidenote: CHARACTERS OF SCORIA-CONES.]
When masses of scoriæ in a semi-fluid condition are thrown to only a
little distance above the volcanic vent, so that they have not time to
assume a perfectly solid condition before they fall round the vent, the
rugged masses of lava unite to form heaps of most irregular shape. In
such cases, the falling fragments being in a semi-plastic state, stick
to the masses below, and do not tend to roll down the sides of the
heap. Irregular heaps of such volcanic scoriæ abound on the surfaces
of lava-streams, being piled up around each 'bocca' or vent which the
steam-jets escaping from the lava-currents form at their surfaces.
Such irregular accumulations of scoriæ were observed on the lavas of
Vesuvius during the eruptions of 1822, 1855, and 1872, and have also
been described in the case of many other volcanoes. In fig. 26 (p. 101)
we have given representations of a group of such irregular scoria-cones
which was observed by Schmidt on the Vesuvian lava of 1855. It will
be seen from this drawing that there is scarcely any limit to the
steepness of the sides of such scoria-heaps, in which the materials are
in an imperfectly solidified condition when they reach the ground.
But in the majority of cases, the scoriæ ejected from volcanic vents
are thrown to a great height, and are in a more or less perfectly
solidified condition when they fell to the ground again. In such cases
the fragments obey the ordinary mechanical laws of falling bodies,
rolling and sliding over one another, till they acquire a slope which
varies according to the size and form of the fragments. In this way
the great conical mounds are formed which are known as 'cinder-cones,'
or more properly as 'scoria-cones.' Scoria-cones usually vary in the
slope of their sides from 35° to 40° and may differ in size from mere
monticules to hills a thousand feet or more in height. Scoria-cones
of this character abound in many volcanic districts, as the Auvergne,
where they may be numbered by thousands. The materials forming such
scoria-cones vary in size from that of a nut to masses as large as a
man's head, and fragments of even larger dimensions are by no means
uncommon.
When the lava in a volcanic vent is perfectly glassy, instead of being
partially crystalline in structure, we find not scoriæ but pumice
ejected. In such cases, as in the Lipari Islands for example, we see
cones entirely built up of pumice. Such pumice-cones resemble in
the angle of their slope (see fig. 41, facing p. 124), the ordinary
scoria-cones, but are of a brilliant white colour, appearing as if
covered with snow.
[Sidenote: PRESERVATION OF SCORIA-CONES.]
Ordinary scoriæ are usually of a black colour when first ejected, but
after a short time the black oxide of iron (magnetite) which they
contain, attracts the oxygen of the air and moisture, and assumes the
reddish-brown colour of iron-rust. Under such circumstances the heaps
of black material gradually acquire the red-brown colour which is
characteristic of so many of the scoria-cones around Etna, and in the
Auvergne and the Eifel. The moisture of the air, and the rain falling
upon these loose cindery heaps, cause them to decompose upon their
surfaces; the action is facilitated by the growth of the lower forms
of vegetation, such as mosses and lichens, and thus at last a soil is
produced on the surfaces of these conical piles of loose materials
which may support an abundant vegetation. Cinder- or scoria-cones are
not uncommonly found retaining in a most perfect manner their regular,
conical form, the lips of their craters being sharp and unbroken as if
the cone were formed but yesterday, while their slopes may nevertheless
be covered with a rich soil supporting abundant grass and forest-trees.
It may at first sight seem difficult to understand how a loose mass of
scoriæ could have so long withstood the action of the rain and floods,
retaining so perfectly its even slopes and sharp ridges. A little
consideration will, however, convince us that it is the very loose and
pervious nature of the materials of which scoria-cones are composed,
which tends to their perfect preservation. The rain at once sinks into
their mass, before it has time to form rivulets and streams which would
wear away their surfaces and destroy the regularity of their outlines.
Scoria- and pumice-cones are frequently found to be acted upon by acid
vapours to such an extent that the whole of the materials is reduced
to a white pulverulent mass. In these cases the oxides of iron and the
alkalis have united with the sulphuric or hydrochloric or carbonic
acids, the compounds being carried away in solution by the rain-water
falling on the mass; the materials left are silica, the hydrated
silicate of alumina, and hydrated sulphate of lime (gypsum), all of
which are of a white colour.
Cinder- or scoria-cones, and pumice-cones, are often found raised
by the action of winds to a greater elevation on one side than the
other, in the manner already described. One side of the cone is often
seen to be more or less completely swept away by an outwelling stream
of lava, and thus breached cones are formed (see fig. 40, p. 123).
Not unfrequently we find a number of cones which are united more or
less completely at their bases, as in Vulcanello (fig. 6, p. 43), the
several vents being so near together that their ejections have mingled
with one another. Cones composed entirely of fragmentary materials
often show an approach to the beautifully curved slopes which we have
described as being so characteristic of volcanoes, as may be seen in
fig. 41, facing p. 124. In the case of scoria- and pumice-cones this
curvature is probably due to the rolling downnwards and outwards of the
larger fragments.
We have already pointed out that with the scoriæ there are often
ejected fragments torn from the sides of the volcanic vents. Sometimes
such fragments are so numerous as to make up a considerable portion of
the mass of the volcanic cones. Thus in the Eifel we find hills, of
by no means insignificant size, completely built up of small scoriæ
and broken fragments of slate torn from the rocks through which the
volcanic fissures have been opened. Occasionally we see that few or no
scoriæ have been ejected, and the volcanic vents are surrounded simply
by heaps of burnt slate.
The smaller fragmentary materials ejected from volcanic vents--such
as lapilli and dust--rest in heaps, having a different angle of slope
from those formed by scoriæ. In many cases, as we have seen, such
finely-divided materials descend in the condition of mud, which flows
evenly over the surface of the growing cone and consolidates in beds of
very regularly stratified 'tufa' or 'tuff.'
[Sidenote: CHARACTERS OF TUFF-CONES.]
The 'tuff-cones' thus formed differ in many important respects from the
scoria-cones already described. The slope of their sides varies from
15° to 30°, and is almost always considerably less than in scoria- and
pumice-cones. The tuff-cones undergo much more rapid degradation from
rain and moisture than do the scoria-cones; for, though the materials
of the former 'set,' as we have seen, into a substance of considerable
hardness, yet this substance, being much less pervious to water than
the loose scoria heaps, permits of the formation of surface-streams
which furrow and wear away the sides of the cones. Sometimes the sides
of the crater are found to be almost wholly removed by atmospheric
denudation, and only a shallow depression is found occupying the
site of the crater; such a case is represented in fig. 59. We not
unfrequently find the whole slopes of such cones to be traversed by a
series of radiating grooves passing from the summit to the base of the
mountains, these channels being formed by water, which has collected
into streams, flowing down the slopes of the mountains. The volcanic
cone, under these circumstances, frequently presents the appearance of
a partially opened umbrella. Owing to the impervious character of the
materials composing tuff-cones, their craters are frequently found to
be occupied by lakes.
[Illustration: Fig. 59.--Summit of the volcano of Monte Sant' Angelo in
Lipari exhibiting a crater with walls worn down by denudatioh.]
Tufas have usually a white or yellowish-brown colour, and these are the
colours exhibited by the cones composed of this material before they
become covered by vegetation. Tufas scoriæ, and lavas usually crumble
down to form a very rich soil, and many of the choicest wines are
produced from grapes grown on the fertile slopes of volcanic mountains.
When, however, as not unfrequently happens, the materials are finely
divided and incoherent, they are so easily driven about by the winds
that cultivation of any kind is rendered almost impossible. In the
Islands of Stromboli and Vulcano the gardens have to be surrounded by
high fences to prevent them from being overwhelmed by the ever-shifting
masses of volcanic sand.
[Sidenote: CHARACTERS OF LAVA-CONES.]
There are some cones which are composed in part of scoriæ and in part
of tufa. Hence we are sometimes at a loss whether to group them with
the one class of cones or the other. But in the majority of cases,
scoria- and tuff-cones present the sufficiently well-marked and
distinctive characters which we have described.
Lava-cones differ quite as greatly in their forms as do the cones
composed of fragmentary materials, the variations being principally
determined by the degree of liquidity of the lavas.
We sometimes find that outwelling masses of lava, when issuing in small
quantities from a vent, accumulate in cauliflower-shaped masses, or
sometimes in the form of a column, or bottle. Professor J. D. Dana
describes many such fantastically-formed masses of lava as being found
in Hawaii, one of which is represented in fig. 25 (p. 100).
When the lava issues from the vent in great quantities it tends to
flow on all sides of it, and to build up a great conical heap above
the orifice. If the lava be very liquid it flows to great distances,
resting at a very slight slope. Thus we find that the volcanoes of
Hawaii have been built up of successive ejections of very liquid lava,
which have formed cones having a slope of only 6° to 8°, but of such
enormous dimensions that the diameter of their bases is seventy miles
and their height 14,000 feet.
[Illustration: Fig. 60.--Outlines of Lava-cones.
1. Mauna Loa, in Hawaii. Composed of very fluid lava. 3. The
Schlossberg of Teplitz, Bohemia. Composed of very imperfectly fluid or
viscid lava.]
If, on the other hand, the lava be viscid, or very imperfectly liquid
in character, it tends to accumulate immediately around the vent; fresh
ejections force the first extruded matter outwards, in the manner so
well illustrated by Dr. Reyer's experiments, and at last a more or less
steep-sided bulbous mass is formed over the vent. Such bulbous masses,
composed of imperfectly fluid lavas, occur in many volcanic districts,
and constitute hills of considerable size. From the tendency of matters
thus extruded to choke up the vents, however, these volcanoes composed
of viscid lavas cannot be expected to attain the vast dimensions
reached by some of those composed of very liquid lavas. The difference
in the forms of lava-cones composed of very fluid or of somewhat
viscid materials is illustrated in fig. 58. When the interior of such
steep-sided volcanic mountains composed of viscid materials is exposed
by the action of denuding forces, the peculiar internal structure we
have described is displayed by them. In the Chodi-Berg of Hungary,
a great bulbous mass of andesitic rock, this endogenous structure is
admirably displayed. It is also well seen in the excavation of the hill
of the Grand Sarcoui, a similar mass, composed of altered trachyte,
which has been erupted in the midst of a scoria-cone in the Auvergne.
See fig. 44 (p. 126).
[Sidenote: CHARACTERS OF COMPOSITE CONES.]
Most of the great volcanic mountains of the globe belong to the
class of 'composite cones,' and are built up by alternate ejections
of fluid lava and fragmentary materials. The slope of the sides in
such composite cones is subject to a wide range of variation, being
determined in part by the degree of liquidity of the lavas, in part
by the nature of the fragmentary materials ejected, and in part by
the proportions which the fragmentary and lava-ejections bear to one
another.
But there is another set of causes which tends to modify the form
and character of these composite, volcanic cones. As we have already
pointed out, the sides of such cones are liable to be rent asunder from
time to time, and the fissures so produced are injected with masses of
liquid lava from below. These fissures, rent in the sides of volcanic
cones, often reach the surface and eruptive action takes place, giving
rise to the formation of a cone, or series of cones, upon the line
of the fissure (fig. 61). Such small cones thrown up on the flanks
of a great volcanic mountain are known as 'parasitic cones'; though
subordinate to the great mountain mass, they may be in themselves of
considerable dimensions. Among the hundreds of parasitic cones which
stud the flanks of Etna, there are some which are nearly 800 feet in
height.
[Illustration: Fig. 61.--Diagram illustrating the formation of
Parasitic Cones along lines of fissure formed on the flanks of a great
volcanic mountain.]
[Illustration: Fig. 62.--Outline of Etna, as seen from Catania.]
[Sidenote: FORMATION OF PARASITIC CONES.]
The building up of parasitic cones upon the flanks of a volcanic
mountain tends, of course, to destroy its regular conical form. This
may be well seen in Etna, which, by the accumulation of materials
upon its flanks, has become a remarkably 'round-shouldered' mountain.
(See figs. 62 and 63.) At the same time it must be remembered that
materials erupted from the central vent tend to fill up the hollows
between these parasitic cones, and thus to restore to the mountain its
regularly conical form.
[Illustration: Fig. 63.--Outline of Etna, as seen from the Val del
Bronte.]
[Illustration: Fig. 64.--Plan of the Volcano forming the Island of
Ischia.
_a, a, a._ The semi-circular crater-ring of Epomeo.
_b, c, d._ Lava-currents which have flowed from the principal crater.
_e, f, g, h._ Plateaux formed by ancient lava-currents.
_k._ Montagnone. }
_l._ Monte Rotaro. }
_m._ Monte Tabor. } Parasitic cones and craters on the slopes
_n._ Castiglione. } of the mountain.
_o._ Lago di Bagno. }
_p._ The Cremate. }
_r._ Lava-stream of the Arso, which flowed from the Cremate in 1301.
_x, x, x._ Raised beaches on the shores of the island, showing that it
has recently undergone elevation.
]
The Island of Ischia is a good example of a great volcanic cone the
flanks of which are covered with numerous small parasitic cones. While
the great central volcano has evidently been long extinct, and one
side of its crater-wall is completely broken down, some of the small
parasitic cones around its base have been formed within the historical
period--one of them as recently as the year 1302. Fig. 64 is a plan of
the Island of Ischia, showing the numerous parasitic cones scattered
over the slopes of the principal cone.
[Illustration: Fig, 65.--A primary Parasitic Cone with a secondary one
at its base--Ischia.
_a._ Monte Rotaro. _b._ Monte Tabor. _c._ Lava-stream flowing from the
latter.]
In one case we find that a parasitic cone, the Monte Rotaro, has itself
a similar smaller cone, which is parasitic to it, at its foot; this
secondary parasitic cone gives off a small lava-stream of trachyte,
which has flowed down to the sea. (See fig. 65.)
[Illustration: Fig. 66.--Scoria-cone near Auckland, New Zealand, with a
lava-current flowing from it.
The strata beneath the volcanic cone are exposed in the sea-cliff, and
exhibit proofs of depression having taken place.]
[Illustration: Fig. 67.--Section of rocks below the ancient triassic
volcano of Predazzo in the Tyrol.
The position of the strata _a b c_, etc., indicates a central
subsidence.]
[Sidenote: SUBSIDENCE BENEATH VOLCANIC VENTS.]
Most great volcanic mountains exhibit a tendency towards a subsidence
of their central portions, which may take place either during or
subsequently to their period of activity. When we examine the strata
upon which a volcano has been built up, but which are now exposed to
our study by denuding forces, we usually find that they incline towards
the centre of the eruptive activity. (See figs. 66 and 67.) Two causes
may contribute to bring about this result. In the first instance,
we must remark that the piling up of materials around the volcanic
vent causes the subjacent strata to be subjected to a degree of
pressure far is excess of that which acts upon the surrounding rocks.
And secondly, it must be borne in mind that the continual removal
of material from below the mountain must tend to the production of
hollows, into which the overlying strata will sink. The effect of this
central subsidence is to give to the flanks of volcanic cones those
beautifully curved outlines which constitute so striking a feature in
Vesuvius (see fig. 17, p. 87), Fusiyama (see fig. 77, No. 1, facing p.
178), and many other volcanic mountains.
There seems, at first sight, to be scarcely any limit to the dimensions
which these great composite volcanic cones may attain: the lateral
eruptions tending to enlarge the area of the base of the mountain,
and, by the injection of the fissures, to knit together and strengthen
its structure, while the central eruptions continually increase the
elevation of the mass. Great, however, as is the force which is
concerned in the production of our terrestrial volcanoes, it has its
limits; and, at last, the piling up of materials will have gone on
to such an extent, that the active forces beneath the volcano are no
longer competent either to raise materials to the elevated summit of
the mountain or to tear asunder its strengthened and fortified flanks.
Under these circumstances, the volcanic forces, if they have not
already exhausted themselves, will be compelled to find weak places in
the district surrounding the volcano, at which fissures may be produced
and the phenomena of eruption displayed.
[Sidenote: SHIFTING OF VOLCANIC FOCI.]
Some volcanic cones exhibit evidence that during the series of
eruptions by which they have gradually been built up, the centre of
volcanic action has shifted to another point within the mountain. Thus
Lyell has shown, in the case of Etna, that during the earlier periods
in the history of the mountain the piling up of materials went on
around a centre which is now situated at a distance of nearly four
miles from the present focus of eruptive activity. Some of our old
British volcanoes, of which the denuded wrecks exist in the Western
Isles of Scotland, show similar evidence of a shifting of the axis of
eruption.
One of the most conspicuous features of a volcanic cone is the great
depression or crater found at its summit. In describing the internal
structure of volcanic cones, we have seen how these craters are
produced and acquire their inverted conical form, by the slipping and
rolling back of materials towards the centre of eruptive action.
Almost all volcanic cones exhibit craters, but in those which are
formed entirely by the outwelling of viscid lavas the central
depression is often slight and inconspicuous, and occasionally
altogether wanting. It frequently happens, however, that eruptive
action has ceased at the centre of a volcano, and its summit-crater
may by denudation be entirely destroyed, while new and active craters
are formed upon its flanks. Stromboli furnishes us with an admirable
example of this kind (see fig. 1, facing p. 10). Other volcanoes may
exhibit several craters, one at the summit of the mountain and others
upon its flanks. Of this we find a good example in Vulcano (fig. 6, p.
43).
[Illustration: Fig. 68.--Cotopaxi (19,600 feet), as seen from a
distance of ninety miles.]
When a volcano has been built up by regular and continuous eruptions
from the same volcanic vent, the size of the crater remains the same,
while the volcano continually grows in height and in the diameter of
its base. The size of the crater will be determined by the eruptive
force at the volcanic centre, the size of the mountain by the duration
of the volcanic activity and the quantity of material ejected. In the
earliest stage of its history, such a volcano will resemble Monte
Nuovo, which has a crater reaching down almost to the base of the
mountain; in the later stages of its history, such a volcano will
resemble Cotopaxi (fig. 68) and Citlaltepetl (fig. 69), in which the
crater, though of far greater absolute dimensions than that of Monte
Nuovo, bears but a small proportion to the vast cone at the summit of
which it is situated.
[Illustration: Fig. 69.--Citlaltepetl, or the Pic d'Orizaba, in Mexico
(17,370 feet), as seem from the forest of Xalapa.]
[Sidenote: ORIGIN OF VOLCANIC CRATERS.]
In the great majority of volcanoes, however, eruptive action does not
go on by any means regularly and continuously, but terrible paroxysmal
outbursts occur, which suddenly enlarge the dimensions of the crater to
an enormous extent.
In the year 1772, there occurred a volcanic eruption in the Island
of Java, which is perhaps the most violent and terrible that has
happened within the historical period. A lofty volcanic cone, called
Papandayang, 9,000 feet high, burst into eruption, and, in a single
night, 30,000,000,000 cubic feet of materials were thrown into the
atmosphere, falling upon the country around the mountain where no less
than forty villages were buried. After the eruption, the volcano was
found to have been reduced in height from 9,000 to 5,000 feet, and
to present a vast crater in its midst, which had been formed by the
ejection of the enormous mass of materials.
Many similar cases might be cited of the removal of a great part of a
mountain-mass by a sudden, paroxysmal outburst. In some cases, indeed,
the whole mass of a mountain has been blown away during a terrific
eruption, and the site of the mountain is now occupied by a lake. This
is said to have been the case with the Island of Timor, where an active
volcano, which was visible from a distance of 300 miles at sea, has
entirely disappeared.
The removal of the central portion of great volcanic mountains by
explosive action, gives rise to the formation of those vast, circular,
crater-rings of which such remarkable examples occur in many volcanic
districts. These crater-rings present a wall with an outer slope
agreeing with that of the volcanic cone of which they originally formed
a part, but with steep inner cliffs, which exhibit good sections of the
beds of tuff, ash, and lava with the intersecting dykes of which the
original volcano was built up. Near Naples, one of these crater-rings,
with sloping outer sides and steep inner ones, is employed to form the
royal game-preserve of Astroni, the only entrance to the crater being
closed by gates.
[Sidenote: FORMATION OF CRATER-LAKES.]
As these crater-rings are usually composed of materials more or
less impervious to water, they often become the site of lakes. The
beautiful circular lake of Laach, in the Rhine Provinces, with the
numerous similar examples of Central Italy--Albano, Nemi, Bracciano,
and Bolsena--the lakes of the Campi Phlegræi (Agnano, Avernus, &c.),
and some similar lakes in the Auvergne, may be adduced as examples of
crater-rings which have become the site of lakes.
[Illustration: Fig. 70.--Lac Paven, in the Auvergne.
_a._ Scoriæ. _b._ Basalt.]
One of the most beautiful of the crater-lakes in the Auvergne is
Lac Paven (fig. 70), which lies at the foot of a scoria-cone, Mont
Chalme, and is itself surrounded by masses of ejected materials. The
crater-lake of Bagno, in Ischia (fig. 71), has had a channel cut
between it and the sea, so that it serves as a natural harbour. The
lake of Gustavila, in Mexico (fig. 72), is an example of a crater-lake
on a much larger scale.
In many of these crater-rings the diameter of the circular space
enclosed by them is often very great indeed as compared with the height
of the walls.
[Illustration: Fig. 71.--The crater-lake called Lago del Bagno, in
Ischia, converted into a harbour.]
[Illustration: Fig. 72.--Lake of Gustavila, in Mexico.
(The terraces round the lake have been artificially formed.)]
[Sidenote: DIMENSIONS OF CRATER-LAKES.]
Two of the largest crater-rings in the world are found in Central
Italy, and are both occupied by lakes, the circular forms of which
must strike every observer. The Lago di Bracciano, which lies to
the north-west of Rome, is a circular lake six and a half miles in
diameter, surrounded by hills which at their highest point rise to the
height of 1,486 feet above the sea, while the surface of the waters
of the lake is 640 feet above the sea-level. The Lago di Bolsena is
somewhat less perfectly circular in outline than the Lago di Bracciano;
it has a length from north to south of ten-and-a-quarter miles and a
breadth from east to west of nine miles; the surface of the waters of
this lake is 962 feet above that of the waters of the Mediterranean.
The lake of Bolsena, like that of Bracciano, is surrounded by hills
composed of volcanic materials; the highest points of this ring of
hills rise to elevations of 684, 780, and 985 feet respectively above
the waters of the lake.
In these great circular lakes of Bolsena and Bracciano, as well as in
the smaller ones of Albano, Nemi, and the lakes of Frascati in the same
district, the vast circular spaces enclosed by them, the gradual outer
slope of the ring, and the inner precipices which bound the lake, all
afford evidence of the explosive action to which they owe their origin.
But while the vast crater-rings we have mentioned are frequently found
to be occupied by lakes, there are many other similar crater-rings
which remain dry, either from the materials of which they are composed
being of more pervious character, or from rivers having cut a channel
through the walls of the crater, in this way draining off its waters.
Thus in the Campi Phlegræi, while we have the craters of Agnano and
Avernus forming complete circular lakes, Astroni has only a few
insignificant lakelets on its floor, and the Pianura, the Piano di
Quarto, which have each a diameter of three or four miles, with many
others, remain perfectly dry. In the vicinity of the great crater-lakes
of Central Italy we find the crater-ring of the Vallariccia, which has
evidently once been a lake but is now drained, its floor being covered
with villages and vineyards.
[Sidenote: CRATER-RINGS SURROUNDING CONES.]
A comparison of these vast crater-rings leads us to the conclusion that
in the majority of cases, if not in every instance, they are composed
almost entirely of volcanic tuff and dust. In the case of the more
solidly-built composite volcanic cones, the volcanic forces, as we have
seen, produce fissures in the mass, and along these fissures parasitic
cones are thrown up, the tension of the mass of imprisoned vapours
below the mountain being thus from time to time relieved. But in the
case of a volcanic cone composed of loose fragmentary materials, such
temporary relief is impossible. The cracks, as soon as they originate,
will be filled up and choked by the falling in of materials from above
and at their sides. In this way the eruptive action will be continually
repressed, till at last the imprisoned vapours acquire such a high
state of tension that the outburst, when it occurs, is of the most
terrible character, and the whole central mass of the volcano is blown
into the air. It may often seem surprising that the ejection of such
vast masses of material from the centre of a volcanic cone does not
effect more in the way of raising the height of the crater-walls. But
it must be remembered that, in the case of craters of such vast area,
the majority of the ejected materials must fall back again within
its circumference. By repeated ejections these materials will at last
be reduced to such an extreme state of comminution that they can be
borne away by the winds, and spread over the country to the distance of
hundreds or thousands of miles. After great volcanic outbursts enormous
areas are thus found covered with fine volcanic dust to the depth of
many inches or feet.
[Illustration: Fig. 73.--Peak of Teneriffe in the Canary Islands
(12,182 ft.), surrounded by great crater-rings.]
Sometimes, as in the case of the Lago di Bracciano, the eruptive forces
appear to have entirely exhausted themselves in the prodigious outburst
by which the great crater was produced. But in other cases, as in that
of the Lago di Bolsena, the eruptive action was resumed at a later
date, and small tuff-cones were thrown up upon the floor of the crater;
these now rise as islands above the surface of the lake. In other
cases, again, the eruptive action was resumed after the formation of
the great crater-ring, with such effect that bulky volcanic cones were
built up in the midst of the crater-ring which surrounds them like a
vast wall; examples of this are exhibited in the extinct volcanoes of
Rocca Monfina and Monte Albano. Some of the grandest volcanoes of the
globe, such as Teneriffe (fig. 73), the volcanoes of Mauritius and
Bourbon (figs. 74 and 75), and many others that might be cited, are
thus found to be surrounded by vast crater-rings. Vesuvius itself is
surrounded by the crater-ring of Somma (fig. 76).
[Illustration: Fig. 74.--The volcano of Bourbon, rising in the midst of
a crater-ring four miles in diameter.]
[Illustration: Fig. 75.--The volcano of Bourbon, as seen from another
point of view, with three concentric crater-rings encircling its base.]
[Sidenote: BASALTIC CONES IN TRACHYTIC CRATER-RINGS.]
This formation of cone within crater, often many times repeated, is
very characteristic of volcanoes. The craters mark sudden and violent
paroxysmal outbursts, the cones are the result of more moderate but
long-continued ejection. Sometimes, as at Vesuvius in 1767 (fig. 15,
p. 85), we find a nest of craters and cones which very strikingly
exemplifies this kind of action.
[Illustration: Fig. 76.--Vesuvius, as seen from Sorrento, half
encircled by the crater-ring of Somma.]
We shall point out, hereafter, that at most volcanic centres the
ejection of trachytic lavas precedes that of the basaltic lavas. Now it
is these trachytic lavas which principally give rise to the formation
of the light lapilli of which tuff-cones are formed. Hence it is that
we so frequently find, as in the case of Vesuvius, Rocca-Monfina, and
many other volcanoes, that a great crater-ring, largely composed of
tuffs, encloses a cone built up of more basic lavas.
In fig. 77 we have shown by a series of outline sections the various
forms assumed by volcanoes in consequence of the different kinds of
eruptive action going on in them:--
1. Is an outline of Fusiyama, an almost perfect cone, with a small
crater at its summit. The sides of this volcano admirably illustrate
the beautiful double curves characteristic of volcanic cones.
2. Hverfjall in Iceland, a volcanic cone with a large crater, reaching
almost to its base.
3. The crater-lake of Bracciano, in which the area of the crater is out
of all proportion to the height of the crater-walls.
4. Rocca-Monfina, in Southern Italy, a tuff-cone of large dimensions,
in the midst of which an andesitic lava-cone has been built up.
5. Teneriffe, in the Canary Islands, in which a perfect volcanic cone
has been built up in the centre of an encircling crater-ring.
6. Vulcano, in the Lipari Islands, in which, by the shifting of the
centre of volcanic activity along a line of fissure, a series of
overlapping volcanic cones has been produced.
[Illustration: Fig. 77.--Outlines of various Volcanoes, illustrating
the different relations of the craters to cones.]
[Sidenote: SUBMARINE VOLCANOES.]
While speaking of the varieties of form assumed by volcanic cones and
craters, we must not forget to notice the effects which are produced
by denuding forces upon them. In the case of submarine volcanoes, like
the celebrated island called by the English Graham Isle, by the French
Isle Julie, and by the Germans the Insel Ferdinandez (fig. 78), which
was thrown up off the coast of Sicily in 1831, it was evident that
volcanic outbursts taking place at some depth below the level of the
sea gradually piled up a cone of scoriæ with a crater in its midst.
By constant accessions to its mass, this scoria-cone was eventually
raised above the sea-level, but the action of the waves upon the loose
materials soon destroyed the crater-walls and eventually reduced the
island to a shoal. It is evident that in all cases in which eruptions
take place beneath the sea-level, and the loose materials are exposed
during their accumulation to the beating of the sea-waves, the form
of the volcanic cone so produced will be greatly modified by the
interaction of the two sets of opposed causes, the eruptive forces from
below and the distributive action of the sea-waves.
[Illustration: Fig. 78.--Island thrown up in the Mediterranean Sea in
July and August 1881.
(The view was taken in the month of September, after the sides of the
crater had been washed away by the waves.)]
Craters when once formed are often rent across, along the line of the
fissure above which they are thrown up. Thus the crater of Vesuvius was
in 1872 rent completely asunder on one side, so that it was possible
to climb through the fissure thus produced and reach the bottom of the
crater. Streams flowing down the sides of the crater, and escaping
through such a rent, may in the end greatly modify the form and
disguise the characters of a volcanic crater. Of this kind of action we
have a striking example in the Val del Bove of Etna.
Volcanoes, as we shall point out in the sequel, are after their
extinction frequently submerged beneath the waters of the ocean. The
sea entering the craters, eats back their cliff-like sides and enlarges
their areas. Such denuded waters are called 'calderas,' the channels
into them 'barrancos.'
Sometimes the action of the waves upon a partially submerged volcano
has led to the cutting back of its slopes into steep cliffs, at the
same time that the crater-ring is enlarged. In such cases we have left
a more or less complete rocky ring, composed of alternating lavas and
fragmentary materials. Of such a ruined crater-ring, the Island of St.
Paul in the South Atlantic affords an admirable example.
When the action of denudation has gone still further, all the lavas and
tuffs composing the cone may be completely removed and nothing left but
masses of the hard and highly-crystalline rocks which have cooled down
slowly in the heart of the volcano. An example of this kind is afforded
to us by St. Kilda, the remotest member of the British Archipelago.
But although the majority of volcanic craters are clearly formed by
explosive action, there are some craters, like those of Kilauea in
Hawaii, which probably owe their origin to quite a different set of
causes. In this case the explosive action at the vent is but slight,
and the crater, which is of very irregular form, appears to have
originated in a fissure, which has been slowly enlarged by the liquid
lavas encroaching upon and eating away its sides. Such craters as
these, however, appear to be comparatively rare.
Besides the great volcanic mountains composed of lava, scoriæ, tuff and
ash, there are other structures which are formed around volcanic vents
even when these do not eject molten rock-masses. The water which issues
in these cases either as steam or in a more or less highly heated
condition frequently carries materials in suspension or solution, and
these sometimes accumulate in considerable quantities around the vent.
[Sidenote: FORMATION OF MUD-VOLCANOES.]
When fissures are formed in the midst of loose argillaceous materials,
such as are frequently produced by the decomposition of volcanic rocks,
the waters which issue through them are sometimes so charged with muddy
matter that this accumulates to form cones having all the general
characters of volcanic mountains, and which occasionally rise to the
height of 250 feet. The gases and vapours which issue from these
'mud-volcanoes' are those which are known to be emitted from volcanic
vents at which the action going on is not very intense. Daubeny and
others have suggested that these mud-volcanoes may be the result of
actions which have little or no analogy with those which take place
at ordinary volcanic vents, and that the combustion of subterranean
beds of sulphur and similar causes would be quite competent to their
production. But inasmuch as these mud-volcanoes are almost always
situated in regions in which the more powerful volcanic action has
only recently died away, and the gases and vapours emitted by them are
very similar in character to those which issue from volcanoes, there
does not appear to be any good reason for doubting that they should be
classed as truly volcanic phenomena.
Mud-volcanoes are found in Northern Italy near Modena, in Sicily near
Girgenti, on the shores of the Sea of Azof and the Caspian, in Central
America, and in other parts of the globe. The gas frequently escapes
from them with such violence that mud is thrown into the air to the
height of several hundreds of feet. Sometimes this gas is inflammable,
consisting of sulphuretted hydrogen, hydrogen, or some hydrocarbons,
and these gases occasionally take fire, so that true flames issue from
these mud-volcanoes. In other cases the mud-volcanoes appear to be
formed by either hot or cold springs containing large quantities of
suspended materials, and the liquid mud issues from the vent without
any violent eruptive action.
[Illustration: Fig. 79.--Sinter-cones surrounding the orifices of
Geysers.
1. Basin of the Great Geyser, Iceland. 2. Hot spring cone. 3. Old
Faithful. 4. The Great Geyser. 5. Liberty Cap. (2, 3, 4 and 5 are in
the Yellowstone Park district of the Rocky Mountains.)]
[Sidenote: FORMATION OF SINTER-CONES.]
The soluble materials which waters issuing from volcanic vents deposit
on their sides are chiefly silica and carbonate of lime.
Hot springs, whether intermittent or constant, often contain large
quantities of silica in solution. The solution of this silica is
effected, at the moment of its separation from combination with the
alkali or alkaline earths, during the decomposition of volcanic rocks,
and is favoured by the presence of alkaline carbonates in the water,
and the high temperature and the pressure under which it exists in the
subterranean regions. When the water reaches the surface and, being
relieved from pressure, begins to cool down the silica is deposited.
By this deposited silica the basins around the geysers of Iceland are
formed. Sometimes conical structures are built up around the vents of
hot springs by the deposition of silica from their waters. Examples
of this kind abound in the National Park of Colorado, where they have
received fanciful names, such as the Beehive, Liberty Gap, &c. This
deposited silica is known to geologists as sinter. The forms of some of
the structures which surround the orifices of geysers is shown in fig.
79. The 'Liberty Cap' is an extinct geyser-cone fifty feet high and
twenty feet in diameter.
Hot and cold springs rising in volcanic regions are often highly
charged with carbonic acid, and in passing through calcareous rocks
dissolve large quantities of carbonate of lime. Upon exposure to the
atmosphere, the free carbonic acid escapes and the carbonate of lime
is deposited in the form known as 'travertine.' Such springs occur
in great numbers in many volcanic regions. In the Auvergne great
rock-masses occur formed of carbonate of lime deposited from a state
of solution and taking the form of natural aqueducts and bridges. In
Carlsbad the numerous hot springs have deposited masses of pisolitic
rock (Strudelstein) which have filled up the whole bottom of the
valley, and upon these deposits the town itself is mainly built.
In Central Italy the deposits of travertine formed by calcareous
springs are of enormous extent and thickness: St. Peter's and all the
principal buildings of Rome being constructed of this travertine or
'Tibur-stone.'
[Sidenote: FORMATION OF SINTER-TERRACES.]
When springs charged with silica or carbonate of lime rise upon the
slope of a hill composed of loose volcanic materials, they give rise
to the remarkable structures known as sinter- and travertine-terraces
(see fig. 80). The water flowing downwards from the vent forms a
hard deposit upon the lower slope of the hill, while the continual
deposition of solid materials within the vent tends to choke it up.
As a new vent cannot be forced by the waters through the hard rock
formed below, it is originated a little higher up. Thus the site of the
spring is gradually shifted farther and farther back into the hill. As
deposition takes place along the surfaces over which this water flows,
terraces are built up enclosing basins. Of structures of this kind we
have remarkable examples in the sinter-terraces of Rotomahana in New
Zealand and the travertine-terraces of the Gardiner's River in the
Yellowstone Park district of the Rocky Mountains.
[Illustration: Fig. 80.--Diagram illustrating the mode of formation of
Travertine and Sinter Terraces on the sides of a hill of tuff.]
CHAPTER VII.
THE SUCCESSION OF OPERATIONS TAKING PLACE AT VOLCANIC CENTRES.
That a volcanic vent, when once established, may display intense
activity during enormous periods of time, there cannot be the smallest
reason for doubting; for the accumulation of materials around some
existing volcanic centres must certainly have been going on during
many thousands, perhaps millions, of years. To us, whose periods of
observation are so circumscribed, it may therefore at first sight
appear a hopeless task to trace the 'life-history of a volcano,' to
discover the stages of its development, and to indicate the various
episodes which have occurred during the long periods it has been in
existence. But when it is remembered that we have the opportunity of
studying and comparing hundreds of such volcanoes, exhibiting every
varying phase of their development, we shall see that such an attempt
is by no means so unpromising as it at first sight appears to be. In
the present chapter, we shall give an account of the results which have
already been obtained by inquiries directed to this object.
[Sidenote: CYCLES OF VOLCANIC PHENOMENA.]
There is not the smallest room for doubt that during the past history
of our globe, exhibitions of subterranean energy have occurred at many
different parts of its surface. There is further evidence that at the
several sites where these displays of the volcanic forces have taken
place, the succession of the outbursts has run through a regular cycle,
gradually increasing in intensity to a maximum, and then as gradually
dying away.
A little consideration will show that the first portion of this cycle
of events is the one which it is most difficult to examine and study.
The products of the earlier and feeble displays of volcanic activity,
at any particular centre, are liable to be destroyed, or masked, during
the ejection of overwhelming masses of materials in the later stages
of its more matured energy. That the feeble displays of volcanic
force now exhibited in some localities will gradually increase in
intensity in the future, and eventually reach the grandest stage of
development, there can be no reason for doubting. But, unfortunately,
we are quite unable to discriminate these feeble manifestations, which
are the embryonic stages in the development of grand exhibitions of
the volcanic forces, from slight outbursts which die away and make no
farther sign.
From what has been proved concerning the true nature of volcanic
action, however, it is certain that the first step towards the
exhibition of such action, at any particular locality, must be the
production of an aperture in the earth's crust. Only by means of
such an aperture can the vapours, gases, and rocky materials reach
the surface, and give rise to the phenomena there displayed. There is
reason to believe that all such apertures are really of the nature of
fissures, or cracks, which have been opened through the superjacent
strata by the efforts of the repressed subterranean forces.
Some recent writers have, it is true, endeavoured to draw a distinction
between what they call 'fissure-eruptions,' and eruptions taking
place from volcanic cones. But all volcanic outbursts are truly
'fissure-eruptions'--the subterranean materials finding their way to
the surface through great cracks, which, in a more or less vertical
position, traverse the overlying rock-masses. It is true that in many
cases portions of these cracks soon get choked up, while other portions
become widened, and the volcanic energy is concentrated at such spots.
Thus the materials ejected from these fissures are usually emitted in
greatest quantities at one or more points along the fissure, and a
single great volcanic vent, or a row of smaller vents, is established
upon the line at which the fissure reaches the surface.
We have seen that the amount of explosive action taking place at
different volcanic vents varies according to the proportion of
imprisoned water contained in the lava. In the cases where there is
much explosive action, vast accumulations of scoriæ, lapilli, and
dust take place, and cones of great size are built up; but in those
cases where the explosive action is small the lavas flow quietly from
the vent, and only small scoriæ-cones are thrown up, these being
probably soon swept away by the lava-currents themselves or by denuding
agencies. But both kinds of eruption have equal claims to be called
'fissure-eruptions.'
[Sidenote: FORMATION OF VOLCANIC FISSURES.]
In the expansive force of great masses of imprisoned vapour, we have
a competent cause for the production of the fissures through which
volcanic outbursts take place. Such fissures are found traversing the
rocks lying above volcanic foci, and often extending to distances of
many miles, or even hundreds of miles, from the centres of activity.
Some of these cracks are found to be injected with fused materials from
below, others have been more or less completely filled with various
minerals that have been volatilized, or carried by superheated waters
from the deeper regions of the earth's crust. That many of the cracks
thus produced in the superjacent rocks, by the heaving forces of
imprisoned vapour seeking to escape, never reached the surface, we have
sufficient proof in many mining regions.
If we now transfer our attention from the deeper portions of the
earth's crust to the surface, we can well understand how the attempts
of the imprisoned vapours to force a passage for themselves through the
solid rock-masses would lead to shocks and jars among the latter. Each
of these shocks or jars would give rise, in the surrounding portions
of the earth's crust, to those vibrations which we know as earthquakes.
The close connection between most earthquakes and volcanic phenomena is
a fact that does not admit of the smallest doubt; and though it would
be rash to define all earthquakes as 'uncompleted efforts to establish
a volcano,' yet, in the efforts of the repressed subterranean forces to
find a vent by the production of fissures in the overlying rock-masses,
we have a cause competent to the production of those shocks which are
transmitted to such enormous distances as waves of elastic compression.
We have seen that the production of the fissure upon which the small
volcano of Monte Nuovo was thrown up was preceded by a succession
of earthquakes, which for a period of over two years terrified the
inhabitants of the district, and might have warned them of the
coming event. In the same manner, doubtless, the period before the
appearance of volcanic phenomena in a new area would be marked by
powerful subterranean disturbances within it, due to the efforts of the
imprisoned vapours to force for themselves a channel to the surface.
[Sidenote: NATURE OF FIRST EJECTIONS FROM FISSURES.]
In the case of Monte Nuovo, we have seen that the fissure, when
produced, emitted water--at first in a cold, then in a boiling
condition--and, eventually, steam and scoriæ. It is probable that
through the first cracks which reached the surface, during the heaving
of the subterranean forces, water, charged with carbonic acid, flowed
abundantly, and that these cold springs, charged with carbonic acid
and carbonate of lime, would be succeeded by others which were hot
and contained silica in solution. In Hungary, the Western Isles of
Scotland, and many other volcanic districts, we find abundant evidence
that, before the eruption of lavas in the area, great masses of
travertine and siliceous sinter were formed by the action of cold and
hot springs.
As the volcanic action became more intense by the more perfect opening
of the fissures, the evolution of carbonic add gas would be succeeded
by the appearance of sulphurous acid, sulphuretted hydrogen, boracic
acid, and hydrochloric acid, which recent studies have shown to be
successively emitted from volcanic vents as the temperature within them
rises. At last lava or molten rock becomes visible within the fissures,
and the ejection of the frothy masses--scoriæ, pumice, lapilli and
dust--commences, and this is sometimes succeeded by the outflow of
currents of lava.
That volcanoes originate upon lines of fissure in the earth's crust
we have the most convincing proofs. Not only have such fissures been
seen in actual course of formation at Vesuvius, Etna, and other active
volcanoes, but a study of the volcanoes dissected by denudation affords
the most convincing evidence of the same fact. The remarkable linear
arrangement seen in groups of volcanoes, which is conspicuous to the
most superficial observer, confirms this conclusion.
[Illustration: Fig. 81.--Map of the volcanic group of the Lipari
Islands, illustrating the position of the lines of fissure on which the
volcanoes have been built up.]
[Sidenote: SHIFTING OF VENTS ALONG FISSURES.]
We have described the action going on at Stromboli as typical of that
which occurs at all volcanic vents. Stromboli is, however, one among
a group of islands all of which are entirely of volcanic origin. The
volcanoes of this group of islands, the Æolian or Lipari Islands, are
arranged along a series of lines which doubtless mark fissures in the
earth's crust. These fissures, as will be seen by the accompanying map
(fig. 81), radiate from a centre at which we have proofs of the former
existence of a volcano of enormous dimensions. It is a very interesting
fact, which the studies of Prof. Suess have established, that the
earthquakes which have so often desolated Calabria appear to have
originated immediately beneath this great centre of volcanic activity.
[Illustration: Fig. 82.--The Puy de Pariou in the Auvergne,
illustrating the shifting of the centre of eruption along a line of
fissures.]
When two volcanic cones are thrown up on the same line of fissure,
their full development is interfered with, and irregularities in their
form and characters are the consequence. In the plan (fig. 82) and the
section (fig. 83) an example is given of the results of such a shifting
of the centre of eruption along a line of fissure. By the second
outburst, one-half of the first-formed cone has been removed, and the
second-formed overlaps the first.
[Illustration: Fig. 83.--Ideal section of the Puy de Pariou.]
Sometimes a number of scoria- or tuff-cones are thrown up in such close
proximity to one another along a line of fissure, that they merge into
a long irregular heap on the summit of which a number of distinct
craters can be traced. An example of this kind was furnished by the
line of scoria-cones formed above the fissure which opened on the
flanks of Etna in 1865 (see fig. 84).
[Illustration: Fig. 84.--Fissure formed on the flanks of Etna during
the eruption of 1865.
_a._ Monte Frumento, an old parasitic cone. _b._ Line of fissure. _c,
c, c._ New scoria-cones thrown up on line of fissure. _d._ Lava from
same.]
[Illustration: Fig. 85.--Plan of the Island of Vulcano, based on the
map of the Italian Government.]
[Sidenote: SHIFTING OF ERUPTIONS ALONG FISSURES.]
Even in the case of great composite cones, however, we sometimes find
proofs of the centre of eruption having shifted its place along the
line of fissure. No better example of this kind could possibly be
adduced than that of the Island of Vulcano, with the peninsula of
Vulcanello, which is joined to it by a narrow isthmus (see the map,
fig. 81, p 192). In fig. 85 we have given an enlarged plan of this
island which will make its peculiar structure more intelligible (see
also the section given in fig. 77, No. 6, facing p. 178).
The south-eastern part of the island consists of four crater-rings,
one half of each of Which has been successively destroyed, through
the shifting of the centre of eruption towards the north-west, along
the great line of fissure shown in the general map (fig. 81). The
last formed of these four crater-rings is the one which is now most
complete, and culminates in Monte Saraceno (1581 ft.), _a_ in the
plan, the highest point in the island. The older crater-rings have
been in part removed by the inroads of the waters of the Mediterranean
on the shores of the island. In the centre of the great crater,
_b_, which we have just described, rises the present active cone of
Vulcano, 1,266 feet high, and having a crater, _c_, about 600 yards
in diameter and more than 500 feet in depth. From this cone, a great
stream of obsidian, _e_, flowed in the year 1775, and a small crater,
_d_, the Fossa Anticha, has been opened in the side of the cone. The
continuation of the same line of fissure is indicated by a ruined
tuff-cone, _f_, known as the Faraglione, and the three scoria-cones of
Vulcanello, _g, h_, which have been thrown up so close to one another
as to have their lower portions merged in one common mass, as shown in
fig. 86.
[Sidenote: SYSTEMS OF VOLCANIC FISSURES.]
Even in volcanoes of the largest dimensions we sometimes find proofs of
the centre of eruption having shifted along the line of fissure. Lyell
showed that such a change in the position of the central axis of the
volcano had taken place in Etna, and the same phenomenon is exhibited
in the clearest manner' by some of the ancient volcanoes of the Inner
Hebrides, which have been dissected by the denuding forces.
[Illustration: Fig. 86.--Vulcanello, with its three craters.
_a._ The most recently-formed and perfect crater, _b_ and _c_. Older
craters, the walls of which have been partly removed by denudation,
_e._ Lava-currents proceeding from _b_. The section exposed in the
cliff at _d_ is represented in fig. 35, p. 116.]
In the case of the Lipari Islands, the fissures along which the
volcanic mountains have been thrown up radiate from a common centre,
and a similar arrangement can be traced in many volcanic regions,
especially those in which a great central volcano has existed. In other
cases, however, as in the Campi Phlegræi, the volcanic vents appear
to be formed along lines which assume a parallel arrangement, and
this doubtless marks the relative position of the original fissures
produced in the earth's crust when these volcanoes were formed. In some
other cases we find evidences of the existence of a principal fissure
from the sides of which smaller cracks originated. These three kinds of
arrangements of volcano-producing fissures are equally well illustrated
when we study those denuded districts, in which, as we have seen, the
ground-plans of volcanic structures are revealed to our view.
There is now good ground for believing that in volcanic vents, at which
long-continued eruptive action takes place, the lavas of different
chemical composition make their appearance in something like a definite
order. It had been remarked by Scrope and other geologists at the
beginning of the present century, that in many volcanic areas the acid
or trachytic lavas were erupted before the basic or basaltic.
Von Richthofen, by his studies in Hungary and the volcanic districts
of the Rocky Mountains, has been able to enunciate a law governing the
natural order of succession of volcanic products; and although some
exception to this law may be mentioned, it is found to hold good for
many other districts than those in which it was first determined.
In a great number of cases it has been found that the first erupted
rocks in a volcanic district are those of intermediate composition
which are known as andesites. These andesites, which are especially
characterised by the nature of their felspar, sometimes contain free
quartz and are then known as quartz-andesites or dacites, from their
abundance in Transylvania, the old Roman province of Dacia.
[Sidenote: ORDER OF ERUPTION OF VOLCANIC PRODUCTS.]
Von Richthofen suggests that another class of volcanic rocks, to which
he gives the name of 'propylites,' were in every case erupted before
the andesites, and in support of his views adduces the fact that in
many instances propylites are found underlying andesites. But the
propylites are, in chemical composition, identical with the andesites,
and like them present some varieties in which quartz occurs, and others
in which that mineral is absent. In their microscopic characters the
propylites differ from the andesites and dacites only in the fact that
the former are more perfectly crystalline in structure, being indeed in
many cases quite undistinguishable from the diorites or the plutonic
representatives of the andesites. The propylites also contain liquid
cavities, which the andesites and dacites as a rule do not, and the
former class of rocks, as Prof. Szabo well points out, are usually much
altered by the passage of sulphurous and other vapours, in consequence
of which they frequently contain valuable metallic ores.
The extrusion of these andesitic lavas is sometimes accompanied, and
sometimes preceded or followed, by eruptions of trachytic lavas--that
is, of lavas of intermediate composition which have a different kind of
felspar from that prevailing in the andesites.
In the final stages of the eruptive action in most volcanic districts
the lavas poured forth belong to the classes of the rhyolitic or acid,
and the basaltic or basic lavas.
These facts are admirably illustrated in the case of the volcanic
district of the Lipari Islands, to which we have had such frequent
occasion to refer. The great central volcano of this district, which
now in a ruined condition constitutes a number of small islets (see
the map, fig. 81, p. 192), is composed of andesitic lavas. The other
great volcanoes thrown up along the three radiating lines of fissure
are composed of andesitic and trachytic rocks. But all the more recent
ejections of the volcanoes of the district have consisted either of
rhyolites, as in Lipari and Vulcano, or of basalts, as in Stromboli and
Vulcanello.
Von Richthofen and the geologists who most strongly maintain the
generalisations which he has made concerning the order of appearance
of volcanic products, go much farther than we have ventured to do,
and insist that in all volcanic districts a constant and unvarying
succession of different kinds of lavas can be made out. It appears to
us, however, that the exceptions to the law, as thus precisely stated,
are so numerous as to entirely destroy its value.
The generalisation that in most volcanic districts the first ejected
lavas belong to the intermediate group of the andesites and trachytes,
and that subsequently the acid rhyolites and the basic basalts made
their appearance, is one that appears to admit of no doubt, and is
found to hold good in nearly all the volcanic regions of the globe
which have been attentively studied.
The Tertiary volcanic rocks of our own country, those of North
Germany, Hungary, the Euganean Hills, the Lipari Islands, and many
other districts in the Old World, together with the widespread volcanic
rocks of the Rocky Mountains in the New World, all seem to conform to
this general rule.
[Sidenote: THEORY OF VOLCANIC MAGMAS.]
In connection with this subject, it may be well to refer to the
ideas on the composition of volcanic rocks which were enunciated by
Bunsen, and the theoretic views based on them by Durocher. Bunsen
justly pointed out that all volcanic rocks might be regarded as
mixtures in varying proportions of two typical kinds of materials,
which he named the 'normal trachytic' and the 'normal pyroxenic'
elements respectively. The first of these corresponds very closely in
composition with the acid volcanic rocks or rhyolites, and the second
with the basic volcanic rocks or basalts. Durocher pointed out that
if quantities of these different materials existed in admixture, the
higher specific gravity of the basic element would cause it gradually
to sink to the bottom, while the acid element would rise to the top.
Carrying out this idea still further, he propounded the theory that
beneath the earth's solid crust there exist two magmas, the upper
consisting of light acid materials, the lower of heavy basic ones; and
he supposed that by the varying intensity of the volcanic forces we may
have sometimes one or the other magma erupted and sometimes varying
mixtures of the two.
The study of volcanic rocks in recent years has not lent much support
to the theoretic views of Durocher concerning the existence of two
universal magmas beneath the earth's crust; and there are not a few
facts which seem quite irreconcilable with such a theory. Thus we
find evidence that in the adjacent volcanic districts of Hungary and
Bohemia, volcanic action was going on during the whole of the latter
part of the Tertiary period. But the products of the contemporaneous
volcanic outbursts in adjacent areas were as different in character
as can well be imagined. The volcanic rocks all over Hungary present
a strong family likeness; the first erupted were trachytes, then
followed andesites and dacites in great abundance, and lastly rhyolites
and basalts containing felspar. But in Bohemia, the lavas poured out
from the volcanoes during the same period were firstly phonolites
and then basalts containing nepheline and leucite. It is scarcely
possible to imagine that such very different classes of lavas could
have been poured out from vents which were in communication with the
same reservoirs of igneous rock, and we are driven to conclude that the
Hungarian and Bohemian volcanoes were supplied from different sources.
[Sidenote: SEPARATION OF LAVAS IN RESERVOIRS.]
But the undoubted fact that in so many volcanic regions the eruption of
andesitic and trachytic rocks, which are of intermediate composition,
is followed by the appearance of the differentiated products, rhyolite
and basalt, which are of acid and basic composition respectively, lends
not a little support to the view that under each volcanic district a
reservoir of more or less completely molten rock exists, and that in
these reservoirs various changes take place during the long periods
of igneous activity. During the earlier period of eruption the heavier
and lighter elements of the contents of these subterranean reservoirs
appear to be mingled together; but in the later stages of the volcanic
history of the district, the lighter or acid elements rise to the top,
and the heavier or basic sink to the bottom, and we have separate
eruptions of rhyolite and basalt. We even find some traces of this
action being carried still further. Among the basalts ejected from the
volcanoes of Northern Germany, Bohemia, Styria, Auvergne, and many
other regions, we not unfrequently find rounded masses consisting of
olivine, enstatite, augite, and other heavier constituents of the rock.
These often form the centre of volcanic bombs, and are not improbably
portions of a dense mass which may have sunk to the bottom of the
reservoirs of basaltic materials.
In consequence of the circumstance that the eruption of lavas of
intermediate composition usually precedes that of other varieties, we
usually find the central and older portions of great volcanoes to be
formed of andesites, trachytes, or phonolites, while the outer and
newer portions of the mass are made up of acid or basic lavas. This is
strikingly exemplified in the great volcanoes of the Auvergne and the
Western Isles of Scotland, in all of which we find that great mountain
masses have, in the first instance, been built up by extrusions of lava
of the intermediate types, and that through this central core fissures
have been opened conveying basic lavas to the surface. From these
fissures great numbers of basaltic lava-streams have issued, greatly
increasing the height and bulk of the volcanic cones and deluging the
country all around.
The lavas of intermediate composition--the andesites, trachytes, and
phonolites--possess, as we have already seen, but very imperfect
liquidity as they flow from the volcanic vents. Hence we find them
either accumulating in great dome-shaped masses above the vent or
forming lava-streams which are of great bulk and thickness, but do not
flow far from the orifices whence they issue. The more fusible basaltic
lavas, on the other hand, spread out evenly on issuing from a vent, and
sometimes flow to the distance of many miles from it. This difference
in the behaviour of the intermediate and basic lavas is admirably
illustrated in the volcanic districts of the Auvergne and the Western
Isles of Scotland.
In other cases, like Vesuvius, we find that great volcanic cones of
trachytic tuff have been built up, and that these masses of fragmentary
trachytic materials have been surrounded and enclosed by the ejection,
at a later date, of great outbursts of basaltic lavas. In still
other cases, of which Rocca Monfina in Southern Italy constitutes an
excellent example, we find that a great crater-ring of trachytic tuffs
has been formed in the first instance, and in the midst of this a cone,
composed of more basic materials, has been thrown up.
[Sidenote: EXCEPTIONS TO THE GENERAL LAW.]
In all these volcanoes we see the tendency towards the eruption
of intermediate lavas in the first instance, and of basaltic and
acid lavas at a later date. Valuable, however, as are the early
generalisations of Scrope, and the more precise law enunciated by Von
Richthofen concerning the 'natural order of succession of volcanic
products,' we must not forget that there are to be found a considerable
number of exceptions to them. There are some volcanic centres from
which only one kind of lava has been emitted, and this may be either
acid, basic, or intermediate in composition; and on the other hand,
there are districts in which various kinds of lava have been ejected
from the same vents within a short period of time, in such a way as to
defy every attempt to make out anything like a law as to the order of
their appearance. Nevertheless the rules which we have indicated appear
to hold good in so great a number of cases that they are well worthy
of being remembered, and may serve as a basis on which we may reason
concerning the nature of the action going on beneath volcanic vents.
From the study of the external appearances of volcanic mountains,
combined with investigations of those which have been dissected by
denudation, we are able to picture to our minds the series of actions
by which the great volcanic mountains of the globe have been slowly and
gradually built up.
In the first instance the eruptions appear to have taken place at
several points along a line of fissure, but gradually all of these
would become choked up except one which became the centre of
habitual eruption. From this opening, ejections, firstly of lavas of
intermediate composition, and afterwards of basic materials, would take
place, until a volcano of considerable dimensions was built up around
it. But at last a point would be reached in the piling up of this
cone, when the volcanic forces below would be inadequate to the work
of raising the liquid lava through the whole length of the continually
upward-growing tube of the volcano. Under these circumstances the
expansive force of the imprisoned steam would find it easier to rend
asunder the sides of the volcanic cone than to force the liquid
material to the summit of the mountain. If these fissures reached the
surface explosive action would take place, in consequence of the escape
of steam from the glowing mass, and scoria-, tuff-, and lava-cones
would be formed above the fissure. In this way, as we have already
pointed out, the numerous 'parasitic cones' which usually abound on
the flanks of the greater volcanic mountains have been formed. The
extrusion of these masses of scoriæ and lava on the flanks of the
mountain tends, not only to increase the bulk of the mass, but to
strengthen and fortify the sides. For by the powerful expansive force
at work below, every weak place in the cone is discovered and a fissure
produced there; but by the extrusion of material at this fissure, and
still more by the consolidation of the lava in the fissure, the weak
place is converted into one of exceptional strength.
[Sidenote: INTRUSIVE MASSES BENEATH VOLCANOES.]
As the sides of the cone are thus continually repaired and strengthened
they are rendered more capable of withstanding the heaving forces
acting from below, and these forces can then only find vent for
themselves by again raising the liquefied lava to the central orifice
of the mountain. Many volcanoes, like Etna, exhibit this alternation of
eruptive action from the crater at the summit of the mountain, and from
fissures opened upon its flanks, the former tending to raise the height
of the volcanic pile, the latter to increase its bulk.
But at last a stage will be reached when the volcanic forces are
no longer able either to raise the lava up the long column of the
central vent on the one hand, or to rend asunder the strongly-built
and well-compacted flanks of the mountain on the other. It is probably
under these conditions, for the most part, that the lavas find their
way between the masses of surrounding strata and force them asunder in
the way that we have already described.
In the case of the more fluid basaltic lavas, as was pointed out so
long ago by Macculloch, the liquefied materials may find their way
between the strata to enormous distances from the volcanic centre. Such
extended flat sheets of igneous rock retain their parallelism with the
strata among which they are intruded over large areas, and did not
probably produce any marked phenomena at the surface.
But in the case of less fluid lavas, such as those of intermediate or
acid composition, for example, the effect would be far otherwise. Such
lavas, not flowing readily from the centre of eruption, would tend to
form great bulky lenticular masses between the strata which they forced
asunder, and, in so doing, could not fail to upheave and fissure the
great mountain-mass above. Vast lenticular masses of trachytic rock,
thus evidently forced between strata, have been described by Mr. G.
K. Gilbert, as occurring in the Henry Mountains of Southern Utah, and
by him have been denominated 'laccolites,' or stone-cisterns. Whether
the great basaltic sheets, like those described by Macculloch, and
those more bulky lenticular reservoirs of rock of which Mr. Gilbert
has given us such an admirable account, were in all cases connected
with the surface, may well be a matter for doubt. It is quite possible
that, in some cases, liquefied masses of rocky materials in seeking
to force their way to the surface only succeeded in thus finding a
way for themselves between the strata, and their energy was expended
before the surface was reached and explosive action took place. But it
is an undoubted fact that beneath many of the old volcanoes, of which
the internal structure is now revealed to us by the action of denuding
forces, great intrusive sheets and laccolites abound; and we cannot
doubt that beneath volcanoes now in a state of eruption, or in those
which have but recently become extinct, similar structures must be in
course of formation.
[Sidenote: EFFECTS OF INTRUSION BENEATH CONES.]
That great upheaving forces have operated on volcanoes, subsequently
to the accumulation of their materials, we have sufficient evidence in
the Val del Bove of Etna, the Caldera of Palma, the Corral of Madeira,
&c. In all of these cases we find a radial fissure ('barranco') leading
into a great crateral hollow; and these radial fissures are of such
width and depth that their origin can only be referred to a disruptive
force like that which would be exercised by the intrusion of masses of
more or less imperfectly fluid material between the subjacent strata.
These facts, of course, lend no countenance to the views formerly held
by many geologists, both in Germany and France, that the materials
of which volcanoes are built up were deposited in an approximately
horizontal position, and were subsequently blown up like a gigantic
bubble. In Etna, Palma, and Madeira we find abundant proofs that the
mass existed as a great volcanic cone before the production of the
fissures (barrancos), which we have referred to the force exercised
during the intrusion of great igneous masses beneath them.
But besides the horizontally-disposed intrusive sheets and laccolites,
great, radiating, vertical fissures are produced by the heaving forces
acting beneath those volcanic centres which have been closed up and
'cicatrised' by the exudation from them of subterranean materials.
These vertical intrusions, which we call dykes, like the horizontal
ones, differ in character, according to the nature of the materials
of which they are composed. Dykes of acid and intermediate lava are
usually of considerable width, and do not extend to great distances
from the centres of eruption. Dykes composed of the more-liquid, basic
lavas, on the other hand, may extend to the distance of hundreds of
miles from the central vent. The way in which comparatively narrow,
basaltic dykes are found running in approximately straight lines
for such enormous distances is a very striking fact, and bears the
strongest evidence to the heaving and expanding forces at work at
volcanic centres, during and subsequently to the extrusion of the
igneous products at the surface.
These basaltic dykes occur in such prodigious numbers around some
volcanic vents, that the whole of the stratified rocks in the immediate
vicinity are broken up by a complete network of them, crossing and
interlacing in the most complicated fashion. Farther away from the
vents, similar dykes are found in smaller numbers, evidently radiating
from the same centre, and sometimes extending to a distance of more
than a hundred miles from it. Nowhere can we find more beautiful
illustrations of such dykes than in the Western Isles of Scotland. When
composed of materials which do not so easily undergo decomposition as
the surrounding rocks, they stand up like vast walls; but when, on the
other hand, they are more readily acted on by atmospheric moisture than
are the rocks which enclose them, they give rise to deep trenches with
vertical sides, which render the country almost impassable.
[Sidenote: STRUCTURE OF INTRUSIVE MASSES.]
The lava consolidating in these horizontal intrusions (sheets and
laccolites), and the vertical intrusions (dykes), is usually more
crystalline in structure than the similar materials poured out at the
surface. In the same dyke or sheet, when it is of great width, we
often find every variation--from a glassy material formed by the rapid
cooling of the mass where it is in contact with other rocks, to the
perfectly crystalline or granitic varieties which form the centre of
the intrusion. It is in these dykes and other intrusions that we find
the most convincing evidence of the truth of the conclusions, which
we have enunciated in a former chapter, concerning the dependence of
the structure of an igneous rock upon the conditions under which it
has consolidated. One material is found, under varying conditions,
assuming the characters of obsidian, rhyolite, quartz-felsite, or
granite; another, under the same set of conditions, taking the form of
tachylyte, basalt, dolerite, and gabbro.
That these great intrusive masses, sheets and dykes, in their passage
between the sedimentary rocks sometimes find places where the overlying
strata are of such thinness or incoherence that the liquefied rocks are
able to force a way for themselves to the surface, we have the clearest
proof. In some dykes we find the rock in their upper portions losing
its compact character and becoming open and scoriaceous, showing that
the pressure had been so far diminished as to allow of the imprisoned
water flashing into steam.
All round great volcanoes which have become extinct we frequently find
series of small volcanic cones, which have evidently been thrown up
along the lines where the great lava-filled fissures, which we have
been describing, have reached the surface and given rise to explosive
action there. The linear arrangement of these small cones, which are
thrown up in the plains surrounding vast volcanic mountains that have
become extinct, is very striking. The numerous 'puys' of the Auvergne
and adjoining volcanic regions of Central France are for the most part
small scoria- and lava-cones which were thrown up along great lines
of fissure radiating from the immense, central, volcanic mountains of
the district, after they had become extinct. These scoria-cones and
the small lava-streams which flow from them, as was so well shown by
Mr. Scrope, mark the latest efforts of the volcanic forces beneath the
district before they finally sank into complete extinction. In the
Western Isles of Scotland, as I have elsewhere shown, we can study the
formation of these later-formed cones in the plains around extinct
volcanic mountains, with the additional advantage of having revealed
to us, by the action of the denuding forces, their connection with the
great radiating fissures.
It has been shown that the several stages in the decline of each
volcanic outburst is marked by the appearance at the vent of certain
acid gases. In the same way, after the ejection of solid materials
from a volcanic vent has come to an end, certain gaseous substances
continue to be evolved; and as the temperature at the vents declines,
the nature of the volatile substances emitted from them undergoes a
regular series of changes.
[Sidenote: ORDER OF EMISSION OF VOLCANIC GASES.]
M. Fouqué, by a careful series of analyses of the gases which he
collected at different gaseous vents, or fumaroles as they are called,
in the crater of Vulcano, has been able to define the general relations
which appear to exist between the temperature at a volcanic orifice
and the volatile substances which issue from it. He found that in
fumaroles, in which the temperature exceeded 360° centigrade, and
in which in consequence strips of zinc were fused by the stream of
issuing gas, the analysis of the products showed sulphurous acid and
hydrochloric add to be present in large quantities, and sulphuretted
hydrogen and carbonic acid in much smaller proportions. Around these
excessively heated fumaroles, the lips of which often appear at night
to be red-hot, considerable deposits of sulphide of arsenic, chloride
of iron, chloride of ammonium, boracic acid, and sulphur were taking
place.
It was found, however, that as the temperature of the vent declined,
the emission of the sulphurous acid and hydrochloric acid diminished,
and the quantity of sulphuretted hydrogen and carbonic acid mingled
with them was proportionately increased.
In the same way it appears to be a universal rule that when a volcanic
vent sinks into a condition of temporary quiescence or complete
extinction the powerfully acid gases, hydrochloric acid and sulphurous
acid, make their appearance in the first instance, and at a later stage
these are gradually replaced by sulphuretted hydrogen and carbonic acid.
Of these facts we find a very beautiful illustration in the Campi
Phlegræi near Naples. With the exception of Monte Nuovo, the volcano
which has most recently been in a state of activity in that district
is the Solfatara. From certain apertures in the floor of the crater of
the Solfatara there issue continually watery vapours, sulphurous acid,
sulphuretted hydrogen, hydrochloric acid, and chloride of ammonium.
The action of these substances upon one another, and upon the volcanic
rocks through which they pass, gives rise to the formation of certain
chemical products which, from a very early period, have been collected
on account of their commercial value. The action of these add gases
upon the surrounding rocks is very marked; efflorescent deposits of
various sulphates and chlorides take place in all the crevices and
vesicles of the rock; sulphur and sulphide of arsenic are also formed
in considerable quantities; and the trachytic tuffs, deprived of their
iron-oxide, alkaline earths and alkalies, which are converted into
soluble sulphates and chlorides, are reduced to a white, powdery,
siliceous mass. Many volcanoes, which have sunk into a state of
quiescence or extinction like the Solfatara of Naples, exhibit the same
tendency to give off great quantities of the powerfully-acid gases
which act upon the surrounding rocks, and deprive them of their colour
and consistency. Such volcanoes are said by geologists to have sunk
into the 'solfatara stage.'
[Sidenote: SOLFATARA-STAGE OF VOLCANOES.]
At the Lake of Agnano and some other points in the Campi Phlegræi,
however, we find fissures from which the less-powerfully acid gases,
sulphuretted hydrogen and carbonic acid, issue. These gases as they,
are poured forth from the vents are found to be little, if at all,
above the temperature of the atmosphere. Sulphuretted hydrogen is an
inflammable gas, and in the so-called salses and mud-volcanoes, at
which it is ejected in considerable quantities, it not unfrequently
takes fire and bums with a conspicuous flame. Carbonic acid on account
of its great density tends to accumulate in volcanic fissures and
craters rather than to mingle with the surrounding atmosphere. At the
so-called Grotto del Cane, beside the Lago Agnano, it is the custom to
show the presence of this heavy and suffocating gas by thrusting a dog
into it, the poor animal being revived, before life is quite extinct,
by pouring cold water over it. At the Büdos Hegy or 'stinking hill' of
Transylvania, carbonic acid and sulphuretted hydrogen are emitted in
considerable quantities, and it is possible to take a bath of the heavy
gas, the head being kept carefully above the constant level of the
exhalations.
Although the stories of the ancient Avernian lake, across which no bird
could fly without suffocation, and of the Guevo Upas, or Poison Valley
of Java, which it has been said no living being can cross, may not
improbably be exaggerations of the actual facts, yet there is a basis
of truth in them in the existence of old volcanic fissures and craters
which evolve the poisonous sulphuretted hydrogen and carbonic acid
gases.
Besides the gases which we have already named, and which are the most
common at and characteristic of volcanic vents, there are some others
which are not unfrequently emitted. First among these we must mention
boracic acid, which, though not a remarkably volatile substance, is
easily carried along in a fine state of division in a current of steam.
At Monte Cerboli and Monte Rotondo in Tuscany, great quantities of
steam jets accompanied by sulphuretted hydrogen and boracic acid issue
from the rocks, and these jets being directed into artificial basins of
water, the boracic acid is condensed and is recovered by evaporation.
We have already noticed that boracic add is evolved with the gases at
Vulcano and other craters; and the part which this substance plays in
volcanic districts is shown by the fact that many of the rocks, filling
old subterranean volcanic reservoirs, are found to be greatly altered
and to have new minerals developed in their midst through the action
upon them of boracic acid.
Ammonia and various compounds of carbon, nitrogen, and hydrogen are
among the gases evolved from volcanic vents. In some cases these gases
may be produced by the destructive distillation of organic materials
in the sedimentary rocks through which volcanic outbursts take place.
But it is far from impossible that under the conditions of temperature
and pressure which exist at the volcanic foci, direct chemical union
may take place between substances, which at the surface appear to be
perfectly inert in each other's presence.
When the temperature at volcanic fissures is no longer sufficiently
high to cause water to issue in the condition of vapour or steam, as is
the case at the 'stufas' which we have described, it comes forth in the
liquid state. Water so issuing from old volcanic fissures may vary in
its temperature, from the boiling point downwards.
[Sidenote: GEYSERS AND HOT-SPRINGS.]
When the water issues at a temperature little removed from the boiling
point, it is apt to give rise to intermittent springs or geysers, the
eruptions of which exhibit a remarkable analogy with those of ordinary
volcanoes. Geysers may indeed be described as volcanoes in which heated
water, instead of molten rock, is forced out from the vent by the
escaping steam. They occur in great abundance in districts in which the
subterranean action is becoming dormant or extinct, such as Iceland,
the North Island of New Zealand, and the district of the National Park
in the Rocky Mountains.
Many attempts have been made to explain the exact mechanism by which
the intermittent action of geysers is produced, but it is not at all
probable that any one such explanation will cover all the varied
phenomena exhibited by them. Like volcanic outbursts, geyser eruptions
doubtless originate in the escape of bubbles of steam through a liquid
mass, and this liberation of steam follows any relief of pressure.
In districts where vast masses of lava are slowly cooling down from
a state of incandescence, and surface waters are finding their way
downwards while subterranean waters are finding their way upwards,
there can be no lack of the necessary conditions for such outbursts.
Sometimes the eruptions of geysers take place at short and regular
intervals, at other times they occur at wide and irregular intervals
of time. In some cases the outbursts take place spontaneously, and at
others the action can be hastened by choking up the vent with stones or
earth.
Other hot springs, like the Strudel of Carlsbad, rise above the surface
in a constant jet, while most of them issue quietly and flow like
ordinary springs.
Although the violent and paroxysmal outbursts of volcanic mountains
arrest the attention, and powerfully impress us with a sense of the
volcanic activity going on beneath the earth's surface, yet it may
well be doubted whether the quantity of heat, which the earth gets rid
of by their means, at all approaches in amount that which is quietly
dissipated by means of the numerous 'stufas,' gaseous exhalations, and
thermal springs which occur in such abundance all over its surface.
For while the former are intermittent in their action, and powerful
outbursts are interrupted by long periods of rest, the action of the
latter, though feeble, is usually continuous.
[Sidenote: EFFECTS OF HOT-SPRINGS.]
Most people may regard the hot spring of Bath as a very slight
manifestation of volcanic activity. This spring issues at a constant
temperature of 49° C, or 120° Fahr. As, however, no less than 180,000
gallons of water issue daily from this source, we may well understand
how great is the amount of heat of which the earth's crust is relieved
by its agency. It may indeed be doubted whether its action in this way
is not at least equal to that of a considerable volcano which, though
so much more violent, is intermittent in its action.
Nor are thermal springs by any means ineffective agents in bringing
materials from the interior of the earth's crust and depositing it
at the surface. The Bath spring contains various saline substances,
principally sulphates and chlorides, in solution in its waters. These
are quietly carried by rivers to the sea, and are lost to our view. The
spring has certainly maintained its present condition since the time
of the Romans, and I find that if the solid materials brought from the
interior of the earth during the last 2,000 years had been collected,
they would form a solid cone equal in height to Monte Nuovo. Yet we
usually regard the Campi Phlegræi as a powerfully-active volcanic
district, and the subterranean action in our own country as quite
unworthy of notice.
When we remember the fact that on the continent of Europe the hot and
saline springs may be numbered by thousands, and that they especially
abound in districts like Hungary, the Auvergne, the Rhine provinces,
and Central Italy, where volcanic action has recently become extinct,
we shall be able to form some slight idea of the work performed
by these agents, not only in relieving the earth's crust of its
superfluous heat, but in transporting materials in a state of solution
from the interior of that crust and depositing them at the surface. The
vast deposits of siliceous sinter and of travertine also bear witness
to the effects produced by hot and mineral springs.
Nor is the work of these springs confined to the surface. Mr. John
Arthur Phillips has shown that metallic gold and the sulphide of
quicksilver (cinnabar) have been deposited with the silica and other
minerals formed on the sides of a fissure from which hot springs issue
at the surface. There cannot be any doubt that the metallic veins or
lodes, which are the repositories of most of the metals employed in the
arts, have been formed in cracks connected with great volcanic foci,
the transfer of the various sulphides, oxides, and salts which fill
the vein having been effected either by solution, sublimation, or the
action of powerful currents of steam.
As the igneous activity of the district declines, the temperature
of the issuing gases and waters diminishes with it, until at last
the volcanic forces appear to wholly abandon that region and to be
transferred to another.
Yet even after all or nearly all indications of the volcanic agencies
cease to make themselves visible at the surface, occasional tremblings
of the earth's crust show that perfect equilibrium has not been
restored below, but that movements are taking place which result in
shocks that are transmitted through the overlying and surrounding
rock-masses as earthquake vibrations.
[Sidenote: NATURE OF VOLCANIC CYCLES.]
Such is the cycle of changes which appears to take place at each
district of the earth's surface, as it successively becomes the scene
of volcanic activity.
The invasion of any particular area of the earth's surface by the
volcanic forces appears to be heralded by subterranean shocks causing
earthquake vibrations. Presently the origination of fissures is
indicated by the rise of saline and thermal springs, and the issuing
of carbonic acid and other gases at the surface. As the subterranean
activity becomes more pronounced, the temperature of the springs and
emitted gases is found to increase, and at last a visible rent is
formed at the surface, exposing the incandescent materials below.
From this open fissure which has thus been formed, the gas and vapours
imprisoned in the incandescent rock-materials escape with such violence
as to disperse the latter in scoriæ and dust, or to cause them to
well out in great streams as lava-flows. Usually the action becomes
concentrated at one or several points at which the ejected materials
accumulate to form volcanic cones.
Sometimes the volcanic activity dies away entirely after these cones
are thrown up along the line of fissure, but at others some such
centre becomes for a longer or shorter time the habitual vent for the
volcanic forces in the district, and by repeated ejections of lavas and
fragmentary materials at longer or shorter intervals the cone increases
both in height and bulk.
When the height of the cone has grown to a certain extent, it becomes
more easy for the volcanic energies below to rend the sides of the
cone than to raise the molten materials to its summit. In this way
lateral or parasitic cones are thrown up on the flanks of the volcanic
mountain, the mass being alternately elevated and strengthened by the
ejections from the summit and sides respectively.
When the volcanic energies no longer suffice to raise the fluid
materials to the summit, nor to rend the sides of the volcano, fissures
with small cones may be formed in the plains around the great central
volcano.
At last, however, this energy diminishes so far that rock materials
can no longer be forced to the surface, the fissures become sealed up
by consolidating lava, and the volcanic cones fall into a condition of
extinction and decay.
The existence of heated materials at no great depth from the surface
is indicated by the outburst of gases and vapours, the formation of
geysers, mud-volcanoes, and ordinary thermal springs. But as the
underlying rocks cool down, the issuing jets of gas and vapour lose
their high temperature and diminish in quantity, the geysers and
mud-volcanoes become extinct, and the thermal springs lose their
peculiar character or disappear, and thus all manifestations of the
igneous energies in the district gradually die away.
[Sidenote: DURATION OF VOLCANIC CYCLES.]
Such a cycle of changes probably requires many hundreds of thousands,
or even many millions, of years for its accomplishment; but by the
study of volcanoes in every stage of their growth and decline we are
able to reconstruct even the minutest details of their history.
CHAPTER VIII.
THE DISTRIBUTION OF VOLCANOES UPON THE SURFACE OF THE GLOBE.
It is not by any means an easy task to frame an estimate of the number
of volcanoes in the world. Volcanoes, as we have seen, vary greatly
in their dimensions--from vast mountain masses, rising to a height
of nearly 25,000 feet above the sea-level, to mere molehills; the
smaller ones being in many cases subsidiary to larger, and constituting
either parasitic cones on their flanks, or 'puys' around their bases.
Volcanoes likewise exhibit every possible stage of development and
decay: while some are in a state of chronic active eruption, others
are reduced to the condition of solfataras, and others again have
fallen into a more or less complete state of ruin through the action of
denuding forces.
Even if we confine our attention to the larger volcanoes, which merit
the name of 'mountains,' and such of these as we have reason to
believe to be in a still active condition, our difficulties will be
diminished, but not by any means removed. Volcanoes, as we have seen,
may sink into a dormant condition that may endure for hundreds or
even thousands of years, and then burst forth into a state of renewed
activity; and it is quite impossible, in many cases, to distinguish
between the conditions of dormancy and extinction. Concerning certain
small areas in Southern Europe, Western Asia, and Northern Africa,
historical records, more or less reliable, extend back over periods of
several thousands of years; but with regard to the greater part of the
rest of the world we have no information beyond a few hundred years,
and there are considerable areas which have been known only for far
shorter periods, while some are as yet quite unexplored. In districts
almost wholly uninhabited, or roamed over by nomadic tribes, legend
and tradition constitute our only guides--and very unsafe ones they
are--in the attempt to determine what volcanoes have recently been in a
condition of activity.
[Sidenote: NUMBER OF ACTIVE VOLCANOES.]
We shall, however, probably be within the limits of truth in stating
that the number of great habitual volcanic vents upon the globe,
which we have reason to believe are still in an active condition, is
somewhere between 300 and 350. Most of these active volcanic vents
are marked by more or less considerable mountains, composed of the
materials ejected from them. If we include the mountains which exhibit
the external conical form, the crateral hollows, and other features
of volcanoes, but concerning the activity of which we have no record
or tradition, the number will fall little, if anything, short of
1,000. The mountains composed of volcanic materials, but which have
lost through denudation the external form of volcanoes, are still more
numerous. The smaller temporary openings which are usually subordinate
to the habitual vents, that have been active during the periods
covered by history and tradition, must be numbered by thousands and
tens of thousands. The still feebler manifestations of the volcanic
forces--such as are exhibited in 'stufas,' or steam-jets, geysers,
or intermittent hot springs, thermal and mineral waters, fumaroles,
emitting various gases, salses or spouting saline and muddy springs,
and mud volcanoes--may be reckoned by millions. It is not improbable
that these less powerful manifestations of the volcanic forces, to a
great extent make up in number what they want in individual energy; and
the relief which they afford to the imprisoned activities within the
earth's crust may be scarcely less than that which results from the
occasional outbursts at the 300 or 350 great habitual volcanic vents.
In taking a general survey of the volcanic phenomena of the globe,
no fact comes out more strikingly than that of the very unequal
distribution, in different districts, both of the great habitual
volcanic vents, and of the minor exhibitions of subterranean energy.
[Sidenote: VOLCANOES OF THE CONTINENTS.]
Thus, on the whole of the continent of Europe, there is but one
habitual volcanic vent--that of Vesuvius--and this is situated upon
the shores of the Mediterranean. In the islands of the Mediterranean,
however, there are no less than six volcanoes; namely, Stromboli and
Vulcano, in the Lipari Islands; Etna, in Sicily; Graham's Isle, a
submarine volcano, off the Sicilian coast; and Santorin and Nisyros, in
the Ægean Sea.
The African continent is at present known to contain about ten active
volcanoes--four on the west coast, and six on the east coast; about ten
other active volcanoes occur on islands close to the African coasts. In
Asia, twenty-four active volcanoes are known, but no less than twelve
of these are situated in the peninsula of Kamtschatka. No volcanoes are
known to exist in the Australian continent.
The American continent contains a greater number of volcanoes than
the divisions of the Old World. There are twenty in North America,
twenty-five in Central America, and thirty-seven in South America.
Thus, taken altogether, there are about one hundred and seventeen
volcanoes situated on the great continental lands of the globe, while
nearly twice as many occur upon the islands scattered over the various
oceans.
Upon examining further into the distribution of the continental
volcanoes, another very interesting fact presents itself. The volcanoes
are in almost every case situated either close to the coasts of the
continent, or at no great distance from them. There are, indeed, only
two exceptions to this rule. In the great and almost wholly unexplored
table-land lying between Siberia and Tibet four volcanoes are said to
exist, and in the Chinese province of Mantchouria several others. More
reliable information is, however, needed concerning these volcanoes,
situated, unlike all others, at a great distance from the sea.
It is a remarkable circumstance that all the oceanic islands which
are not coral-reefs are composed of volcanic rocks; and many of these
oceanic islands, as well as others lying near the shores of the
continents, contain active volcanoes.
Through the midst of the Atlantic Ocean runs a ridge, which, by the
soundings of the various exploring vessels sent out in recent years,
has been shown to divide the ocean longitudinally into two basins.
Upon this great ridge, and the spurs proceeding from it, rise numerous
mountainous masses, which constitute the well-known Atlantic islands
and groups of islands. All of these are of volcanic origin, and among
them are numerous active volcanoes. The Island of Jan Mayen contains
an active volcano, while Iceland contains thirteen, and not improbably
more; the Azores have six active volcanoes, the Canaries three; while
about eight volcanoes lie off the west coast of Africa. In the West
Indies there are six active volcanoes; and three submarine volcanoes
have been recorded within the limits of the Atlantic Ocean. Altogether,
no less than forty active volcanoes are situated upon the great
submarine ridges which traverse the Atlantic longitudinally.
[Sidenote: VOLCANOES ON THE OCEANIC ISLANDS.]
But along the same line the number of extinct volcanoes is far greater,
and there are not wanting proofs that the volcanoes which are still
active are approaching the condition of extinction. At a somewhat
earlier period of the earth's history the whole line of the present
Atlantic Ocean was in all probability traversed by a chain of volcanoes
on the very grandest scale; but submergence has taken place, and only a
few portions of this great mountain range now rise above the sea-level,
forming the isolated islands and island-groups of the Atlantic. Here
and there among these a still active volcano exists.
But if the great medial chain of the Atlantic presents us with an
example of a chain of volcanic mountains verging on extinction, we have
in the line of islands separating the Pacific and Indian Oceans an
example of a similar range of volcanic vents which are in a condition
of the greatest activity. In the peninsula of Kamtschatka there are
twelve active volcanoes, in the Aleutian Islands thirty-one, and in
the peninsula of Alaska three. The chain of the Kuriles contains at
least ten active volcanoes; the Japanese Islands and the islands lying
to the south of Japan twenty-five. The great group of islands lying
to the south-east of the Asiatic continent is at the present time the
grandest focus of volcanic activity upon the globe. No less than fifty
active volcanoes occur here. Farther south, the same chain is probably
continued by the four active volcanoes of New Guinea, one or more
submarine volcanoes, and several vents in New Britain, the Solomon
Isles, and the New Hebrides, the three active volcanoes of New Zealand,
and possibly by Mount Erebus and Mount Terror in the Antarctic region.
Altogether, no less than 150 active volcanoes exist in the chain of
islands which stretch from Behring's Straits down to the Antarctic
circle; and if we include the volcanoes on Indian and Pacific islands
which appear to be situated on lines branching from this particular
band, we shall not be wrong in the assertion that this great system of
volcanic mountains includes at least one half of the habitually active
vents of the globe.
A third series of volcanoes starts from near the last in the
neighbourhood of Behring's Straits, and stretches along the whole
western coast of the American continent. In this great range there are
about eighty active volcanoes.
[Sidenote: LINEAR ARRANGEMENT OF VOLCANOES.]
In considering the facts connected with the distribution of volcanoes
upon the globe, the one which, by its striking character, seems to
demand our attention in the first instance is that of the remarkable
linear arrangement of volcanic vents. We have already seen that small
scoria-cones are often thrown up on the flanks, or at the base, of a
great volcanic mountain, along lines which are manifestly lines of
fissure. In the eruption of Etna, in 1865, and again in that of 1874,
Professor Silvestri, of Catania, witnessed the actual opening of great
fissures on the north-east and north sides of the mountain: and
along the bottom of these cracks the glowing lava was clearly visible
(fig. 84, page 194). In the course of a few days, there were thrown
up a number of small scoria-cones along these lines of fissure--those
formed on the fissure of 1865 being seven in number, and those on the
fissure of 1874 being no less than thirty-six in number. Precisely
familiar phenomena were witnessed upon the slopes of Vesuvius, in 1760,
when a fissure opened on the south side of the mountain, and fifteen
scoria-cones, which are still visible, were thrown up along it.
We have already considered the evidence pointing to the conclusion that
systems of volcanoes, like that of the Lipari Islands, are similarly
ranged along lines of fissures, and there is equally good ground for
believing that the great linear bands of volcanoes, which, as we
have seen, stretch for thousands of miles, have had their positions
determined by great lines of fissure in the earth's crust. While,
however, the smaller fissures, upon which rows of scoria-cones are
thrown up, seem to have been in many cases opened by a single effort
of the volcanic forces, the enormous fissures, which traverse so large
a portion of the surface of the globe, are doubtless the result of
numerous manifestations of energy extending over vast periods of time.
The greatest of these bands along which the volcanic forces are so
powerfully exhibited at the present day, is the one which stretches
from near the Arctic circle at Behring's Straits to the Antarctic
circle at South Victoria. The line followed by this volcanic band,
which, as we have seen, includes more than one half of the active
volcanoes of the globe, is a very sinuous one, and it gives off
numerous offshoots upon either side of it. The great focus of this
intense volcanic action may be regarded as lying in the district
between the islands of Borneo and New Guinea. From this centre there
radiate a number of great lines, along which the volcanic forces are
exhibited in the most powerful manner. The first of these extends
northwards through the Philippine Isles, Japan, the Kurile Islands, and
Kamtschatka, giving off a branch to the east, which passes through the
Aleutian Islands and the peninsula of Alaska. This band, along which
the volcanic forces are very powerfully active, is continued towards
the south-east in the New Britain, the Solomon Islands, Santa Cruz, the
New Hebrides, New Zealand, and South Victoria. East and west from the
great central focus there proceed two principal branches. The former
of these extends through the Navigator Islands and Friendly Islands
as far as Elizabeth Islands. The latter passes through Java, and then
turns north-westward through Sumatra, the Nicobar Islands, the Andaman
Islands up to the coast of Burmah.
The great band which we have been describing exhibits the most
striking examples of volcanic activity to be found upon the globe.
Besides the 150 or more volcanoes which are known to have been in a
state of activity during the historical period, there are several
hundred very perfect volcanic cones, many of which appear to have but
recently become extinct, if indeed, they are not simply in a dormant
condition. For long distances these chains of volcanic cones are almost
continuous, and the only very considerable breaks in the series are
those between New Zealand and the New Hebrides on the one hand, and
between the former islands and South Victoria on the other.
[Sidenote: GREAT VOLCANIC BANDS OF THE GLOBE.]
Much less continuous, but nevertheless very important, is the great
band of volcanoes which extends along the western side of the great
American continent, and contains, with its branches, nearly a hundred
active volcanoes. On the north this great band is almost united with
the one we have already described by the chain of the Aleutian and
Alaska volcanoes. In British Columbia about the parallel of 60° N.
there exist a number of volcanic mountains, one of which, Mount St.
Elias, is believed to be 18,000 feet in height, and several of these
have certainly been seen in a state of eruption. Farther south in the
part of the United States, territories drained by the Columbia River,
a number of grand volcanic mountains exist, some of which are probably
still active, for geysers and other manifestations of volcanic activity
abound. From the southern extremity of the peninsula of California
an almost continuous chain of volcanoes stretches through Mexico and
Guatemala, and from this part of the volcanic band a branch is given
off which passes through the West Indies, and forms a connection with
the great volcanic band of the Atlantic Ocean. In South America the
line is continued by the active volcanoes of Ecuador, Bolivia and
Chili, but at many intermediate points in the chain of the Andes
extinct volcanoes occur, which to a great extent fill up the gaps
in the series. A small offshoot to the westward passes through the
Galapagos Islands. The great band of volcanoes which stretches through
the American continent is second only in importance, and in the
activity of its vents, to the band which divides the Pacific from the
Indian Ocean.
The third volcanic band of the globe is that which traverses the
Atlantic Ocean from north to south. This series of volcanic mountains
is much more broken and interrupted than the other two, and a greater
proportion of its vents are extinct. This chain, as we shall show in a
future chapter, attained its condition of maximum activity during the
distant period of the Miocene, and now appears to be passing into a
state of gradual extinction. Beginning in the north with the volcanic
rocks of Greenland and Bear Island, we pass southwards, by way of Jan
Mayen, Iceland, and the Faroe Islands, to the Hebrides and the north
of Ireland. Thence by way of the Azores, the Canaries and the Cape de
Verde Islands, with some active vents, we pass to the ruined volcanoes
of St. Paul, Fernando de Noronha, Ascension, St. Helena, Trinidad and
Tristan d'Acunha. From this great Atlantic band two branches proceed
to the eastward, one through Central Europe, where all the vents are
now extinct, and the other through the Mediterranean to Asia Minor,
the great majority of the volcanoes along the latter line being now
extinct, though a few are still active. The vol canoes on the eastern
coast of Africa may be regarded as situated on another branch from this
Atlantic volcanic band. The number of active volcanoes on this Atlantic
band and its branches, exclusive of those in the West Indies, does not
exceed fifty.
[Sidenote: LENGTH OF THE VOLCANIC BANDS.]
From what has been said, it will be seen that, not only do the
volcanoes of the globe usually assume a linear arrangement, but nearly
the whole of them can be shown to be thrown up along three well-marked
bands and the branches proceeding from them. The first and most
important of these bands is nearly 10,000 miles in length, and with its
branches contains more than 150 active volcanoes; the second is 8,000
miles in length, and includes about 100 active volcanoes; the third is
much more broken and interrupted, extends to a length of nearly 1,000
miles, and contains about 50 active vents. The volcanoes of the eastern
coast of Africa, with Mauritius, Bourbon, Rodriguez, and the vents
along the line of the Red Sea, may be regarded as forming a fourth and
subordinate band.
Thus we see that the surface of the globe is covered by a network
of volcanic bands, all of which traverse it in sinuous lines with a
general north-and-south direction, giving off branches which often
run for hundreds of miles, and sometimes appear to form a connection
between the great bands.
These four bands of volcanic vents, running in a general
north-and-south direction, separate four unequal areas within which the
exhibitions of volcanic activity are feeble or quite unknown. The two
grandest of the bands of volcanic activity, with their branches, form
an almost complete series encircling the largest of the oceans.
To this rule of the linear arrangement of the volcanic vents of the
globe and their accumulation along certain well-marked bands, there are
two very striking exceptions, which we must now proceed to notice.
In the very centre of the continent formed by Europe and Asia, the
largest unbroken land-mass of the globe, there rises from the great
central plateau the remarkable volcanoes of the Thian Shan Range.
The existence of these volcanoes, of which only obscure traditional
accounts had reached Europe before the year 1858, appears to be
completely established by the researches of the Russian traveller
Semenof. Three volcanic vents appear to exist in this region: the
active volcanoes of Boschan and Turfan or Hot-schen, and the solfatara
of Urumtsi. At a point situated about half-way between these three
volcanoes and the sea, another active vent, that of Ujung-Holdongi, is
said to exist. Other volcanic phenomena have been stated to occur in
the great plateau of Central Asia, but the existence of some at least
of these appears to rest on very doubtful evidence. The only accounts
which we have of the eruptions of these Thian Shan volcanoes are
contained in Chinese histories and treatises on geography; and a great
service would be rendered to science could they be visited by some
competent explorer.
[Sidenote: EXCEPTIONALLY-SITUATED VOLCANOES.]
The second exceptionally-situated volcanic group is that of the
Sandwich Islands. While the Thian Shan volcanoes rise in the centre of
the largest unbroken land-mass, and stand on the edge of the loftiest
and greatest plateau in the world, the volcanoes of the Sandwich
Islands rise almost in the centre of the largest ocean and from almost
the greatest depths in that ocean. All round the Sandwich Islands the
sea has a depth of from 2,000 to 3,000 fathoms, and the island-group
culminates in several volcanic cones which rise to the height of nearly
14,000 feet above the sea-level. The volcanoes of the Sandwich Islands
are unsurpassed in height and bulk by those of any other part of the
globe.
With the exception of the two isolated groups of the Thian Shan and
the Sandwich Islands, nearly all the active volcanoes of the globe
are situated near the limits which separate the great land- and
water-masses of the globe--that is to say, they occur either on the
parts of continents not far removed from their coast-lines, or on
islands in the ocean not very distant from the shores.
The fact of the general proximity of volcanoes to the sea, is one which
has frequently been pointed out by geographers, and may now be regarded
as being thoroughly established. Even the apparently anomalous case of
the Thian Shan volcanoes is susceptible of explanation if we remember
the fact, now well ascertained by geological researches, that as
late certainly as Pliocene times, a great inland sea spread over the
districts where the Caspian, the Sea of Aral, and many other isolated
lakes are now found. Upon the southern shore of this sea rose the
volcanoes of the Thian Shan, some of which have not yet fallen into a
state of complete extinction.
But although the facts concerning the general proximity of volcanoes to
the ocean may be admitted to be thoroughly established, yet inferences
are sometimes hastily drawn from these facts which the latter, if
fairly considered, will not be found to warrant. It is frequently
assumed that we may refer all the remarkable phenomena of volcanic
action to the penetration of sea-water to a mass of incandescent lava
in the earth's crust, and to the chemical or mechanical action which
would result from this meeting of sea-water and molten rock. And this
conclusion is supposed to find support in the circumstance that many
of the gases and volatile substances emitted from volcanic vents are
such as would be produced by the decomposition of the various salts
contained in sea-water.
This argument in favour of the production of volcanic outbursts by
the irruption of sea-water into subterranean reservoirs, involves, as
Mr. Scrope long ago pointed out, a curious example of reasoning in a
circle. It is assumed, on the one hand, that the heaving subterranean
movements, which give rise to the fissures by which steam and other
gases escape to the surface, are the result of the passage of water
to heated masses in the earth's crust. But, on the other hand, it is
supposed that it is the production of these fissures which leads to
the influx of water to the heated materials. If it is the passage of
water through these fissures which produces the eruptions, it may be
fairly asked, what is it that gives rise to the fissures? And if, on
the other hand, there exist subterranean forces competent to produce
the fissures, may they not also give rise to the eruptions through the
openings which they have originated? Nor does the chemical argument
appear to rest upon any surer ground. It is true that many of the
volatile substances emitted from volcanic vents are such as might be
produced by the decomposition of sea-water, but, upon the other hand,
there are not a few substances which cannot possibly be regarded as so
produced, and, all the materials may equally well be supposed to have
been originally imprisoned in the masses of subterranean lava.
[Sidenote: CAUSE OF PROXIMITY OF VOLCANOES TO SEA.]
The problem before us is this. Granting that it is proved that active
volcanoes are always in close proximity to the ocean, are we to explain
the fact by supposing that the agency of sea-water is necessary to
volcanic outbursts, or by regarding the position of the coast-lines
as to some extent determined by the distribution of volcanic action
upon the surface of the globe? The first supposition is the one which
perhaps most readily suggests itself, but the latter, as we shall
hereafter show, is one in favour of which not a few weighty arguments
may be advanced.
Another problem which suggests itself in connection with the
distribution of volcanoes is the following. Are the great depressed
tracts which form the bottom of the oceans, like the elevated tracts
which constitute the continents, equally free from exhibitions of
volcanic energy?
When we remember the fact that the area of the ocean beds is two and
three-quarter times as great as that of the continents, it will be seen
how important this question of the existence of volcanoes at the bottom
of the ocean really is.
The fact that recent deep-sea soundings have shown the deepest parts
of the ocean to be everywhere covered with volcanic _débris_ is by
no means conclusive upon this question; for, as we have seen, the
ejections of sub-aerial volcanoes are by the wind and waves distributed
over every part of the earth's surface.
[Sidenote: SUBMARINE ERUPTIONS.]
Submarine volcanic outbursts have occurred in many parts of the
globe, but it may well be doubted whether any such outburst has ever
commenced at the bottom of a deep ocean, and has succeeded in building
up a volcanic cone reaching to the surface. Most, if not all, of the
recorded submarine outbursts have occurred in the midst of volcanic
districts, and the volcanic cones have been built up in water of no
great depth. Indeed, when it is remembered that the pressure of each
1,000 fathoms of water is equivalent to a weight of more than one ton
on every square inch of the ocean-bottom, it is difficult to imagine
the ordinary explosive action of volcanic vents taking place at abysmal
depths. If, however, fissures were opened in the beds of the ocean,
quiet outwellings of lava might possibly occur.
The solution of this problem of the probable existence of volcanic
outbursts on the floor of the ocean can only be hoped for from the
researches of the geologist. The small specimens of the ocean-beds
brought up by deep-sea sounding-lines, taken at wide distances apart,
and including but a few inches from the surface, can certainly afford
but little information upon the question. But the geologist has the
opportunity of studying the sea-bottoms of various geological periods
which have been upheaved and are now exposed to his view. It was at
one time supposed by geologists that in the so-called 'trap-rocks' we
have great lava-sheets which must have been piled upon one another,
without explosive action. But the more accurate researches of recent
years have shown that between the layers of 'trap-rock,' in every part
of the globe, traces of terrestrial surfaces and freshwater deposits
are found; and the supposed proofs of the absence of explosive action
break down no less signally upon re-examination; for the loose,
scoriaceous materials would either be removed by denudation, or
converted into hard and solid rocks by the infilling of their vesicles
and air-cavities with crystalline minerals. It is not possible, among
the representatives of former geological periods, to point to any rocks
that can be fairly regarded as having issued from great submarine
fissures, and it is therefore fair to conclude that no such great
outbursts of the volcanic forces take plane at the present day on the
deep ocean-floors.
In connection with the question of the relation between the position of
the volcanic bands of the globe and the areas covered by the ocean, we
may mention a fact which deep-sea soundings appear to indicate, namely,
that the deepest holes in the ocean-floor are situated in volcanic
areas. Near Japan, the soundings of the U.S. ship 'Tuscarora' showed
that at two points the depth exceeded 4,000 fathoms; and the deepest
sounding obtained by H.M.S. 'Challenger,' amounting to 4,575 fathoms,
was taken in the voyage from New Gruinea to Japan, in the neighbourhood
of the Ladrone Islands. Depths nearly as great were found in the
soundings carried on in the neighbourhood of the volcanic group of the
West Indian Islands. It must be remembered, however, that at present
our knowledge of the depths of the abysmal portions of the ocean is
very limited. A few lines of soundings, often taken at great distances
apart, are all we have to guide us to any conclusions concerning the
floors of the great oceans, and between these lines are enormous areas
which still remain altogether unexplored. It may be wise, therefore, to
suspend our judgment upon such questions till more numerous facts have
been obtained.
[Sidenote: RELATIONS TO MOUNTAIN-CHAINS.]
Another fact concerning the distribution of volcanoes which is worthy
of remark is their relation to the great mountain-ranges of the globe.
Many of the grandest mountain-chains have bands of volcanoes
lying parallel to them. This is stinkingly exhibited by the great
mountain-masses which lie on the western side of the American
continent. The Rocky Mountains and the Andes consist of folded and
crumpled masses of altered strata which, by the action of denuding
forces, have been carved into series of ridges and summits. At many
points, however, along the sides of these great chains, we find that
fissures have been opened and lines of volcanoes formed, from which
enormous quantities of lava have flowed and covered great tracts of
country. At some parts of the chain, however, the volcanoes are of such
height and dimensions as to overlook and dwarf the mountain-ranges by
the side of which they lie. Some of the volcanoes lying parallel to the
great American axis appear to be quite extinct, while others are in
full activity.
In the Eastern continent we find still more striking examples of
the parallelism between great mountain-chains and the lands along
which volcanic activity is exhibited. Stretching in a more or less
continuous chain from east to west, through Europe and Asia, we find
the mountain-masses known in different parts of their course as the
Pyrenees, the Alps, the Balkan, the Caucasus, which form the axis
of the Eastern continent. These chains consist of numerous parallel
ridges, and give off branches on either side of them. They are
continued to the eastward by the Hindoo Koosh and the Himalaya, with
the four parallel ranges that cross the great Central-Asian plateau.
Now, on either side of this grand axial system of mountains, we find
a great parallel band of volcanoes. The northern volcanic band is
constituted by the eruptive rocks of the Auvergne, the Eifel, the
Siebengebirge, Central Germany, Bohemia, Hungary, and Transylvania,
few, if any, of the vents along this northern band being still active.
The remarkable volcanoes of the Thian Shan range and of Mantchouria may
not improbably be regarded as a continuation of the same great series.
The southern band of volcanoes, lying parallel to the great mountain
axis of the Old World, also consists for the most part of extinct
volcanoes, but includes not a few vents which are still active. In
this band we include the extinct volcanoes of Spain and Sardinia, the
numerous extinct and active vents of the Italian peninsula and islands,
and those of the Ægean Sea and Asia Minor. We may, perhaps, consider
the scattered volcanoes of Arabia and the northern part of the Indian
Ocean as a continuation of the same series. Both of these bands may
be regarded as offshoots from the great mid-Atlantic volcanic chain,
and the condition of the vents, both in the principal band and its
offshoots, is such as to indicate that they form parts of a system
which is gradually sinking into a state of complete extinction.
There are some other volcanic bands which exhibit a similar parallelism
with mountain chains; but, on the other hand, there are some volcanoes
between which and the nearest mountain axes no such connection can be
traced.
[Sidenote: RELATION TO AREAS OF UPHEAVAL.]
There is yet one other fact concerning the mode of distribution of
volcanoes upon the surface of the globe, to which we must allude. It
was first established by Mr. Darwin as one of the conclusions derived
from the valuable series of observations made by him during the voyage
of H.M.S. 'Beagle,' and relates to the position of active volcanoes
with respect to the portions of the earth's crust which are undergoing
upheaval or subsidence.
From the relative position of the different kinds of coral-reefs, and
the fact that reef-forming corals cannot live at a depth of more than
twenty fathoms beneath the sea-level, or above tide-mark, we are led to
the conclusion that certain areas of the earth's surface are undergoing
slow elevation, while other parts are as gradually subsiding. This
conclusion is confirmed by the occurrence of raised beaches, which
are sometimes found at heights of hundreds, or even thousands, of
feet above the sea-level, and of submerged forests, which are not
unfrequently found beneath the waters of the ocean.
By a study of the evidences presented by coral-reefs, raised beaches,
submerged forests, and other phenomena of a similar kind, it can be
shown that certain wide areas of the land and of the ocean-floor are
at the present time in a state of subsidence, while other equally
large areas are being upheaved. And the observations of the geologist
prove that similar upward and downward movements of portions of the
earth's crust have been going on through all geological times. Now,
as Mr. Darwin has so well shown in his work on 'Coral-Reefs,' if we
trace upon a map the areas of the earth's surface which are undergoing
upheaval and subsidence respectively, we shall find that nearly all the
active volcanoes of the globe are situated upon rising areas, and that
volcanic phenomena are conspicuously absent from those parts of the
earth's crust which can be proved at the present day to be undergoing
depression.
CHAPTER IX.
VOLCANIC ACTION AT DIFFERENT PERIODS OF THE EARTH'S HISTORY.
It is only in comparatively recent times that the important doctrine of
geological continuity has come to be generally accepted, as furnishing
us with a complete and satisfactory explanation of the mode of origin
of the features of our globe. The great forces, which are ever at
work producing modifications in those features, operate so silently
and slowly, though withal so surely, that without the closest and
most attentive observation their effects may be easily overlooked;
while, on the other hand, there are so many phenomena upon our globe
which seem at first sight to bear testimony to the action of sudden
and catastrophic forces, very different to any which appear to be at
present at work, that the tendency to account for all past changes by
these violent actions is a very strong one. In spite of this tendency,
however, the real potency of the forces now at work upon the earth's
crust has gradually made its way to recognition, and the capability of
these forces, when their effects are accumulated through sufficiently
long periods of time, to bring about the grandest changes, is now
almost universally admitted. The modern science of geology is based
upon the principle that the history of the formation and development
of the earth's surface-features, and of the organisms upon it, has
been continuous during enormous periods of time, and that in the study
of the operations taking place upon the earth at the present day, we
may find the true key to the changes which have occurred during former
periods.
In no branch of geological science has the doctrine of continuity
had to encounter so much opposition and misconception as in that
which relates to the volcanic phenomena of the globe. For a long time
students of rocks utterly failed to recognise any relation between the
materials which have been ejected from active volcanic vents and those
which have been formed by similar agencies at earlier periods of the
earth's history. And what was far worse, the subject became removed
from the sphere of practical scientific inquiry to that of theological
controversy, those who maintained the volcanic origin of some of the
older rocks being branded as the worst of heretics.
[Sidenote: CONTROVERSY CONCERNING ORIGIN OF BASALT.]
With the theological aspects of the great controversy concerning the
origin of basalt and similar rocks--a controversy which was carried
on with such violence and acrimony during the latter half of the
eighteenth century--we have here nothing to do. But it may not be
uninstructive to notice the causes of the strange misconceptions which
for so long a period stood in the way of the acceptance of rational
views upon the subject.
At this period but little had been done in studying the chemical
characters of aqueous and igneous rock-masses respectively; and while,
on the one hand, the close similarity in chemical composition between
the ancient basalts and many modern lavas was not recognised, the
marked distinction between the composition of such materials and most
aqueous sediments remained, on the other hand, equally unknown. Nor
had anything been yet accomplished in the direction of the study of
rock-masses by the aid of the microscope. Hence there could be no
appeal to those numerous structural peculiarities that at once enable
us to distinguish the most crystalline aqueous rocks from the materials
of igneous origin.
On the other hand, there undoubtedly exist rocks of a black colour
and crystalline structure, sometimes presenting a striking similarity
in general appearance to the basalts, which contain fossils and are
undoubtedly of aqueous origin. Thus on the shore near Portrush, in the
North of Ireland, and in the skerries which lie off that coast, there
occur great rock-masses, some of which undoubtedly agree with basalt in
all their characters, while others are dark-coloured and crystalline,
and are frequently crowded with _Ammonites_ and other fossils. We now
know that the explanation of these facts is as follows. Near where
the town of Portrush is now situated, a volcanic vent was opened in
Miocene times through rocks of Lias shale. From this igneous centre,
sheets and dykes of basaltic lava were given off, and in consequence
of their contact with these masses of lava, the Lias shales were baked
and altered, and assumed a crystalline character, though the traces of
the fossils contained in them were not altogether obliterated. In the
last century the methods which had been devised for the discrimination
of rocks were so imperfect that no distinction was recognised between
the true basalt and the altered shale, and specimens of the latter
containing _Ammonites_ found their way to almost every museum in
Europe, and were used as illustrations of the 'origin of basalt by
aqueous precipitation.'
Another source of the widely-spread error which prevailed concerning
the origin of basalt, was the failure to recognise the nature of
the alterations which take place in the character of rock-masses in
consequence of the passage through them, during enormous periods of
time, of water containing carbonic acid and other active chemical
agents. The casual observer does not recognise the resemblance which
exists between certain ornamental marbles and the loose accumulations
of shells and corals which form many sea-beaches; but close examination
shows that the former consist of the same materials as the latter,
bound together by a crystalline infilling of carbonate of lime, which
has been deposited in all the cavities and interstices of the mass. In
the same way, as we have already seen, the vesicles and interstices
of heaps of scoriæ may, by the percolation of water through the mass,
become so filled with various crystalline substances, that its original
characters are entirely masked.
But the progress of chemical and microscopic research has effectually
removed these sources of error. Many rocks of aqueous origin, formerly
confounded with the basalts, have now been relegated to their proper
places among the different classes of rocks; while, on the other
hand, it has been shown that the chemical and physical differences
between the ancient basalts and the modern basic lavas are slight
and accidental, and their resemblances are of the closest and most
fundamental character.
[Sidenote: VOLCANIC ORIGIN OF 'TRAP ROCKS.']
The notion of the aqueous origin of basalt, which was so long
maintained by the school of Werner, has now been entirely abandoned,
and the so-called 'trap-rocks' are at the present day recognised as
being as truly volcanic in their origin as the lavas of Etna and
Vesuvius.
There is, however, a vestige of this doctrine of Werner, which still
maintains its ground with obstinate persistence. Many geologists in
Germany who admit that volcanic phenomena, similar to those which are
going on at the present day, must have occurred during the Tertiary and
the later Secondary periods, nevertheless insist that among the earlier
records of the world's history we find no evidence whatever of such
volcanic action having taken place. By the geologists who hold these
views it is asserted that while the granites and other plutonic rocks
were formed during the earlier periods of the world's history, true
volcanic products are only known in connection with the sediment of the
later geological periods.
Some geologists have gone farther even than this, and asserted that
each of the great geological periods is characterised by the nature
of the igneous ejections which have taken place in it. They declare
that granite was formed only during the earliest geological periods,
and that at later dates the gabbros, diabases, porphyries, dolerites
and basalts, successively made their appearance, and finally that the
modern lavas were poured out.
A little consideration will suffice to convince us that these
conclusions are not based upon any good evidence. The plutonic rocks,
as we have already seen, exhibit sufficient proofs in their highly
crystalline character, and in their cavities containing water,
liquefied carbonic acid, and other volatile substances, that they must
have been formed by the very slow consolidation of igneous materials
under enormous pressure. Such pressures, it is evident, could only
exist at great depths beneath the earth's surface. Mr. Sorby and
others have endeavoured to calculate what was the actual thickness of
rock under which certain granites must have been formed, by measuring
the amount of contraction in the liquids which have been imprisoned
in the crystals of these rocks. The conclusions arrived at are of a
sufficiently startling character. It is inferred that the granites
which have been thus examined must have consolidated at depths varying
from 30,000 to 80,000 feet beneath the earth's surface. It is true
that in arriving at these results certain assumptions have to be made,
and to these exception may be taken, but the general conclusion that
granitic rocks could only have been formed under such high pressures as
exist at great depths beneath the surface, appears to be one which is
not open to reasonable doubt.
If, then, granites and similar rocks were formed at the depth of some
miles, it is evident that they can only have made their appearance at
the surface by the removal of the vast thickness of overlying rocks;
and the sole agency which we know of that is capable of effecting the
removal of such enormous quantities of rock-materials, is denudation.
But the agents of denudation--rain and frost, rivers and glaciers, and
sea-waves--though producing grand results, yet work exceeding slowly;
and almost inconceivably long periods of time must have elapsed before
masses of rock several miles in thickness could have been removed, and
the subjacent granites and other highly crystalline rocks have been
exposed at the surface.
[Sidenote: ANCIENT AND MODERN VOLCANIC ROCKS.]
It is an admitted fact that among the older geological formations, we
much more frequently find intrusions of granitic rocks than in the case
of younger ones. It is equally true that among the sediments formed
during the most recent geological periods, no true granitic rocks have
been detected. But if, as we insist is the case, granitic rocks can
only be formed at a great depth from the surface, the £acts we have
described are only just what we might expect to present themselves
under the circumstances. The older a mass of granitic rock, the greater
chance there is that the denuding forces operating upon the overlying
masses, will have had an opportunity of so far removing the latter
as to expose the underlying crystalline rocks at the surface. And,
on the other hand, the younger crystalline rocks are still, for the
most part, buried under such enormous thicknesses of superincumbent
materials that it is hopeless for us to search for them. Nevertheless,
it does occasionally happen that, where the work of denudation has been
exceptionally rapid in its action, such crystalline rocks formed during
a comparatively recent geological period, are exposed at the surface.
This is the case in the Western Isles of Scotland and in the Pyrenees,
where masses of granite and other highly crystalline rocks are found
which were evidently formed during the Tertiary period.
The granites which were formed in Tertiary times present no essential
points of difference from those which had their origin during the
earlier periods of the earth's history. The former, like the latter,
consist of a mass of crystals with no imperfectly crystalline base or
groundmass between them; and these crystals include numerous cavities
containing liquids.
Between the granites and the quartz-felsites every possible
gradation may be found, so that it is impossible to say where the one
group ends and the other begins; indeed, many of the rocks called
'granite-porphyries' have about equal claims to be placed in either
class. Nor is the distinction between the quartz-felsites and rhyolites
any more strongly marked than that between the former class of rocks
and the granites; some of the more crystalline rhyolites of Hungary
being quite undistinguishable, in their chemical composition, their
mineralogical constitution, and their microscopic characters, from
the quartz-felsites. The more crystalline rhyolites are in turn found
passing by insensible gradations into the glassy varieties and finally
into obsidian.
[Sidenote: RELATIONS BETWEEN GRANITE AND PUMICE.]
A piece of granite and a piece of pumice may at first sight appear to
present so many points of difference, that it would seem quite futile
to attempt to discover any connection between them. Yet, if we analyse
the two substances, we may find that in ultimate chemical composition
they are absolutely identical. There is nothing irrational, therefore,
in the conclusion that the same materials under different conditions
may assume either the characters of granite on the one hand, or of
pumice on the other; the former being consolidated under circumstances
in which the chemical and crystalline forces have had the freest play
and have used up the whole of the materials to form crystallised
minerals, while the latter has cooled down and solidified rapidly at
the surface, in such a way that only incipient crystallisation has
occurred, and the glassy mass has been reduced to a frothy condition
by the escape of steam-bubbles from its midst This conclusion receives
the strongest support from the fact that examples of every stage of
the change, between the glassy condition of pumice and the crystalline
condition of granite, may be detected among the materials of which the
globe is built up.
There is still another class of facts which may be adduced in
support of the same conclusion. Many lavas, as we have seen, contain
crystals of much larger dimensions than those constituting the mass
of the rock, which is then said to be 'porphyritic' in structure.
The porphyritically embedded crystals, when carefully examined, are
often seen to be broken and injured, and to exhibit rounded edges,
with other indications of having undergone transport. When examined
microscopically, too, they often present the cavities containing
liquids which distinguish the crystals of plutonic rocks. All the
facts connected with these porphyritic lavas point to the conclusion
that while the crystals in their groundmass have separated from the
liquefied materials near the surface, the large embedded crystal, have
been floated up from great depths within the earth's crust, where they
had been originally formed.
[Sidenote: GRANITIC REPRESENTATIVES OF OTHER LAVAS.]
The careful consideration of all the facts of the case leads to the
conclusion that where pumice, obsidian, and rhyolite are now being
ejected at the surface, the materials which form these substances
are, at various depths in the earth's interior, slowly consolidating
in the form of quartz-felsite, granite-porphyry and granite. It may
be that we can nowhere point to the example of a mass of rock which
can be traced from subterranean regions to the surface, and is, under
such conditions, actually seen to pass from the dense and crystalline
condition of granite to the vesicular and glassy form of pumice;
but great granitic masses often exhibit a more coarsely crystalline
condition in their interior, and the offshoots and dykes which they
give off not infrequently assume the form of quartz-felsite; while,
on the other hand, the more slowly consolidated rocks found in the
interior of some rhyolite masses are not distinguishable in any way
from some of the true quartz-felsites.
That which is true of the lavas of acid composition is equally true
of the lavas of intermediate and basic character. The andesites,
the trachytes, the phonolites, and the basalts have all their exact
representatives among the plutonic rocks, and these have a perfectly
crystalline or granitic structure. The plutonic and the volcanic
representatives of each of these groups are identical in their chemical
composition, and numerous intermediate gradations can be found between
the most completely granitic and the most perfectly vitreous or glassy
types. In illustration of this fact, we may again refer to the series
of microscopic sections of rocks given in the frontispiece.
Another objection to the conclusion that the volcanic products of
earlier periods of the earth's history were identical in character with
those which are being ejected at the present day is based on the fact
of the supposed non-existence of the scoriaceous and glassy materials
which abound in the neighbourhood of the active volcanic vents. Where,
it is asked, do we find among the older rocks of the globe the heaps of
lapilli, dust, and scoriæ, with the glassy and pumiceous rocks that now
occur so abundantly in all volcanic districts?
In reply to this objection, we may point out that these accumulations
of loose materials are of such a nature as to be capable of easy
removal by denuding agents, and that as they are formed upon the land
they will, if not already washed away by the action of rain, floods,
rivers, &c., run great risk of having their materials distributed,
when the land sinks beneath the waters of the ocean and the surface
is covered by new deposits. With respect to the glassy rocks it must
be remembered that the action of water, containing carbonic acid and
other substances, in percolating through such masses has a tendency
to set up crystalline action, and these glassy rocks easily undergo
'devitrification'; it would therefore be illogical for us to expect
glassy rock-masses to retain their vitreous character through long
geological periods, during which they have been subjected to the action
of water and acid gases.
But careful observation has shown that the scoriaceous and vitreous
rocks are by no means absent among the igneous materials ejected during
earlier periods of the earth's history. Their comparative infrequency
is easily accounted for when we remember, in the first place, the ease
with which such materials would be removed by denuding forces, and in
the second place, the tendency of the action of percolating water to
destroy their characteristic features, by filling up their vesicles
with crystalline products and by effecting devitrification in their
mass.
[Sidenote: SIMILARITY OF ANCIENT AND RECENT LAVAS.]
If we go back to the very oldest known rock-masses of the globe, those
which are found underlying the fossiliferous Cambrian strata, we find
abundant evidence that volcanic action took place during the period
in which these materials were being accumulated. Thus, in the Wrekin,
as Mr. Allport has so well shown, we find clear proofs that before
the long-distant period of the Cambrian, there existed volcanoes
which ejected scoriæ, lapilli, and volcanic dust, and also gave rise
to streams of lava exhibiting the characteristic structures found in
glassy rocks. In these rocks, which have undergone a curious alteration
or devitrification, we still find all those peculiar structures--the
sphærulitic, the perlitic, and the banded--so common in the rhyolites
of Hungary, with which rocks the Wrekin lavas, in their chemical
composition, precisely agree. Prof. Bonney, too, has shown that the
rocks of Charnwood Forest, which are also probably of pre-Cambrian age,
contain great quantities of altered volcanic agglomerates, tuffs, and
ashes. I have found the sphærulitic, perlitic, and banded structures
exhibited by British lavas of the Cambrian, Silurian, Devonian and
Carboniferous periods, as well as in those of Tertiary age; and in
connection with these different lavas we find vast accumulations,
sometimes thousands of feet in thickness, of volcanic agglomerates and
tuffs which have undergone great alteration.
All these facts point to one conclusion--namely, that during all past
geological periods, materials similar to those which are now being
extruded from volcanic vents were poured out on the earth's surface
by analogous agencies. If we could trace the lava-streams of the
present day down to the great subterranean reservoirs from which their
materials have been derived, we should doubtless find that at gradually
increasing depths, where the pressure would be greater and the escape
of heat from the mass slower, the rocky materials would by degrees
assume more and more crystalline characters. We should thus find
obsidian or rhyolite insensibly passing into quartz-felsite and finally
into granite; trachyte passing into orthoclase-porphyry and syenite;
and basalt passing into dolerite, augite-porphyry, and gabbro.
On the other hand, if we could replace the great masses of stratified
rocks which must once have overlain the granites, syenites, diorites,
and gabbros, we should find that, as we approached the original
surface, these igneous materials would gradually lose their crystalline
characters, and when they were poured out at the surface would take the
forms of rhyolite, trachyte, andesite, and basalt--all of which might
occasionally assume the glassy forms known as obsidian or tachylyte.
[Sidenote: ALTERED FORMS OF ANCIENT LAVAS.]
But while we insist on the essential points of similarity between
the lavas poured out upon the surface of the earth during earlier
geological periods and those which are being extruded at the present
day, we must not forget that by the action of percolating water and
acid gases, the mineral constitution, the structure, and sometimes
even the chemical composition of these ancient lavas may undergo
a vast amount of change. In not a few cases these changes in the
characters of a lava may be carried so far that the altered rock bears
but little resemblance to the lava from which it was formed, and it may
be found desirable to give it a new name. Among the rocks of aqueous
origin we find similar differences in the materials deposited at
different geological periods. Clay, shale and clay-slate have the same
composition, and the two latter are evidently only altered forms of the
first mentioned, yet so great is the difference in their characters
that it is not only allowable, but desirable, to give them distinctive
names.
In the same way, among the deposits of the earlier geological periods
we find rocks which were doubtless originally basalts, but in which
great alterations have been produced by the percolation of water
through the mass. The original rock has consisted of crystals of
felspar, augite, olivine, and magnetite distributed through a glassy
base. But the chemical action of water and carbonic acid may have
affected all the ingredients of the rock. The outward form of the
felspar crystals may be retained while their substance is changed to
kaolinite, various zeolites, and other minerals; the olivine maybe
altered to serpentine and other analogous minerals; the magnetite
changed to hydrous peroxide of iron; the augite may be changed to
uralite or hornblende; and the surrounding glassy mass more or
less devitrified and decomposed. The hard, dense, and black rock
known as basalt has under these circumstances become a much softer,
earthy-looking mass of a reddish-brown tint, and its difference from
basalt is so marked that geologists have agreed to call it by another
name, that of 'melaphyre.' Even in their ultimate chemical compositions
the 'melaphyres' differ to some extent from the basalts, for some of
the materials of the latter may have been removed in solution, and
water, oxygen, and carbonic acid have been introduced to combine with
the remaining ingredients.
But if we carefully study, by the aid of the microscope, a large series
of basalts and melaphyres, we shall find that many rocks of the former
class show the first incipient traces of those changes which would
reduce them to the latter class. Indeed, it is quite easy to form a
perfect series from quite unaltered basalts to the most completely
changed melaphyres. Hence we are justified in concluding that all the
melaphyres were originally basalts, just as we infer that all oaks were
once acorns.
Now changes, similar to those which we have seen to take place in
the case of basaltic lavas, are exhibited by the lavas of every
other class, which have been exposed to the influence of the same
agencies,--namely, the passage of water and acid gases. But inasmuch as
the minerals composing the basic lavas are for the most part much more
easily affected by such agencies than are the minerals of acid lavas,
the ancient basic rocks are usually found in a much more highly altered
condition than are the acid rocks of equivalent age.
[Sidenote: NAMES GIVEN TO ALTERED LAVAS.]
We thus see that each of the classes of modern lavas has its
representative in earlier geological periods, in the form of rocks
which have evidently been derived from these lavas, through alterations
effected by the agency of water and acid-gases that have permeated
their mass. Thus, while the basalts are represented among the ancient
geological formation by the melaphyres, the andesites are represented
by the porphyrites, and the trachytes and rhyolites by different
varieties of felstones. And, as we can form perfect series illustrating
the gradual change from basalt to melaphyre, so we can arrange other
series demonstrating the passage of andesites into porphyrites, and of
trachytes and rhyolites into felsites.
It must be remembered, however, that these changes do not take place
in anything like determinate periods of time. Occasionally we may
find lavas of ancient date which have undergone surprisingly little
alteration, and in other cases there occur lavas belonging to a
comparatively recent period which exhibit very marked signs of change.
The alteration of the lavas and other igneous rocks does not, however,
stop with the production of the melaphyres, porphyrites, and felstones.
By the further action of the water and carbonic acid of the atmosphere,
the basic lavas are reduced to the soft earthy mass known as 'wacke,'
and the intermediate and acid lavas to the similar material known as
'claystone.' As the passage of water and carbonic acid gas through
these rock-masses goes on, they are eventually resolved into two
portions, one of which is insoluble in water and the other is soluble.
The insoluble portion consists principally of quartz, the crystals
of which are almost unattacked by water and carbonic acid, and the
hydrated silicate of alumina. All the sands and clays, which together
make up more than nine-tenths of the stratified rocks of the globe,
are doubtless derived, either directly or indirectly, from these
insoluble materials separated during the decomposition of volcanic and
plutonic rocks. The soluble materials, which consist of the carbonates,
sulphates and chlorides of lime, magnesia, soda, potash, and iron,
give rise to the formation of the limestones, gypsum, rock-salt,
ironstones, and other stratified masses of the earth's crust. We thus
see how the igneous materials of the globe, by their decomposition,
famish the materials for the stratified rock-masses. The relations of
the different plutonic and volcanic rocks to one another and to the
materials which are derived from them are illustrated in the following
table.
[Sidenote: RELATIONS OF ALTERED TO UNALTERED LAVAS.]
Unaltered Altered Decomposed
Plutonic Rocks lavas lavas Rocks
Granite {Quartz-felsite } Rhyolite and }
{ ('quartz-porphyry')} Obsidian } Felstone }
} }
Syenite {Orthoclase-porphyry } Trachyte } } Claystones
}
Diorite {Hornblende-porphyry } Andesite Porphyrite }
Miascite {Liebnerite porphyry } Phonolite ? --
Gabbro {Augite-porphyry and } Basalt Melaphyre Wacke
{Dolerite }
Some petrographers, indeed, have maintained the principle that rocks
belonging to widely separated geological periods, even when they
exhibit no essential points of difference, should nevertheless be
called by distinct names. But such a system of classification is
calculated rather to hinder than to advance the cause of science. If
the palæontologist were to adopt the same principle and give distinct
names to the same fossil, when it was found to occur in two different
geological formations, we can easily understand what confusion would
be occasioned, and how the comparison of the fauna and flora of the
different formations would be thereby rendered impossible. But the
naturalist, in his diagnosis of a species, wisely confines himself
to the structure and affinities of the organism before him; and in
the same way the petrographer, in giving a name to a rock, ought
to be guided only by his studies of its chemical composition, its
mineralogical constitution, and its structure, putting altogether
out of view its geographical distribution and geological age. Only
by strict attention to this principle can we hope to arrive at such
comparisons of the rocks of different areas and different periods, as
may serve as the basis for safe inductions.
Before leaving this question of the relation which exists between the
igneous rocks of different ages, it may be well to notice several facts
that have been relied upon, as proving that the several geological
periods are distinguished by characteristic igneous products.
It has frequently been asserted that the acid igneous rocks are present
in much greater quantities in connection with the older geological
formations than axe the basic; while, on the other hand, the basic
igneous rocks are said to have been extruded in greater abundance in
the more recent geological periods. But in considering this question it
must not be forgotten that, as a general rule, the basic rocks undergo
decomposition and disintegration far more rapidly than do the acid
rocks. In consequence of this circumstance the chance of our finding
their recognisable representatives among the older formations, is much
less in the case of the former class of rocks than in the latter. As a
matter of fact, however, we do find great masses of gabbro, diabase,
and melaphyre associated even with the oldest geological formations,
while trachytes and rhyolites abound in many volcanic districts where
active vents exist at the present day. Upon a general review of the
subject, it may well be doubted whether the supposed preponderance of
acid igneous materials in the earlier periods of the earth's history,
and of basic igneous materials during the later periods, rests on any
substantial basis of observation.
[Sidenote: AUGITIC AND HORNBLENDIC ROCKS.]
Another difference which has frequently been relied upon, as
distinguishing the older igneous rocks from those of more recent date,
is the supposed fact that the former are characterised by the presence
of hornblende, the latter by the presence of augite. It may be admitted
that this distinction is a real one, but its significance and value are
greatly diminished when we remember the relations which exist between
the two minerals in question. Hornblende and augite are interesting
examples of a dimorphous substance; in chemical composition they are
identical, or rather they are liable to variation between the same
limits, but in their crystalline forms and optical characters they
differ from one another. It has been proved that hornblende is the
stable, and augite the unstable condition of the substance in question.
If hornblende be fused and allowed to cool, it crystallises in the form
of augite. On the other hand, augite-crystals in rocks of ancient date
are found undergoing gradual change and passing into hornblende. The
mineral uralite has the outward form of augite, but the cleavage and
optical properties of hornblende; and there are not wanting many facts
pointing to the conclusion that rocks which now contain hornblende were
originally augitic masses, in which the unstable mineral in their
midst has been gradually converted into the stable one.
There are, however, two minerals which up to the present time have
been found in association only with the older and newer rock-masses
respectively. These are _muscovite_, or the white form of mica, which
occurs in so many granites, but has not yet been discovered in any
modern representative of that rock; and leucite, which is not yet known
in rocks of older date than the Tertiary.
When we remember that muscovite would appear to be a product of
deep-seated igneous action, and is only found in rock-masses that have
been formed under such conditions, we shall be the less surprised at
its non-occurrence in rocks of recent date, especially if we bear in
mind the fact that very few of the younger granitic rocks have as yet
been exposed at the surface by denudation.
With respect to leucite, on the other hand, it must be remembered that
it is a very unstable mineral which appears to be easily changed into
felspar. It is by no means improbable, therefore, that some ancient
igneous rocks which now contain felspar were originally leucitic rocks.
To the view that the action of volcanic forces upon the globe during
past geological times was similar in kind to that which we now observe
going on around us, still another objection has been raised. It has
been asserted that some of the deposits of igneous rock associated
with the older geological formations are of such a nature that they
could not possibly have been accumulated around volcanic vents of the
kind which we see in operation around us.
[Sidenote: VOLCANIC ORIGIN OF ANCIENT IGNEOUS ROCKS.]
Mr. Mallet has declared that the igneous products of the Palæozoic
period differ fundamentally in character from those materials formed
by volcanic action during the later Secondary and the Tertiary
periods. Upon what observations these generalisations are based he
has given us no information, and the enormous mass of facts which
have been collected in recent years concerning the structure of the
lavas and fragmental volcanic deposits of the pre-Cambrian, Cambrian,
Silurian, Devonian and Carboniferous periods, all point to a directly
opposite conclusion. The more carefully we carry on our investigations
concerning these ancient lavas, by the aid of chemical analysis and
microscopic study, the more are we convinced of the essential identity
of the ancient and modern volcanic rocks, both in their composition
and their minute structure. Of great masses of dust produced by
crushing, such as Mr. Mallet has supposed to have been formed during
the earlier geological periods, there is not the smallest evidence;
but we everywhere find proofs, when the rocks are minutely examined,
of the vesicular structure so characteristic of materials produced by
explosive volcanic action.
It has frequently been asserted that in the great districts covered by
basaltic lavas which we find in the Rocky Mountains of North America,
in the Deccan of India, in Abyssinia, and even in the Western Isles of
Scotland, we have proofs of the occurrence, during earlier geological
periods, of volcanic action very different in character from that
which at present takes place on our globe. It has been asserted that
the phenomena observed in these districts can only be accounted for by
supposing that great fissures have opened in their midst, from which
lavas have issued in enormous floods unaccompanied by the ordinary
explosive phenomena of volcanoes.
It must be remembered, however, that none of the districts in question
have been subjected to careful and systematic examination with a view
to the discovery of the vents from which these masses of lava have
issued, with the exception of that which occurs in our own islands. In
this case, in which superficial observers have spoken of the district
as being covered with horizontal lava-sheets piled upon one another to
the depth of 3,000 feet, careful study of the rock-masses has shown
that the accumulations of basalt really consist of a great number of
lava-currents which have issued at successive epochs covering enormous
periods of time. During the intervals between the emission of these
successive lava-currents the surfaces of the older ones have been
decomposed, and formed soils upon which forests have grown up; they
have been eroded by streams, the valleys so formed being filled with
gravels; and lakes have been originated on their surfaces in which
various accumulations have taken place.
[Sidenote: TRULY-VOLCANIC ORIGIN OF LAVA PLATEAUX.]
It has been demonstrated, moreover, that the basal-wrecks of no less
than five volcanic mountains, each of which must have rivalled Etna in
its proportions, existed within this area, and the connection of the
lava-currents, which have deluged the surrounding tracts, with these
great volcanoes has been clearly proved. It is probable that when
more careful and systematic researches are carried on in the other
districts, in which widely-spread sheets of basaltic rocks exist,
similar volcanic vents will be discovered. It must also be remembered
that if such a country as Iceland were subjected to long-continued
denudation, the mountain peaks and cones of loose materials would be
worn away, the whole island being thus reduced to a series of plateaux
composed of lava-sheets, the connection of which with the crystalline
materials filling the great volcanic vents, a superficial observer
might altogether fail to recognise.
But even where we cannot trace the former existence of great volcanic
mountains, like those which once rose in the Hebrides, it would
nevertheless be very rash to conclude that the vast plateaux of
lava-rock must have been formed as gigantic floods unaccompanied by
ordinary volcanic action. Mr. Darwin has pointed out that in crossing
districts covered by lava, he was frequently only able to determine
the limits of the different currents of which it was made up, by an
examination of the age of the trees and the nature of the vegetation
which had sprung up on them. And everyone who has travelled much in
volcanic districts can confirm this observation; what appears at first
sight to be a great continuous sheet of lava proves upon more careful
observation to be composed of a great number of distinctly different
lava-currents, which have succeeded one another at longer or shorter
intervals.
We must remember, too, how various in kind are the volcanic
manifestations which present themselves under different circumstances.
Sometimes the amount of explosive action at a volcanic vent is very
great, and only fragmental ejections take place, composed of the frothy
scum of the lava produced by the escape of gases and vapours from its
midst. But in other cases the amount of explosive action may be small,
and great volumes of igneous materials may issue as lava-streams. In
such cases, only small scoriæ-cones would be formed around the vents,
and one half of such cones is commonly swept away by the efflux of the
lava-currents, while the remainder may be easily removed by denuding
action or be buried under the lava-currents issuing from other vents
in the neighbourhood. Thus it may easily come to pass that what a
superficial observer takes for an enormous mass of basaltic lava poured
out from a great fissure at a single effort, may prove upon careful
observation to be made up of innumerable lava-currents, each of which
is of moderate dimensions; and it may further be found that these
lava-currents, instead of being the product of a single paroxysmal
effort from one great fissure, have been accumulated by numerous small
outbursts taking place at wide intervals, from a great number of minor
orifices.
[Sidenote: SHIFTING OF VOLCANIC ACTION IN DIFFERENT AREAS.]
Having then considered the arguments which have been adduced in support
of the view that the volcanic phenomena of former geological periods
differ from those which are still occurring upon the globe, we may
proceed to state the general conclusions which have been drawn from the
study of the volcanic rocks of the different geological periods.
From a survey of the volcanic rocks of different ages, we are led to
the interesting and important conclusion that the scene of volcanic
action has been continually shifting to fresh areas at different
periods of the earth's history. We find repeated proofs that the
volcanic energy has made its appearance at a certain part of the
earth's crust, has gradually increased in intensity to a maximum, and
then as slowly declined. But as these manifestations have died away
at one part of the earth's surface, they have gradually made their
appearance at another. In every district which has been examined, we
find abundant proofs that volcanic energy has been developed at certain
periods, has disappeared during longer or shorter periods, and then
reappeared in the same area. And on the other hand, we find that there
is no past geological period in which we have not abundant evidence
that volcanic outbursts took place at some portion of the earth's
surface.
To take the case of our own islands for example. We know that during
the pre-Cambrian periods volcanic outbursts occurred, traces of which
are found both in North and South Wales, in the Wrekin Chain in
Shropshire, in Charnwood Forest, and in parts of Scotland and Ireland.
In Cambro-Silurian times we have abundant proofs, both in North Wales
and the Lake district, that volcanic action on the very grandest
scale was taking place during the Arenig and the older portion of
the Llandeilo periods, and again during the deposition of the Bala
or Caradoc beds. The lavas, tuffs, and volcanic agglomerates ejected
during these two periods have built up masses of rock many thousands of
feet in thickness. Snowdon and Cader Idris among the Welsh mountains,
and some of the higher summits of the Lake district, have been carved
by denudation from the vast piles of volcanic materials ejected during
these periods.
In Devonian or Old-Red-Sandstone times, volcanic activity was renewed
with fresh violence upon that part of the earth's surface now occupied
by the British Islands. Along the line which now forms the Grampians
there rose a series of volcanoes of the very grandest dimensions. Ben
Nevis, and many others among the higher Scotch mountains, have been
carved by denudation from the hard masses of granite, quartz-felsite,
and other plutonic rocks which formed the central cores of these
ancient volcanic piles. The remains of the great lava-sheets, and of
the masses of volcanic agglomerate ejected from these grand Devonian
volcanoes, make up hill-ranges of no mean altitude, like the Sidlaws,
the Ochils, and the Pentlands.
[Sidenote: ANCIENT BRITISH VOLCANOES.]
The volcanic action of the Devonian period was prolonged into
Carboniferous times, but was then evidently diminishing gradually in
violence. Instead of great central volcanoes, such as existed in the
earlier period, we find innumerable small vents which threw out tuffs,
agglomerates and lavas, and were scattered over the districts lying
around the bases of the now extinct Devonian volcanoes. In the central
valley of Scotland and in many parts of England, we find abundant
proofs of the existence of these small and scattered volcanic vents
during Carboniferous times. The well-known hill of Arthur's Seat, which
overlooks the city of Edinburgh, and many castle-crowned crags of the
Forth and Clyde valleys, are the worn and denuded relics of these small
volcanoes. There are some indications which point to the conclusion
that the volcanic action of the Newer Palæozoic epoch had not entirely
died out in Permian times, but the evidence upon this point is not
altogether clear and satisfactory.
During nearly the whole of the Secondary or Mesozoic periods the
volcanic forces remained dormant in the area of the British Isles.
Some small volcanic outbursts, however, appear to have occurred in
Triassic times in Devonshire. But in other areas, such as the Tyrol,
South-eastern Europe and Western America, the Triassic, Jurassic, and
Cretaceous periods were marked by grand manifestations of volcanic
activity.
The volcanic forces which had during the long Mesozoic periods deserted
our part of the earth's surface, appear to have returned to it in
full rigour in the Tertiary epoch. In the Newer-Palæozoic periods
the direction of the great volcanic band which traversed our islands
appears to have been from north-east to south-west; but in Tertiary
times a new set of fissures were opened running from north to south.
There is evidence that during the Eocene or Nummulitic period, the
first indications of the subterranean forces having gathered strength
below the district were afforded by the issue of calcareous and
siliceous springs, and soon fissures were opened which emitted scoriæ,
tuffs, and lavas. The intensity of the volcanic action gradually
increased till it attained its maximum in the Miocene period, when
a great chain of volcanic mountains stretched north and south along
the line of the Inner Hebrides, the north-east of Ireland, and the
sea which separates Great Britain from Ireland. The basal-wrecks of a
number of these volcanoes can be traced in the islands of Skye, Mull,
Rum, and parts of the adjoining mainland. We have already seen that
along this great band of volcanic action, which traverses the Atlantic
Ocean from north to south, a number of active vents still exist, though
their energy is now far less intense than was the case in former times.
The only vestiges of the action of these now declining volcanic forces,
at present found in our islands, are the hot springs of Bath and a few
other warm and mineral springs; but in connection with this subject it
must be remembered that our country occasionally participates in great
earthquake-vibrations, like that which destroyed Lisbon in the year
1759.
[Sidenote: ANCIENT VOLCANOES IN OTHER DISTRICTS.]
If we were to study any other part of the earth's surface, we should
arrive at precisely the same conclusion as those to which we have
been conducted by our examination of the British Islands--namely,
that during past geological times the subterranean forces had made
themselves felt in the area, had gradually attained a maximum, and then
as gradually declined, passing through all those varied cycles which we
have described in a former chapter. And we should also find that these
periods of volcanic activity alternated with other periods of complete
quiescence which were of longer or shorter duration. But on comparing
two different districts, we should discover that what was a period of
volcanic activity in the one was a period of repose in the other, and
_vice versâ_.
From these facts geologists have been led to the conclusion which we
have already enunciated--namely, that the subterranean forces are in a
state of continual flux over the surface of the globe. At one point of
the earth's crust these forces gradually gather such energy as to rend
asunder the superincumbent rock-masses and make themselves manifest
at the surface in the series of phenomena characteristic of volcanic
action. But after a longer or shorter interval of time--an interval
which must probably be measured by millions of years--the volcanic
forces die out in that area to make their appearance in another.
Hence, although we may not be able to prove the fact by any
mathematical demonstration, a strong presumption is raised in favour
of the view that the subterranean energy in the earth's crust is a
constant quantity, and that the only variations which take place are in
the locality of its manifestation.
Upon this question whether the amount of this subterranean energy
within the earth's crust is at the present time increasing, stationary,
or declining, we are not altogether destitute of evidence. There are
some considerations connected with certain astronomical hypotheses, to
which we shall hereafter have to refer, that might lead us to entertain
the view that the subterranean activity was once far greater than it
is at present, and that during the long periods of the earth's past
history it has been slowly and gradually declining. And those who
examine the vast masses of igneous materials which have been poured
out from volcanic vents during the earlier periods of the earth's
history may be inclined, at first sight, to point to them as affording
conclusive proof of this gradual decline.
[Sidenote: SUPPOSED DECLINE OF VOLCANIC ACTION.]
But a more careful study of the rocks in question will probably cause
a geologist to pause before jumping to such a conclusion. If we look
at the vast masses of volcanic materials erupted in Miocene times in
our own island and in Ireland, for example, we might be led to imagine
that we have the indications of a veritable 'Reign of Fire,' and that
the evidence points to a condition of things very different indeed
from that which prevails at the present day. But in arriving at such
a conclusion we should be neglecting a most important consideration,
the disregard of which has been the fertile parent of many geological
errors. Many independent lines of evidence all point to the inference
that these volcanic ejections are not the result of one violent effort,
but are the product of numerous small outbreaks which have been
scattered over enormous periods of time.
When we examine with due care the lavas, tuffs, and other volcanic
ejections which constitute such mountain-masses as those of the
Hebrides, of the Auvergne, and of Hungary, we find clear proofs
that the ancient Miocene volcanoes of these districts were clothed
with luxuriant forests, through which wild animals roamed in the
greatest abundance. The intervals between the ejections of successive
lava-streams were often so great, that soils were formed on the
mountain-slope, and streams cut deep ravines and valleys in them.
The island of Java is situated near the very heart of what is at the
present day the most active volcanic centre on the face of the globe,
yet vegetable and animal life flourish luxuriantly there, and the
island is one of the richest and most fertile spots upon the face of
the globe. Not all the terrors of occasional volcanic outbreaks will
ever drive the Neapolitan vine-dressers from the fertile slopes of
Vesuvius, for its periods of repose are long, and its eruptions are of
short duration.
These considerations lead the geologist to conclude that the evidence
afforded by the ancient volcanic rocks is clear and positive in support
of the view that the manifestations of the subterranean forces in the
past agree precisely in their nature and in their products with those
taking place around us at the present time. On the question of great
secular changes having occurred in the amount of volcanic energy in
past geological periods, the evidence must be pronounced negative, or
at the best doubtful.
But even if the geologist confesses himself unable to establish the
fact of any decline in the subterranean energies during the vast
periods of which he takes cognisance, it must be remembered that such
decline may really be going on; for vast as was the duration of the
geological epochs, they probably constitute but a fraction of those far
grander periods which are required by the speculations of the physical
astronomer.
CHAPTER X.
THE PART PLAYED BY VOLCANOES IN THE ECONOMY OF NATURE.
The first impression which is produced upon the mind, when the
phenomena of volcanic action are studied, is that here we have
exhibitions of destructive violence the effects of which must be
entirely mischievous and disastrous to the living beings occupying the
earth's surface. A little consideration will convince us, however, that
the grand and terrible character of the displays of volcanic energy
have given rise to exaggerated notions concerning their destructive
effects. The fact that districts situated over the most powerful
volcanic foci, like Java and Japan, are luxuriant in their productions,
and thickly inhabited, may well lead us to pause ere we condemn
volcanic action as productive only of mischief to the living beings
on the earth's surface. The actual slopes of Vesuvius and Etna, and
many other active volcanoes, are abundantly clothed with vineyards and
forests and are thickly studded with populous villages.
As a matter of fact, the actual amount of damage to life and property
which is effected by volcanic eruptions is small. Usually, the
inhabitants of the district have sufficient warning to enable them
to escape with their lives and to carry away their most valuable
possessions. And though fertile tracts are covered by loose dust and
ashes, or by lava- and mud-currents, yet the sterility thus produced
is generally of short duration, for by their decomposition volcanic
materials give rise to the formation of the richest and most productive
soils.
Earthquakes, as we have already seen, are far more destructive in
their effects than are volcanoes. Houses and villages, nay even entire
cities, are, by vibrations of portions of the earth's crust, reduced to
heaps of ruins, and famines and pestilences too frequently follow, as
the consequence of the disorganisation of our social systems by these
terrible catastrophes.
It may well be doubted, however, whether the annual average of
destruction to life and property caused by all kinds of subterranean
action, exceeds that produced either by floods or by hurricanes. Yet
we know that the circulation of water and air over our globe are
beneficial and necessary operations, and that the mischief occasionally
wrought by the moving bodies of water and air is quite insignificant
compared with the good which they effect.
In the same way, we shall be able to show that the subterranean
energies are necessary to the continued existence of our globe as
a place fitted for the habitation of living beings, and that the
mischievous and destructive effects of these energies bear but a small
and insignificant proportion to the beneficial results with which they
must be credited.
[Sidenote: LEVELLING ACTION OF DENUDING FORCES.]
We have had frequent occasion in the preceding pages to refer to the
work--slow but sure, silent but effective--wrought by the action of
the denuding forces ever operating upon the surface of our globe. The
waters condensing from the atmosphere and falling upon the land in
the form of rain, snow, or hail, are charged with small quantities of
dissolved gases, and these waters penetrating among the rock-masses
of which the earth's crust is composed, give rise to various
chemical actions of which we have already noticed such remarkable
illustrations in studying the ancient volcanic products of our globe.
By this action the hardest and most solid rock-masses are reduced
to a state of complete disintegration, certain of their ingredients
undergoing decomposition, and the cementing materials which hold
their particles together being removed in a state of solution. In the
higher regions of the atmosphere this work of rock-disintegration
proceeds with the greatest rapidity; for there the chemical action is
reinforced by the powerful mechanical action of freezing water. On
high mountain-peaks the work of breaking up rock-masses goes on at the
most rapid rate, and every craggy pinnacle is swathed by the heaps
of fragments which have fallen from it. The Alpine traveller justly
dreads the continual fusillade of falling rock-fragments which is
kept up by the ever-active power of the frost in these higher regions
of the atmosphere; and fears lest the vibrations of his footsteps
should loosen, from their position of precarious rest, the rapidly
accumulating piles of detritus. No mountain-peak attains to any very
great elevation above the earth's surface, for the higher we rise in
the atmosphere the greater is the range of temperature and the more
destructive are the effects of the atmospheric water. The moon, which
is a much smaller planet than our earth, has mountains of far greater
elevation; but the moon possesses neither an atmosphere nor moisture on
its surface, to produce those levelling effects which we see everywhere
going on around us upon the earth.
The disintegrated materials, produced by chemical and mechanical
actions of the atmospheric waters upon rock-masses, are by floods,
rivers, and glaciers, gradually transported from higher to lower
levels; and sooner or later every fragment, when it has once been
separated from a mountain-top, must reach the ocean, where these
materials are accumulated and arranged to form new rocks.
Over every part of the earth's surface these three grand operations of
the disintegration of old rock-masses, the transport of the materials
so produced to lower levels, and the accumulation of these materials
to form new rocks, is continually going on. It is by the varied action
of these denuding agents upon rocks of unequal hardness, occupying
different positions in relation to one another, that all the external
features of hills, and plains, and mountains owe their origin.
It is a fact, which is capable of mathematical demonstration, that by
the action of these denuding forces the surface of all the lands of the
globe is being gradually but surely lowered; and this takes place at
such a rate that in a few millions of years the whole of the existing
continents must be washed away and their materials distributed over the
beds of the oceans.
[Sidenote: NECESSITY FOR COMPENSATING AGENCIES.]
It is evident that there exists some agency by which this levelling
action of the denuding forces of the globe is compensated; and a little
consideration will show that such compensating agency is found in the
subterranean forces ever at work within the earth's crust. The effects
of these subterranean forces which most powerfully arrest our attention
are volcanic outbursts and earthquake shocks, but a careful study of
the subject proves that these are by no means the most important of the
results of the action of such forces. Exact observation has proved that
almost every part of the earth's surface is either rising or falling,
and the striking and destructive phenomena of volcanoes and earthquakes
probably bear only the same relation to those grand and useful
actions of the subterranean forces, which floods do to the system of
circulating waters, and hurricanes to the system of moving air-currents.
If we ride in a well-appointed carriage with good springs, upon
a railway which is in excellent order, the movement is almost
imperceptible to us; and the rate of speed may be increased
indefinitely, without making itself apparent to our senses. The
smallest impediment to the evenness of the movement--such as that
produced by a small object placed upon the rails--at once makes itself
felt by a violent jar and vibration. How perfectly insensible we may
be of the grandest and most rapid movements is taught us by the facts
demonstrated by the astronomer. By the earth's daily rotation, we are
borne along at a rate which in some places amounts to over 1,000 miles
an hour; and by its annual revolution we are every hour transported
through a distance of 70,000 miles; yet concerning the fact and
direction of these movements we are wholly unconscious.
In the case both of the railway train and of our planet, we can only
establish the reality of the movement, and its direction and rate, by
means of observations upon external objects, which appear to us to
have a movement in the opposite direction. In the same way we can only
establish the fact of the movement of portions of the earth's crust
by noticing the changing positions of parts of the earth's surface in
relation to the constant level of the ocean. When this is done we find
abundant proof that while some parts of the earth's crust are rising,
others are as undoubtedly undergoing depression.
[Sidenote: POTENCY OF THE SUBTERRANEAN FORCES.]
We shall be able to form some idea of the vastness of the effects
produced by the subterranean forces, by a very simple consideration.
It is certain that during the enormous periods of time of which the
records have been discovered by the geologist, there have always been
continents and oceans upon the earth's surface, just as at present,
and it is almost equally certain that the proportions of the earth's
surface occupied by land and water respectively, have not varied very
widely from those which now prevail. But, at the same time, it is an
equally well-established bet that the denuding forces ever at work
upon the earth's surface would have been competent to the removal
of existing continents many times over, in the vast periods covered
by geological records. Hence we are driven to conclude that the
subterranean movements have in past times entirely compensated for the
waste produced by the denuding forces ever at work upon our globe. But
this is not all. The subterranean forces not only produce upheaval;
in a great many cases the evidences of subsidence are as clear and
conclusive as are those of upheaval in others. Hence we are driven to
conclude that the forces producing upheaval of portions of the earth's
crust are sufficient, not only to balance those producing subsidence,
but also to compensate for the destructive action of denuding agents
upon the land-masses of the globe.
It is only by a careful and attentive study and calculation of the
effects produced by the denuding agents at work all around us, aided
by an examination of the enormous thicknesses of strata formed by the
action of such causes during past geological times, that we are able
to form any idea of the reality and vastness of the agents of change
which are ever operating to modify the earth's external features. When
we have clearly realised the grand effects produced on the surface
of the globe by these external forces, through the action of its
investing atmosphere and circulating waters, then, and only then,
shall we be in a position to estimate the far greater effects resulting
from the internal forces, of which the most striking, but not the most
important, results are seen in the production of volcanic eruptions and
earthquake-shocks.
Another series of facts which serve to convince the geologist of the
reality and potency of the forces ever at work within the earth's
crust, and the way in which these have operated during past geological
periods, is found in the disturbed condition of many of the stratified
rock-masses of which it is composed. Such stratified rock-masses, it is
clear, must have been originally deposited in a position of approximate
horizontality; but they are now often found in inclined and even
vertical positions; they are seen to be bent, crumpled, puckered, and
folded in the most remarkable manner, and have not unfrequently been
broken across by dislocations--'faults'--which have sometimes displaced
masses, originally in contact, to the extent of thousands of feet.
The slate-rocks of the globe, moreover, bear witness to the fact that
strata have been subjected to the action of lateral compression of
enormous violence and vast duration; while in the metamorphic rocks
we see the effects of still more extreme mechanical strains, which
have been in part transformed into chemical action. No one who has
not studied the crushed, crumpled, fractured, and altered condition
of many of the sedimentary rocks of the globe, can form the faintest
idea of the enormous effects of the internal forces which have been in
operation within the earth's crust during earlier geological periods.
And it is only by such studies as these that we at last learn to regard
the earthquake and volcanic phenomena of our globe, not as the grandest
and most important effects of these forces, but as their secondary
and accidental accompaniments. 'Volcanoes,' it has been said, 'are
the safety-valves of the globe;' and when we come to realise the real
extent and nature of the internal forces ceaselessly working in the
earth's crust we shall scarcely be disposed to regard the simile as an
overstrained one.
[Sidenote: RELATION TO CONTINENTAL MOVEMENTS.]
The first geologist who attempted to show the exact relations existing
between those subterranean forces which cause the movements of
continental masses of land, and those more startling displays of energy
which are witnessed in volcanic outbursts, was the late Mr. Poulett
Scrope. At a somewhat later date Mr. Darwin, in his remarkable paper
'On the Connexion of certain Volcanic Phenomena in South America, and
on the Formation of Mountain-chains and Volcanoes as the effect of
Continental Elevations,' threw much new and important light upon the
question.
While, on the one hand, we are led by recent geological investigations
to reject the notions which were formerly accepted, by which
mountain-ranges were supposed to be suddenly and violently upheaved by
volcanic forces, we are, on the other hand, driven to conclude that
without the action of these subterranean forces, the irregularities
which are exhibited on the earth's surface could not have had any
existence.
It is true that the actual forms of the mountain-ranges are due
directly to the action of denuding forces, which have sculptured
out from the rude rocky masses all the varied outlines of peaks and
crags, of ravines and valleys. But it is none the less true that
the determining causes which have directed and controlled all this
earth-sculpture, are found in the relative positions of hard and soft
masses of rock; but these rock-masses have acquired their hardness and
consistency, and have assumed their present positions, in obedience
to the action of subterranean forces. Hence we see that though the
formation of mountain-ranges is proximately due to the denuding
forces, which have sculptured the earth's surface, the primary cause
for the existence of such mountain-chains must be sought for in the
fact that subterranean forces have been at work, folding, crumpling,
and hardening the soft sediments, and placing them in such positions
that, by the action of denudation, the more indurated portions are left
standing as mountain-masses above the general surface.
The old notion that mountain-chains are due to a vertical upthrust
from below, finds but little support when we come to study with due
care the positions of the rock-masses composing the earth's crust. On
the contrary, we find that mountain-ranges are usually carved out of
the crushed and crumpled edges of strata which have along certain lines
been influenced by great mechanical strains, and subjected to more or
less induration and chemical alteration. When we compare these folded
and contorted portions of the strata with those parts of the same beds
which are not so affected, we find the effects produced in the former
are not such as would result from an upthrust from below, but from
movements by which a tangential strain would be brought about. If we
imagine certain lines of weakness to exist in the solid crust of the
earth, then any movements in the portions of the crust between these
lines of weakness would cause crushing and crumpling of the strata
along the latter.
[Sidenote: FORMATION OF MOUNTAIN-CHAINS.]
Recent investigations of Dana and other authors have thrown much new
light upon the question of the mode of formation of mountain-chains,
and the relation between the movements by which they are produced and
the sudden and violent manifestations of force witnessed in volcanic
outbursts. We cannot, perhaps, better illustrate this subject than by
giving a sketch of the series of operations to which the great Alpine
chains owe their origin.
There are good grounds for believing that the great mountain-axis of
Southern Europe, with its continuation in Asia, had no existence
during the earlier geological periods. Indeed, it has been proved that
all the higher among the existing mountain-chains of the globe have
been almost entirely formed in Tertiary times. The reason of this
remarkable fact is not far to seek. So rapid is the work of denudation
in the higher regions of the atmosphere, that the elevated crags and
pinnacles are being broken up by the action of moisture and frost at
an exceedingly rapid rate. This fact is attested by the existence
of those enormous masses of angular rock-fragments which are found
lodged on every vantage-ground among the mountain-summits, as well as
by the continually descending materials which are borne by glaciers
and mountain-torrents to the valleys below. Where such a rate of
disintegration as this is maintained, no elevated mountain-crests
could exist through long geological periods. It is true we find in all
parts of the globe relics of many mountain-chains which were formed
before the Tertiary period; but these have by long-continued denudation
been worn down to 'mere stumps.' Of such worn-down and degraded
mountain-ranges we have examples in the Scandinavian chains, and some
of the low mountain-regions of Central Europe and North America.
Let us now proceed to illustrate this subject by briefly sketching the
history of that series of operations by which the great mountain-chains
of the Alpine system have been formed.
The first stage of that grand series of operations appears from recent
geological researches to have consisted in the opening of a number
of fissures running along a line near to that at which, in a long
subsequent period, the elevation of the mountain-masses took place.
This betrayal of the existence of a line of weakness in this part of
the earth's crust occurred in the Permian period, and from that time
onward a series of wonderful movements and changes have been going
forward, which have resulted in the production of the Alpine chains as
we now see them.
[Sidenote: VOLCANIC FISSURES OF PERMIAN PERIOD.]
From the great fissures opened in Permian times along this line of
weakness, great quantities of lava, scoriæ, and tuff were poured out,
and these accumulated to form great volcanic mountains, which we can
now only study at a few isolated spots, as in the Tyrol, Carinthia,
and about Lake Lugano. Everywhere else, these Permian volcanic rocks
appear to be deeply buried under the later-formed sediments, from which
the Alpine chains have been carved. Few and imperfect, however, as are
the exposures of these ancient rhyolite and quartz-andesite lavas and
agglomerates formed at the close of the Palæozoic epoch, their greatly
denuded relics form masses which are in places more than 9,000 feet in
thickness. From this fact we are able to form some slight idea of the
scale upon which the volcanic outbursts in question must have taken
place during Permian times.
The second stage in the series of operations by which the Alpine chains
have been formed, consisted in a general sinking of the surface along
that line of weakness in the earth's crust, the existence of which had
been betrayed by the formation of fissures and the eruption of volcanic
rocks. We have already had occasion to remark how frequently such
subsidences follow upon the extrusion of volcanic masses at any part
of the earth's surface; and we have referred these downward movements
in part to the removal of support from below the portion of the crust
affected, and in part to the weight of the materials piled upon its
surface by the volcanic forces.
The volcanic energy which had been manifested with such violence
during the Permian period, does not appear to have died out altogether
during the succeeding Triassic period. A number of smaller volcanic
vents were opened from time to time, and from these, lavas, tuffs, and
agglomerates, chiefly of basic composition, were poured out. The relics
of these old Triassic volcanoes are found at many points along the
Alpine chain, but it is evident that the igneous forces were gradually
becoming exhausted during this period, and before the close of it they
had fallen into a state of complete extinction.
But the great subsidence which had commenced in the Triassic period,
along what was to become the future line of the Alpine chain, was
continued almost without interruption during the Rhætic, the Jurassic,
the Tithonian, the Neocomian, the Cretaceous and the Nummulitic
periods. With respect to the strata formed during all these periods,
it is found that their thiknesses, which away from the Alpine axis
may be measured by hundreds of feet, is along that axis increased
to thousands of feet. The united thickness of sediments accumulated
along this great line of subsidence between the Permian and Nummulitic
periods probably exceeds 60,000 feet, or ten miles. The subsidence
appears to have been very slow and gradual, but almost uninterrupted,
and the deposition of sediments seems to have kept pace with the
sinking of the sea-bottom, a fact which is proved by the circumstance
that nearly the whole of these sediments were such as must have been
accumulated in comparatively shallow water.
[Sidenote: FORMATION OF ALPINE GEOSYNCLINAL.]
By the means we have described there was thus formed a 'geosynclinal,'
as geologists have called it, that is, a trough-like hollow filled
with masses of abnormally thickened sediments, which had been piled
one upon another during the long periods of time in which almost
uninterrupted subsidence was going on along the Alpine line of
weakness in the earth's crust. In this way was brought together that
enormous accumulation of materials from which the hard masses of the
Alpine chains were subsequently elaborated, and out of which the
mountain-peaks were eventually carved by denudation.
The third stage in this grand work of mountain-making commenced in the
Oligocene period. It consisted of a series of movements affecting the
parts of the earth's crust on either side of the line of weakness
which had first exhibited itself in Permian times. By these movements
a series of tangential strains were produced, which resulted in the
violent crushing, folding, and crumpling of the sedimentary materials
composing the geosynclinal.
One effect of this action was the violent flexure and frequent fracture
of these stratified masses, which are now found in the Alpine regions
assuming the most abnormal and unexpected positions and relations to
one another. Sometimes the strata are found tortured and twisted into
the most complicated folds and puckerings; at others they are seen to
be completely inverted, so that the older beds are found lying upon the
newer; and in others, again, great masses of strata have been traversed
by numerous fractures or faults, the rocks on either side of which are
displaced to the extent of thousands of feet.
Another effect of the great lateral thrusts by which the thick
sedimentary masses of the geosynclinal were being so violently
disturbed, was the production of a great amount of induration and
chemical change in these rocks. Masses of soft clay, of the age of
that upon which London is built, were by violent pressure reduced to
the condition of roofing-slate, similar to that of North Wales. One
of the most important discoveries of modern times is that which has
resulted in the recognition of the fact of the mutual convertibility
of different kinds of energy. We now know that mechanical force
may be transformed into heat-force or chemical force; and of such
transformations we find abundant illustrations in the crushed and
crumpled rock-masses of the Alpine chains.
Under the influence of these several kinds of force, not only was
extreme consolidation and induration produced among the rock-masses,
but chemical affinity and crystalline action had the fullest play
among the materials of which they were composed. In many cases we find
the originally soft muds, sands, and shell-banks converted into the
most highly crystalline rocks, which retain their primary chemical
composition, but have entirely lost all their other original features.
[Sidenote: FORMATION OF ALPINE GEANTICLINAL.]
To the mass of folded, crumpled, and altered strata, formed from a
geosynclinal by lateral pressure, geologists have given the name
of a 'geanticlinal.' The formation of the Alpine geanticlinal was
due to movements which commenced in the Oligocene period, attained
their maximum in the Miocene, and appear to have declined and almost
altogether died out in the Pliocene period.
The movements which resulted in the crushing and crumpling of the
thickened mass of sediments along the Alpine line of weakness, also
gave rise to the formation of a series of fissures from which volcanic
action took place. These fissures were not, however, formed along the
original line of weakness, for this had been strengthened and repaired
by the deposition of ten-miles' thickness of sediments upon it, but
along new fissures opened in directions parallel to the original lines
of weakness, and in areas where a much less considerable amount of
deposition had taken place since Permian times.
We have abundant evidence that, just at the period when those great
movements were commencing which resulted in the formation of the great
Alpine and Himalayan geanticlinal, earth-fissures were being opened
upon either side of the latter from which volcanic outbursts took
place. At the period when the most violent mountain-forming movements
occurred, these fissures were in their most active condition, and at
this time two great volcanic belts stretched east and west, on either
side of, and parallel to, the great Alpine chain. The Northern volcanic
band was formed by the numerous vents, now all extinct, in Auvergne,
Central Germany, Bohemia, and Hungary, and was probably continued in
the volcanoes of the Thian Shan and Mantchouria. The Southern volcanic
band was formed by the numerous vents of the Iberian and Italian
peninsulas, and the islands of the Mediterranean, and were continued
to the eastward by those of Asia Minor, Arabia, and the North Indian
Ocean. As the earth-movements which produced the geanticlinal died
away, the volcanic energy along these parallel volcanic bands died
away at the same time. In studying the geology of Central and Southern
Europe, no fact comes out more strikingly than that of the synchronism
between the earth-movements by which the geanticlinal of the Alps was
formed, and the volcanic manifestations which were exhibited along
lines of fissure parallel to that geanticlinal. The earth-movements
and the volcanic outbursts both commenced in the Oligocene period,
gradually attained their maximum in the Miocene, and as slowly declined
in the Pliocene.
[Sidenote: SCULPTURING OF ALPS BY DENUDATION.]
The fourth stage in the great work of mountain-building in the case
of the Alps consisted in the operation of the denuding forces, the
disintegrating action of rain and frost, the transporting action
of rivers and glaciers, by which the Alpine peaks were gradually
sculptured out of the indurated and altered masses constituting the
geanticlinal. The action of this fourth stage went on to a great
extent side by side with that of the third stage. So soon as the
earth-movements had brought the submerged sedimentary masses of the
geosynclinal under the action of the surface tides and currents of the
ocean, marine denudation would commence; and, as the work of elevation
went on, the rock-masses would gradually be brought within the reach
of those more silently-working but far more effective agents which
are ever operating in the higher regions of the atmosphere. It is
impossible to say what would have been the height of the Alpine chain
if the work of denudation had not to a great extent kept pace with
that of elevation. Only the harder and more crystalline masses have
for the most part escaped destruction, and stand up in high craggy
summits; while flanking hills, like the well-known Rigi, are Been to
be composed of conglomerates thousands of feet in thickness, composed
of their disintegrated materials. It is a remarkable fact, as showing
how enormous was the work of elevation daring the formation of the
geanticlinal, that some of the youngest and least consolidated rocks of
the Nummulitic period are still found at a height of 11,000 feet in the
Alps, and of 16,000 feet in the Himalaya.
From what has been said, it will be seen that mountain-chains may be
regarded as cicatrised wounds in the earth's solid crust. A line of
weakness first betrays itself at a certain part of the earth's surface
by fissures, from which volcanic outbursts take place; and thus the
position of the future mountain-chain is determined. Next, subsidence
during many millions of years permits of the accumulation of the raw
materials out of which the mountain-range is to be formed; subsequent
earth-movements cause these raw materials to be elaborated into the
hardest and most crystalline rock-masses, and place them in elevated
and favourable positions; and lastly, denudation sculptures from these
hardened rock-masses all the varied mountain forms. Thus the work of
mountain-making is not, as was formerly supposed by geologists, the
result of a simple upheaving force, but is the outcome of a long and
complicated series of operations.
[Sidenote: ORIGIN OF OTHER MOUNTAIN-CHAINS.]
The careful study of other mountain-chains, especially those of the
American continent, has shown that the series of actions which we
have described as occurring in the Alps, took place in the same order
in the formation of all mountain-masses. It is doubtful whether the
line of weakness is always betrayed in the first instance by the
formation along its course of volcanic fissures. But in all cases we
have evidence of the production of a geosynclinal, which is afterwards,
by lateral pressure, converted into a geanticlinal, and from this the
mountain-chains have been carved by denudation. Professor Dana has
shown that the geosynclinal of the Appalachian chain was made up of
sediments attaining a thickness of 40,000 feet, or eight miles; while
Mr. Clarence King has shown that a part of the geosynclinal of the
Rocky Mountains was built up of no less than 60,000 feet, or twelve
miles of strata.
It has thus been established that a very remarkable relation exists
between the forces by which continental masses of land are raised
and depressed, and mountain-ranges have been developed along lines
of weakness separating such moving continental masses, and those
more sudden and striking manifestations of energy which give rise to
volcanic phenomena. It is in this relation between the widespread
subterranean energies and the local development of the same forces
at volcanic vents, that we must in all probability seek for the
explanation of those interesting peculiarities of the distribution
of volcanoes upon the face of the globe which we have described
in a former chapter. The parallelism of volcanic bands to great
mountain-chains is thus easily accounted for; and in the same way
we may probably explain the position of most volcanoes with regard
to coast-lines. We have already pointed out the objections to the
commonly-received view that volcanoes depend for their supplies of
water on the proximity of the ocean. This proximity of the ocean to
volcanic vents we are thus inclined to regard, not as the cause, but as
the effect of the subterranean action. The positions of both volcanoes
and coast-lines are determined by the limits of those great areas of
the earth's crust which are subjected to slow vertical movements, often
in opposite directions.
Terrible and striking, then, as are the phenomena connected with
volcanic action, such sudden and violent manifestations of the
subterranean energy must not be regarded as the only, or indeed the
chief, effects which they produce. The internal forces continually
at work within the earth's crust perform a series of most important
functions in connection with the economy of the globe, and were the
action of these forces to die out, our planet would soon cease to be
fit for the habitation of living beings.
There is no fact which the geological student is more constantly
called upon to bear in mind than that of the potency of seemingly
insignificant causes which continue in constant operation through long
periods of time. Indeed these small and almost unnoticed agencies at
work upon the earth's crust are often found, in the long ran, to
produce far grander effects than those of which the action is much
more striking and obvious. It is to the silent and imperceptible
action of atmospheric moisture and frost that the disintegration of
the solid rock-masses must be mainly ascribed; and the noisy cataract
and ocean-billow produce effects which are quite insignificant
compared with those which must be ascribed to the slight and almost
unnoticed forces. Great masses of limestone are built up of the remains
of microscopic organisms, while the larger and higher life-forms
contribute but little to the great work of rock-building.
[Sidenote: EFFECTS OF SLOW CONTINENTAL MOVEMENTS.]
In the same way it is to the almost unnoticed action of the
subterranean forces in raising some vast areas of the earth's crust,
in depressing others, and in bringing about the development of
mountain-chains between them, that we must ascribe a far more important
part in the economy of our globe than to the more conspicuous but less
constant action of volcanoes.
A few simple considerations will serve to convince us, not only of the
beneficial effects of the action of the subterranean energies within
the earth's crust, but of the absolute necessity of the continued
operation of those energies to the perpetuation of that set of
conditions by which our planet is fitted to be the habitation of living
beings.
We have already referred to the prodigious effects which are constantly
being produced around us by the action of the external forces at work
upon the globe. The source of these external forces is found in the
movements and changes which are ever going on within the aqueous and
atmospheric media in which the globe is enveloped. The circulation of
the air, influencing the circulation of the waters in the shape of
clouds, rain, snow, rivers, glaciers, and oceans, causes the breaking
up of even the hardest rock-masses, and the continual removal of their
disintegrated fragments from higher to lower levels. This work goes on
with more or less regularity over every part of the land raised above
the level of the ocean, but the rate of destruction in the higher
regions of the atmosphere is far more rapid than at lower levels. Hence
the circulating air and water of the globe are found to be continually
acting as levellers of the land-masses of the earth.
It is by no means a difficult task to calculate the approximate rate
at which the various continents and islands are being levelled down,
and such calculations prove that in a very few millions of years the
existing forces operating upon the earth's surface would reduce the
whole of the land-masses to the level of the ocean.
But a little consideration will convince us that the circulation of
the air and waters of the globe are themselves dependent upon the
existence of those irregularities of the land-surfaces which they are
constantly tending to destroy. Without elevated mountain ridges the
regular condensation of moisture, and its collection and distribution
in streams and rivers over every part of the land surfaces, could
not take place. Under these circumstances the unchecked evaporation
of the oceanic waters would probably go on, till the proportion
of water-vapour increased to such an extent in the atmosphere as
effectually to destroy those nicely-balanced conditions upon which the
continued existence of both vegetable and animal life depend.
But the repeated upward and downward movements which have been shown
to be going on in the great land-masses of the globe, giving rise
in turns to those lateral thrusts and tangential strains to which
mountain-chains owe their formation, afford a perfect compensation
to the action of the external forces ever operating upon the earth's
surface.
If, however, the uncompensated effect of the external forces acting on
the earth's crust is calculated to bring about the destruction of those
conditions upon which the existence of life depends, the uncompensated
effect of the internal forces acting on the earth's crust are fraught
with at least equal dangers to those necessary conditions.
[Sidenote: CONTRAST BETWEEN THE EARTH AND MOON.]
In our nearest neighbour among the planets--the moon--the telescope has
revealed to us the existence of a globe, in which the internal forces
have not been checked and controlled by the operation of any external
agencies--for the moon appears to be destitute of both atmosphere and
water.
Under these circumstances we find its surface, as we might expect, to
be composed of rocks which appear to be entirely of igneous origin;
the mountain-masses, unworn by rain or frost, river or glacier, being
of most prodigious dimensions as compared with those of our own globe,
while no features at all resembling valleys, or plains, or alluvial
flats are anywhere to be discerned upon the lunar surface.
But by the admirable balancing of the external and internal forces
on our own globe, the conditions necessary to animal and vegetable
existence are almost constantly maintained, and those interruptions
of such conditions, produced by hurricanes and floods, by volcanic
outbursts and earthquakes, may safely be regarded as the insignificant
accidents of what is, on the whole, a very perfectly working piece of
machinery.
The ancients loved to liken the earth to a living being--the macrocosm
of which man was the puny representative or microcosm; and when we
study the well-adapted interplay of the forces at work upon the
earth's crust, both from within and without, the analogy seems a
scarcely strained one. In the macrocosm and the microcosm alike, slight
interferences with the regular functions occasionally take place, and
both of them exhibit the traces of a past evolution and the germs of an
eventual decay.
CHAPTER XI.
WHAT VOLCANOES TEACH US CONCERNING THE NATURE OF THE EARTH'S INTERIOR.
In entering upon any speculations or enquiries concerning the nature of
the interior of our globe, it is necessary before all things that we
should clearly realise in our minds how small and almost infinitesimal
is that part of the earth's mass which can be subjected to direct
examination. The distance from the surface to the centre of our globe
is nearly 4,000 miles, but the deepest mines do not penetrate to much
more than half a mile from the surface, and the deepest borings fall
far short of a mile in depth. Sometimes, it is true, the geologist
finds means for drawing inferences as to the nature of the rocks at
depths of ten or fifteen miles below the surface; but the last-named
depth must be regarded as the utmost limit of that portion of our
globe which can be made the object of direct observation and study.
This thin exterior film of the earth's mass, which the geologist is
able to investigate, we call the 'crust of the globe'; but it must be
remembered that in using this term, it is not intended to imply that
the outer part of our globe differs in any essential respect from the
interior. The term 'crust of the globe' is employed by geologists as
a convenient way of referring to that portion of the earth which is
accessible to their observation.
But if we are unable to make direct investigations concerning the
nature of the internal portions of the globe, there are nevertheless a
number of facts from which we may draw important inferences upon the
subject. These facts and the inferences based upon them we shall now
proceed to consider.
First in importance among these we may mention the results which have
been obtained by weighing our globe. Various methods have been devised
for accomplishing this important object, and the conclusions arrived
at by different methods agree so closely with one another, that there
is no room for doubt as to the substantial accuracy of those results.
It may be taken as proved beyond the possibility of controversy that
our globe is equal in weight to five and a half globes of the same size
composed of water, or, in other words, that the average density of the
materials composing the globe is five and a half times as great as that
of water.
Now the density of the materials which compose the crust of the globe
is very much less than this, varying from about two-and-one-third to
three times that of water. Hence we are compelled to conclude that the
interior portions of the globe are of far greater density than the
exterior portions; that, as a matter of fact, the mass of the globe is
composed of materials having twice the density of the rocks exposed at
the surface.
[Sidenote: DENSITY OF EARTH'S INTERIOR.]
It has been sometimes argued that as all materials under intense
pressure appear to yield to an appreciable extent, and to allow their
particles to be packed into a smaller compass, we may find in this fact
an explanation of the great density of the internal parts of the globe.
It has in fact been suggested that under the enormous pressure which
must be exerted by masses of rock several thousand feet in thickness,
the materials of which our earth is composed may be compelled to pack
themselves into less than one-half the compass which they occupy at the
surface. But the ascription of such almost unlimited compressibility to
solid substances can be supported neither by experiment nor analogy.
Various considerations point to the probability that solid bodies yield
to pressure up to a certain limit and no farther, and that when this
limit is reached an increase in pressure is no longer attended with a
reduction in bulk.
If then we are compelled to reject the idea of the unlimited
compressibility of solid substances, we must conclude that the interior
portions of our globe are composed of _materials of a different kind_
from those which occur in its crust. And this conclusion, as we shall
presently see, is borne out by a number of independent facts.
The study of the materials ejected from volcanic vents proves that even
at very moderate depths there exist substances differing greatly in
density, as well as in chemical composition. The lightest lavas have a
specific gravity of 2·3, the heaviest of over 3. And that materials of
even greater density are sometimes brought by volcanic action from the
earth's interior, we have now the clearest proofs.
[Sidenote: RELATION BETWEEN EARTH AND OTHER PLANETS.]
But in considering a question of this kind, it will be well to remember
that analogy may furnish us with hints upon the subject which may prove
to be by no means unimportant. There is no question upon which modern
science has wrought out a more complete revolution in our ideas, than
that of the relation of our earth to the other bodies of the universe.
We know, as the result of recent research, that our globe is one of
a great family of bodies, moving through space in similar paths and
in obedience to the same laws. A hundred years ago the primary and
secondary planets of the solar system could be almost numbered upon
the fingers; now we recognise the fact that they exist in countless
millions, presenting every variety of bulk from masses 1,400 times
as large as our earth down to the merest planetary dust. Between the
orbits of Mars and Jupiter, more than 200 small planets have been
recognised as occurring, and every year additions are made to the
number of these asteroids. Comets have now been identified with streams
of such planetary bodies, of minute size, moving in regular orbits
through our system. The magnificent showers of 'shooting-stars' have
been proved to be caused by the passage of the earth through such
bands of travelling bodies, and 'the zodiacal light' finds its most
probable explanation in the supposition that the sun is surrounded by
a great mass of such minute planets. Every increase in the power of
the telescope reveals to us the existence of new secondary planets
or moons, revolving about the primaries; and the wonderful system
of the Saturnian rings is now explained by the proved existence of
great streams of such secondary planets circling around it. The solar
system was formerly conceived of as a vast solitude through which a
few gigantic bodies moved at awful distances from one another. Now we
know that the supposed empty void is traversed by countless myriads of
bodies of the most varied dimensions, all moving in certain definite
paths, in obedience to the same laws, ever acting and reacting upon
each other, and occasionally coming into collision.
There are not wanting further facts to prove that the other planets are
like our own in many of their phenomena and surroundings. In some of
them atmospheric phenomena have been detected, such as the formation of
clouds and the deposition of snow, so that the external forces at work
on our globe act upon them also. And that internal forces, like those
we have been considering in the case of our earth, are at work in our
neighbours, is proved by the great solar storms and the condition of
the moon's surface.
But the results of spectrum-analysis in recent years have furnished new
facts in proof of the close relationship of our earth to the numerous
similar bodies by which it is surrounded. So far as observation has yet
gone we have reason for believing that not only the members of the
solar system, but the more distant bodies of the universe, are all
composed of the same elementary substances as those which enter into
the composition of our globe.
The most satisfactory information concerning the composition and nature
of other planetary bodies is derived from the study of those small
planets which occasionally come into collision with our globe, and
which have their own proper motion in space thereby arrested. These
meteorites, as such falling planetary bodies are called, have justly
attracted great attention, and their fragments are treasured as the
most valuable objects in our museums.
[Sidenote: COMPOSITION OF METEORITES.]
The first fact concerning these meteorites, which it is necessary
to notice, is that they are composed of the same chemical elements
as occur in the earth's crust. No element has yet been found in any
meteorite which was not previously known as existing in the earth, and
of the sixty-five or seventy known terrestrial elements no less than
twenty-two have already been detected in meteorites.
There are, however, a dozen elements which occur in overwhelming
proportions in the earth's crust. We shall probably not be going too
far in saying that these twelve elements--namely, oxygen, silicon,
aluminium, calcium, magnesium, sodium, potassium, iron, carbon,
hydrogen, sulphur, and chlorine--make up amongst them not less than 999
out of 1,000 parts of the earth's crust, and that all the other fifty
or sixty elements are 80 comparatively rare that they do not constitute
when taken altogether more than one part in 1,000 of the rocks of the
globe. Now all of these twelve common terrestrial elements occur in
meteorites, and the fact that the rarer terrestrial elements have not
as yet been found in them will not surprise anyone, who remembers how
small is the bulk of all the specimens of these meteorites existing in
our museums.
We have hitherto insisted on the points of resemblance in the chemical
composition of meteorites and that of the rocks of the globe, but we
shall now have to indicate some very important points in which they
differ.
While in the rocks composing the earth's crust oxygen forms one-half of
their mass, and silicon another quarter, we find that in the meteorites
these elements, though present, play a much less important part. The
most abundant element in the meteorites is iron; and nickel, chromium,
cobalt, manganese, sulphur, and phosphorus, are much more abundant in
these extra-terrestrial bodies than they are in the earth's crust.
We have already referred to the remarkable fact that in our earth's
crust nearly all the other elementary substances are found combined
in the first instance with oxygen, and that most rocks consist of the
oxide of silicon combined with the oxides of various metals. But this
is by no means the case with the meteorites. In them we find metals
like iron, nickel, cobalt, &c., in their uncombined condition, and
forming alloys with one another. The same and other metals also occur
in combination with carbon, phosphorus, chlorine, and sulphur, and
some of the substances thus formed are quite unknown among terrestrial
rocks. Compounds of the oxide of silicon with the oxides of the metals
such as form the mass of the crust of the globe do occur in meteorites,
but they play a much less important part than in the case of the
terrestrial rocks.
Among the substances found in meteorites are several which do not exist
among the terrestrial rocks--some, indeed, which it seems impossible
to conceive of as being formed and preserved under terrestrial
conditions. Among these we may mention the phosphide of iron and nickel
(Schreibersite), the sulphide of chromium and iron (Daubréelite), the
protosulphide of iron (Troilite), the sulphide of calcium (Oldhamite),
the protochloride of iron (Lawrencite), and a peculiar form of
crystallised silica, called by Professor Maskelyne 'Asmanite.'
[Sidenote: DIFFERENT KINDS OF METEORITES.]
There are other phenomena exhibited by meteorites which indicate that
they must have been formed under conditions very different to those
which prevail upon the earth's surface. Thus we find that fused iron
and molten slag-like materials have remained entangled with each other,
and have not separated as they would do if a great body like the earth
were near to exercise the varying force of gravity upon the two
classes of substances. Again, meteorites are found to have absorbed
many times their bulk of hydrogen gas, and to exhibit peculiarities in
their microscopic structure which can probably be only accounted for
when we remember that they were formed in the interplanetary spaces,
far away from any great attracting body.
But in recent years a number of very important facts have been
discovered which may well lead us to devote a closer attention to the
composition and structure of meteorites. It has been shown, on the
one hand, that some meteorites contain substances precisely similar
to those which are sometimes brought from the earth's interior during
volcanic outbursts; and, on the other hand, there have been detected,
among some of the ejections of volcanoes, bodies which so closely
resemble meteorites that they were long mistaken for them. Both kinds
of observation seem to point to the conclusion that the earth's
interior is composed of similar materials to those which we find in the
small planets called meteorites.
M. Daubrée has proposed a very convenient classification for
meteorites, dividing them into the following four groups:--
I. _Holosiderites_; consisting almost entirely of metallic iron, or of
iron alloyed with nickel, stony matter being absent; but sulphides,
phosphides, and carbides of several metals are often diffused through
the mass. The polished surfaces of these meteoric irons, when etched
with acid, often exhibit a remarkable crystalline structure.
II. _Syssiderites_; in which a network of metallic iron encloses a
number of granular masses of stony materials.
III. _Sporadosiderites_; which consist of a mass of stony materials,
through which particles of metallic iron are disseminated.
IV. _Asiderites_; containing no metallic iron, but consisting entirely
of stony materials.
There are, besides the meteorites belonging to these principal groups,
a few of peculiar and exceptional composition, which we need not notice
further for our present purpose.
From the above classification it will be seen that most meteorites
consist of a mixture in varying proportions of metallic and stony
materials. Sometimes the metallic constituents are present in greater
proportions than the stony, at other times the stony materials
predominate, while occasionally one or other of these elements may be
wholly wanting.
The stony portions of meteorites, upon careful examination, prove to be
built up of certain minerals, agreeing in their chemical composition
and their crystalline forms with those which occur in the rocks of the
earth's crust. Among the ordinary terrestrial minerals occurring in
the stony portions of meteorites, we may especially mention olivine,
enstatite, augite, anorthite, chromite, magnetite, and pyrrhotite.
[Sidenote: METEORITES AND ULTRA-BASIC ROCKS.]
The minerals which occur in meteorites are in every case such as are
found in the more basic volcanic rocks--quartz, and the acid felspars,
with the other minerals which occur in acid rocks, being entirely
absent in the 'extra-terrestrial' rocks.
Now, besides the three great classes of lavas which we have described
as being ejected from volcanic vents, there are some rarer materials
occasionally brought from the earth's interior by the same agency,
that present a most wonderful resemblance to the stony portions of
meteorites. These materials we may call 'ultra-basic rocks.' Their
specific gravity is very high, usually exceeding 3, and they contain
a very low percentage of silica; on the other hand, the proportion of
iron and magnesia is often much greater than in ordinary terrestrial
rocks. But the most remarkable fact about these ultra-basic rocks is,
that they are almost entirely composed of the minerals which occur in
meteorites; namely, olivine, enstatite, augite, anorthite, magnetite,
and chromite.
The ultra-basic rocks often occur under very peculiar conditions.
Sometimes they are found forming ordinary volcanic protrusions
through the sedimentary rocks. The rocks named pikrites, lherzolites,
dunites, &c., are examples of such igneous protrusions composed of
these ultra-basic materials, and probably all the true serpentines are
rocks of the same class which have absorbed water and undergone great
alteration. The ultra-basic rocks sometimes contain platinum and other
metals in the free or uncombined state. But not unfrequently we find
among the ordinary ejections of volcanoes, nodules and fragments of
such ultra-basic materials, which have clearly been carried up with the
other lavas from great depths in the earth's crust. Thus in Auvergne,
the Eifel, Bohemia, Styria, and many other volcanic districts, the
basaltic lavas and tuffs are found to contain nodules composed of
the minerals which are so highly characteristic of meteorites. Such
nodules, too, often form the centres of the volcanic bombs which are
thrown out of craters during eruptions.
We thus see that materials identical in composition and character with
the stony portions of meteorites, exist within the earth's interior,
and are thrown out on its surface by volcanic action. A still more
interesting discovery has been made in recent years; namely, that
materials similar to the metallic portion of meteorites, and consisting
of nickeliferous iron, also occur in deep-seated portions of the
earth's crust, and are brought to the surface during periods of igneous
activity.
In the year 1870, Professor Nordenskiöld made a most important
discovery at Ovifak, on the south side of the Island of Disko, off
the Greenland coast. On the shore of the island a number of blocks of
iron were seen, and the chemical examination of these proved that,
like ordinary metallic meteorites, they consisted of iron alloyed with
nickel and cobalt.
[Sidenote: IRON-MASSES OF OVIFAK.]
Now, when the facts concerning the masses of native iron of Ovifak were
made known, the first and most natural explanation which presented
itself to every mind was, that these were a number of meteorites which
at some past period had fallen upon the earth's surface.
Metallic iron.
Opaque crystals of magnetite (black oxide of iron).
Transparent crystals of felspar, augite, and olivine.
[Illustration: Fig. 87.--Section of basalt from Ovifak, Greenland, with
particles of metallic iron diffused through its mass.]
But a further examination of the locality revealed a number of facts
which, as Professor Steenstrup pointed out, it is very difficult
to reconcile with the theory that the Ovifak masses of iron are of
meteoric origin. The district of Western Greenland, where these masses
were discovered, has been the scene of volcanic outbursts on the
grandest scale during the Miocene period. In close proximity to the
great iron masses, there are seen a number of basaltic dykes; and, when
these dykes are carefully examined, the basaltic rock of which they are
composed is seen to be full of particles of metallic iron. In fig. 87,
we have a drawing made from a section of the Ovifak basalts magnified
four or five diameters. The rock-mass is seen to be composed of black,
opaque magnetite, and transparent crystals of augite, labradorite,
olivine, &c.; while, through the whole, particles of metallic iron are
found entangled among the different crystals in the most remarkable
manner.
It has been suggested that this singular rock might have been formed by
a meteorite falling, in Miocene times, into a lava-stream in a state
of incandescence. But the relation of the metallic particles to the
stony materials is such as to lend no support whatever to this rather
strained hypothesis.
A careful study of all the facts of the case by Lawrence Smith,
Daubrée, and others well acquainted with the phenomena exhibited by
meteorites, has led to the conclusion that the large iron-masses of
Ovifak, as well as the particles of metallic iron diffused through
the surrounding basalts, are all of terrestrial origin, and have been
brought by volcanic action from the earth's interior. It is probable
that, just as we find in many basaltic lavas nodules of ultra-basic
materials similar to the stony parts of meteorites, so in these basalts
of Ovifak we have masses of iron alloyed with nickel, similar to the
metallic portions of meteorites. Both the stony and metallic enclosures
in the basalt are in all probability derived from deeper portions of
the earth's crust. By the weathering away of the basalt of Ovifak, the
larger masses of metallic iron have been left exposed upon the shore
where they were found.
There are a number of other facts which seem to support this startling
conclusion. Thus it has been shown by Professor Andrews that certain
basalts in our own islands contain particles of metallic iron of
microscopic dimensions, and it is not improbable that some of the
masses of nickeliferous iron found in various parts of the earth's
surface, which have hitherto been regarded as meteorites, are, like
those of Ovifak, of terrestrial origin.
[Sidenote: MATERIALS FILLING METALLIC-VEINS.]
Another piece of evidence pointing in the same direction, is derived
from those great fissures communicating with the interior of our globe
which become filled with metallic minerals, and are known to us as
mineral-veins. In these mineral-veins the native metals, their alloys,
and combinations of these with sulphur, chlorine, phosphorus, &c., are
frequently present. But oxides of the metals, except as products of
subsequent alteration, occur far less frequently than in the earth's
crust generally. Hence we are led to conclude that the substances which
in the outer part of the earth's crust always exist in combination
with oxygen, are at greater depths in a free and uncombined condition.
Nor is it a circumstance altogether unworthy of attention that the
researches of Mr. Norman Lockyer and other astronomers, based on the
known facts of the relative densities of the several members of the
solar system, and the ascertained relations of the different solar
envelopes, have led to conclusions closely in accord with those arrived
at by geologists. These researches appear to warrant the hypothesis
that the interior of our globe consists of metallic substances
uncombined with oxygen, and that among these metallic substances iron
plays an important part. Our globe, as we know, is a great magnet,
and the remarkable phenomena of terrestrial magnetism may also not
improbably find their explanation in the fact that metallic iron forms
80 large a portion of the earth's interior.
The interesting facts which we have been considering may be made
clearer by the accompanying diagram (fig. 88). The materials ejected
from volcanic vents (lavas) are in almost all cases compounds of
silicon and the various metals with oxygen. In the lighter or acid
lavas oxygen constitutes one-half of their weight, and the proportion
of metals of the iron-group is very small. As we pass to the heavier
intermediate and basic lavas, we find the proportion of oxygen
diminishing, and the metals of the alkaline earths (magnesium and
calcium) with the metals of the iron-group increasing, in quantity. In
the small and interesting group of the ultra-basic lavas the proportion
of oxygen is comparatively small, and the proportion of magnesium and
iron very high. So much for the terrestrial rocks.
[Illustration: Fig. 88.--Diagram illustrating the relation between the
Terrestrial and the Extra-Terrestrial Rock.]
[Sidenote: TERRESTRIAL AND EXTRA-TERRESTRIAL ROCKS.]
Now let us turn our attention to the extra-terrestrial rocks or those
found in meteorites. The Asiderites are quite identical in composition
with the ultra-basic lavas of our globe, but in the Sporadosiderites
and the Syssiderites we find the proportion of oxygen rapidly
diminishing, and that of metallic iron increasing. Finally, in the
Holosiderites the oxygen entirely disappears, and the whole mass
becomes metallic.
From the Holosiderites at one end of the chain to the add lavas
at the other, we find there is a complete and continuous series;
the rocks of terrestrial origin overlapping, in their least
oxydized representatives, the most highly oxydized representatives
of the extra-terrestrial rocks. But the discovery at Ovifak of
the iron-masses, and the basalts with iron disseminated, has
afforded another very important link, placing the terrestrial and
extra-terrestrial rocks in closer relations with one another.
All these facts appear to point to the conclusion that the earth's
interior consists of metallic substances either quite uncombined
or simply alloyed with one another, and among these iron is very
conspicuous by its abundance. The outer crust, which is probably of no
great thickness, contains an enormous proportion of oxygen and silicon
combined with the materials which constitute the interior portions of
our globe. It may be, as has been suggested by astronomers, that our
earth consisted at one time of a solid metallic mass surrounded by a
vaporous envelope of metalloids, and that the whole of the latter, with
the exception of the constituents of the atmosphere and ocean, have
gradually entered into combination with the metals of the nucleus to
form the existing crust of the globe. But of this period the geologist
can take no cognisance. The records which he studies evidently
commenced at a long subsequent period, when the conditions prevailing
at the earth's surface differed but little, if at all, from those which
exist at the present day. Equally little has the geologist to do with
speculations concerning a far distant future when, as some philosophers
have suggested, the work of combination of the waters and atmosphere of
the earth's surface with the metallic substances of its interior shall
be completed, and our globe, entirely deprived of its fluid envelopes,
reduced to the condition in which we find our satellite, the moon.
* * * * *
[Sidenote: PHYSICAL CONDITION OF EARTH'S INTERIOR.]
There is another class of enquiries concerning the earth's interior to
which the attention of both geologists and astronomers has long been
directed--that, namely, which deals with the problem of the _physical
condition_ of the interior of our globe.
The fact that masses of molten materials are seen at many points of the
earth's surface to issue from figures in the crust of our globe, seems
at first sight to find a simple explanation if we suppose our planet
to consist of a fluid central mass surrounded by a solid crust. Hence
we find that among those who first thought upon this subject, this
hypothesis of a liquid centre and a solid crust was almost universally
accepted. This hypothesis was supposed to find further support in the
fact that, as we penetrate into the earth's crust by mines or boring
operations, the temperature is found to continually increase. It was
imagined, too, that this condition of our planet would best agree with
the requirements of the nebular hypothesis of Laplace, which explains
the formations and movements of the bodies of the solar system by the
cooling down of a nebulous mass.
But a more careful and critical examination of the question has led
many geologists and astronomers to reject the hypothesis that the earth
consists of a great fluid mass surrounded by a comparatively thin shell
of solid materials.
Volcanic outbursts and earthquake tremors, though so terrible and
destructive to man and his works, are but slight and inconsiderable
disturbances in a globe of such vast dimensions as that on which we
live. The condition of the crust of the globe is, in spite of volcanic
and earthquake manifestations, one of general stability; and this
general stability has certainly been maintained during the vast periods
covered by the geological record. Such a state of things seems quite
irreconcilable with the supposition that, at no great depth from the
surface, the whole mass of the globe is in a liquid condition. If, on
the other hand, it be supposed that the solid crust of the globe is
several hundreds of miles in thickness, it is difficult to understand
how the local centres of volcanic activity could be supplied from such
deep-seated sources.
There are other facts which seem equally irreconcilable with the
hypothesis of a fluid centre and a thin solid crust in our globe. If
all igneous products were derived from one central reservoir, we might
fairly expect to find a much greater uniformity of character among
those products than really exists. But in some cases, materials of
totally different composition are ejected at the same time from closely
adjoining volcanic districts. Thus in Hungary and Bohemia, as we have
seen, lavas of totally different character were being extruded during
the Miocene period. In the island of Hawaii, as Professor Dana has
pointed out, igneous ejections have taken place at a crater 14,000 feet
above the sea-level, while a closely adjoining open vent at a level
10,000 feet lower exhibited no kind of sympathy with the disturbance.
Whatever may be the cause of volcanic action, it seems clear that it
does not originate in a universal mass of liquefied material situated
at no great depth from the earth's surface.
The conclusions arrived at by astronomers and physicists is one quite
in accord with those which geologists have reached by totally different
methods. It is now very generally admitted that if the earth were not
a rigid mass, its behaviour under the attract live influences of the
surrounding members of the solar system would be very different to what
is found to be the case.
[Sidenote: ARGUMENTS AGAINST LIQUID INTERIOR.]
That the earth is in a solid condition to a great depth from the
surface, and possibly quite to the centre, is a conclusion concerning
which there can be little doubt; and in the next chapter we shall
endeavour to show that such a condition of thirds is by no means
incompatible with those manifestations of internal energy, the
phenomena of which we are considering in this work. The question,
therefore, of the complete solidity of our globe, or of its consisting
of a solid and a liquid portion, is one of speculative interest only,
and is in no way involved in our investigations concerning the nature
and origin of volcanic activity. We may conclude this chapter by
enumerating the several hypotheses which have at different times been
maintained concerning the nature of the interior of our globe.
_First._ It has been suggested that the earth consists of a fluid or
semi-fluid nucleus surrounded and enclosed in a solid shell. Some
have maintained this shell to be of such insignificant thickness, as
compared with the bulk of the interior liquid mass, that portions of
the latter are able to reach the earth's surface through movements
and fractures of the outer shell, and that in this manner volcanic
manifestations originate. Others, impressed with the general stability
and rigidity of the globe as a whole, have maintained that the outer
solid shell must have a very considerable thickness, amounting
probably to not less than several hundreds of miles. But through a
shell of such thickness it is difficult to conceive of the liquid
masses of the interior finding their way to the surface, and those who
have held this view are driven to suggest some other means by which
local developments of volcanic action might be brought about.
_Secondly._ Some physicists have asserted that a globe of liquid
matter radiating its heat into space, would tend to solidify both at
the surface and the centre, at the same time. The consequence of this
action would be the production of a sphere with a solid external shell
and a solid central nucleus, but with an interposed layer in a fluid
or semi-fluid condition. It has been pointed out that if we suppose
the solidification to have gone so far, as to have caused the partial
union of the interior nucleus and the external shell, we may conceive a
condition of things in which the stability and rigidity is sufficient
to satisfy both geologists and astronomers, but that in still
unsolidified pockets or reservoirs, filled with liquefied rock, between
the nucleus and the shell, we should have a competent cause for the
production of the volcanic phenomena of the globe. In this hypothesis,
however, it is assumed that the cooling at the centre and the surface
of the globe would go on at such rates that the reservoirs of liquid
material would be left at a moderate depth from the surface, so that
easy communication could be opened between them and volcanic vents.
[Sidenote: REVIEW OF THE SEVERAL HYPOTHESES.]
_Thirdly._ It has been maintained that the earth may have become
perfectly solid from the centre to the surface. Those who hold this
view endeavour to account for the phenomena of volcanoes in one of two
ways. It may be, they say, that the deep-seated rock-masses, though
actually solid, are in a state of _potential_ liquidity; that though
reduced to a solid state by the intense pressure of the superincumbent
masses, yet such is the condition of unstable equilibrium in the whole
mass, that the comparatively slight movements and changes taking place
at the earth's surface suffice to bring about the liquefaction of
portions of its crust and consequent manifestations of volcanic energy.
But It may be, as other supporters of the doctrine of the earth's
complete solidity have maintained, that the phenomena of volcanoes
have no direct connection with a supposed incandescent condition of
our planet at all, and that there are chemical and mechanical forces
at work within our globe which are quite competent to produce at the
surface all those remarkable phenomena which we identify with volcanic
action.
From this summary of the speculative views which have been entertained
upon the subject of the physical condition of the earth's interior,
it will be clear that at present we have not sufficient evidence for
arriving at anything like a definite solution of the problem. The
conditions of temperature and pressure which exist in the interior of
a globe of such vast dimensions as our earth, are so far removed from
those which we can imitate in our experimental enquiries, and it is so
unsafe to push the application of laws arrived at by the latter to the
extreme limits required by the former, that we shall do well to pause
before attempting to dogmatise on such a difficult question.
In the next chapter we shall endeavour to grapple with a somewhat more
hopeful task, to point out how far observation and experiment have
enabled us to offer a reasonable explanation of the wonderful series of
phenomena which are displayed during outbursts of volcanic activity.
CHAPTER XII.
THE ATTEMPTS WHICH HAVE BEEN MADE TO EXPLAIN THE CAUSES OF VOLCANIC
ACTION.
Every completed scientific investigation must consist of four series
of operations. In the first of these an attempt is made to collect the
whole of the facts bearing on the question, by means of observation
and experiment; the latter being only observation under conditions
determined by ourselves. In the second stage of the enquiry, the
attention is directed to classifying and grouping the isolated facts,
so as to determine their bearings upon one another, and the general
conclusions to which they appear to point. In the third stage, it is
sought to frame an hypothesis which shall embrace all the observed
facts, and shall be in harmony with the general conclusions derived
from them. In the fourth stage, this hypothesis is put to the most
rigid test; comparing the results which must follow, if it be true,
with the phenomena actually observed, and rejecting or amending our
hypothesis accordingly. Every great scientific theory has thus been
established by these four processes--observation, generalisation,
hypothesis, and verification.
The enquiry concerning the nature and causes of volcanic action is far
from being a completed one. It is true that many hypotheses upon the
subject have been framed, but in too many instances these have not been
based on accurate observations and careful generalisations, and can be
regarded as little better than mere guesses. Indeed, the state of the
enquiry at the present time would seem to be as follows. Although much
remains to be done in the direction both of observation and experiment,
the main facts of the case have been established upon irrefragable
evidence. The classification and comparison of these facts have led to
the recognition of certain laws, which seem to embrace all the known
facts. To account for these facts and their demonstrated relations to
one another, certain tentative hypotheses have been suggested; but in
no case can it be truly said that these latter have so far stood the
test of exact enquiry as to deserve to rank as demonstrated truths. A
complete and consistent theory of volcanic action still remains to be
discovered.
[Sidenote: VALUE AND LIMITS OF HYPOTHESES.]
In accordance with the plan which we have sketched out for ourselves
at the commencement of this work, we shall aim at following what has
been the order of investigation and discovery in our study of volcanic
action; and in this concluding chapter we shall indicate the different
hypotheses by which it has been proposed to account for the varied
phenomena, which we have discussed in the preceding pages, and their
remarkable relations to one another. We shall endeavour, in passing,
to indicate how far these several hypotheses appear to be probable,
as satisfying a larger or smaller number of those conditions of the
problem which have been established by observation, experiment, and
careful reasoning; but we shall at the same time carefully avoid such
advocacy of any particular views as would tend to a prejudgment of
the question. Hypothesis is, as we have seen, one of the legitimate
and necessary operations in scientific investigation. It only becomes
a dangerous and treacherous weapon when it is made to precede rather
than to follow observation and experiment, or when being regarded
with paternal indulgence, an attempt is made to shield it from the
relentless logic of facts. Good and bad hypotheses must be allowed to
'grow together till the harvest;' such as are unable to accommodate
themselves to the surrounding conditions imposed by newly-discovered
facts and freshly-established laws will assuredly perish; and in this
'struggle for existence' the true hypothesis will in the end survive,
while the false ones perish.
It may well happen, however, that among the hypotheses which have up to
the present time been framed, none will be found to entirely satisfy
all the conditions of the problem. New discoveries in physics and
chemistry have suggested fresh explanations of volcanic phenomena in
the past, and may continue to do so in the future; and the true theory
of volcanic action, when it is at last discovered, may combine many of
the principles which now seem to be peculiar to different hypotheses.
Let us, in the first place, enquire what are the facts which must be
accounted for in any theory of volcanic action. We have already been
led to the conclusion that the phenomena exhibited by volcanoes were
entirely produced by the escape of imprisoned water and other gases
from masses of incandescent and fluid rock. Our subsequent examination
of the problem confirmed the conclusion that in all cases of volcanic
outburst we have molten rock-materials from which water and other
gases issue with greater or less violence. The two great facts to
be accounted for, then, in any attempted explanation of volcanic
phenomena, are the existence of this high temperature at certain points
within the earth's crust, and the presence of great quantities of water
and gas, imprisoned in the rocks. We shall perhaps simplify the enquiry
if we examine these two questions separately, and, in the first place,
review those hypotheses which have been suggested to account for high
temperatures in the subterranean regions, and, in the second place,
examine those which seek to explain the presence of large quantities of
imprisoned water and gases.
[Sidenote: INCREASE OF TEMPERATURE WITH DEPTH.]
That a high temperature exists in the earth's crust at some depth from
the surface is a £act which does not admit of any doubt. Every shaft
sunk for mining operations, and every deep boring made for the purpose
of obtaining water, proves that a more or less regular increase of
temperature takes place as we penetrate downwards. The average rate
of this increase of temperature has been estimated to be about 1°
Fahrenheit for every 50 or 60 feet of depth.
Now if it be assumed that this regular increase of temperature
continues to great depths, a simple calculation proves that at a depth
of 9,000 feet a temperature of 212° Fahrenheit will be found--one
sufficient to boil water at the earth's surface--while at a depth of 28
miles the temperature will be high enough to melt cast-iron, and at 34
miles to fuse platinum.
So marked is this steady increase of temperature as we go downwards,
that it has been seriously proposed to make very deep borings in order
to obtain supplies of warm water for heating our towns. Arago and
Walferdin suggested this method for warming the Jardin des Plantes at
Paris; and now that such important improvements have been devised in
carrying borings to enormous depths, the time may not be far distant
when we shall draw extensively upon these supplies of subterranean
heat. At the present time the city of Buda-Pesth is extensively
supplied with hot-water from an underground source. Should our
coal-supply ever fail it may be well to remember that we have these
inexhaustible supplies of heat everywhere beneath our feet.
But although we may conclude that at the moderate depths we have
indicated such high temperatures exist, it would not be safe to
infer, as some have done, that at a distance of only 40 or 50 miles
from the surface the materials composing our globe are in a state
of actual fusion. Both theory and experiment indicate that under
increased pressure the fusing point of solid bodies is raised; and
just as in a Papin's digester we may have water retained by high
pressure in a liquid condition at a temperature far above 212° F.,
so in the interior of the earth, masses of rock may exist in a solid
state, at a temperature far above that at which they would fuse at
the earth's surface. We may speak of such rock-masses, retained in a
solid condition by intense pressure, at a temperature far above their
fusing point at the earth's surface, as being in a 'potentially liquid
condition.' Upon any relief of pressure such masses would at once
assume the liquid state, just as the superheated water in a Papin's
digester immediately flashes into steam upon the fracture of the strong
vessel by which it is confined. We have already seen how the action
at volcanic vents often appears to indicate just such a manifestation
of elastic forces, as would be exhibited by the relief of superheated
masses from a state of confinement by pressure.
In reasoning upon questions of this kind, however, we must always be
upon our guard against giving undue extension to principles and laws
which seem to be clearly established by experiment at the earth's
surface. It is well to remember how exceedingly limited is our command
of extreme pressures and high temperatures, when compared with those
which may exist within a body of the dimensions of our globe.
[Sidenote: EFFECT OF PRESSURE ON FUSION-POINT.]
If we were to imagine a set of intelligent creatures, who were able to
command only a range of temperatures from 50° to 200° F., engaged upon
an investigation of the properties of water, we shall easily understand
how unsafe it may be to extend generalisations far beyond the limits
covered by actual experiment. Such beings, from their observation of
the regular changes of volume of water at all the temperatures they
could command, might infer that at still higher and lower temperatures
the same rates of expansion and contraction would be maintained. Yet,
as we well know, such an inference would be quite wide of the truth;
for a little above 200° F. water suddenly expands to 1,700 times its
volume, and not far below 50° F. the contraction is suddenly changed
for expansion.
It has been argued by the late Mr. David Forbes and others that,
inasmuch as experiment has shown that--though the fusing points of
solids are raised by pressure, yet that this rise of the fusing
points goes on in a diminishing ratio as compared with the pressures
applied--a limit will probably be reached at which the most intense
pressure will not be sufficient to retain substances at a high
temperature in their solid state. The fact that gases cannot be
retained in a liquid condition by the most intense pressure at a
temperature above their critical point, may seem by analogy to favour
the same conclusion. Hence, David Forbes, Dana, and other authors, have
argued in favour of the existence of a great liquid nucleus in our
globe covered by a comparatively thin, solid crust. And if we accept
the supposed proofs of a constant increase of temperature from the
surface to the centre of the globe, such a conclusion appears to be at
least as well founded as that which regards the central masses of the
earth as maintained in a solid condition by intense pressure.
A little consideration will, however, convince us that the facts which
have been relied upon as proving the intensely heated condition of the
central masses of our globe, are by no means so conclusive as has been
supposed.
The earth's form, which mathematicians have shown to be exactly that
which would be acquired by a globe composed of yielding materials
rotating on its axis at the rate which our planet does, has often
been adduced as proving that the latter was not always in a rigid and
unyielding condition. In the same way, all the remarkable facts and
relations of the bodies of the solar system, which have been shown by
astronomers to lend such support to the nebular hypothesis, have been
thought, at the same time, to favour the view that our earth is still
in a condition of uncompleted solidification.
But it is quite admissible to accept the nebular hypothesis and the
view that our globe attained its present form while still in a state
of fluidity, and at the same time to maintain that our earth has long
since reached its condition of complete solidification. And there are
not a few facts which appear to lend support to such a conclusion.
[Sidenote: SUPPOSED PROOFS OF LIQUID NUCLEUS.]
If the rapid rate of increase in temperature which has been
demonstrated to occur at so many parts of the earth's surface be
maintained to the centre, then, as argued by David Forbes and Dana, it
is difficult to conceive of our earth as being in any other condition
than that of a liquid mass covered by a comparatively thin crust. The
objection to this view, both upon geological and astronomical grounds,
we have pointed out in the previous chapter.
Before accepting as a demonstrated conclusion this notion of a constant
increase of temperature from the surface to the centre of our globe, it
may be well to re-examine the facts which are relied upon as proving it.
That there is a general increase of temperature so far as we are able
to go downwards in the earth's crust, there can, as we have seen,
be no doubt whatever. Yet it may be well to bear in mind how very
limited is the range of our observation on the subject. The deepest
mines extend to little more than half-a-mile from the surface, and the
deepest borings to little more than three-quarters of a mile, while the
distance from the earth's surface to its centre is nearly 4,000 miles.
We may well pause before we extend conclusions, derived from such very
limited observations, to such enormous depths.
But when we examine critically these observations themselves, we
shall find equal grounds for caution in generalising from them. There
is the greatest and most startling divergence in the results of the
observations which have been made at different points at the earth's
surface. Even when every allowance is made for errors of observation,
these discrepancies still remain. In some places the increase of
temperature as we go downwards is so rapid that it amounts to 1°
Fahrenheit for every 20 feet in depth, while in other cases, in order
to obtain the same increase in temperature of 1° Fahrenheit, we have to
descend as much as 100 feet.
Now if, as is so often assumed, this increase of temperature as we
go downwards be due to our approach to incandescent masses forming
the interior portions of the globe, it is difficult to understand
why greater uniformity is not exhibited in the rate of increase in
different areas. No difference in the conducting powers of the various
rock-materials is sufficient to account for the fact that in some
places the rate of increase in temperature in going downwards is no
less than five times as great as it is in others.
[Sidenote: VARIATIONS IN UNDERGROUND TEMPERATURES.]
Again, there are some remarkable facts concerning the variation in
the rate of increase in temperature with depth which seem equally
irreconcilable with the theory that the heat in question is directly
derived from a great, central, incandescent mass. M. Walferdin, by a
series of careful observations in two shafts at Creuzot, proved that
down to the depth of 1,800 feet the increase of temperature amounted to
1° Fahrenheit for every 55 feet of descent, but below the depth named,
the rate of increase was as much as 1° Fahrenheit for every 44 feet. On
the other hand, in the great boring of Grenelle at Paris, the increase
in temperature down to the depth of 740 feet amounted to 1° Fahrenheit
for every 50 feet of descent, but from 740 feet down to 1,600 feet,
the rate of increase diminished to 1° for 75 feet of descent. The
same remarkable fact was strikingly shown in the case of the deepest
boring in the world--that of Sperenberg, near Berlin, which attained
the great depth of 4,052 feet. In this case, the rate of increase in
temperature for the first 1,900 feet, was 1° Fahrenheit for every 55
feet of descent, and for the next 2,000, it diminished to 1° Fahrenheit
for every 62 feet of descent. In the deep well of Buda-Pesth there was
actually found a decline in temperature below the depth of 3,000 feet.
Perhaps the most interesting fact in connection with this question
which has been discovered of late years, is that in districts which
have recently been the seat of volcanic agencies, the rate of increase
in temperature, as we go downwards in the earth's crust, is abnormally
high. Thus at Monte Massi in Tuscany, the temperature was found to
increase at the rate of 1° Fahrenheit for every 24 feet of descent.
In Hungary several deep wells and borings have been made, which prove
that a very rapid increase of temperature occurs. The deep boring at
Buda-Pesth penetrates to a depth of 3,160 feet, and a temperature of
178° Fahrenheit has been observed near the bottom. The rate of increase
of temperature in this boring was about 1° for every 23 feet of
descent. In the mines opened in the great Comstock lode, in the western
territories of the United States, an abnormally high temperature has
been met with amounting in some cases to 157° Fahrenheit. Although
this is the richest mineral-vein in the world, having yielded since
1859, when it was first discovered, 60,000,000_l._ worth of gold and
silver, this rapid increase in temperature in going downwards threatens
in the end to entirely baffle the enterprise of the miner. The rate
of increase in temperature in the case of the Comstock mines has been
estimated at 1° Fahrenheit for every 46 feet of descent, between 1,000
and 2,000 feet from the surface, but as much as 1° Fahrenheit for every
25 feet, at depths below 2,000 feet.
The facts which we have stated, with others of a similar kind, have
led geologists to look with grave feelings of doubt upon the old
hypothesis which regarded the increase of temperature found in making
excavations into the earth's crust as a proof that we are approaching a
great incandescent nucleus. They have thus been led to enquire whether
there are any conceivable sources of high temperatures at moderate
depths--temperatures which would be quite competent to produce locally
all the phenomena of volcanic action.
There are not wanting other facts which seem to point to the same
conclusion: namely, that volcanic action is not due to the existence
of a universal reservoir of incandescent material occupying the central
portion of our globe, but to the local development of high temperatures
at moderate depths from the surface.
[Sidenote: DEPTHS AT WHICH EARTHQUAKES ORIGINATE.]
The close connection between the phenomena of volcanoes and earthquakes
cannot be doubted. It is true that some of those vibrations or tremors
of the earth's crust, to which we apply the name of earthquakes,
occur in areas which are not now the seat of volcanic action; and it
is equally true that the stratified rock-masses of our globe, far
away from any volcanic centres, exhibit proofs of violent movement
and fracture, in the production of which, concussions giving rise
to earthquake vibrations, could scarcely fail to have occurred. But
it is none the less certain that earthquakes as a rule take place
in those areas which are the seats of volcanic action, and that
great earthquake-shocks precede and accompany volcanic outbursts.
Sometimes, too, it has been noticed that the manifestation of
activity at a volcanic centre is marked by the sudden decline of the
earthquake-tremors of the district around, as though a safety-valve had
been opened at that part of the earth's surface.
Mr. Mallet has shown that by the careful study of the effects
produced at the surface by earthquake-vibrations, we may determine
with considerable accuracy the point at which the shock or concussion
occurred which gave rise to the vibration. Now it is a most remarkable
fact that such calculations have led to the conclusion that, so far
as is at present known, earthquake shocks never originate at greater
depths than thirty miles from the surface, and that in some cases
the focus from which the waves of elastic compression producing an
earthquake proceed is only at the depth of seven or eight miles.
As we have already seen, there can be no doubt that in the great
majority of instances the forces originating earthquake-vibrations and
volcanic outbursts are the same, and independent lines of reasoning
have conducted us to the conclusion that these forces operate at very
moderate distances from the earth's surface.
Under these circumstances, geologists have been led to enquire
whether there are any means by which we can conceive of such an
amount of heat, as would be competent to produce volcanic outbursts,
being locally developed at certain points within the earth's crust.
Recent discoveries in physical science which have shown the close
relation to one another of different kinds of force, and their mutual
convertibility, have at least suggested the possibility of the
existence of causes by which such high temperatures within certain
portions of the earth's crust may be originated.
[Sidenote: DAVY'S CHEMICAL THEORY.]
When, at the commencement of the present century, Sir Humphry Davy
discovered the remarkable metals of the alkalies and alkaline earths,
and at the same time demonstrated the striking phenomena which are
exhibited if these metals be permitted to unite with oxygen, he at
once perceived that if such metals existed in an uncombined condition
within the earth's crust, the access of water and air to the mass might
give rise to the development of such an amount of heat, as would be
competent to produce volcanic phenomena at the surface. It is true that
at a later date Davy recognised the chemical theory of volcanoes as
being beset with considerable difficulties, and was disposed to abandon
it altogether. It was argued, with considerable show of reason, that if
the heat at volcanic centres were produced by the access of water to
metallic substances, great quantities of hydrogen would necessarily be
evolved, and this gas ought to be found in prodigious quantities among
the emanations of volcanoes. The fact that such enormous quantities of
hydrogen gas are not emitted from volcanic vents has been held by many
authors to be fatal to the chemical theory of volcanoes.
But the later researches of Graham and others have made known facts
which go far towards supplying an answer to the objections raised
against the chemical theory of volcanoes. Various solids and liquids
have been shown to possess the power of absorbing many times their
volume of certain gases. Among the gases thus absorbed in large
quantities by solids and liquids, hydrogen is very conspicuous. In
some cases gases are absorbed by metals or other solids in a state of
fusion, and yielded up again by them as they cool.
It is a very remarkable circumstance that some meteorites are found
to have absorbed large quantities of hydrogen gas, and this is given
off when they are heated in vacuo. Thus it has been demonstrated that
certain meteorites have contained as much as forty seven times their
own volume of hydrogen gas.
We have already pointed out that there are reasons for believing the
internal portions of our globe to be composed of materials similar to
those found in meteorites. If such be the case, the access of water
to these metallic substances may result in the formation of oxides,
attended with a great local development of heat, the hydrogen which
is liberated being at once absorbed by the surrounding metallic
substances. That this oxidation of the metallic substances in the
interior of our globe by the access of water and air from the surface
is continually going on, can scarcely be doubted. We may even look
forward to a far-distant period when the whole of the liquid and
gaseous envelopes of the globe shall have been absorbed into its
substance, and our earth thereby reduced to the condition in which we
now find the moon to be.
There is a second method by which high temperatures might be locally
developed within the earth's crust, which has been suggested by Vose,
Mallet, and other authors.
We have good grounds for believing that the temperature of our globe
is continually diminishing by its radiation of heat into space. This
cooling of our globe is attended by contraction, which results in
movements of portions of its crust. It may at first sight appear that
such movements would be so small and insignificant as to be quite
unworthy of notice. But if we take into account the vast size of our
earth it will be seen that the movements of such enormous masses may be
attended with the most wonderful results.
It has been shown that if a part of the earth's crust fifty miles in
thickness were to have its temperature raised 200° Fahrenheit, its
surface would be raised to the extent of 1,000 or 1,500 feet Le Conte
has pointed out that if we conceive the conduction of heat to take
place at slightly different rates along different radii of our globe,
we should at once be able to account for the existing inequalities of
the earth's surface, and for all those continental movements which can
be shown to have taken place in past geological periods.
[Sidenote: DYNAMICAL THEORIES.]
But if we admit, as we have good grounds for doing, that the loss of
heat from the external portions of our globe goes on more rapidly than
in the case of the central masses, we have thereby introduced another
powerful agent for the production of high temperatures within the
earth's crust. The external shell of the globe will tend to contract
upon the central mass, and in so doing a series of tangential strains
will result which will be capable of folding and crumpling the rocks
along any lines of weakness. That such crushing and crumpling has
during all geological periods taken place along lines of weakness
in the earth's crust, is proved, as we have seen, by the phenomena
presented by mountain-ranges. Now these crushings, crumplings, and
other violent movements of great rock-masses must result in the
development of a vast amount of heat, just as the forcing down of a
break upon a moving wheel produces heat. This conclusion is strikingly
confirmed by the well-known geological fact that nearly all rocks which
have undergone great movement and contortion are found to present
evidence of having been subjected to such chemical and crystalline
actions, as would result from the development of a high temperature
within their mass.
[Sidenote: RECAPITULATION OF SEVERAL THEORIES.]
Let us sum up briefly the various methods which have been suggested to
account for the high temperatures within certain parts of the earth's
crust by which volcanic phenomena are produced.
Our globe may be conceived of as an incandescent liquid mass surrounded
by a cooler, solid shell. If we regard this liquid interior mass as
supplying directly the various volcanic vents of the earth, it must be
conceded that the outer shell is of comparatively slight thickness.
But astronomers are almost universally agreed that such a thin outer
shell and inner liquid mass are quite incompatible with that rigidity
which our planet exhibits under the attractions of its neighbours.
Geologists are almost equally unanimous in regarding this hypothesis
of a liquid nucleus and thin, solid shell as contradicted by the
stability of the conditions which have been maintained during such
long past periods, and which exist at the present day. The extent and
character of volcanic action do not indicate a condition of general
instability in our earth, but one of stability subject to small and
local interferences The grandest volcanic disturbances appear small and
insignificant, if we take into account the vast dimensions of the globe
upon which they are displayed.
If, on the other hand, we consider the outer solid shell to be of
great thickness, we are met by the difficulty of accounting for the
upheaval of liquid matter through such vast thicknesses of a solid
shell. The differences in character of lavas extruded from closely
adjoining volcanic districts seem equally difficult of explanation on
any theory of a central, fluid nucleus and a solid, outer shell. Nor is
the distribution of heat within the earth's crust so uniform as might
be anticipated, if the source of that heat be a great central mass of
highly heated materials.
Under these circumstances, geologists and physicists have enquired
whether any other conditions can be imagined as existing in the earth's
interior, which would better account for the observed phenomena than
does the hypothesis of a liquid nucleus and a solid outer shell. Two
such alternative hypotheses have been suggested.
Mr. Hopkins, adopting the theory that the earth has solidified both at
the centre and its outer surface, endeavoured to explain the occurrence
of volcanoes and earthquakes by supposing that cavities of liquid
material have been left between the solid nucleus and the solid shell,
and these cavities full of liquid material constitute the sources from
which the existing volcanoes of the globe draw their supplies. But
this hypothesis is found to be beset with many difficulties when we
attempt to apply it to the explanation of the phenomena of volcanic
action. It entirely fails, among other things, to account for the
remarkable fact that during past geological periods the scene of
volcanic action has been continually shifting over the surface of the
earth, so that there is probably no considerable area of our globe
which has not at one time or other been invaded by the volcanic forces.
By some other theorists, who have felt the full force of this last
objection, an attempt has been made to explain the phenomena of
volcanoes by supposing that the globe is solid from its surface to its
centre, but that the internal portions of the globe are at such a high
temperature that they are only retained in a solid condition by the
enormous pressure to which they are subjected. The central masses of
the globe are thus regarded as being in an _actually_ solid, but in a
_potentially_ liquid condition, and any local relief of pressure is at
once followed by the conversion of solid to liquefied materials, in the
district where the relief takes place, resulting in the manifestation
of volcanic phenomena at the spot. It may be granted that this
hypothesis better accords with the known facts of Vulcanology than any
of those which we have previously described, but it is impossible to
shut our eyes to the fact that not a few serious difficulties still
remain. Thus it is based upon the assumption that the law of the
elevation of the point of fusion by pressure is true at temperatures
and pressures almost infinitely above those at which we are able to
conduct observations; but neither experiment nor analogy warrant this
conclusion, for the former shows that the elevation of the point of
fusion by pressure goes on in a continually diminishing ratio, and the
latter famishes us with the example of volatile liquids which, above
their critical points, obstinately remain in a gaseous condition under
the highest pressures. Nor is it easy upon this hypothesis to account
for the very irregular distribution of temperatures within the earth's
crust, as demonstrated by observations in mines, wells, and borings.
The hypothesis further requires the assumption that, at such very
moderate depths as are required for the reservoirs of volcanoes, the
effects of pressure and temperature on the condition of rock-materials
are so nicely balanced that the smallest changes at the surface lead to
a disturbance of the equilibrium.
[Sidenote: DIFFICULTIES NOT YET EXPLAINED.]
It is the weight of these several objections that has led geologists
in recent years to regard with greater favour those hypotheses which
seek to account for the production of high temperatures within parts of
the earth's crust, without having recourse to a supposed incandescent
nucleus. If it can be shown that there are any chemical or mechanical
forces at work within the crust of the globe which are capable of
producing local elevations of temperature, then we may conceive of a
condition of things existing in the earth's interior which is free
from the objections raised by the astronomer on the score of the
earth's proved rigidity, and by the geologist on the ground of its
general stability, and which at the same time seems to harmonise better
with the observed facts of the distribution of temperature within the
earth's crust. How far the existence of such chemical and mechanical
agencies capable of producing high temperatures within the crust of the
globe have been substantiated, we have already endeavoured to point out.
It must be admitted, then, that the questions of the nature of the
earth's interior and the cause of the high temperatures which produce
volcanic phenomena, are still open ones. We have not yet got beyond the
stage of endeavouring to account for the facts observed by means of
tentative hypotheses. Some of these, as we have seen, agree with the
facts, so far as they are at present known, much better than others;
but the decision between them or the rejection of the whole of them in
favour of some new hypothesis, must depend on the results of future
observation and enquiry.
It may be well, before leaving this subject, to remark that they are
all equally reconcilable with the nebular theory of Kant and Laplace.
Granting that the matter composing our globe has passed successively
through the gaseous and liquid conditions, it is open to us to imagine
the earth as now composed of a liquid nucleus with either a thick or
a thin solid shell; of a solid nucleus and a solid shell with more or
less liquid matter between them; or, lastly, to conceive of it as
having become perfectly solid from the centre to the surface.
[Sidenote: CAUSE OF THE PRESENCE OF WATER IN LAVAS.]
But it is not upon the existence of a high temperature within certain
parts of the earth's crust that the production of volcanic activity
alone depends. The presence of water and other liquid and gaseous
substances in a state of the most intimate admixture with the fused
rock-masses, is, as we have seen, the main cause of the violent
displays of energy exhibited at volcanic centres. And We shall now
proceed to notice the hypotheses which have been suggested to account
for the presence of these liquid and gaseous bodies in the midst of the
masses of incandescent materials poured out from volcanic vents.
There is an explanation of this presence of water and various gases in
the masses of molten rock-materials within the earth's crust which at
once suggests itself, and which was formerly very generally accepted.
Volcanoes, as we have seen, are usually situated near coast-lines, and
if we imagine fissures to be produced by which sea-water finds access
to masses of incandescent rock-materials, then we can regard volcanic
outbursts as resulting from this meeting of water with rock-masses in a
highly healed condition. This supposition has been thought to receive
much support from the fact that many of the gases evolved from volcanic
vents are such as would be produced by the decomposition of substances
present in sea-water.
But it frequently happens that an explanation which at first sight
appears to be very simple and obvious, turns out on more critical
examination to be quite the reverse, and this is the case with the
supposed origination of volcanic outbursts by the access of sea-water
to incandescent rock-material by means of earth-fissures. It is
difficult to understand how, by such means, that wonderfully intimate
union between the liquefied rock and the water, evolved in such
quantities during volcanic outbursts, could be brought about; and
moreover, we can scarcely regard the production of fissures in the
earth's crust as being at the same time both the cause and the effect
of this influx of water to the deep-seated rock-masses at a high
temperature.
[Sidenote: ABSORPTION OF GASES BY LIQUIDS AND SOLIDS.]
During recent years the attention of both geologists and physicists
has been directed to a remarkable property exhibited by many liquids
and solids, as supplying a possible explanation of the phenomena of
volcanic action. The property to which we refer is that whereby some
liquid and solid substances are able to absorb many times their volume
of certain gases--which gases under different conditions may be given
off again from the liquids or solids. This power of absorption is a
very remarkable one; it is not attended with chemical combination, but
the amount of condensation which gases must undergo within the solid or
liquid substances is sometimes enormous. Water may be made to absorb
more than 1,000 times its volume of ammonia, and more than 500 times
its volume of hydrochloric acid. Alcohol may absorb more than 300
times its volume of sulphurous acid. Charcoal may absorb 100 times its
volume of ammonia, 85 times its volume of hydrochloric acid, 65 times
its volume of sulphuretted hydrogen, 55 times its volume of sulphurous
acid, and 35 times its volume of carbonic acid. Platinum-black absorbs
many times its volume of oxygen and other gases.
This power of absorption of gases varies in different solids and
liquids according to the conditions to which they are subjected. Dr.
Henry showed it to be a general law in liquids that, as the pressure is
augmented, the weight of the gas absorbed is proportionately increased.
Sometimes this absorption of gases takes place only at high
temperatures. Thus silver in a state of fusion is able to absorb 22
times its volume of oxygen gas. When the metal is allowed to cool this
gas is given off, and if the cooling takes place suddenly a crust is
formed on the surface, and the phenomenon known as the 'spitting of
silver' is exhibited. Sometimes during this operation miniature cones
and lava-streams are formed on the surface of the cooling mass, which
present a striking resemblance to those formed on a grand scale upon
the surface of the globe. Similar phenomena are exhibited by several
other metals and by the oxide of lead.
The researches of Troost and others have shown that molten iron and
steel possess the property of absorbing considerable quantities of
oxygen, hydrogen, carbonic acid, and carbonic oxide, and that these
gases are given off in the operation known as 'seething,' when either
the pressure or the temperature is diminished.
Hochstetter has shown that in the process of extracting sulphur from
the residues obtained during the manufacture of soda, some very
interesting phenomena are manifested. The molten sulphur is exposed
to a temperature of 262° Fahrenheit, and a pressure of two or three
atmospheres, in the presence of steam; under these circumstances it
is found that the sulphur absorbs a considerable quantity of water,
which is given off again with great violence from the mass as it
undergoes solidification. The hardened crust which forms on the surface
of the molten sulphur is agitated and fissured, miniature cones and
lava-streams being formed upon it, which have a striking resemblance to
the grander phenomena of the same kind exhibited upon the crust of the
globe.
The observations which we have described prove conclusively that many
liquids and solids in a molten condition have the power of absorbing
many times their volume of certain gases, and that this action is aided
by heat and pressure.
That the molten materials which issue from volcanic vents have
absorbed enormous quantities of steam and other gases, we have the
most undisputable evidence. The volume of such gases given off
during volcanic outbursts, and while the lava-streams are flowing
and consolidating, is enormous, and can only be accounted for by
supposing that the masses of fluid rock have absorbed many times their
volume of the gases. But we have another not less convincing proof
of the same fact in the circumstance that volcanic materials which
have consolidated under great pressure--such as granites, gabbros,
porphyries, &c.--exhibit in their crystals innumerable cavities
containing similar gases in a liquefied state.
It is to the violent escape of these gases from the molten rock-masses,
as the pressure upon them is relieved, that nearly all the active
phenomena of volcanoes must be referred; and it was the recognition of
this bet by Spallanzani, while he was watching the phenomena displayed
in the crater of Stromboli, which laid the foundations of the science
of Vulcanology.
[Sidenote: SOURCE OF THE ABSORBED GASES.]
But here another question presents itself to the investigator of the
phenomena of volcanoes: it is this. At what period did the molten
rock-masses issuing from vents absorb those gaseous materials which
are given off so violently from their midst during eruptions? Two
different answers to this question have been suggested. It may be that
the original materials of which our globe was composed consisted of
metallic substances in a state of fusion which had absorbed many gases,
and that, in the fluid masses below the solid crust, vast quantities of
vapour and gas are stored up, which are being gradually added to the
atmosphere during volcanic outbursts. The fact that meteorites, which,
as we have seen, in all probability closely resemble the materials
forming the earth's interior, sometimes yield many times their volume
of hydrogen and other gases, may be thought to lend some support to
this idea. If it be the correct one, we must regard our globe as
gradually parting with its pent-up stores of energy, in those absorbed
gases and vapours held in bondage by the solid and fluid materials of
its interior.
But there is another hypothesis which is, to say the least, equally
probable. Water containing various gases in solution is continually
finding its way downwards by infiltration into the earth's crust.
Much of this water, after passing through pervious beds, reaches
some impervious stratum and is returned to the surface in the form
of springs. But that some of this percolating water penetrates to
enormous depths is shown by the fact that the deepest mines and borings
encounter vast underground supplies of water. When we remember that
nearly three-fourths of the earth's surface is covered by the waters
of the ocean, and that the average depth of these oceanic waters is
more than 10,000 feet, we may easily understand how great a portion of
the earth's crust must be penetrated by infiltrating waters which can
find no outlet in springs. The penetration of the waters of the ocean
into the earth's crust will be aided, too, by the enormous pressure
amounting to not less than several tons to the square-inch upon the
greater part of the ocean-floor. It might be thought that this downward
penetration of water would be counteracted by the upward current of
steam that would be produced as these subterranean waters reach the
hotter portions of the earth's crust. But the experiments of Daubrée
have conclusively shown that the penetration of water through rocks
takes place in opposition to the powerful pressure of steam in the
contrary direction. Hence, we may assume that certain quantities of
water, containing various gases and solids in solution, are continually
finding their way by capillary infiltration from the surface to
the deeply seated portions of the earth's crust, there to undergo
absorption by the incandescent rock-masses and to produce oxidation of
some of their materials.
[Sidenote: POSITION OF THE ISOGEOTHERMS.]
The deep-sea soundings of the 'Challenger' have shown that the floor of
the ocean is constantly maintained at a temperature but little above
that of the freezing point of water. This low temperature is probably
produced by the absorption of heat from the earth's crust by the waters
of the ocean, which distribute it by means of convection currents on
the grandest scale. Hence, the isogeotherms, or lines indicating the
depths at which the same mean temperature is found within the earth's
crust, are probably depressed beneath the great ocean-floors, and rise
towards the land-masses. It is to this circumstance, combined with
that of the enormous pressure of water on the ocean-beds, that we must
probably ascribe the general absence of volcanoes in the deep seas and
their distribution near coast-lines.
We have thus briefly reviewed the chief hypotheses which have been
suggested in order to account for the two great factors in all volcanic
phenomena--namely, the presence of highly heated rock-masses within
the earth's crust, and the existence of various vapours and gases in a
state of most intimate mechanical, but not chemical, union with these
incandescent materials. It must be admitted that we do not at present
appear to have the means for framing a complete and consistent theory
of volcanic action, but we may hopefully look forward to the time when
further observation and experiment shall have removed many of the
existing difficulties which beset the question, and when by the light
of such future researches untenable hypotheses shall be eliminated and
the just ones improved and established.
But if we are constrained to admit that a study of the observed
phenomena and established laws of volcanic action have not as yet
enabled us to frame any complete and satisfactory theory on the
subject, we cannot lose sight of the fact that all modern speculation
upon this question appears to be tending in one definite direction. It
is every day becoming more and more clear that our earth is bound by
ties of the closest resemblance to the other members of that family of
worlds to which it belongs, and that the materials entering into their
constitution, and the forces operating in all are the same.
We have had occasion in a previous chapter to point out that there
are the strongest grounds for believing the interior of our globe to
consist of similar materials to those found in the small planetary
bodies known as meteorites. That the comets are merely aggregations
of such meteorites, and that the planets differ from them only in
their greater dimensions, may be regarded as among the demonstrated
conclusions of the astronomer. The materials found most abundantly in
meteorites and in the interior of our globe are precisely the same as
those which are proved to exist in an incandescent state in our sun.
Hence we are led to conclude that the whole of the bodies of the solar
system are composed of the same chemical elements.
[Sidenote: ERUPTIVE ACTION IN THE SUN.]
That the forces operating in each of these distant bodies present
striking points of analogy is equally clear. The sun is of far greater
dimensions than our earth, and is still in great part, if not entirely,
in a gaseous condition. The great movements in the outer envelopes of
the sun exhibited in the 'sun-spots' and 'solar prominences,' recall
to the mind the phenomena of volcanic activity upon our globe. But
the vast energy still existing in the intensely heated mass of the
sun, and the wonderful mobility of its gaseous materials, give rise to
appearances beside which all terrestrial outbursts seem to sink into
utter insignificance. Vast cavities of such dimensions that many globes
of the size of our earth might be swallowed up in them are formed
in the solar envelopes in the course of a few days or hours. Within
these cavities or sun-spots incandescent vapours are observed, rushing
upwards and downwards with almost inconceivable velocity.
The drawings made by Secchi, and reproduced in figs. 89 and 90, will
give some idea of the appearances presented by these great holes in the
solar envelopes.
[Illustration: Fig. 89.--A group of Sun-spots. (After Secchi.)]
In fig. 89 a group of sun-spots is represented and, in their circular
outlines and tendency to a linear arrangement, they can scarcely fail
to remind anyone familiar with volcanic phenomena of terrestrial
craters, though their dimensions are so much greater.
In fig. 90 the sun-spot represented shows the presence of large
floating masses of incandescent materials rushing upwards and downwards
within the yawning gulf.
[Sidenote: PHENOMENA OF SUN-SPOTS.]
[Illustration: Fig. 90.--A Sun-spot, showing the great masses of
incandescent vapour rising or falling within it. (After Secchi.)]
[Illustration: Fig. 91.--The edge of a Sun-spot, showing a portion of
the prominent masses of incandescent gas (A), which detached itself at
E and floated into the midst of the cavity.]
From fig. 91, taken from a drawing by Mr. Norman Lockyer, we may
understand the movements of these great protuberances of incandescent
gas which are seen on the sides of the sun-spots.
The so-called solar prominences present even more striking resemblances
to the volcanic outbursts of our globe.
Two drawings made by Mr. Norman Lockyer will serve to give some idea
of the vast dimensions of these solar prominences, and of the rapid
changes which take place in their form.
[Illustration: Fig. 92.--Drawing of a Solar prominence, made by Mr.
Norman Lockyer on March 14, 1869, at 11 H. 5 M. A.M.]
The masses of incandescent gas were estimated as being no less than
27,000 feet in height, yet in ten minutes they had totally changed
their form and appearance, as shown in fig. 93.
Even still more striking are the changes recorded by Professor Young,
of New-Haven, in a solar prominence, which he observed on September 7,
1871.
[Illustration: Fig. 93.--The same object, as seen at 11 H. 15 M. on the
same day.]
[Sidenote: SOLAR PROMINENCES.]
That astronomer described a mass of incandescent gas rising from
the surface of the sun to the height of 54,000 miles. In less than
twenty-five minutes he saw the whole mass torn to shreds and blown
upwards, some of the fragments being in ten minutes hurled to the
height of 200,000 miles above the sun's surface. The masses of
incandescent gas thus hurled upwards were of enormous dimensions, the
smallest being estimated as having a greater area than the whole of the
British Islands, and the force with which they were urged upwards was
so great that they acquired a velocity of 166 miles per second. The
accompanying woodcut shows the successive appearances presented by this
grand eruptive outburst on the surface of the sun.
[Illustration: Fig. 94.--Drawings of a Solar prominence at four
different periods on Sept. 7, 1871. (After Young.)]
[Sidenote: EXTINCT VOLCANOES OF THE MOON.]
The moon, which is of far smaller size than our earth, exhibits on
its surface sufficiently striking evidences of the action of volcanic
forces. Indeed the dimensions of the craters and fissures which cover
the whole visible lunar surface are such that we cannot but infer
volcanic activity to have been far more violent on the moon than it
is at the present day upon the earth. This greater violence of the
volcanic forces on the moon is perhaps accounted for by the fact that
the force of gravity on the surface of the moon is only one-sixth of
that at the surface of the earth; and thus the eruptive energy will
have a much less smaller resistance to overcome in bursting asunder the
solid crust and accumulated heaps of ejected materials on its surface.
But the volcanic action on the moon appears now to have wholly ceased,
and the absence of both water and atmosphere in our satellite suggests
that this extinction of volcanic energy may have been caused by the
complete absorption of its gaseous envelope. The appearance presented
by a portion of the moon's surface is shown in fig. 95.
The sun and the moon appear to exhibit two widely separated extremes
in the condition assumed during the cooling down from a state
of incandescence of great globes of vaporised materials. The
several planets, our own among the number, probably exhibit various
intermediate stages of consolidation.
[Illustration: Fig. 95.--A group of Lunar craters (Maurolycus,
Barocius, etc.), the largest being more than 60 miles in diameter.]
[Sidenote: ERUPTIVE ACTION IN THE SUN, EARTH AND MOON.]
Our earth is, as we have seen, closely allied to the other bodies of
the solar system in its movements, its relations, and its composition;
and a true theory of terrestrial vulcanicity, when it is discovered,
may be expected not only to afford an explanation of the phenomena
displayed on our own globe, but to account for those displays of
internal energy which have been manifested in other members of the same
great family of worlds.
INDEX.
[The subjects illustrated in the engravings are indicated by _italics_,
the names of authors are in Capitals.]
ABICH, cited, 122
-- researches of, 4
Absorption of gases by liquids and solids, 354, 355
Acid lavas, 48
Æolian Islands. _See_ Lipari Islands
Æolus, origin of myth, 35
Africa, volcanoes of, 227
-- South, diamonds of, 147
Agates, formation of, 150
Allport, Mr., cited, 259
Alps, formation of, 292
Altered lavas, names given to, 261
America, volcanoes of, 227
Amygdaloids, formation of, 140, 141
Andesites, 50, 59
Andesite-volcanoes, 126
Andrews, Professor, cited, 321
Anne Boleyn and Etna, 3
Armstrong, Sir W., hydro-electric machine, 29
Arthur's Seat, 275
Artificial stone, 55
Asia, volcanoes of, 227
Asiderites, 316
Asmanite, 314
Astroni, crater-ring of, 170
Atlantic, volcanoes in, 223
Auvergne, _breached cones of_, 123, fig. 40
-- _denuded cones in_, 124, fig. 42
-- incrusting springs of, 184
-- puys of, 152, 212
-- volcanic cones in, 79
BALL-AND-SOCKET structure in basaltic columns, 107
Barrancos, formation of, 209
Basalt, controversy concerning origin of, 249
Basalts, 49, 50, 59
Basaltic columns of Bohemia, 107
-- -- of Central Germany, 107
-- -- of Monte Albano, 107
-- -- _from the Giant's Causeway_, 107, fig. 29
Basic lavas, 48
Bath, hot spring of, 219
Ben Nevis, 274
Bohemia, volcanoes of, 126
-- lavas of, 103
Boiling. _See_ Ebullition
Bonney, Professor, cited, 69, 109, 259
Boracic acid at volcanic vents, 216
_Bourbon, volcano of_, 176, figs. 74, 75
_Bracciano, crater-lake of_, 178, fig. 77
_Breached cones_, 123, fig. 40
Babbles of steam, escape from lava, 21
_Bubbles, spontaneous movement of, in liquid cavities_, 62, fig. 8
-- -- cause of, 65
Buch, Von, researches of, 4
Buda-Pesth, deep well of, 335, 341
Büdos Hegy, Transylvania, 215
Bunsen, cited, 201
Burning, does not take place at volcanoes, 2
CADER IDRIS, 274
'Calderas,' formation of, 180
Caldera of Palma, 209
Cambro-Silurian volcanoes of British Islands, 274
_Campi-Phlegræi, map of_, fig. 11
-- -- volcanoes of, 79
-- -- tuff-cones of, 118
-- -- fissures in, 197
Carbonic acid in cavities of crystals, 63
Carboniferous volcanoes of British Islands, 275
Carlsbad, Strudel of, 218
-- Strudelstein of, 184
Caspian Sea, mud-volcanoes of, 182
Catacecaumene, volcano cones in, 79
Cause of proximity of volcanoes to sea, 239
Central Asia, volcanoes of, 236
-- America, mud-volcanoes, 182
-- Pacific, volcanoes of, 236
'Challenger,' H.M.S., voyage of, 73
-- -- soundings of, 359
Chance, Messrs., of Birmingham, 55
Charnwood Forest, ancient volcanic rocks of, 259
Chemical deposits at Vulcano, 44
-- -- on surfaces of lavas, 110
-- elements present in lavas, 46
-- theory of volcanoes, 344, 346
Chiaja di Luna, 108
Chimborazo, size of, 44
-- 151
Chodi-Berg, Hungary, 161
_Citlaltepetl, view of_, 169, fig. 69
Coast-lines, proximity of volcanoes to, 228
Cole, Mr. Grenville, 110
Colours of lavas, 49
Columns in lava, 105
-- -- dimensions of, 105
-- radiating in intrusive masses, 136
Columnar structure in lavas, 104
-- -- origin of, 105
_Columnar lava-stream on the Ardèche_, 107, fig. 28
Combustion, does not take place at volcanoes, 2
Composite cones, 128, 161
Comstock mines, temperature of, 342
_Concentric jointing in lava_, 108, fig. 30
_Cones composed of viscid lava_, 129, fig. 43
-- miniature on lava-streams, 100, 101, figs. 25, 26
-- natural sections of, 129
-- shifting of axis in, 167
Coolin Hills, Skye, 144
Cotopaxi, volcanic dust of, 69
-- _view of_, 168, fig. 68
Craters, formation of, 82
-- origin of, 167
-- position of, 167
-- fissuring of sides, 180
Crater of Stromboli, aperture at bottom of, 15
Crater-lakes, formation of, 171
-- of Agnano, 171
-- of Albano, 171
-- of Avernus, 171
-- _of Bagno_, 171, fig. 71
-- of Bolsena, 171
-- of Bracciano, 171
-- of Frascati, 173, 175
-- _of Gustavila_, 171, fig. 72
-- of Laach, 171
-- of Nemi, 171
Crater-rings, formation of, 170
_Crater-ring of Somma_, 177, fig. 76
Crater-ring of Pianura, 174
-- -- of Piano di Quarto, 174
-- -- of Vallariccia, 174
Creuzot, shafts at, 340
'Critical point' of liquids, 63
Crust of globe, definition of, 308
Crystals in lavas, 51
-- -- formed of crystallites, 54-57
-- -- formed in subterranean reservoirs, 60
-- -- interruption in growth of, 60
-- pressure under which formed, 65
-- deposited on surface of lava, 110
-- porphyritic, origin of, 256
Crystalline minerals formed beneath volcanoes, 146, 147
-- -- ejected from volcanoes, 147
Crystallised minerals of volcanoes, 46
Crystallites, aggregates of, 54, _Frontispiece_
Crystallites in lavas, 53, _Frontispiece_
Crypto-crystalline base, 57
'Cupolas,' 135
Corral of Madeira, 209
DACITES, 198
Dana, Professor, J. D., cited, 100, 159, 291, 301, 327, 338, 339
Darwin, Mr., cited, 245, 246, 271, 289
Daubeny, cited, 182
Daubrée, M., cited, 147, 315, 320, 358
Daubréelite, 314
Davy, Sir Humphry, chemical theory of volcanoes, 344, 345
Deccan of India, 103
Density of the earth, 306
_Denuded cones and craters_, 158, fig. 59
Denudation, effects of, on volcanoes, 114
Deposits about volcanic fissures, 42
Detonations at Vesuvius, 26
Devonian volcanoes of British Islands, 274
Diorite, 59
Dolomieu, cited, 4, 39
Durocher, cited, 201
Dykes, formation of, 116, 117, 209, 210
-- structure of rock in, 211
-- pseudo-, 119
Dynamical theory of volcanoes, 347, 348
EARTH'S interior, nature of, 309
-- -- physical condition of, 325
-- -- hypothesis concerning, 328-330
-- relation to other planets, 310, 311
Earthquakes, depth of origin of, 343, 344
-- connection with volcanoes, 343
-- accompanying Vesuvian eruption of 1872, 27
Ebullition, compared to volcanic eruptions, 19, 20
Eifel, volcanic cones of, 45
Ejected blocks, 45
-- materials, height to which thrown, 72
-- -- stratification of, 117-119
Elements, pyroxenic and trachytic, theory of, 201
Elevation-craters, theory of, 135, 200
Erroneous opinions, sources of, in regard to volcanoes, 2
Eruptions, feeble and violent compared, 31
-- prediction of, not possible, 32
-- intervals between, 33
-- of varying intensity, 33
-- and barometric pressure, 36
-- effects of repetition of from same fissure, 80
Eruptive action in sun and moon, 360-369
Etna, ideas of ancients concerning, 3
-- and Anne Boleyn, 3
-- observatory on, 37
-- size of, 44
--, 151
-- eruptions at summit and on flanks, 207
_Etna, dyke and lava-stream in_, 133, fig. 54
_Etna, views of_, 162, 163, figs. 62, 63
Euganean Hills, 139
-- -- volcanoes of, 201
Europe, volcanoes of, 227
Extra-terrestrial rocks, 316
_Extra-terrestrial rocks, relation to ultra-basic rocks_, 322, fig. 88
FELSTONES, 263
Ferric-chloride, mistaken for sulphur, 41
_Fissure on flanks of Etna_, 194, fig. 84
Fissure-eruptions, 188
Fissures, volcanic cones on, 194
-- systems of, 198
Fingal's Cave, 106
Flames, phenomena mistaken for, 2
-- at volcanoes, feebly luminous, 17
-- false appearance of, in volcanoes, 17
Flames at volcanic vents, 41
Flashing lighthouse, compared to Stromboli, 10
Floods, accompanying volcanic outbursts, 30
Forbes, Mr. David, cited, 337, 339
Fossils, from beneath Vesuvius, 45
-- supposed in basalt, 250
Fouqué, M., cited, 110, 213
Fumaroles, gases emitted from, 213
Fusiyama, form of, 90, 166
_Fusiyama_, 178, fig. 77
GABBRO, 59
Gardiner's river, travertine terraces of, 185
Gases emitted from volcanoes, 40
-- -- volcanic vents, 212-216
Geanticlinals, formation of, 297
Gems, formation of, 147
-- mode of occurrence, 148
Geological continuity, doctrine of, 247
Geosynclinals, formation of, 294
Geysers, formation of, 217
-- intermittent action of, 218
-- of Colorado, 184, 217
-- of Iceland, 184, 217
Giant's Causeway, 108
Gilbert, Mr. G. K., cited, 208
Girgenti, mud-volcanoes of, 182
Glass, formed by fusion of lavas, 52
Glasses, composed of certain silicates, 58
Glassy base, 57
Goethe, cited, 112
_Graham Isle_, 178, 179, fig. 78
Graham, cited, 345
Grand Sarcoui, Auvergne, 161
Granite, 59
Granite of Secondary and Tertiary ages, 254
Granitic rocks, position beneath volcanoes, 145
Great earth movements, nature of, 286
Great volcanic bands of the globe, 232-234
Grenelle, boring of, 341
Greystones, 49
Groundmass of lavas, 52
Grotto del Cane, 215
Guevo Upas, Java, 215
Guiscardi, Professor, referred to, 45
_Gustavila, crater-lake of_, 172, fig. 72
HAMILTON, Sir W., researches of, 4, 75, 84
-- -- observations on Vesuvius, 80
Hannay, Mr., referred to, 147
Hartley, Mr. Noel, referred to, 65
Hawaii, volcanoes of, 100, 125
-- -- lava-masses of, 159
-- -- volcanic eruptions at different levels, 327
Hebrides, volcanoes of, 271
Henry, Dr., cited, 355
Henry Mountains, Southern Utah, 208
Hephæstus, forge of, 3
Hochstetter, cited, 135, 356
Holosiderites, 315
Hopkins, Mr., cited, 349
Hot springs, numbers of, 219
Humboldt, researches of, 4
Hungary, lavas of, 96, 103
-- volcanoes of, 126, 201
-- deep wells of, 341
_Hverfjall, Iceland_, 178, fig. 77
Hydro-electric machine of Sir W. Armstrong, 29
Hypothesis, value of, 331-333
ICE under lava of Vesuvius in 1872, and of Etna, 110
Iceland, volcanic dust of, carried to Norway, 72
Indian Ocean, volcanoes in, 229
_Insel Ferdinandez_, 178, 179, fig. 78
Intermediate lavas, 48
Intervals between Eruptions, 33
Ireland, north-east of, 103
_Iron in Ovifak-basalts_, 319, fig. 87
Iron, seething of, 356
-- of Ovifak, terrestrial origin of, 320
Ischia, eruption in 1301, 164
-- _crater-lake of Bagno in_, 172, fig. 71
-- _plan of_, 163, fig. 64
-- _parasitic cones in_, 164, fig. 65
Island of Bourbon, 93
_Isle Julie_, 178, 179, fig. 78
Isogeotherms, 359
JANSSEN, referred to, 42
Joint-structures in lava, 104-110
_KAMMERBÜHL_, 112-114, fig. 33
-- _section of_, 114, fig. 34
-- _section in side of_, 118, fig. 36
Kant, nebular hypothesis of, 352
Kilauea, volcano of, 71, 138
-- crater of, 181
King, Mr. Clarence, cited, 301
LAACHER SEE, minerals ejected at, 149
_Lac Paven, Auvergne_, 171, fig. 70
'Laccolites,' formation of, 208
Lago di Bolsena, 173, 175
Lago di Bracciano, dimensions of, 172, 173
Lake Avernus, 215
Lapilli, 70
Laplace, nebular hypothesis of, 325, 352
Lavas, action of acid gases on, 41
-- resemblance to slags, 46
-- chemical elements in, 46
-- oxygen in, 47
-- silicon in, 47
-- proportion of silica and other oxides in, 47
-- silicates in, 47
-- acid, intermediate, basic, 48
-- specific gravities of, 49
-- colours of, 49
-- microscopic study of, 50
-- fusibility of, 51
-- minerals in, 51
-- artificially fused, 51
-- crystals in, 51, 93
-- ground mass of, 52
-- crystalline forms of, 59
-- of Bohemia, 103
-- of Hungary, 96, 103
-- of Kilauea, 95
-- of Lipari, 96
-- of Niedermendig, 103
-- of Vesuvius, 104
-- of Volvic, 95
-- of Volcano, 95
-- presence of water In, 102
-- chemical deposits on, 110
-- different fluidity of, 204
-- augite and hornblende in, 267
_Lava, cascade of_, 93, fig. 18
Lava-cones, composed of liquid lava, 125
-- -- of viscid lava, 126, 127
-- characters of, of liquid lava, 159
-- -- of viscid lava, 160
_Lava-cones, outlines of_, 160, fig. 60
Lava, in deep-seated reservoirs, 138
-- consolidation of, at great depths, 139
Lava-fountains, 94
_Lava-sheets, intrusive_, 136, 137, fig. 56
'Lava' ornaments of Naples, 45
'Lava,' slow-cooling of, 110
-- a bad conductor of heat, 110
-- ice under, 110
Lava-streams, nature of movements, 92
-- difference in liquidity of, 92
-- miniature cones on, 100, 101
-- vast dimensions of, 102
-- structure of, 103
-- position of columns in, 106
-- sinking of surface of, 111
'Lave di fango,' 30
'Lave di fuoco,' 30
Lawrencite, 314
Laws of volcanic action, 38
Le Conte, cited, 347
Leucite, absence from ancient lavas, 268
Lightning, accompanying volcanic outbursts, 28
Linear arrangement of volcanic vents, 191
-- -- of volcanoes, 231
Lipari Islands, 3, 39
-- -- fissures in, 197
-- -- pumice-cones in, 154
-- -- order of appearance of lavas in, 200
-- -- _breached pumice-cones in_, 124, fig. 41
-- -- _map of_, 192, fig. 81
-- -- _lavas of_, 96, figs. 20, 21
Liquids in cavities of crystals, 63
_Liquid cavities in lavas_, 60, fig. 7
-- -- _spontaneous movement of bubbles in_, 62, fig. 8
-- -- spontaneous movement of bubbles in, cause of, 65
Lockyer, Mr. Norman, cited, 322, 363, 364
_Lunar craters_, 368, fig. 95
Lyell, Sir Charles, cited, 135, 167, 197
MACCULLOCH, cited, 207, 208
_Madeira, cliff-section in_, 128, fig. 47
Magmas, theory of, 201
-- objections to, 202, 203
Mallet, Mr., cited, 269, 343, 346
_Mamelons of Bourbon_, 126, 127, figs. 45, 46
Maskelyne, Professor, cited, 314
Massa di Somma, destruction of, 26
Mauna Loa, 138
Metamorphism around volcanic vents, 145
Meteorites, nature of, 312
-- composition of, 313
-- minerals of, 314
-- classification of, 315
Melaphyres, 262
Miascite, 59
Michel Lévy, M., 110
Micro-crystalline base, 58
Microliths. _See_ Crystallites
Microscopic study of lavas, 50
Minerals in lavas, 51
-- of Vesuvius, 46
Mineral-veins, formation of, 149
-- connection with volcanoes, 220
-- nature of materials in, 321
_Misenum, Cape of, section of tuff-cone of_, 121, fig. 38
Modena, mud-volcanoes of, 182
_Mont Dore, section at_, 130, fig. 48
Monte Cerboli, Tuscany, 216
Monte Massi, Tuscany, well at, 341
Monte Nuovo, history of formation of, 76
-- -- _description of_, 77, 78, fig. 10
-- -- 152
-- -- crater of, 168
-- -- production of fissure at, 190
Monte Rotondo, Tuscany, 216
Moon, effect of internal forces on, 305
Mountains, all volcanoes not, 2
Mountain-chains, formation of, 291
-- -- all of recent date, 292
Mud-streams at volcanoes, 30
Mud-volcanoes, formation of, 181, 182
_Mull, dissected volcano of_, 142-4, figs. 57, 58
Muscovite, absence of, from modern lavas, 268
NEBULAR hypothesis of Laplace, 325, 352
-- -- of Kant, 352
New Zealand, geysers of, 217
-- -- volcanoes of, 135
-- -- volcanic cones in, 79
Niedermendig, lava of, 103
Nordenskiöld, Professor, cited, 318
OBSERVATORY on Vesuvius, 24, 37
-- on Etna, 37
Obsidian, 59
Oceans, depth of, in volcanic areas, 242
Oceanic islands, volcanoes in, 228
Oliver, Capt. S. P., 92
Oldhamite, 314
_Outlines of Vesuvius_, 87, fig. 17
Ovifak, iron-masses of, 319
Oxidation of materials of globe, 324
Oxygen, proportion in lavas, 47
PACIFIC, volcanoes in, 229
Palmieri, Professor, cited, 25, 37
Papandayang, eruption of, 169
Papin's digester, nature of action in, 22
_Parasitic cones, formation of_, 161, 162, fig. 61
Pele's Hair, 71
Perlitic structure, 109
Phillips, Mr. J. A., cited, 220
Phonolites, 50, 59
Phonolite-volcanoes, 126
_Photograph of Vesuvius eruption_, 24, fig. 5
'Pine-tree, appendage of Vesuvius, 29
Pitchstones, porphyritic, 60
Plateaux formed of lava-sheets, 270
Pliny, Elder, death of, 7
Plombières, hot springs of, 147
Plutonic rocks, 61
Pompeii, nature of materials covering, 117
Ponza Islands, 39
_Ponza, sections in_, 131, 132, figs. 51, 52
Porphyrites, 263
Porphyritic pitchstones, 60
Potentially liquid rock, 250
Pre-Cambrian volcanoes of British Islands, 274
Presence of water in lavas, 353
Pressure under which crystals were formed, 65
_Predazzo, ancient volcano of_, 165, fig. 67
Propylites, 199
Pseudo-dykes, 119
Pumice, how formed, 68
-- cause of white colour of, 71
-- floating on ocean, 73
-- on ocean-beds, 73
Pumice-cones, 154
_Puy de Pariou, Auvergne_, 193, 194, figs. 82, 83
Puzzolana, 89
RAIN, accompanying volcanic outbursts, 30
Rate of movement of lava-streams, 97
Rath, Professor Vom, 72
Red clay of ocean-beds, 74
Red Mountains, Skye, 144
Reservoirs beneath volcanoes, 145
Reyer, Dr. Ed., experiments of, 125, 160
Reykjanes, eruption of, in 1783, 102
Rhyolites, 50, 59
Richthofen, Von, cited, 196, 199, 200, 205
_Rocca-Monfina_, 178, fig. 77
-- --, 204
Rock-masses, movements of, 288
Rocky Mountains, 103
-- -- volcanoes of, 201
Rotomahana, sinter-terraces of, 185
_Ropy-lavas_, 98, fig. 24
_SALINA, section in_, 132, fig. 53
Sandwich Islands, lavas of, 125
San Sebastiano, destruction of, 26
_San Stephano, section in_, 131, fig. 50
Santorin, 42
_Sarcoui, Grand Puy of_, 126, fig. 44
Sciarra del fuoco, 13
Scoria, how formed, 68, 70
Scoria-cones, altered by acid gas, 155
-- breached, 156
-- characters of, 153
-- preservation of, 155
-- red colour of, 154
_Scoria-cone in Vesuvius_, 122, fig. 39
_Scoria-cone near Auckland, N. Z._, 1656, fig. 66
Schmidt, referred to, 153
Schreibersite, 314
Scrope, Mr. Poulett, cited, 5, 69, 106, 135, 198, 205, 212, 238, 289
Sea of Azof, mud-volcanoes of, 182
Secchi, Father, cited, 362
Shiant Isles, 105
Silica, presence in lavas, 47
Silicates in lavas, 47
Silicon, proportion in lavas, 47
Siliceous sinter, deposits of, 220
Silver, spitting of, 355
Silvestri, Professor, cited, 230
Similarity of lavas of different ages, 260
_Sinter-cones, forms of_, 183, fig. 79
Skye, dissected volcano of, 144
Slags, compared with lavas, 46
Smith, Lawrence, cited, 320
Smoke, appearance of, due to steam, 2
Snowdon, 274
_Solar prominences_, 364-366, figs. 92, 93, 94
Solfatara of Naples, 214
Solfatara-stage of volcanoes, 215
Somma, 133
-- crater-ring of, 83
Sorby, Mr. H. C., referred to, 59, 252
Spallanzani, early researches of, 4
-- observations on Stromboli, 8
-- cited, 39, 367
Specific gravities of lavas, 49
-- -- of glassy and crystalline rocks, 59
Spectroscope in vulcanology, 41
Spectrum-analysis, results of, 311
Specular-iron, deposited on lava-streams, 110
Sperenberg, boring of, 341
_Sphærulites_, 54, _Frontispiece_
Sporadosiderites, 316
Stability of crust of globe, 326
Staffa, Isle of, 106
Steenstrup, cited, 319
Steam-engine compared to volcano, 8
Steam, emitted by lava of Vesuvius, 27
Sternberg, referred to, 113
St. Kilda, 181
Stokes, Professor, 65
St. Paul, Island of, 180
Stromboli, 42, 158
-- apertures at bottom of crater, 15
-- appearances in crater of, 16
-- -- at night, 10
-- compared with Vesuvius, 23
-- crater of, 13
-- dependence of eruptions on atmospheric conditions, 34
-- eruption of, 14, fig. 4
-- general features of, 11
-- map of, 11, fig. 2
-- observations by Spallanzani, 8
-- resemblance to flashing light, 10
-- _section of_, 13, fig. 8
-- soundings around, 12
-- vapour-cloud above, 9
-- violent eruptions of, 23
Strombolian stage, 23
Stufas, nature of, 217
Submarine volcanoes, 179
Subterranean forces, beneficial effects of, 303
Subsidence in centre of volcanoes, 165
Sulphur, absorption of water by molten, 356
-- deposited on lava-streams, 110
-- how formed at volcanoes, 18
-- not the cause of volcanic outbursts, 18
_Surfaces of lava-streams_, 97-99, figs. 22, 23
_Sun-spots_, 361-363, figs. 89, 90, 91
Syenite, 59
Syssiderites, 316
Szabo, Professor, cited, 199
TACHYLYTE, 59
Tertiary volcanoes of British Islands, 276
_Terraces, sinter- and travertine-formation of_, 185, fig. 80
Temperature, increase in deeper parts of earth's crust, 335
-- rate of increase in different areas, 340
Teneriffe, 44, 151
_Teneriffe_, 178, fig. 77
-- _peak of_, 175, fig. 73
Tenon-and-mortise structure in basaltic columns, 107
Theodosius and Vulcano, 3
Thunder, accompanying volcanic outbursts, 28
Trachytes, 49, 50, 69
Trap-rocks, origin of, 241
Trass, 90
Travertine or Tibur-stone, 184
-- deposits of, 220
Triassic volcanoes of British Islands, 275
Tridymite deposited on lava-streams, 110
Troilite, 314
Troost, cited, 355
Tufa, or tuff, 90
Tuff-cones, character of, 157
-- denudation of, 157, 158
Typhon, fable of, 3
ULTRA-BASIC lavas, 50, 66
-- rocks, 317
VAL DEL BOVE, Etna, 133, 180, 209
-- -- _dykes in_, 134, fig. 55
Vapour-cloud over Vesuvius, 26, 29
-- -- Stromboli, 9
Ventotienne, Island of, section at, 130, fig. 49
Vesuvius, 37
-- changes in form of, 81
-- compared with Stromboli, 23
-- _crater of in 1756_, 84, fig. 14
-- -- _of in 1767_, 85, fig. 16
-- -- _of in 1822_, 82, fig. 13
-- -- _of in 1843_, 86, fig. 16
-- detonations at, 26
-- early history of, 83
-- eruption of year 79, 84
-- -- of 1822, 69
-- -- of April 1872, 24
-- -- of October 1822, 24
-- ejected blocks of, 45
-- first eruption of, 7
-- form of, 166
-- fossils of, 45
-- growth of cone of, 80
-- history of, 204
-- last eruption of, 7
-- lava-stream of 1855, 101
-- lava-streams of 1858, 1872, 97
-- lavas of, 104
-- minerals of, 46
-- -- ejected at, 149
-- observatory on, 24, 37
-- outlines of, 87
-- pine-tree appendage of, 29
-- scoria-cones in lava, 122
-- -- on lava of 1855, 153
-- steam emitted by lava of, 27
-- vapour-cloud over, 26, 29
Vesuvian stage, 23
-- _eruption, photograph of_, 24, fig. 5
Viscid lavas of Lipari Islands, 94-96
Vitreous lavas, devitrification of, 259
Volcanic action, laws of, 32
-- bombs, 70, 71
-- cycles, nature of, 221, 222
-- -- duration of, 223
-- cones, internal structure of, 115-122
-- -- _experimental illustration of formation of_, 120, fig. 37
-- -- limits to height of, 166
-- -- form of, 152
-- -- dimensions of, 152
-- -- irregular development of, 90
-- -- slopes of sides of, 91
-- -- composed of ejected rock-fragments, 156
-- -- curved slopes of, 156
-- _débris_ on sea-bottom, 240
-- dust, fineness of, 69
-- districts, areas of upheaval, 245
-- ejections, alteration of, 258
-- eruptions, compared to ebullition, 19, 20
-- forces, compensate for denudation, 283
-- -- intensity at former periods, 278
-- -- necessity for action of, 285
-- -- shifting of from one area to another, 277
-- mountains, origin of conical forms, 89
-- -- mode of growth, 89
-- phenomena of the past similar to those at present, 273
-- products, order of appearance of, 198, 199
Volcanic rocks, 61
-- -- similarity of ancient and modern, 253
Volcano, origin of name, 3
-- craters of, 167
-- Island of. _See_ Vulcano.
-- compared to steam-engine, 8
Volcanoes, blocks, ejected from, 45
-- built up of ejected fragments, 74
-- destruction caused by, 281
-- dissected by denudation, 115, 139
-- erroneous ideas concerning, 1
-- ejection of different materials from, 205
-- known to ancients, 3
-- life-history of, 186
-- number of, 224, 225
-- of Africa, 227
-- of America, 236
-- of Asia, 236
-- of Bohemia, 126
-- of Central Asia, 236
-- of Central Pacific, 236
-- of Europe, 227
-- of Hungary, 126
-- position in relation to mountain chains, 243
-- popular ideas concerning, 1
-- reservoirs beneath, 145
Volvic lava of, 103
Vose, cited, 346
Vulcan, forge of, 3
Vulcano, island of, 3, 158
Vulcano and Theodosius, 3
_Vulcano_, 178, fig. 77
-- _and Vulcanello, view of_, 43, fig. 6
-- chemical deposits at, 44
-- eruption in 1786, 43
-- -- in 1873, 43
-- -- _lava-stream in_, 95, fig. 19
-- --, 103, fig. 27
-- _plan of_, 195, fig. 85
-- _section of volcanic cone in_, 116, fig. 35
-- section in, 129
-- shifting of centre of eruption in, 196
_Vulcanello, craters of_, 197, fig. 86
Vulcanology, origin of the science, 4
-- earliest treatise on, 5
Walferdin, M., cited, 340
Water in lavas, 353
-- penetration through rocks, 358
-- presence of in lavas, 102
-- and saline solutions in cavities of crystals, 63
Werner, cited, 201
Western Isles of Scotland, 103, 139, 142
-- -- volcanoes of, 212
Whymper, Mr., 69
Woodward, Mr., experiments of, 119
Wrekin, ancient volcanic rocks of, 259
YOUNG, Professor, cited, 365
ZEOLITES, formation of, 150
PRINTED BY
SPOTTISWOODE AND CO. LTD., NEW-STREET SQUARE
LONDON
THE
INTERNATIONAL SCIENTIFIC SERIES.
Each Book Complete in One Volume. Crown 8vo. cloth, 5_s._
unless otherwise described.
I. FORMS of WATER: in Clouds and Rivers, Ice and Glaciers. By J.
Tyndall, LL.D., F.R.S. With 26 Illustrations. Twelfth Edition.
II. PHYSICS and POLITICS; or, Thoughts on the Application of
the Principles of 'Natural Selection' and 'Inheritance' to
Political Society. By Walter Bagehot. Tenth Edition.
III. FOODS. By Edward Smith, M.D., L.L.B., F.R.S. With 156
Illustrations. Tenth Edition.
IV. MIND and BODY: the Theories of their Relation. By Alexander
Bain, LL.D. With Four Illustrations. Ninth Edition.
V. The STUDY of SOCIOLOGY. By Herbert Spencer. Eighteenth Edition.
VI. The CONSERVATION of ENERGY. By Balfour Stewart, M.A., LL.D.,
F.R.S. With 14 Illustrations. Eighth Edition.
VII. ANIMAL LOCOMOTION; or, Walking, Swimming, and Flying. By J. B.
Pettigrew, M.D., F.R.S., &c. With 130 Illustrations. Fourth
Edition.
VIII. RESPONSIBILITY in MENTAL DISEASE. By Henry Maudsley, M.D.
Fifth Edition.
IX. The NEW CHEMISTRY. By Professor J. P. Cooke, of the Harvard
University. With 31 Illustrations. Eleventh Edition.
X. The SCIENCE of LAW. By Professor Sheldon Amos. Eighth Edition.
XI. ANIMAL MECHANISM: a Treatise on Terrestrial and Aërial
Locomotion. By Professor R. J. Marey. With 117 Illustrations.
Fourth Edition.
XII. The DOCTRINE of DESCENT and DARWINISM. By Professor Oscar
Schmidt (Strasburg University). With 26 Illustrations. Eighth
Edition.
XIII. The HISTORY of the CONFLICT between RELIGION and SCIENCE. By
J. W. Draper, M.D., LL.D. Twenty-second Edition.
XIV. FUNGI: their Nature, Influences, Uses, &c. By M. C. Cooke,
M.A., LL.D. Edited by the Rev. M. J. Berkeley, M.A., F.R.S.
With Illustrations. Fifth Edition.
XV. The CHEMISTRY of LIGHT and PHOTOGRAPHY. By Dr. Hermann Vogel.
With 100 Illustrations. Sixth Edition.
XVI. The LIFE and GROWTH of LANGUAGE. By William Dwight Whitney.
Sixth Edition.
XVII. MONEY and the MECHANISM of EXCHANGE. By W. Stanley Jevons,
M.A., F.R.S. Eleventh Edition.
XVIII. The NATURE of LIGHT, with a General Account of PHYSICAL
OPTICS. By Dr. Eugene Lommel. With 188 Illustrations and a
Table of Spectra In Chromo-lithography. Sixth Edition.
XIX. ANIMAL PARASITES and MESSMATES. By Monsieur Van Beneden. With
88 Illustrations. Fourth Edition.
London: KEGAN PAUL, TRENCH, TRÜBNER, & CO., Ltd.
_The International Scientific Series_--continued.
XX. FERMENTATION. By Professor Schützenberger. With 98
Illustrations. Fourth Edition.
XXI. The FIVE SENSES of MAN. By Professor Bernstein. With 91
Illustrations. Seventh Edition.
XXII. The THEORY of SOUND in its RELATION to MUSIC. By Professor
Pietro Blaserna. With numerous Illustrations. Sixth Edition.
XXIII. STUDIES in SPECTRUM ANALYSIS. By J. Norman Lockyer, F.R.S.
With Six Photographic Illustrations of Spectra, and numerous
Engravings on Wood. Fifth Edition. _6s, 6d._
XXIV. A HISTORY of the GROWTH of the STEAM ENGINE. By Professor R.
H. Thurston. With numerous Illustrations. Fifth Edition.
XXV. EDUCATION as a SCIENCE. By Alexander Bain, LL.D. Ninth Edition.
XXVI. The HUMAN SPECIES. By Professor A. de Quatrefages, Membre de
l'Institut. Fifth Edition.
XXVII. MODERN CHROMATICS. With Application to Art and Industry. By
Ogden N. Rood. Third Edition. With 130 original Illustrations.
XXVIII. The CRAYFISH: an Introduction to the Study of Zoology. By
T. H. Huxley, F.R.S. Sixth Edition. With 83 Illustrations.
XXIX. The BRAIN as an ORGAN of MIND. By H. Charlton Bastian, M.D.
Fourth Edition. With 184 Illustrations.
XXX. The ATOMIC THEORY. By Professor A. Wurtz. Translated by E.
Cleminshaw, F.R.S. Seventh Edition.
XXXI. The NATURAL CONDITIONS of EXISTENCE as they affect Animal
Life. By Karl Semper. Fifth Edition. With 3 Maps and 106
Woodcuts.
XXXII. GENERAL PHYSIOLOGY of MUSCLES and NERVES. By Prof. J.
Rosenthal. Fourth Edition. With 75 Illustrations.
XXXIII. SIGHT: an Exposition of the Principles of Monocular and
Binocular Vision. By Joseph Le Conte, LL.D. Third Edition.
With 132 Illustrations.
XXXIV. ILLUSIONS: a Psychological Study. By James Sully. Fourth
Edition.
XXXV. VOLCANOES: what they are and what they teach. By John W.
Judd, F.R.S. Fifth Edition. With 96 Illustrations.
XXXVI. SUICIDE: an Essay on Comparative Moral Statistics. By
Professor H. Morselli. Third Edition.
XXXVII. The BRAIN and its FUNCTIONS. By J. Luys, Physician to the
Hospice de la Salpêtrière. With numerous Illustrations. Third
Edition.
XXXVIII. MYTH and SCIENCE: an Essay. By Tito Vignoli. Fourth
Edition.
XXXIX. The SUN. By C. A. Young, Ph.D., LL.D. Fifth Edition. With
numerous Illustrations.
XL. ANTS, BEES, and WASPS. A Record of Observations on the Habits
of the Social Hymenoptera. By Sir John Lubbock, Bart., M.P.
Fourteenth Edition. With 5 Chromo-lithographic Plates.
XLI. ANIMAL INTELLIGENCE. By George J. Romanes. LL.D., F.R.S. Sixth
Edition.
XLII. The CONCEPTS and THEORIES of MODERN PHYSICS. By J. B. Stallo.
Third Edition.
London: KEGAN PAUL, TRENCH, TRÜBNER, & CO., Ltd.
_The International Scientific Series_--continued.
XLIII. DISEASES of MEMORY. An Essay in the Positive Psychology. By
Th. Ribot. Fourth Edition.
XLIV. MAN BEFORE METALS. By N. Joly, Correspondent de l'Institut de
France. Fifth Edition. With 148 Illustrations.
XLV. The SCIENCE of POLITICS. By Prof. Sheldon Amos. Third Edition.
XLVI. ELEMENTARY METEOROLOGY. By Robert H. Scott. With 11 Plates
and 40 Figures in Text. Seventh Edition.
XLVII. The ORGANS of SPEECH. By Georg Hermann von Meyer. With 47
Illustrations.
XLVIII. FALLACIES: a View of Logic from the Practical Side. By
Alfred Sidgwick. Second Edition.
XLIX. The ORIGIN of CULTIVATED PLANTS. By Alphonse De Candolle.
Second Edition.
L. JELLY FISH, STAR FISH, and SEA URCHINS. Being a Research on
Primitive Nervous Systems. By G. J. Romanes, LL.D., F.R.S.
Second Edition.
LI. The COMMON SENSE of the EXACT SCIENCES. By the late William
Kingdon Clifford. Third Edition. With 100 Figures.
LII. PHYSICAL EXPRESSION: its Modes and Principles. By Francis
Warner, M.D., F.R.C.P. Second Edition. With 50 Illustrations.
LIII. ANTHROPOID APES. By Robert Hartmann. With 63 Illustrations.
Second Edition.
LIV. The MAMMALIA in their RELATION to PRIMEVAL TIMES. By Oscar
Schmidt. Second Edition. With 51 Woodcuts.
LV. COMPARATIVE LITERATURE. By H. Macaulay Posnett, LL.D.
LVI. EARTHQUAKES and other EARTH MOVEMENTS. By Prof. John Milne.
With 88 Figures. Fourth Edition, revised.
LVII. MICROBES, FERMENTS, and MOULDS. By E. L. Trouessart. With 107
Illustrations. Third Edition.
LVIII. GEOGRAPHICAL and GEOLOGICAL DISTRIBUTION of ANIMALS By Prof.
A. Heilprin. Second Edition.
LIX. WEATHER: a Popular Exposition of the Nature of Weather Changes
from Day to Day. By the Hon. Ralph Abercromby. With 96
Figures. Fourth Edition.
LX. ANIMAL MAGNETISM. By Alfred Binet and Charles Fére. Fourth
Edition.
LXI. MANUAL of BRITISH DISCOMYCETES, with descriptions of all the
Species of Fungi hitherto found in Britain included in the
Family, and Illustrations of the Genera. By William Phillips,
F.L.S. Second Edition.
LXII. INTERNATIONAL LAW. With Materials for a Code of International
Law. By Professor Leone Levi.
LXIII. The GEOLOGICAL HISTORY of PLANTS. By Sir J. William Dawson.
With 80 Illustrations.
LXIV. The ORIGIN of FLORAL STRUCTURES THROUGH INSECT and other
AGENCIES. By Prof. G. Henslow. Second Edition.
LXV. On the SENSES, INSTINCTS, and INTELLIGENCE of ANIMALS, with
special reference to INSECTS. By Sir John Lubbock, Bart., M.P.
With 118 Illustrations. Third Edition.
London: KEGAN PAUL, TRENCH, TRÜBNER, & CO., Ltd.
_The International Scientific Series_--continued.
LXVI. The PRIMITIVE FAMILY in its ORIGIN and DEVELOPMENT. By C. N.
Starcke. Second Edition.
LXVII. PHYSIOLOGY of BODILY EXERCISE. By Fernand Lagrange, M.D.
Second Edition.
LXVIII. The COLOURS of ANIMALS: their Meaning and Use, especially
considered in the case of Insects. By E. B. Poulton, F.R.S.
With Chromolithographic Frontispiece and upwards of 60 Figures
in Text. Second Edition.
LXIX. INTRODUCTION to FRESH-WATER ALGÆ. With an Enumeration of all
the British Species. By M. C. Cooke, LL.D. With 13 Plates
Illustrating all the Genera.
LXX. SOCIALISM: NEW and OLD. By William Graham, M.A., Professor of
Political Economy and Jurisprudence, Queen's College, Belfast.
Second Edition.
LXXI. COLOUR-BLINDNESS and COLOUR-PERCEPTION. By F. W.
Edridge-Green, M.D. With 3 Coloured Plates.
LXXII. MAN and the GLACIAL PERIOD. By G. F. Wright, D.D. With 111
Illustrations and Maps. Second Edition.
LXXIII. HANDBOOK of GREEK and LATIN PALÆOGRAPHY. By Sir E. Maunde
Thompson, K.C.B. With Tables of Alphabets and Facsimiles.
Second Edition.
LXXIV. A HISTORY of CRUSTACEA: Recent Malacostraca. By Thomas R. R.
Stebbing, M.A. With 19 Plates and 32 Figures in Text.
LXXV. The DISPERSAL of SHELLS: an Inquiry into the means of
Dispersal possessed by Fresh Water and Land Mollusca. By H.
Wallis Kew, F.Z.S. With Preface by A. R. Wallace, F.B.S., and
Illustrations.
LXXVI. RACE and LANGUAGE. By André Lefèvre, Professor in the
Anthropological School, Paris.
LXXVII. The ORIGIN of PLANT STRUCTURES by SELF-ADAPTATION TO THE
ENVIRONMENT. By Rev. G. Henslow. M.A., F.L.S., F.G.S., &c.,
author of 'The Origin of Floral Structures,' &c.
LXXVIII. ICE-WORK PRESENT and PAST. By Rev. T. G. Bonney, D.Sc.,
LL.D., F.R.S., &c., Professor of Geology at University
College, London; Fellow of St. John's College, Cambridge.
LXXIX. A CONTRIBUTION to our KNOWLEDGE of SEEDLINGS. By Rt. Hon.
Sir John Lubbock, Bart., M.P., F.R.S.
LXXX. The ART of MUSIC. By Sir C. Hubert H. Parry, Mus. Doc.
LXXXI. The POLAR AURORA. By Alfred Angot. Illustrated.
LXXXII. WHAT is ELECTRICITY? By J. Trowbridge. Illustrated.
LXXXIII. MEMORY. By F. W. Edridge-Green, M.D. With Frontispiece.
LXXXIV. The ELEMENTS of HYPNOTISM. By B. Harry Vincent. With
Diagrams. Second Edition.
LXXXV. SEISMOLOGY. By John Milne, F.R.S., F.G.S., &c., Author of
'Earthquakes.' With 63 Figures.
LXXXVI. On BUDS and STIPULES. By the Right Hon. Sir John Lubbock,
Bart, M.P., F.R.S., D.C.L., LL.D. With 4 Coloured Plates and
340 Figures in the Text.
LXXXVII. EVOLUTION by ATROPHY, in Biology and Sociology. By Jean
Demoor, Jean Massart, and Emile Vandervelde. Translated by
Mrs. Chalmers Mitchell. With 84 Figures.
London: KEGAN PAUL, TRENCH, TRÜBNER, & CO., Ltd.
* * * * *
Transcriber Note
Minor typos corrected. Listing of "The International Scientific Series"
was split between the front and end of book but are here moved to the
end. The list was also reformatted. The captions for the map key in
Figure 58 were reformatted into a table. Figure 41 was relocated as it
followed Figure 42 in the printed version.
*** End of this LibraryBlog Digital Book "Volcanoes: What They are and What They Teach" ***
Copyright 2023 LibraryBlog. All rights reserved.