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Title: The Geological History of Plants
Author: Dawson, Sir J. William
Language: English
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THE INTERNATIONAL SCIENTIFIC SERIES
VOLUME LXI
THE
INTERNATIONAL SCIENTIFIC SERIES.
-------------------
=Each book complete in One Volume, 12mo, and bound in Cloth.=
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59. ANIMAL MAGNETISM. By Alfred Binet and Charles Féré, Assistant
Physician at the Salpêtrière.
* * * * *
[Illustration: DIAGRAM OF THE HISTORY OF PLANTS IN GEOLOGICAL TIME.
(Adapted from Ward.)]
THE INTERNATIONAL SCIENTIFIC SERIES
THE
GEOLOGICAL HISTORY
OF PLANTS
BY
SIR J. WILLIAM DAWSON C. M. G., LL. D., F. R. S., &c.
_WITH ILLUSTRATIONS_
NEW YORK APPLETON AND COMPANY 1888
Copyright, 1888, By D. APPLETON AND COMPANY.
PREFACE.
The object of this work is to give, in a connected form, a summary of the
development of the vegetable kingdom in geological time.
To the geologist and botanist the subject is one of importance with
reference to their special pursuits, and one on which it has not been
easy to find any convenient manual of information. It is hoped that its
treatment in the present volume will also be found sufficiently simple
and popular to be attractive to the general reader.
In a work of so limited dimensions, detailed descriptions cannot be
given, except occasionally by way of illustration; but references to
authorities will be made in foot-notes, and certain details, which may be
useful to collectors and students, will be placed in notes appended to
the chapters, so as not to encumber the text.
The illustrations of this work are for the most part original; but some
of them have previously appeared in special papers of the author.
J. W. D.
_February, 1888._
CONTENTS.
PAGE
CHAPTER I.
Preliminary Ideas of Geological Chronology and of the
Classification of Plants 1
CHAPTER II.
Vegetation of the Laurentian and Early Paleozoic--Questions
as to Algæ 8
CHAPTER III.
The Erian or Devonian Forests--Origin of Petroleum--The
Age of Acrogens and Gymnosperms 45
CHAPTER IV.
The Carboniferous Flora--Culmination of the
Acrogens--Formation of Coal 110
CHAPTER V.
The Flora of the Early Mesozoic--Reign of Pines and Cycads 175
CHAPTER VI.
The Reign of Angiosperms in the Later Cretaceous and Early
Tertiary or Kainozoic 191
CHAPTER VII.
Plants from the Tertiary to the Modern Period 219
CHAPTER VIII.
General Laws of Origin and Migrations of Plants--Relations
of Recent and Fossil Floras 237
APPENDIX.
I. Comparative View of Paleozoic Floras 273
II. Heer's Latest Statements on the Greenland Flora 281
III. Mineralisation of Fossil Plants 284
IV. General Works on Palæobotany 286
LIST OF ILLUSTRATIONS.
PAGE
Table of Chronology of Plants (Frontispiece.)
Protannularia Harknessii 21
Nematophyton Logani (three Figures) 22, 23
Trail of King-Crab 28
Trail of Carboniferous Crustacean 28
Rusichnites 29
Palæophycus 30
Astropolithon 31
Carboniferous Rill-mark 33
Cast of Shrinkage Cracks 34
Cone-in-cone 36
Buthotrephis 37
Silurian Vegetation 40
Erian Plants 49
Protosalvinia 54
Ptilophyton (two Figures) 62, 63
Psilophyton (two Figures) 64, 66
Sphenophyllum 65
Lepidodendron 66
Various Ferns 72, 73
Archæopteris 74
Caulopteris 75
Megalopteris 76
Calamites 77
Asterophyllites 78
Dadoxylon 79
Cordaites 81
Erian Fruits 82
Foliage from the Coal-formation 111
Sigillariæ (five Figures) 112-114
Stigmariæ (two Figures) 115
Vegetable Tissues 117
Coals and Erect Trees (two Figures) 118, 119
Lepidodendron 120
Lepidophloios 121
Asterophyllites, &c. 122
Calamites (five Figures) 123-125
Ferns of the Coal-formation (six Figures) 126-129
Noeggerathia dispar 130
Cordaites 131
Fruits of Cordaites, &c. 132
Conifers of the Coal-formation (four Species) 135
Trigonocarpum 136
Sternbergia 137
Walchia imbricatula 138
Foliage of the Jurassic Period 177
Podozamites 178
Salisburia 180
Sequoia 181
Populus primæva 191
Stercalia and Laurophyllum 194
Vegetation of the Cretaceous Period 195
Platanus 198
Protophyllum 199
Magnolia 200
Liriodendron (two Figures) 201
Brasenia 207
Gaylussaccia resinosa 228
Populus balsamifera 229
Fucus 230
THE
GEOLOGICAL HISTORY OF PLANTS.
CHAPTER I.
PRELIMINARY IDEAS OF GEOLOGICAL CHRONOLOGY AND
OF THE CLASSIFICATION OF PLANTS.
The knowledge of fossil plants and of the history of the vegetable
kingdom has, until recently, been so fragmentary that it seemed hopeless
to attempt a detailed treatment of the subject of this little book. Our
stores of knowledge have, however, been rapidly accumulating in recent
years, and we have now arrived at a stage when every new discovery serves
to render useful and intelligible a vast number of facts previously
fragmentary and of uncertain import.
The writer of this work, born in a district rich in fossil plants, began
to collect and work at these as a boy, in connection with botanical and
geological pursuits. He has thus been engaged in the study of fossil
plants for nearly half a century, and, while he has published much
on the subject, has endeavoured carefully to keep within the sphere
of ascertained facts, and has made it a specialty to collect, as far
as possible, what has been published by others. He has also enjoyed
opportunities of correspondence or personal intercourse with most of the
more eminent workers in the subject. Now, in the evening of his days, he
thinks it right to endeavour to place before the world a summary of facts
and of his own matured conclusions--feeling, however, that nothing can
be final in this matter; and that he can only hope to sketch the present
aspect of the subject, and to point the way to new developments, which
must go on long after he shall have passed away.
The subject is one which has the disadvantage of presupposing
some knowledge of the geological history of the earth, and of the
classification and structures of modern plants; and in order that all
who may please to read the following pages may be placed, as nearly as
possible, on the same level, this introductory chapter will be devoted to
a short statement of the general facts of geological chronology, and of
the natural divisions of the vegetable kingdom in their relations to that
chronology.
The crust of the earth, as we somewhat modestly term that portion of
its outer shell which is open to our observation, consists of many beds
of rock superimposed on each other, and which must have been deposited
successively, beginning with the lowest. This is proved by the structure
of the beds themselves, by the markings on their surfaces, and by the
remains of animals and plants which they contain; all these appearances
indicating that each successive bed must have been the surface before it
was covered by the next.
As these beds of rock were mostly formed under water, and of material
derived from the waste of land, they are not universal, but occur
in those places where there were extensive areas of water receiving
detritus from the land. Further, as the distinction of land and water
arises primarily from the shrinkage of the mass of the earth, and from
the consequent collapse of the crust in some places and ridging of it
up in others, it follows that there have, from the earliest geological
periods, been deep ocean-basins, ridges of elevated land, and broad
plateaus intervening between the ridges, and which were at some times
under water, and at other times land, with many intermediate phases.
The settlement and crumpling of the crust were not continuous, but took
place at intervals; and each such settlement produced not only a ridging
up along certain lines, but also an emergence of the plains or plateaus.
Thus at all times there have been ridges of folded rock constituting
mountain-ranges, flat expansions of continental plateau, sometimes dry
and sometimes submerged, and deep ocean-basins, never except in some of
their shallower portions elevated into land.
By the study of the successive beds, more especially of those deposited
in the times of continental submergence, we obtain a table of geological
chronology which expresses the several stages of the formation of the
earth's crust, from that early time when a solid shell first formed on
our nascent planet to the present day. By collecting the fossil remains
embedded in the several layers and placing these in chronological order,
we obtain in like manner histories of animal and plant life parallel to
the physical changes indicated by the beds themselves. The facts as to
the sequence we obtain from the study of exposures in cliffs, cuttings,
quarries, and mines; and by correlating these local sections in a great
number of places, we obtain our general table of succession; though it is
to be observed that in some single exposures or series of exposures, like
those in the great canons of Colorado, or on the coasts of Great Britain,
we can often in one locality see nearly the whole sequence of beds. Let
us observe here also that, though we can trace these series of deposits
over the whole of the surfaces of the continents, yet if the series could
be seen in one spot, say in one shaft sunk through the whole thickness of
the earth's crust, this would be sufficient for our purpose, so far as
the history of life is concerned.
The evidence is similar to that obtained by Schliemann on the site of
Troy, where, in digging through successive layers of _débris_, he found
the objects deposited by successive occupants of the site, from the time
of the Roman Empire back to the earliest tribes, whose flint weapons and
the ashes of their fires rest on the original surface of the ground.
Let us now tabulate the whole geological succession with the history of
animals and plants associated with it:
ANIMALS. SYSTEMS OF FORMATIONS. PLANTS.
Age of Man and Mammalia.
Kainozoic. { Modern,
{ Pleistocene, Angiosperms and
{ Pliocene, Palms dominant.
{ Miocene,
{ Eocene.
Age of Reptiles.
Mesozoic. { Cretaceous, Cycads and Pines
{ Jurassic, dominant.
{ Triassic.
Age of Amphibians and Fishes.
Age of Invertebrates.
Palæozoic. { Permian, Acrogens and
{ Carboniferous, Gymnosperms
{ Erian, dominant.
{ Silurian,
{ Ordovician,
{ Cambrian,
{ Huronian (Upper).
Age of Protozoa.
Eozoic. { Huronian (Lower), Protogens and Algæ.
{ Upper Laurentian,
{ Middle Laurentian,
{ Lower Laurentian.
It will be observed, since only the latest of the systems of formations
in this table belongs to the period of human history, that the whole
lapse of time embraced in the table must be enormous. If we suppose the
modern period to have continued for say ten thousand years, and each
of the others to have been equal to it, we shall require two hundred
thousand years for the whole. There is, however, reason to believe,
from the great thickness of the formations and the slowness of the
deposition of many of them in the older systems, that they must have
required vastly greater time. Taking these criteria into account, it has
been estimated that the time-ratios for the first three great ages may
be as one for the Kainozoic to three for the Mesozoic and twelve for
the Palæozoic, with as much for the Eozoic as for the Palæozoic. This
is Dana's estimate. Another, by Hull and Houghton, gives the following
ratios: Azoic, 34·3 per cent.; Palæozoic, 42·5 per cent.; Mesozoic and
Kainozoic, 23·2 per cent. It is further held that the modern period
is much shorter than the other periods of the Kainozoic, so that our
geological table may have to be measured by millions of years instead of
thousands.
We cannot, however, attach any certain and definite value in years to
geological time, but must content ourselves with the general statement
that it has been vastly long in comparison to that covered by human
history.
Bearing in mind this great duration of geological time, and the fact that
it probably extends from a period when the earth was intensely heated,
its crust thin, and its continents as yet unformed, it will be evident
that the conditions of life in the earlier geologic periods may have been
very different from those which obtained later. When we further take
into account the vicissitudes of land and water which have occurred, we
shall see that such changes must have produced very great differences of
climate. The warm equatorial waters have in all periods, as superficial
oceanic currents, been main agents in the diffusion of heat over the
surface of the earth, and their distribution to north and south must have
been determined mainly by the extent and direction of land, though it
may also have been modified by the changes in the astronomical relations
and period of the earth, and the form of its orbit.[A] We know by the
evidence of fossil plants that changes of this kind have occurred
so great as, on the one hand, to permit the plants of warm temperate
regions to exist within the Arctic Circle; and, on the other, to drive
these plants into the tropics and to replace them by Arctic forms. It
is evident also that in those periods when the continental areas were
largely submerged, there might be an excessive amount of moisture in
the atmosphere, greatly modifying the climate, in so far as plants are
concerned.
[A] Croll, "Climate and Time."
Let us now consider the history of the vegetable kingdom as indicated in
the few notes in the right-hand column of the table.
The most general subdivision of plants is into the two great series of
Cryptogams, or those which have no manifest flowers, and produce minute
spores instead of seeds; and Phænogams, or those which possess flowers
and produce seeds containing an embryo of the future plant.
The Cryptogams may be subdivided into the following three groups:
1. _Thallogens_, cellular plants not distinctly distinguishable into stem
and leaf. These are the Fungi, the Lichens, and the Algæ, or sea-weeds.
2. _Anogens_, having stem and foliage, but wholly cellular. These are the
Mosses and Liverworts.
3. Acrogens, which have long tubular fibres as well as cells in
their composition, and thus have the capacity of attaining a more
considerable magnitude. These are the Ferns (_Filices_), the Mare's-tails
(_Equisetaceæ_), and the Club-mosses (_Lycopodiaceæ_), and a curious
little group of aquatic plants called Rhizocarps (_Rhizocarpeæ_).
The Phænogams are all vascular, but they differ much in the simplicity
or complexity of their flowers or seeds. On this ground they admit of a
twofold division:
1. _Gymnosperms_, or those which bear naked seeds not enclosed in fruits.
They are the Pines and their allies, and the Cycads.
2. _Angiosperms_, which produce true fruits enclosing the seeds. In this
group there are two well-marked subdivisions differing in the structure
of the seed and stem. They are the _Endogens_, or inside growers, with
seeds having one seed-leaf only, as the grasses and the palms; and
the _Exogens_, having outside-growing woody stems, and seeds with two
seed-leaves. Most of the ordinary forest-trees of temperate climates
belong to this group.
On referring to the geological table, it will be seen that there is a
certain rough correspondence between the order of rank of plants and the
order of their appearance in time. The oldest plants that we certainly
know are Algæ, and with these there are plants apparently with the
structures of Thallophytes but the habit of trees, and which, for want
of a better name, I may call _Protogens_. Plants akin to the Rhizocarps
also appear very early. Next in order we find forests in which gigantic
Ferns and Lycopods and Mare's-tails predominate, and are associated
with pines. Succeeding these we have a reign of Gymnosperms, and in the
later formations we find the higher Phænogams dominant. Thus there is an
advance in elevation and complexity along with the advance in geological
time, but connected with the remarkable fact that in earlier times low
groups attain to an elevation unexampled in later times, when their
places are occupied with plants of higher type.
It is this historical development that we have to trace in the following
pages, and it will be the most simple and at the same time the most
instructive method to consider it in the order of time.
CHAPTER II.
VEGETATION OF THE LAURENTIAN AND EARLY PALÆOZOIC--QUESTIONS AS TO ALGÆ.
Oldest of all the formations known to geologists, and representing
perhaps the earliest rocks produced after our earth had ceased to be a
molten mass, are the hard, crystalline, and much-contorted rocks named
by the late Sir W. E. Logan Laurentian, and which are largely developed
in the northern parts of North America and Europe, and in many other
regions. So numerous and extensive, indeed, are the exposures of these
rocks, that we have good reason to believe that they underlie all the
other formations of our continents, and are even world-wide in their
distribution. In the lower part of this great system of rocks which, in
some places at least, is thirty thousand feet in thickness, we find no
traces of the existence of any living thing on the earth. But, in the
middle portion of the Laurentian, rocks are found which indicate that
there were already land and water, and that the waters and possibly the
land were already tenanted by living beings. The great beds of limestone
which exist in this part of the system furnish one indication of this. In
the later geological formations the limestones are mostly organic--that
is, they consist of accumulated remains of shells, corals, and other
hard parts of marine animals, which are composed of calcium carbonate,
which the animals obtain directly from their food, and indirectly from
the calcareous matter dissolved in the sea-water. In like manner great
beds of iron-ore exist in the Laurentian; but in later formations the
determining cause of the accumulation of such beds is the partial
deoxidation and solution of the peroxide of iron by the agency of
organic matter. Besides this, certain forms known as _Eozoon Canadense_
have been recognised in the Laurentian limestones, which indicate the
presence at least of one of the lower types of marine animals. Where
animal life is, we may fairly infer the existence of vegetable life as
well, since the plant is the only producer of food for the animal. But
we are not left merely to this inference. Great quantities of carbon
or charcoal in the form of the substance known as graphite or plumbago
exist in the Laurentian. Now, in more recent formations we have deposits
of coal and bituminous matter, and we know that these have arisen from
the accumulation and slow putrefaction of masses of vegetable matter.
Further, in places where igneous action has affected the beds, we find
that ordinary coal has been changed into anthracite and graphite, that
bituminous shales have been converted into graphitic shales, and that
cracks filled with soft bituminous matter have ultimately become changed
into veins of graphite. When, therefore, we find in the Laurentian thick
beds of graphite and beds of limestone charged with detached grains and
crystals of this substance, and graphitic gneisses and schists and veins
of graphite traversing the beds, we recognise the same phenomena that are
apparent in later formations containing vegetable _débris_.
The carbon thus occurring in the Laurentian is not to be regarded as
exceptional or rare, but is widely distributed and of large amount. In
Canada more especially the deposits are very considerable.
The graphite of the Laurentian of Canada occurs both in beds and in
veins, and in such a manner as to show that its origin and deposition
are contemporaneous with those of the containing rock. Sir William
Logan states[B] that "the deposits of plumbago generally occur in the
limestones or in their immediate vicinity, and granular varieties of the
rock often contain large crystalline plates of plumbago. At other times
this mineral is so finely disseminated as to give a bluish-grey colour
to the limestone, and the distribution of bands thus coloured seems to
mark the stratification of the rock." He further states: "The plumbago
is not confined to the limestones; large crystalline scales of it are
occasionally disseminated in pyroxene rock, and sometimes in quartzite
and in feldspathic rocks, or even in magnetic oxide of iron." In addition
to these bedded forms, there are also true veins in which graphite
occurs associated with calcite, quartz, orthoclase, or pyroxene, and
either in disseminated scales, in detached masses, or in bands or layers
"separated from each other and from the wall-rock by feldspar, pyroxene,
and quartz." Dr. Hunt also mentions the occurrence of finely granular
varieties, and of that peculiarly waved and corrugated variety simulating
fossil wood, though really a mere form of laminated structure, which also
occurs at Warrensburg, New York, and at the Marinski mine in Siberia.
Many of the veins are not true fissures, but rather constitute a network
of shrinkage cracks or segregation veins traversing in countless numbers
the containing rock, and most irregular in their dimensions, so that they
often resemble strings of nodular masses. It is most probable that the
graphite of the veins was originally introduced as a liquid or plastic
hydrocarbon; but in whatever way introduced, the character of the veins
indicates that in the case of the greater number of them the carbonaceous
material must have been derived from the bedded rocks traversed by these
veins, to which it bears the same relation with the veins of bitumen
found in the bituminous shales of the Carboniferous and Silurian rocks.
Nor can there be any doubt that the graphite found in the beds has been
deposited along with the calcareous matter or muddy and sandy sediment of
which these beds were originally composed.[C]
[B] "Geology of Canada," 1863.
[C] Paper by the author on Laurentian Graphite, "Journal of London
Geological Society," 1876.
The quantity of graphite in the Lower Laurentian series is enormous. Some
years ago, in the township of Buckingham, on the Ottawa River, I examined
a band of limestone believed to be a continuation of that described by
Sir W. E. Logan as the Green Lake limestone. It was estimated to amount,
with some thin interstratified bands of gneiss, to a thickness of six
hundred feet or more, and was found to be filled with disseminated
crystals of graphite and veins of the mineral to such an extent as to
constitute in some places one-fourth of the whole; and, making every
allowance for the poorer portions, this band cannot contain in all a less
vertical thickness of pure graphite than from twenty to thirty feet. In
the adjoining township of Lochaber Sir W. E. Logan notices a band from
twenty-five to thirty feet thick, reticulated with graphite veins to such
an extent as to be mined with profit for the mineral. At another place
in the same district a bed of graphite from ten to twelve feet thick,
and yielding 20 per cent, of the pure material, is worked. As it appears
in the excavation made by the quarrymen, it resembled a bed of coal; and
a block from this bed, about four feet thick, was a prominent object in
the Canadian department of the Colonial Exhibition of 1886. When it is
considered that graphite occurs in similar abundance at several other
horizons, in beds of limestone which have been ascertained by Sir W. E.
Logan to have an aggregate thickness of thirty-five hundred feet, it
is scarcely an exaggeration to maintain that the quantity of carbon in
the Laurentian is equal to that in similar areas of the Carboniferous
system. It is also to be observed that an immense area in Canada appears
to be occupied by these graphitic and _Eozoon_ limestones, and that rich
graphitic deposits exist in the continuation of this system in the State
of New York, while in rocks believed to be of this age near St. John,
New Brunswick, there is a very thick bed of graphitic limestone, and
associated with it three regular beds of graphite, having an aggregate
thickness of about five feet.[D]
[D] Matthew in "Quarterly Journal of the Geological Society," vol. xxi.,
p. 423. "Acadian Geology," p. 662.
It may fairly be assumed that in the present world, and in those
geological periods with whose organic remains we are more familiar than
with those of the Laurentian, there is no other source of unoxidized
carbon in rocks than that furnished by organic matter, and that this
has obtained its carbon in all cases, in the first instance, from the
deoxidation of carbonic acid by living plants. No other source of carbon
can, I believe, be imagined in the Laurentian period. We may, however,
suppose either that the graphitic matter of the Laurentian has been
accumulated in beds like those of coal, or that it has consisted of
diffused bituminous matter similar to that in more modern bituminous
shales and bituminous and oil-bearing limestones. The beds of graphite
near St. John, some of those in the gneiss at Ticonderoga in New York,
and at Lochaber and Buckingham, and elsewhere in Canada, are so pure and
regular that one might fairly compare them with the graphitic coal of
Rhode Island. These instances, however, are exceptional, and the greater
part of the disseminated and vein graphite might rather be likened in
its mode of occurrence to the bituminous matter in bituminous shales and
limestones.
We may compare the disseminated graphite to that which we find in those
districts of Canada in which Silurian and Devonian bituminous shales
and limestones have been metamorphosed and converted into graphitic
rocks not very dissimilar to those in the less altered portions of
the Laurentian.[E] In like manner it seems probable that the numerous
reticulating veins of graphite may have been formed by the segregation
of bituminous matter into fissures and planes of least resistance, in
the manner in which such veins occur in modern bituminous limestones
and shales. Such bituminous veins occur in the Lower Carboniferous
limestone and shale of Dorchester and Hillsborough, New Brunswick, with
an arrangement very similar to that of the veins of graphite; and in
the Quebec rocks of Point Levi, veins attaining to a thickness of more
than a foot, are filled with a coaly matter having a transverse columnar
structure, and regarded by Logan and Hunt as an altered bitumen. These
palæozoic analogies would lead us to infer that the larger part of the
Laurentian graphite falls under the second class of deposits above
mentioned, and that, if of vegetable origin, the organic matter must have
been thoroughly disintegrated and bituminised before it was changed into
graphite. This would also give a probability that the vegetation implied
was aquatic, or at least that it was accumulated under water.
[E] Granby, Melbourne, Owl's Head, &c., "Geology of Canada," 1863, p. 599.
Dr. Hunt has, however, observed an indication of terrestrial vegetation,
or at least of subaërial decay, in the great beds of Laurentian iron-ore.
These, if formed in the same manner as more modern deposits of this kind,
would imply the reducing and solvent action of substances produced in the
decay of plants. In this case such great ore-beds as that of Hull, on the
Ottawa, seventy feet thick, or that near Newborough, two hundred feet
thick,[F] must represent a corresponding quantity of vegetable matter
which has totally disappeared. It may be added that similar demands on
vegetable matter as a deoxidising agent are made by the beds and veins of
metallic sulphides of the Laurentian, though some of the latter are no
doubt of later date than the Laurentian rocks themselves.
[F] "Geology of Canada," 1863.
It would be very desirable to confirm such conclusions as those above
deduced by the evidence of actual microscopic structure. It is to be
observed, however, that when, in more modern sediments, Algæ have been
converted into bituminous matter, we cannot ordinarily obtain any
structural evidence of the origin of such bitumen, and in the graphitic
slates and limestones derived from the metamorphosis of such rocks
no organic structure remains. It is true that, in certain bituminous
shales and limestones of the Silurian system, shreds of organic
tissue can sometimes be detected, and in some cases, as in the Lower
Silurian limestone of the La Cloche Mountains in Canada, the pores
of brachiopodous shells and the cells of corals have been penetrated
by black bituminous matter, forming what may be regarded as natural
injections, sometimes of much beauty. In correspondence with this, while
in some Laurentian graphitic rocks, as, for instance, in the compact
graphite of Clarendon, the carbon presents a curdled appearance due to
segregation, and precisely similar to that of the bitumen in more modern
bituminous rocks, I can detect in the graphitic limestones occasional
fibrous structures which may be remains of plants, and in some specimens
vermicular lines, which I believe to be tubes of Eozoon penetrated by
matter once bituminous, but now in the state of graphite.
When palæozoic land-plants have been converted into graphite, they
sometimes perfectly retain their structure. Mineral charcoal, with
structure, exists in the graphitic coal of Rhode Island. The fronds of
ferns, with their minutest veins perfect, are preserved in the Devonian
shales of St. John, in the state of graphite; and in the same formation
there are trunks of Conifers (_Dadoxylon Ouangondianum_) in which the
material of the cell-walls has been converted into graphite, while
their cavities have been filled with calcareous spar and quartz, the
finest structures being preserved quite as well as in comparatively
unaltered specimens from the coal-formation.[G] No structures so perfect
have as yet been detected in the Laurentian, though in the largest
of the three graphitic beds at St. John there appear to be fibrous
structures, which I believe may indicate the existence of land-plants.
This graphite is composed of contorted and slickensided laminæ, much
like those of some bituminous shales and coarse coals; and in these are
occasional small pyritous masses which show hollow carbonaceous fibres,
in some cases presenting obscure indications of lateral pores. I regard
these indications, however, as uncertain; and it is not as yet fully
ascertained that these beds at St. John are on the same geological
horizon with the Lower Laurentian of Canada, though they certainly
underlie the Primordial series of the Acadian group, and are separated
from it by beds having the character of the Huronian.
[G] "Acadian Geology," p. 535. In calcined specimens the structures
remain in the graphite after decalcification by an acid.
There is thus no absolute impossibility that distinct organic tissues
may be found in the Laurentian graphite, if formed from land-plants,
more especially if any plants existed at that time having true woody or
vascular tissues; but it cannot with certainty be affirmed that such
tissues have been found. It is possible, however, that in the Laurentian
period the vegetation of the land may have consisted wholly of cellular
plants, as, for example, mosses and lichens; and if so, there would be
comparatively little hope of the distinct preservation of their forms or
tissues, or of our being able to distinguish the remains of land-plants
from those of Algæ.
We may sum up these facts and considerations in the following statements:
First, that somewhat obscure traces of organic structure can be detected
in the Laurentian graphite; secondly, that the general arrangement and
microscopic structure of the substance corresponds with that of the
carbonaceous and bituminous matters in marine formations of more modern
date; thirdly, that if the Laurentian graphite has been derived from
vegetable matter, it has only undergone a metamorphosis similar in kind
to that which organic matter in metamorphosed sediments of later age
has experienced; fourthly, that the association of the graphitic matter
with organic limestone, beds of iron-ore, and metallic sulphides greatly
strengthens the probability of its vegetable origin; fifthly, that
when we consider the immense thickness and extent of the Eozoonal and
graphitic limestones and iron-ore deposits of the Laurentian, if we admit
the organic origin of the limestone and graphite, we must be prepared to
believe that the life of that early period, though it may have existed
under low forms, was most copiously developed, and that it equalled,
perhaps surpassed, in its results, in the way of geological accumulation,
that of any subsequent period.
Many years ago, at the meeting of the American Association in Albany, the
writer was carrying into the room of the Geological Section a mass of
fossil wood from the Devonian of Gaspé, when he met the late Professor
Agassiz, and remarked that the specimen was the remains of a Devonian
tree contemporaneous with his fishes of that age. "How I wish I could
sit under its shade!" was the smiling reply of the great zoölogist; and
when we think of the great accumulations of Laurentian carbon, and that
we are entirely ignorant of the forms and structures of the vegetation
which produced it, we can scarcely suppress a feeling of disappointment.
Some things, however, we can safely infer from the facts that are known,
and these it may be well to mention.
The climate and atmosphere of the Laurentian may have been well adapted
for the sustenance of vegetable life. We can scarcely doubt that the
internal heat of the earth still warmed the waters of the sea, and these
warm waters must have diffused great quantities of mists and vapours over
the land, giving a moist and equable if not a very clear atmosphere. The
vast quantities of carbon dioxide afterwards sealed up in limestones and
carbonaceous beds must also have still floated in the atmosphere and must
have supplied abundance of the carbon, which constitutes the largest
ingredient in vegetable tissues. Under these circumstances the whole
world must have resembled a damp, warm greenhouse, and plants loving such
an atmosphere could have grown luxuriantly. In these circumstances the
lower forms of aquatic vegetation and those that love damp, warm air and
wet soil would have been at home.
If we ask more particularly what kinds of plants might be expected to be
introduced in such circumstances, we may obtain some information from the
vegetation of the succeeding Palæozoic age, when such conditions still
continued to a modified extent. In this period the club-mosses, ferns,
and mare's-tails engrossed the world and grew to sizes and attained
degrees of complexity of structure not known in modern times. In the
previous Laurentian age something similar may have happened to Algæ, to
Fungi, to Lichens, to Liverworts, and Mosses. The Algæ may have attained
to gigantic dimensions, and may have even ascended out of the water in
some of their forms. These comparatively simple cellular and tubular
structures, now degraded to the humble position of flat lichens or soft
or corky fungi, or slender cellular mosses, may have been so strengthened
and modified as to constitute forest-trees. This would be quite in
harmony with what is observed in the development of other plants in
primitive geological times; and a little later in this history we shall
see that there is evidence in the flora of the Silurian of a survival of
such forms.
It may be that no geologist or botanist will ever be able to realise
these dreams of the past. But, on the other hand, it is quite possible
that some fortunate chance may have somewhere preserved specimens of
Laurentian plants showing their structure.
In any case we have here presented to us the strange and startling fact
that the remarkable arrangement of protoplasmic matter and chlorophyll,
which enables the vegetable cell to perform, with the aid of solar
light, the miracle of decomposing carbon dioxide and water, and forming
with them woody and corky tissues, had already been introduced upon the
earth. It has been well said that no amount of study of inorganic nature
would ever have enabled any one to anticipate the possibility of the
construction of an apparatus having the chemical powers of the living
vegetable cell. Yet this most marvellous structure seems to have been
introduced in the full plenitude of its powers in the Laurentian age.
Whether this early Laurentian vegetation was the means of sustaining
any animal life other than marine Protozoa, we do not know. It may
have existed for its own sake alone, or merely as a purifier of the
atmosphere, in preparation for the future introduction of land-animals.
The fact that there have existed, even in modern times, oceanic islands
rich in vegetation, yet untenanted by the higher forms of animal life,
prepares us to believe that such conditions may have been general or
universal in the primeval times we are here considering.
If we ask to what extent the carbon extracted from the atmosphere and
stored up in the earth has been, or is likely to be, useful to man, the
answer must be that it is not in a state to enable it to be used as
mineral fuel. It has, however, important uses in the arts, though at
present the supply seems rather in excess of the demand, and it may well
be that there are uses of graphite still undiscovered, and to which it
will yet be applied.
Finally, it is deserving of notice that, if Laurentian graphite indicates
vegetable life, it indicates this in vast profusion. That incalculable
quantities of vegetable matter have been oxidised and have disappeared we
may believe on the evidence of the vast beds of iron-ore; and, in regard
to that preserved as graphite, it is certain that every inch of that
mineral must indicate many feet of crude vegetable matter.
It is remarkable that, in ascending from the Laurentian, we do not at
first appear to advance in evidences of plant-life. The Huronian age,
which succeeded the Laurentian, seems to have been a disturbed and
unquiet time, and, except in certain bands of iron-ore and some dark
slates coloured with carbonaceous matter, we find in it no evidence
of vegetation. In the Cambrian a great subsidence of our continents
began, which went on, though with local intermissions and reversals, all
through the Siluro-Cambrian or Ordovician time. These times were, for
this reason, remarkable for the great abundance and increase of marine
animals rather than of land-plants. Still, there are some traces of land
vegetation, and we may sketch first the facts of this kind which are
known, and then advert to some points relating to the earlier Algæ, or
sea-weeds.
An eminent Swedish geologist, Linnarsson, has described, under the name
of _Eophyton_, certain impressions on old Cambrian rocks in Sweden, and
which certainly present very plant-like forms. They want, however, any
trace of carbonaceous matter, and seem rather to be grooves or marks cut
in clay by the limbs or tails of some aquatic animal, and afterwards
filled up and preserved by succeeding deposits. After examining large
series of these specimens from Sweden, and from rocks of similar age in
Canada, I confess that I have no faith in their vegetable nature.
The oldest plants known to me, and likely to have been of higher grade
than Algæ, are specimens kindly presented to me by Dr. Alleyne Nicholson,
of Aberdeen, and which he had named _Buthotrephis Harknessii_[H] and _B.
radiata_. They are from the Skiddaw rocks of Cumberland. On examining
these specimens, and others subsequently collected in the same locality
by Dr. Gr. M. Dawson, while convinced by their form and carbonaceous
character that they are really plants, I am inclined to refer them not
to Algæ, but probably to Rhizocarps. They consist of slender branching
stems, with whorls of elongate and pointed leaves, resembling the genus
_Annularia_ of the coal formation. I am inclined to believe that both
of Nicholson's species are parts of one plant, and for this I have
proposed the generic name _Protannularia_ (Fig. 1). Somewhat higher in
the Siluro-Cambrian, in the Cincinnati group of America, Lesquereux
has found some minute radiated leaves, referred by him to the genus
_Sphenophyllum_,[I] which is also allied to Rhizocarps. Still more
remarkable is the discovery in the same beds of a stem with rhombic
areoles or leaf-bases, to which the name _Protostigma_ has been given.[J]
If a plant, this may have been allied to the club-mosses. This seems to
be all that we at present know of land-vegetation in the Siluro-Cambrian.
So far as the remains go, they indicate the presence of the families of
Rhizocarps and of Lycopods.
[H] "Geological Magazine," 1869.
[I] See figure in next chapter.
[J] _Protostigma sigillarioides_, Lesquereux.
[Illustration: Fig. 1.--_Protannularia Harknessii_ (Nicholson), a
probable Rhizocarp of the Ordovician period.]
If we ascend into the Upper Silurian, or Silurian proper, the evidences
of land vegetation somewhat increase. In 1859 I described, in "The
Journal of the Geological Society" of London, a remarkable tree from the
Lower Erian of Gaspé, under the name _Prototaxites_, but for which I
now prefer the name _Nematophyton_. When in London, in 1870, I obtained
permission to examine certain specimens of spore-cases or seeds from
the Upper Ludlow (Silurian) formation of England, and which had been
described by Sir Joseph Hooker under the name _Pachytheca_. In the same
slabs with these I found fragments of fossil wood identical with those of
the Gaspé plant. Still later I recognised similar fragments associated
also with _Pachytheca_ in the Silurian of Cape Bon Ami, New Brunswick.
Lastly, Dr. Hicks has discovered similar wood, and also similar fruits,
in the Denbighshire grits, at the base of the Silurian.[K]
[K] "Journal of the Geological Society," August, 1881.
[Illustration: Fig. 2.--_Nematophyton Logani_ (magnified). Vertical
section.]
[Illustration: Fig. 3.--_Nematophyton Logani_ (magnified). Horizontal
section, showing part of one of the radial spaces, with tubes passing
into it.]
[Illustration: Fig. 4.--_Nematophyton Logani_ (magnified).
Restoration.[L]]
[L] Figs. 2, 3, and 4 are drawn from nature by Prof. Penhallow, of McGill
College.
From comparison of this singular wood, the structure of which is
represented in Figs. 2, 3, 4, with the _débris_ of fossil taxine woods,
mineralised after long maceration in water, I was inclined to regard
_Prototaxites_, or, as I have more recently named it, _Nematophyton_, as
a primeval gymnosperm allied to those trees which Unger had described
from the Erian of Thuringia, under the name _Aporoxylon_.[M] Later
examples of more lax tissues from branches or young stems, and the
elaborate examinations kindly undertaken for me by Professor Penhallow
and referred to in a note to this chapter, have induced me to modify
this view, and to hold that the tissues of these singular trees, which
seem to have existed from the beginning of the Silurian age and to have
finally disappeared in the early Erian, are altogether distinct from
any form of vegetation hitherto known, and are possibly survivors of
that prototypal flora to which I have already referred. They are trees
of large size, with a coaly bark and large spreading roots, having the
surface of the stem smooth or irregularly ribbed, but with a nodose or
jointed appearance. Internally, they show a tissue of long, cylindrical
tubes, traversed by a complex network of horizontal tubes thinner walled
and of smaller size. The tubes are arranged in concentric zones, which,
if annual rings, would in some specimens indicate an age of one hundred
and fifty years. There are also radiating spaces, which I was at first
disposed to regard as true medullary rays, or which at least indicate a
radiating arrangement of the tissue. They now seem to be spaces extending
from the centre towards the circumference of the stem, and to have
contained bundles of tubes gathered from the general tissue and extending
outward perhaps to organs or appendages on the surface. Carruthers has
suggested a resemblance to Algæ, and has even proposed to change the
name to _Nematophycus_, or "thread-sea-weed"; but the resemblance is
by no means clear, and it would be quite as reasonable to compare the
tissue to that of some Fungi or Lichens, or even to suppose that a plant
composed of cylindrical tubes has been penetrated by the mycelium or
spawn of a dry-rot fungus. But the tissues are too constant and too
manifestly connected with each other to justify this last supposition.
That the plant grew on land I cannot doubt, from its mode of occurrence;
that it was of durable and resisting character is shown by its state
of preservation; and the structure of the seeds called _Pachytheca_,
with their constant association with these trees, give countenance to
the belief that they are the fruit of Nematophyton. Of the foliage or
fronds of these strange plants we unfortunately know nothing. They seem,
however, to realise the idea of arboreal plants having structures akin
to those of thallophytes, but with seeds so large and complex that they
can scarcely be regarded as mere spores. They should perhaps constitute a
separate class or order to which the name _Nematodendreæ_ may be given,
and of which _Nematophyton_ will constitute one genus and _Aporoxylon_ of
Unger another.[N]
[M] "Palæontologie des Thuringer Waldes," 1856.
[N] See report by the author on "Erian Flora of Canada," 1871 and 1882,
for full description of these fossils.
Another question arises as to the possible relation of these plants
to other trees known by their external forms. The _Protostigma_ of
Lesquereux has already been referred to, and Claypole has described
a tree from the Clinton group of the United States, with large ovate
leaf-bases, to which he has given the name _Glyptodendron_.[O] If the
markings on these plants are really leaf-bases, they can scarcely have
been connected with _Nematophyton_, because that tree shows no such
surface-markings, though, as we have seen, it had bundles of tubes
passing diagonally to the surface. These plants were more probably
trees with an axis of barred vessels and thick, cellular bark, like
the _Lepidodendron_ of later periods, to be noticed in the sequel. Dr.
Hicks has also described from the same series of beds which afforded
the fragments of Nematophyton certain carbonised dichotomous stems,
which he has named _Berwynia_. It is just possible that these plants may
have belonged to the Nematodendreæ. The thick and dense coaly matter
which they show resembles the bark of these trees, the longitudinal
striation in some of them may represent the fibrous structure, and the
lateral projections which have been compared to leaves or leaf-bases
may correspond with the superficial eminences of _Nematophyton_, and
the spirally arranged punctures which it shows on its surface. In
this case I should be disposed to regard the supposed stigmaria-like
roots as really stems, and the supposed rootlets as short, spine-like
rudimentary leaves. All such comparisons must, however, in the mean time
be regarded as conjectural. We seem, however, to have here a type of tree
very dissimilar to any even of the later Palæozoic age, which existed
throughout the Silurian, and probably further back, which ceased to
exist early in the Erian age, and before the appearance of the ordinary
coniferous and lepidodendroid trees. May it not have been a survivor of
an old arboreal flora extending back even to the Laurentian itself?
[O] "American Journal of Science," 1878.
Multitudes of markings occurring on the surfaces of the older rocks have
been referred to the Algæ or sea-weeds, and indeed this group has been
a sort of refuge for the destitute to which palæontologists have been
accustomed to refer any anomalous or inexplicable form which, while
probably organic, could not be definitely referred to the animal kingdom.
There can be no question that some of these are truly marine plants; and
that plants of this kind occur in formations older than those in which
we first find land-plants, and that they have continued to inhabit the
sea down to the present time. It is also true that the oldest of these
Algæ closely resemble in form plants of this kind still existing; and,
since their simple cellular structures and soft tissues are scarcely
ever preserved, their general forms are all that we can know, so that
their exact resemblance to or difference from modern types can rarely
be determined. For the same reasons it has proved difficult clearly to
distinguish them from mere inorganic markings or the traces of animals,
and the greatest divergence of opinion has occurred in recent times
on these subjects, as any one can readily understand who consults the
voluminous and well-illustrated memoirs of Nathorst, Williamson, Saporta,
and Delgado.
The author of this work has given much attention to these remains, and
has not been disposed to claim for the vegetable kingdom so many of them
as some of his contemporaries.[P] The considerations which seem most
important in making such distinctions are the following: 1. The presence
or absence of carbonaceous matter. True Algæ not infrequently present at
least a thin film of carbon representing their organic matter, and this
is the more likely to occur in their case, as organic matters buried in
marine deposits and not exposed to atmospheric oxidation are very likely
to be preserved. 2. In the absence of organic matter, the staining of
the containing rock, the disappearance or deoxidation of its ferruginous
colouring matter, or the presence of iron pyrite may indicate the removal
of organic matter by decay. 3. When organic matter and indications of it
are altogether absent, and form alone remains, we have to distinguish
from Algæ, trails and burrows similar to those of aquatic animals, casts
of shrinkage-cracks, water-marks, and rill-marks widely diffused over the
surfaces of beds. 4. Markings depressed on the upper surfaces of beds,
and filled with the material of the succeeding layer, are usually mere
impressions. The cases of possible exceptions to this are very rare. On
the contrary, there are not infrequently forms in relief on the surfaces
of rocks which are not Algæ, but may be shallow burrows arched upward on
top, or castings of worms thrown up upon the surface. Sometimes, however,
they may have been left by denudation of the surrounding material, just
as footprints on dry snow remain in relief after the surrounding loose
material has been drifted away by the wind; the portion consolidated by
pressure being better able to resist the denuding agency.
[P] "Impressions and Footprints of Aquatic Animals," "American Journal of
Science," 1873.
[Illustration: Fig. 5.--Trail of a modern king-crab, to illustrate
imitations of plants sometimes named _Bilobites_.]
[Illustration: Fig. 6.--Trail of Carboniferous crustacean (_Rusichnites
Acadicus_), Nova Scotia, to illustrate supposed Algæ.]
The footprints from the Potsdam sandstone in Canada, for which the name
_Protichnites_ was proposed by Owen, and which were by him referred to
crustaceans probably resembling _Limulus_, were shown by the writer,
in 1862,[Q] to correspond precisely with those of the American Limulus
(_Polyphemus Occidentalis_) (Fig. 5). I proved by experiment with the
modern animal that the recurring series of groups of markings were
produced by the toes of the large posterior thoracic feet, the irregular
scratches seen in _Protichnites lineatus_ by the ordinary feet, and the
central furrow by the tail. It was also shown that when the Limulus uses
its swimming-feet it produces impressions of the character of those
named _Climactichnites_, from the same beds which afford _Protichnites_.
The principal difference between _Protichnites_ and their modern
representatives is that the latter have two lateral furrows produced by
the sides of the carapace, which are wanting in the former.
[Q] "Canadian Naturalist," vol. vii.
I subsequently applied the same explanation to several other ancient
forms now known under the general name _Bilobites_ (Figs. 6 and 7).[R]
[R] The name Bilobites was originally proposed by De Kay for a bivalve
shell (Conocardium). Its application to supposed Algæ was an error, but
this is of the less consequence, as these are not true plants but only
animal trails.
[Illustration: Fig. 7.--_Rusophycus_ (_Rusichnites_) _Grenvillensis_,
an animal burrow of the Siluro-Cambrian, probably of a crustacean, _a_,
Track connected with it.]
The tuberculated impressions known as _Phymatoderma_ and _Caulerpites_
may, as Zeiller has shown, be made by the burrowing of the mole-cricket,
and fine examples occurring in the Clinton formation of Canada are
probably the work of Crustacea. It is probable, however, that some of
the later forms referred to these genera are really Algæ related to
_Caulerpa_, or even branches of Conifers of the genus _Brachyphyllum_.
_Nereites_ and _Planulites_ are tracks and burrows of worms, with
or without marks of setæ, and some of the markings referred to
_Palæochorda_, _Palæophycus_, and _Scolithus_ have their places here.
Many examples highly illustrative of the manner of formation of the
impressions are afforded by Canadian rocks (Fig. 8).
Branching forms referred to _Licrophycus_ of Billings, and some of
those referred to _Buthotrephis_, Hall, as well as radiating markings
referable to _Scotolithus_, _Gyrophyllites_, and _Asterophycus_, are
explained by the branching burrows of worms illustrated by Nathorst
and the author. _Astropolithon_, a singular radiating marking of the
Canadian Cambrian,[S] seems to be something organic, but of what nature
is uncertain (Fig. 9).
[S] Supplement to "Acadian Geology."
[Illustration: Fig. 8.--_Palæophycus Beverlyensis_ (Billings), a supposed
Cambrian Fucoid, but probably an animal trail.]
_Rhabdichnites_ and _Eophyton_ belong to impressions explicable by the
trails of drifting sea-weeds, the tail-markings of Crustacea, and the
ruts ploughed by bivalve mollusks, and occurring in the Silurian, Erian,
and Carboniferous rocks.[T] Among these are the singular bilobate forms
described as _Rusophycus_ by Hall, and which are probably burrows or
resting-places of crustaceans. The tracks of such animals, when walking,
are the jointed impressions known as _Arthrophycus_ and _Crusiana_.
I have shown by the mode of occurrence of these, and Nathorst has
confirmed this conclusion by elaborate experiments on living animals,
that these forms are really trails impressed on soft sediments by animals
and mostly by crustaceans.
[T] "Canadian Naturalist," 1864.
I agree with Dr. Williamson[U] in believing that all or nearly all the
forms referred to Crossochorda of Schimper are really animal impressions
allied to Nereites, and due either to worms or, as Nathorst has shown to
be possible, to small crustaceans. Many impressions of this kind occur in
the Silurian beds of the Clinton series in Canada and New York, and are
undoubtedly mere markings.
[U] "Tracks from Yoredale Rocks," "Manchester Literary and Philosophical
Society," 1885.
[Illustration: Fig. 9.--_Astropolithon Hindii_, an organism of the Lower
Cambrian of Nova Scotia, possibly vegetable.]
It is worthy of note that these markings strikingly resemble the
so-called _Eophyton_, described by Torell from the Primordial of
Sweden, and by Billings from that of Newfoundland; and which also occur
abundantly in the Primordial of New Brunswick. After examining a series
of these markings from Sweden shown to me by Mr. Carruthers in London,
and also specimens from Newfoundland and a large number _in situ_ at St.
John, I am convinced that they cannot be plants, but must be markings of
the nature of _Rhabdichnites_. This conclusion is based on the absence of
carbonaceous matter, the intimate union of the markings with the surface
of the stone, their indefinite forms, their want of nodes or appendages,
and their markings being always of such a nature as could be produced by
scratches of a sharp instrument. Since, however, fishes are yet unknown
in beds of this age, they may possibly be referred to the feet or spinous
tails of swimming crustaceans. Salter has already suggested this origin
for some scratches of somewhat different form found in the Primordial
of Great Britain. He supposed them to have been the work of species of
_Hymenocaris_. These marks may, however, indicate the existence of some
free-swimming animals of the Primordial seas as yet unknown to us.
Three other suggestions merit consideration in this connection. One
is that Algæ and also land-plants, drifting with tides or currents,
often make the most remarkable and fantastic trails. A marking of this
kind has been observed by Dr. G. M. Dawson to be produced by a drifted
Laminaria, and in complexity it resembled the extraordinary _Ænigmichnus
multiformis_ of Hitchcock from the Connecticut sandstones. Much more
simple markings of this kind would suffice to give species of _Eophyton_.
Another is furnished by a fact stated to the author by Prof. Morse,
namely, that Lingulæ, when dislodged from their burrows, trail themselves
over the bottom like worms, by means of their cirri. Colonies of these
creatures, so abundant in the Primordial, may, when obliged to remove,
have covered the surfaces of beds of mud with vermicular markings. The
third is that the Rhabdichnite-markings resemble some of the grooves in
Silurian rocks which have been referred to trails of Gasteropods, as, for
instance, those from the Clinton group, described by Hall.
Another kind of markings not even organic, but altogether depending on
physical causes, are the beautiful branching rill-marks produced by the
oozing of water out of mud and sand-banks left by the tide, and which
sometimes cover great surfaces with the most elaborate tracery, on
the modern tidal shores as well as in some of the most ancient rocks.
_Dendrophycus_[V] of Lesquereux seems to be an example of rill-mark, as
well as _Aristophycus_, _Cloephycus_, and _Zygopliycus_, of Miller and
Dyer, from the Lower Silurian.
[V] "Coal Flora of Pennsylvania," vol. iii., Plate 88.
Rill-marks occur in very old rocks,[W] but are perhaps most beautifully
preserved in the Carboniferous shales and argillaceous sandstones, and
even more elaborately on the modern mud-banks of the Bay of Fundy.[X]
Some of these simulate ferns and fronds of Laminariæ, and others resemble
roots, fucoids allied to _Buthotrephis_, or the radiating worm-burrows
already referred to (Fig. 10).
[W] "Journal of the Geological Society," vol. xii., p. 251.
[X] "Acadian Geology," 2d ed., p. 26.
[Illustration: Fig. 10.--Carboniferous rill-mark (Nova Scotia), reduced,
to illustrate pretended Algæ.]
_Shrinkage-cracks_ are also abundant in some of the Carboniferous beds,
and are sometimes accompanied with impressions of rain-drops. When finely
reticulated they might be mistaken for the venation of leaves, and,
when complicated with little rill-marks tributary to their sides, they
precisely resemble the _Dictyolites_ of Hall from the Medina sandstone
(Fig. 11).
[Illustration: Fig. 11.--Cast of shrinkage cracks (Carboniferous, Nova
Scotia), illustrating pretended Algæ.]
An entirely different kind of shrinkage-crack is that which occurs in
certain carbonised and flattened plants, and which sometimes communicates
to them a marvellous resemblance to the netted under surface of an
exogenous leaf. Flattened stems of plants and layers of cortical
matter, when carbonised, shrink in such a manner as to produce minute
reticulated cracks. These become filled with mineral matter before the
coaly substance has been completely consolidated. A further compression
occurs, causing the coaly substance to collapse, leaving the little veins
of harder mineral matter projecting. These impress their form upon the
clay or shale above and below, and thus when the mass is broken open we
have a carbonaceous film or thin layer covered with a network of raised
lines, and corresponding minute depressed lines on the shale in contact
with it. The reticulations are generally irregular, but sometimes they
very closely resemble the veins of a reticulately veined leaf. One of
the most curious specimens in my possession was collected by Mr. Elder
in the Lower Carboniferous of Horton Bluff. The little veins which form
the projecting network are in this case white calcite; but at the surface
their projecting edges are blackened with a carbonaceous film.
_Slickensided bodies_, resembling the fossil fruits described by Geinitz
as _Gulielmites_, and the objects believed by Fleming and Carruthers[Y]
to be casts of cavities filled with fluid, abound in the shales of the
Carboniferous and Devonian. They are, no doubt, in most cases the results
of the pressure and consolidation of the clay around small solid bodies,
whether organic, fragmentary, or concretionary. They are, in short, local
slickensides precisely similar to those found so plentifully in the coal
under-clays, and which, as I have elsewhere[Z] shown, resulted from the
internal giving way and slipping of the mass as the roots of Stigmaria
decayed within it. Most collectors of fossil plants in the older
formations must, I presume, be familiar with appearances of this kind in
connection with small stems, petioles, fragments of wood, and carpolites.
I have in my collection petioles of ferns and fruits of the genus
Trigonocarpum partially slickensided in this way, and which if wholly
covered by this kind of marking could scarcely have been recognised. I
have figured bodies of this kind in my report on the Devonian and Upper
Silurian plants of Canada, believing them, owing to their carbonaceous
covering, to be probably slickensided fruits, though of uncertain nature.
In every case I think these bodies must have had a solid nucleus of
some sort, as the severe pressure implied in slickensiding is quite
incompatible with a mere "fluid-cavity," even supposing this to have
existed.
[Y] "Journal of the Geological Society," June, 1871.
[Z] _Ibid._, vol. x., p. 14.
Prof. Marsh has well explained another phase of the influence of hard
bodies in producing partial slickensides, in his paper on _Stylolites_,
read before the American Association in 1867, and the application
of the combined forces of concretionary action and slickensiding to
the production of the cone-in-cone concretions, which occur in the
coal-formation and as low as the Primordial. I have figured a very
perfect and beautiful form of this kind from the coal-formation of Nova
Scotia, which is described in "Acadian Geology"[AA] (Fig. 12).
I have referred to these facts here because they are relatively more
important in that older period, which may be named the age of Algæ, and
because their settlement now will enable us to dispense with discussions
of this kind further on. The able memoirs of Nathorst and Williamson
should be studied by those who desire further information.
[AA] Appendix, p. 676, edition of 1878.
[Illustration: Fig. 12.--Cone-in-cone concretion (Carboniferous, Nova
Scotia), illustrating pretended Algæ.]
But it may be asked, "Are there no real examples of fossil Algæ?" I
believe there are many such, but the difficulty is to distinguish them.
Confining ourselves to the older rocks, the following may be noted:
The genus _Buthotrephis_ of Hall, which is characterised as having
stems, sub-cylindric or compressed, with numerous branches, which are
divaricating and sometimes leaf-like, contains some true Algæ. Hall's
_B. gracilis_, from the Siluro-Cambrian, is one of these. Similar
plants, referred to the same species, occur in the Clinton and Niagara
formations, and a beautiful species, collected by Col. Grant, of
Hamilton, and now in the McGill College collection, represents a broader
and more frondose type of distinctly carbonaceous character. It may be
described as follows:
_Buthotrephis Grantii_, S. N. (Fig. 13).--Stems and fronds smooth and
slightly striate longitudinally, with curved and interrupted striæ.
Stem thick, bifurcating, the divisions terminating in irregularly
pinnate fronds, apparently truncate at the extremities. The quantity
of carbonaceous matter present would indicate thick, though perhaps
flattened, stems and dense fleshy fronds.
[Illustration: Fig. 13.--_Buthotrephis Grantii_, a genuine Alga from the
Silurian, Canada.]
The species _Buthotrephis subnodosa_ and _B. flexuosa_, from the Utica
shale, are also certainly plants, though it is possible, if their
structures and fruit were known, some of these might be referred to
different genera. All of these plants have either carbonaceous matter or
produce organic stains on the matrix.
The organism with diverging wedge-shaped fronds, described by Hall as
_Sphenothallus angustifolius_, is also a plant. Fine specimens, in the
collection of the Geological Survey of Canada, show distinct evidence of
the organic character of the wedge-shaped fronds. It is from the Utica
shale, and elsewhere in the Siluro-Cambrian. It is just possible, as
suggested by Hall, that this plant may be of higher rank than the Algæ.
The genus _Palæophycus_ of Hall includes a great variety of uncertain
objects, of which only a few are probably true Algæ. I have specimens of
fragments similar to his _P. virgatus_, which show distinct carbonaceous
films, and others from the Quebec group, which seem to be cylindrical
tubes now flattened, and which have contained spindle-shaped sporangia
of large size. Tortuous and curved flattened stems, or fronds, from the
Upper Silurian limestone of Gaspé, also show organic matter.
Respecting the forms referred to _Licrophycus_ by Billings, containing
stems or semi-cylindrical markings springing from a common base, I have
been in great doubt. I have not seen any specimens containing unequivocal
organic matter, and am inclined to think that most of them, if not the
whole, are casts of worm-burrows, with trails radiating from them.
Though I have confined myself in this notice to plants, or supposed
plants, of the Lower Palæozoic, it may be well to mention the remarkable
Cauda-Galli fucoids, referred by Hall to the genus _Spirophyton_, and
which are characteristic of the oldest Erian beds. The specimens which
I have seen from New York, from Gaspé, and from Brazil, leave no doubt
in my mind that these were really marine plants, and that the form of a
spiral frond, assigned to them by Hall, is perfectly correct. They must
have been very abundant and very graceful plants of the early Erian,
immediately after the close of the Silurian period.
We come now to notice certain organisms referred to Algæ, and which are
either of animal origin, or are of higher grade than the sea-weeds.
We have already discussed the questions relating to _Prototaxites_.
_Drepanophycus_, of Goeppert,[AB] I suspect, is only a badly preserved
branch or stem of the Erian land-plant known as Arthrostigma. In like
manner, _Haliserites Dechenianus_,[AC] of Goeppert, is evidently
the land-plant known as _Psilophyton_. _Sphærococcites dentatus_
and _S. serra_--the _Fucoides dentatus_ and _serra_ of Brongniart,
from Quebec--are graptolites of two species quite common there.[AD]
_Dictyophyton_ and _Uphantenia_, as described by Hall and the author, are
now known to be sponges. They have become _Dictyospongiæ_. The curious
and very ancient; fossils referred by Forbes to the genus _Oldhamia_ are
perhaps still subject to doubt, but are usually regarded as Zoöphytes,
though it is quite possible they may be plants. Though I have not seen
the specimens, I have no doubt whatever that the plants, or the greater
part of them, from the Silurian of Bohemia, described by Stur as Algæ
and Characeæ,[AE] are really land-plants, some of them of the genus
_Psilophyton_. I may say in this connection that specimens of flattened
_Psilophyton_ and _Arthrostigma_, in the Upper Silurian and Erian of
Gaspé, would probably have been referred to Algæ, but for the fact that
in some of them the axis of barred vessels is preserved.
[AB] "Fossile Flora," 1852, p. 92, Table xli.
[AC] _Ibid._, p. 88, Table ii.
[AD] Brongniart, "Vegeteaux Fossiles," Plate vi., Figs. 7 to 12.
[AE] "Proceedings of the Vienna Academy," 1881. _Hostinella_, of this
author, is almost certainly _Psilophyton_, and his _Barrandiana_ seems to
include _Arthrostigma_, and perhaps leafy branches of _Berwynia_. These
curious plants should be re-examined.
It is not surprising that great difficulties have occurred in the
determination of fossil Algæ. Enough, however, remains certain to prove
that the old Cambrian and Silurian seas were tenanted with sea-weeds
not very dissimilar from those of the present time. It is further
probable that some of the graphitic, carbonaceous, and bituminous
shales and limestones of the Silurian owe their carbonaceous matters
to the decomposition of Algæ, though possibly some of it may have been
derived from Graptolites and other corneous Zoöphytes. In any case,
such microscopic examinations of these shales as I have made, have not
produced any evidence of the existence of plants of higher grade, while
those of the Erian and Carboniferous periods, similar to the naked eye,
abound in such evidence. It is also to be observed that, on the surfaces
of beds of sandstone in the Upper Cambrian, carbonaceous _débris_, which
seems to be the remains of either aquatic or land plants, is locally not
infrequent.
[Illustration: Fig. 14.--Silurian vegetation restored. _Protannularia_,
_Berwynia_, _Nematophyton_, _Sphenophyllum_, _Arthrostigma_,
_Psilophyton_.]
Referring to the land vegetation of the older rocks, it is difficult to
picture its nature and appearance. We may imagine the shallow waters
filled with aquatic or amphibious Rhizocarpean plants, vast meadows or
brakes of the delicate _Psilophyton_ and the starry _Protannularia_ and
some tall trees, perhaps looking like gigantic club-mosses, or possibly
with broad, flabby leaves, mostly cellular in texture, and resembling
Algæ transferred to the air. Imagination can, however, scarcely realise
this strange and grotesque vegetation, which, though possibly copious and
luxuriant, must have been simple and monotonous in aspect, and, though it
must have produced spores and seeds and even fruits, these were probably
all of the types seen in the modern acrogens and gymnosperms.
"In garments green, indistinct in the twilight,
They stand like Druids of old, with voices sad and prophetic."
Prophetic they truly were, as we shall find, of the more varied forests
of succeeding times, and they may also help us to realise the aspect
of that still older vegetation, which is fossilised in the Laurentian
graphite; though it is not impossible that this last may have been of
higher and more varied types, and that the Cambrian and Silurian may have
been times of depression in the vegetable world, as they certainly were
in the submergence of much of the land.
These primeval woods served at least to clothe the nakedness of the
new-born land, and they may have sheltered and nourished forms of
land-life still unknown to us, as we find as yet only a few insects and
scorpions in the Silurian. They possibly also served to abstract from the
atmosphere some portion of its superabundant carbonic acid harmful to
animal life, and they stored up supplies of graphite, of petroleum, and
of illuminating gas, useful to man at the present day. We may write of
them and draw their forms with, the carbon which they themselves supplied.
* * * * *
NOTE TO CHAPTER II.
Examination of Prototaxites (_Nematophyton_), by Prof. Penhallow, of
McGill University.
Prof. Penhallow, having kindly consented to re-examine my specimens, has
furnished me with elaborate notes of his facts and conclusions, of which
the following is a summary, but which it is hoped will be published in
full:
"1. _Concentric Layers._--The inner face of each of these is composed
of relatively large tubes, having diameters from 13·6 to 34·6
micro-millimetres. The outer face has tubes ranging from 13·8 to 27·6 mm.
The average diameter in the lower surface approaches to 34, that in the
outer to 13·8. There is, however, no abrupt termination to the surface of
the layers, though in some specimens they separate easily, with shining
surfaces.
"2. _Minute Structure._--In longitudinal sections the principal part of
the structure consists of longitudinal tubes of indeterminate length,
and round in cross-section. They are approximately parallel, but in some
cases may be seen to bend sinuously, and are not in direct contact. Finer
myceloid tubes, 5·33 mm. in diameter, traverse the structure in all
directions, and are believed to branch off from the larger tubes. In a
small specimen supposed to be a branch or small stem, and in which the
vertical tubes are somewhat distant from one another, this horizontal
system is very largely developed; but is less manifest in the older
stems. The tubes themselves show no structure. The ray-like openings
in the substance of the tissue are evidently original parts of the
structure, but not of the nature of medullary rays. They are radiating
spaces running outward in an interrupted manner or so tortuously that
they appear to be interrupted in their course from the centre towards
the surface. They show tubes turning into them, branching into them,
and approximately horizontal, but tortuous. On the external surface
of some specimens these radial spaces are represented by minute pits
irregularly or spirally arranged. The transverse swellings of the stem
show no difference of structure, except that the tubes or cells may be a
little more tortuous, and a transverse film of coaly matter extends from
the outer coaly envelope inwardly. This may perhaps be caused by some
accident of preservation. The outer coaly layer shows tubes similar to
those of the stem.[AF] The horizontal or oblique flexures of the large
tubes seem to be mainly in the vicinity of the radial openings, and it is
in entering these that they have been seen to branch."
[AF] It is possible that these tubes may be merely part of the stem
attached to the bark, which seems to me to indicate the same dense
cellular structure seen in the bark of _Lepidodendra_, etc.
The conclusions arrived at by Prof. Penhallow are as follows:
"1. The plant was not truly exogenous, and the appearance of rings
is independent of the causes which determine the layers of growth in
exogenous plants.
"2. The plant was possessed of no true bark. Whatever cortical layer
was present was in all probability a modification of the general
structure,[AG]
[AG] On these points I would reserve the considerations: 1. That there
must have been some relation between the mode of growth of these great
stems and their concentric rings; and, 2. That the evidence of a bark is
as strong as in the case of any Palæozoic tree in which the bark is, as
usual, carbonised.
"3. An intimate relation exists between the large tubular cells and the
myceloid filaments, the latter being a system of small branches from
the former; the branching being determined chiefly in certain special
openings which simulate medullary rays.
"4. The specimens examined exhibit no evidence of special decay, and the
structure throughout is of a normal character.
"5. The primary structure consists of large tubular cells without
apparent terminations, and devoid of structural markings, with which is
associated a secondary structure of myceloid filaments arising from the
former.
"6. The structure of _Nematophyton_ as a whole is unique; at least there
is no plant of modern type with which it is comparable. Nevertheless, the
loose character of the entire structure; the interminable cells; their
interlacing; and, finally, their branching into a secondary series of
smaller filaments, point with considerable force to the true relationship
of the stem as being with Algæ or other Thallophytes rather than with
Grymnosperms. A more recent examination of a laminated resinous
substance found associated with the plant shows that it is wholly
amorphous, and, as indicated by distinct lines of flow, that it must
have been in a plastic state at a former period. The only evidence of
structure was found in certain well-defined mycelia, which may have been
derived from associated vegetable matter upon which they were growing,
and over which the plastic matrix flowed."
I have only to add to this description that when we consider that
_Nematophyton Logani_ was a large tree, sometimes attaining a diameter of
more than two feet, and a stature of at least twenty before branching;
that it had great roots, and gave off large branches; that it was
an aërial plant, probably flourishing in the same swampy flats with
_Psilophyton_, _Arthrostigma_, and _Leptophleum_; that the peculiar
bodies known as Pachytheca were not unlikely its fruit--we have evidence
that there were, in the early Palæozoic period, plants scarcely dreamt
of by modern botany. Only when the appendages of these plants are more
fully known can we hope to understand them. In the mean time, I may state
that there were probably different species of these trees, indicated
more particularly by the stems I have described as _Nematoxylon_ and
_Celluloxylon_[AH] There were, I think, some indications that the plants
described by Carruthers as _Berwynia_, may also be found to have been
generically the same. The resinous matter mentioned by Prof. Penhallow is
found in great abundance in the beds containing _Nematophyton_, and must,
I think, have been an exudation from its bark.
[AH] "Journal Geol. Society of London," 1863, 1881.
CHAPTER III.
THE ERIAN OF DEVONIAN FORESTS--ORIGIN OF PETROLEUM--THE AGE OF ACROGENS
AND GYMNOSPERMS.
In the last chapter we were occupied with the comparatively few and
obscure remains of plants entombed in the oldest geological formations.
We now ascend to a higher plane, that of the Erian or Devonian period, in
which, for the first time, we find varied and widely distributed forests.
The growth of knowledge with respect to this flora has been somewhat
rapid, and it may be interesting to note its principal stages, as
an encouragement to the hope that we may yet learn something more
satisfactory respecting the older floras we have just discussed.
In Goeppert's memoir on the flora of the Silurian, Devonian, and Lower
Carboniferous rocks, published in 1860,[AI] he enumerates twenty species
as Silurian, but these are all admitted to be Algæ, and several of them
are remains which may be fairly claimed by the zoologists as zoophytes,
or trails of worms and mollusks. In the Lower Devonian he knows but six
species, five of which are Algæ, and the remaining one a _Sigillaria_,
but this is of very doubtful nature. In the Middle Devonian he gives but
one species, a land-plant of the genus _Lepidodendron_. In the Upper
Devonian the number rises to fifty-seven, of which all but seven are
terrestrial plants, representing a large number of the genera occurring
in the succeeding Carboniferous system.
[AI] Jena, 1860.
Goeppert does not include in his enumeration the plants from the
Devonian of Gaspé, described by the author in 1859,[AJ] having seen only
an abstract of the paper at the time of writing his memoir, nor does
he appear to have any knowledge of the plants of this age described
by Lesquereux in Roger's "Pennsylvania." These might have added ten
or twelve species to his list, some of them probably from the Lower
Devonian. It is further to be observed that a few additional species had
also been recognised by Peach in the Old Red Sandstone of Scotland.
[AJ] "Journal of the Geological Society of London," also "Canadian
Naturalist."
But from 1860 to the present time a rich harvest of specimens has
been gathered from the Gaspé sandstones, from the shales of southern
New Brunswick, from the sandstones of Perry in Maine, and from the
wide-spread Erian areas of New York, Pennsylvania, and Ohio. Nearly
all these specimens have passed through my hands, and I am now able
to catalogue about a hundred species, representing more than thirty
genera, and including all the great types of vascular Cryptogams, the
Gymnosperms, and even one (still doubtful) Angiosperm. Many new forms
have also been described from the Devonian of Scotland and of the
Continent of Europe.
Before describing these plants in detail, we may refer to North America
for illustration of the physical conditions of the time. In a physical
point of view the northern hemisphere presented a great change in the
Erian period. There were vast foldings of the crust of the earth, and
great emissions of volcanic rock on both sides of the Atlantic. In North
America, while at one time the whole interior area of the continent, as
far north as the Great Lakes, was occupied by a vast inland sea, studded
with coral islands, the long Appalachian ridge had begun to assume,
along with the old Laurentian land, something of the form of our present
continent, and on the margins of this Appalachian belt there were wide,
swampy flats and shallow-water areas, which, under the mild climate that
seems to have characterised this period, were admirably suited to nourish
a luxuriant vegetation. Under this mild climate, also, it would seem that
new forms of plants were first introduced in the far north, where the
long continuance of summer sunlight, along with great warm th, seems to
have aided in their introduction and early extension, and thence made
their way to the southward, a process which, as Gray and others have
shown, has also occurred in later geological times.
The America of this Erian age consisted during the greater part of the
period of a more or less extensive belt of land in the north with two
long tongues descending from it, one along the Appalachian line in the
east, the other in the region west of the Rocky Mountains. On the seaward
sides of these there were low lands covered with vegetation, while on the
inland side the great interior sea, with its verdant and wooded islands,
realised, though probably with shallower water, the conditions of the
modern archipelagoes of the Pacific.
Europe presented conditions somewhat similar, having in the earlier and
middle portions of the period great sea areas with insular patches of
land, and later wide tracts of shallow and in part enclosed water areas,
swarming with fishes, and having an abundant vegetation on their shores.
These were the conditions of the Eifel and Devonshire limestones, and of
the Old Red Sandstone of Scotland, and the Kiltorcan beds of Ireland. In
Europe also, as in America, there were in the Erian age great ejections
of igneous rock. On both sides of the Atlantic there were somewhat varied
and changing conditions of land and water, and a mild and equable
climate, permitting the existence of a rich vegetation in high northern
latitudes. Of this latter fact a remarkable example is afforded by the
beds holding plants of this age in Spitzbergen and Bear Island, in its
vicinity. Here there seem to be two series of plant-bearing strata,
one with the vegetation of the Upper Erian, the other with that of the
Lower Carboniferous, though both have been united by Heer under his
so-called "Ursa Stage" in which he has grouped the characteristic plants
of two distinct periods. This has recently been fully established by the
researches of Nathorst, though the author had already suggested it as the
probable explanation of the strange union of species in the Ursa group of
Heer.
In studying the vegetation of this remarkable period, we must take
merely some of the more important forms as examples, since it would be
impossible to notice all the species, and some of them may be better
treated in the Carboniferous, where they have their headquarters. (Fig.
15.)
I may first refer to a family which seems to have culminated in the Erian
age, and ever since to have occupied a less important place. It is that
of the curious aquatic plants known as Rhizocarps,[AK] and referred to in
the last chapter.
[AK] Or, as they have recently been named by some botanists,
"Heterosporous Filices," though they are certainly not ferns in any
ordinary sense of that term.
My attention was first directed to these organisms by the late Sir W.
E. Logan in 1869. He had obtained from the Upper Erian shale of Kettle
Point, Lake Huron, specimens filled with minute circular discs, to which
he referred, in his report of 1863, as "microscopic orbicular bodies."
Recognising them to be macrospores, or spore-cases, I introduced them
into the report on the Erian flora, which I was then preparing, and
which was published in 1871, under the name _Sporangites Huronensis_.
[Illustration: Fig. 15.--Vegetation of the Devonian period, restored.
_Calamites_, _Psilophyton_, _Leptophleum_, _Lepidodendron_, _Cordaites_,
_Sigillaria_, _Dadoxylon_, _Asterophyllites_, _Platyphyllum_.]
In 1871, having occasion to write a communication to the "American
Journal of Science" on the question then raised as to the share of spores
and spore-cases in the accumulation of coal, a question to be discussed
in a subsequent chapter, these curious little bodies were again
reviewed, and were described in substance as follows:
"The oldest bed of spore-cases known to me is that at Kettle Point, Lake
Huron. It is a bed of brown bituminous shale, burning with much flame,
and under a lens is seen to be studded with flattened disc-like bodies,
scarcely more than a hundredth of an inch in diameter, which under
the microscope are found to be spore-cases (or macrospores) slightly
papillate externally (or more properly marked with dark pores), and
sometimes showing a point of attachment on one side and a slit more or
less elongated and gaping on the other. When slices of the rock are
made, its substance is seen to be filled with these bodies, which,
viewed as transparent objects, appear yellow like amber, and show little
structure, except that the walls can be distinguished from the internal
cavity, which may sometimes be seen to enclose patches of granular
matter. In the shale containing them are also vast numbers of rounded,
translucent granules, which may be escaped spores (microspores)." The
bed containing these spores at Kettle Point was stated, in the reports
of the "Geological Survey of Canada," to be twelve or fourteen feet
in thickness, and besides these specimens it contained fossil plants
referable to the species _Calamites inornatus_ and _Lepidodendron
primævum_, and I not unnaturally supposed that the Sporangites might
be the fruit of the latter plant. I also noticed their resemblance
to the spore-cases of _L. corrugatum_ of the Lower Carboniferous (a
Lepidodendron allied to _L. primævum_), and to those from Brazil
described by Carruthers under the name _Flemingites_, as well as to those
described by Huxley from certain English coals, and to those of the
Tasmanite or white coal of Australia. The bed at Kettle Point is shown to
be marine by its holding the sea-weed known as _Spirophyton_, and shells
of _Lingula_.
The subject did not again come under my notice till 1882, when Prof.
Orton, of Columbus, Ohio, sent me some specimens from the Erian shales
of that State, which on comparison seemed undistinguishable from
_Sporangites Huronensis_.[AL] Prof. Orton read an interesting paper on
these bodies, at the meeting of the American Association in Montreal, in
which were some new and striking facts. One of these was the occurrence
of such bodies throughout the black shales of Ohio, extending "from the
Huron River, on the shore of Lake Brie, to the mouth of the Scioto, in
the Ohio Valley, with an extent varying from ten to twenty miles in
breadth," and estimated to be three hundred and fifty feet in thickness.
I have since been informed by my friend Mr. Thomas, of Chicago, that its
thickness, in some places at least, must be three times that amount.
About the same time. Prof. Williams, of Cornell, and Prof. Clarke, of
Northampton, announced similar discoveries in the State of New York,
so that it would appear that beds of vast area and of great thickness
are replete with these little vegetable discs, usually converted into
a highly bituminous, amber-like substance, giving a more or less
inflammable character to the containing rock.
[AL] These shales have been described, as to their chemical and
geological relations, by Dr. T. Sterry Hunt, "American Journal of
Science," 1863, and by Dr. Newberry, in the "Reports of the Geological
Survey of Ohio," vol. i., 1863, and vol. iii., 1878.
Another fact insisted on by Prof. Orton was the absence of Lepidodendroid
cones, and the occurrence of filamentous vegetable matter, to which the
Sporangites seemed to be in some cases attached in groups. Prof. Orton
also noticed the absence of the trigonal form, which belongs to the
spores of many Lepidodendra, though this is not a constant character. In
the discussion on Prof. Orton's paper, I admitted that the facts detailed
by him shook my previous belief of the lycopodiaceous character of these
bodies, and induced me to suspect, with Prof. Orton, that they might have
belonged to some group of aquatic plants lower than the Lycopods.
Since the publication of my paper on Rhizocarps in the Palæozoic period
above referred to, I have received two papers from Mr. Edward Wethered,
F. G. S., in one of which he describes spores of plants found in the
lower limestone shales of the Forest of Dean, and in the other discusses
more generally the structure and origin of Carboniferous coal-beds.[AM]
In both papers he refers to the occurrence in these coals and shales of
organisms essentially similar to the Erian spores.
[AM] "Cotteswold Naturalists' Field Club," 1884; "Journal of the Royal
Microscopical Society," 1885.
In the "Bulletin of the Chicago Academy of Science," January, 1884, Dr.
Johnson and Mr. Thomas, in their paper on the "Microscopic Organisms
of the Boulder Clay of Chicago and Vicinity," notice _Sporangites
Huronensis_ as among these organisms, and have discovered them also in
large numbers in the precipitate from Chicago city water-supply. They
refer them to the decomposition of the Erian shales, of which boulders
filled with these organisms are of frequent occurrence in the Chicago
clays. The Sporangites and their accompaniments in the boulder clay are
noticed in a paper by Dr. G. M. Dawson, in the "Bulletin of the Chicago
Academy," June, 1885.
Prof. Clarke has also described, in the "American Journal of Science"
for April, 1885, the forms already alluded to, and which he finds to
consist of macrospores enclosed in sporocarps. He compares these with my
_Sporangites Huronensis_ and _Protosalvinia bilobata_, but I think it is
likely that one of them at least is a distinct species.
I may add that in the "Geological Magazine" for 1875, Mr. Newton, F.
G. S., of the Geological Survey of England, published a description
of the Tasmanite and Australian white coal, in which he shows that the
organisms in these deposits are similar to my _Sporangites Huronensis_,
and to the macrospores previously described by Prof. Huxley, from the
Better-bed coal. Mr. Newton does not seem to have been aware of my
previous description of _Sporangites_, and proposes the name _Tasmanites
punctatus_ for the Australian form.
Here we have the remarkable fact that the waste macrospores, or larger
spores of a species of Cryptogamous plant, occur dispersed in countless
millions of tons through the shales of the Erian in Canada and the United
States.
No certain clue seemed to be afforded by all these observations as to
the precise affinities of these widely distributed bodies; but this was
furnished shortly after from an unexpected quarter. In March, 1883, Mr.
Orville Derby, of the Geological Survey of Brazil, sent me specimens
found in the Erian of that country, which seemed to throw a new light on
the whole subject. These I described and pointed out their connection
with _Sporangites_ at the meeting of the American Association at
Minneapolis, in 1883, and subsequently published my notes respecting them
in its proceedings, and in the "Canadian Record of Science."
Mr. Derby's specimens contained the curious spiral sea-weed known as
_Spirophyton_, and also minute rounded Sporangites like those obtained
in the Erian of Ohio, and of which specimens had been sent to me some
years before by the late Prof. Hartt. But they differed in showing the
remarkable fact that these rounded bodies are enclosed in considerable
numbers in spherical and oval sacs, the walls of which are composed of
a tissue of hexagonal cells, and which resemble in every respect the
involucres or spore-sacs of the little group of modern acrogens known as
Rhizocarps, and living in shallow water. More especially they resemble
the sporocarps of the genus _Salvinia_. This fact opened up an entirely
new field of investigation, and I at once proceeded to compare the
specimens with the fructification of modern Rhizocarps, and found that
substantially these multitudinous spores embedded in the Erie shales may
be regarded as perfectly analogous to the larger spores of the modern
_Salvinia natans_ of Europe, as may be seen by the representation of them
in Fig. 16.
[Illustration: Fig. 16.--_Sporangites_ (_Protosalvinia_). A, _Sporangites
Braziliensis_, natural size, AX, Same, magnified, B, _Sp. biloba_,
natural size, C, Detached macrospores. D, Spore-cases of Salvinia natans.
DX, Same, magnified. E, Shale with sporangites, vertical section, highly
magnified.]
The typical macrospores from the Erian shales are perfectly circular
in outline, and in the flattened state appear as discs with rounded
edges, their ordinary diameter being from one seventy-fifth to one
one-hundredth of an inch, though they vary considerably in size. This,
however, I do not regard as an essential character. The edges, as seen
in profile, are smooth, but the flat surface often presents minute dark
spots, which at first I mistook for papillæ, but now agree with Mr.
Thomas in recognising them as minute pores traversing the wall of the
disc, and similar to those which Mr. Newton has described in Tasmanite,
and which Mr. Wethered has also recognised in the similar spores of the
Forest of Dean shales. The walls also sometimes show faint indications
of concentric lamination, as if they had been thickened by successive
deposits.
As seen by transmitted light, and either in front or in profile, the
discs are of a rich amber colour, translucent and structureless, except
the pores above referred to. The walls are somewhat thick, or from
one-tenth to one-twentieth the diameter of the disc in thickness. They
never exhibit the triradiate marking seen in spores of Lycopods, nor
any definite point of attachment, though they sometimes show a minute
elongated spot which may be of this nature, and they are occasionally
seen to have opened by slits on the edge or front, where there would seem
to have been a natural line of dehiscence. The interior is usually quite
vacant or structureless, but in some cases there are curved internal
markings which may indicate a shrunken lining membrane, or the remains of
a prothallus or embryo. Occasionally a fine granular substance appears in
the interior, possibly remains of microspores.
The discs are usually detached and destitute of any envelope, but
fragments of flocculent cellular matter are associated with them, and
in one specimen from the corniferous limestone of Ohio, in Mr. Thomas's
collection, I have found a group of eight or more discs partly enclosed
in a cellular sac-like membrane of similar character to that enclosing
the Brazilian specimens already referred to.
The characters of all the specimens are essentially similar, and
there is a remarkable absence of other organisms in the shale. In one
instance only, I have observed a somewhat smaller round body with a
dark centre or nucleus, and a wide translucent margin, marked by a
slight granulation. Even this, however, may indicate nothing more than a
different state of preservation.
It is proper to observe here that the wall or enclosing sac of these
macrospores must have been of very dense consistency, and now appears
as a highly bituminous substance, in this agreeing with that of the
spores of Lycopods, and, like them, having been when recent of a highly
carbonaceous and hydrogenous quality, very combustible and readily
admitting of change into bituminous matter. In the paper already referred
to, on spore-cases in coals, I have noticed that the relative composition
of lycopodium and cellulose is as follows:
Cellulose, C{24}H{20}O{20}.
Lycopodium, C{42}H{19-4/12}NO{5-6/10}.
Thus, such spores are admirably suited for the production of highly
carbonaceous or bituminous coals, etc.
Nothing is more remarkable in connection with these bodies than their
uniformity of structure and form over so great areas and throughout so
great thickness of rock, and the absence of any other kind of spore-case.
This is more especially noteworthy in contrast with the coarse coals and
bituminous shales of the Carboniferous, which usually contain a great
variety of spores and sporangia, indicating the presence of many species
of acrogenous plants, while the Erian shales, on the contrary, indicate
the almost exclusive predominance of one form. This contrast is well
seen in the Bedford shales overlying these beds, and I believe Lower
Carboniferous.[AN] Specimens of these have been kindly communicated to me
by Prof. Orton, and have been prepared by Mr. Thomas. In these we see the
familiar Carboniferous spores with triradiate markings called _Triletes_
by Reinsch, and which are similar to those of Lycopodiaceous plants.
Still more abundant are those spinous and hooked spores or sporangia,
to which the names _Sporocarpon_, _Zygosporites_, and _Traquaria_ have
been given, and some of which Williamson has shown to be spores of
Lycopodiaceous plants.[AO]
[AN] According to Newberry, lower part of Waverly group.
[AO] _Traquaria_ is to be distinguished from the calcareous bodies found
in the corniferous limestone of Kelly's Island, which I have described
in the "Canadian Naturalist" as _Saccamina Eriana_, and believe to be
Foraminiferal tests. They have since been described by Ulrich under a
different name (_Moellerina_: contribution to "American Palæontology,"
1886). See Dr. Williamson's papers in "Transactions of Royal Society of
London."
The true "Sporangites," on the contrary, are round and smooth, with thick
bituminous walls, which are punctured with minute transverse pores. In
these respects, as already stated, they closely resemble the bodies found
in the Australian white coal and Tasmanite. The precise geological age of
this last material is not known with certainty, but it is believed to be
Palæozoic.
With reference to the mode of occurrence of these bodies, we may note
first their great abundance and wide distribution. The horizontal range
of the bed at Kettle Point is not certainly known, but it is merely a
northern outlier of the great belt of Erian shales referred to by Prof.
Orton, and which extends, with a breadth of ten to twenty miles, and of
great thickness, across the State of Ohio, for nearly two hundred miles.
This Ohio black shale, which lies at the top of the Erian or the base of
the Carboniferous, though probably mainly of Erian age, appears to abound
throughout in these organisms, and in some beds to be replete with them.
In like manner, in Brazil, according to Mr. Derby, these organisms are
distributed over a wide area and throughout a great thickness of shale
holding _Spirophyton_, and apparently belonging to the Upper Erian. The
recurrence of similar forms in the Tasmanite and white coal of Tasmania
and Australia is another important fact of distribution. To this we
may add the appearance of these macrospores in coals and shales of the
Carboniferous period, though there in association with other forms.
It is also to be observed that the Erian shales, and the Forest of Dean
beds described by Wethered, are marine, as shown by their contained
fossils; and, though I have no certain information as to the Tasmanite
and Australian white coal, they would seem, from the description of
Milligan, to occur in distinctly aqueous, possibly estuarine, deposits.
Wethered has shown that the discs described by Huxley and Newton in
the Better-bed coal occur in the earthy or fragmentary layers, as
distinguished from the pure coal. Those occurring in cannel coal are
in the same case, so that the general mode of occurrence implies
water-driftage, since, in the case of bodies so large and dense,
wind-driftage to great distances would be impossible.
These facts, taken in connection with the differences between these
macrospores and those of any known land-plant of the Palæozoic, would
lead to the inference that they belonged to aquatic plants, and these
vastly abundant in the waters of the Erian and Carboniferous periods.
It is still further to be observed that they are not, in the Erian beds,
accompanied with any remains of woody or scalariform tissues, such
as might be expected in connection with the _débris_ of terrestrial
acrogens, and that, on the other hand, we find them enclosed in cellular
sporocarps, though in the majority of cases these have been removed by
dehiscence or decay.
These considerations, I think, all point to the probability which I
have suggested in my papers on this subject referred to above, that we
have in these objects the organs of fructification of plants belonging
to the order _Rhizocarpeæ_, or akin to it. The comparisons which I have
instituted with the sporocarps and macrospores of these plants confirm
this suggestion. Of the modern species which I have had an opportunity
to examine, _Salvinia natans_ of Europe perhaps presents the closest
resemblance. In this plant groups of round cellular sporocarps appear
at the bases of the floating fronds. They are about a line in diameter
when mature, and are of two kinds, one containing macrospores, the
other microspores or antheridia. The first, when mature, hold a number
of closely packed globular or oval sporangia of loose cellular tissue,
attached to a central placenta. Each of these sporangia contains a single
macrospore, perfectly globular and smooth, with a dense outer membrane
(exhibiting traces of lamination, and showing within an irregularly
vacuolated or cellular structure, probably a prothallus). I cannot detect
in it the peculiar pores which appear in the fossil specimens. Each
macrospore is about one-seventieth of an inch in diameter when mature.
The sporocarps of the microspores contain a vastly greater number of
minute sporangia, about one two-hundredths of an inch in diameter. These
contain disc-like antheridia, or microspores of very minute size.
The discs from Kettle Point and from the Ohio black shale, and from the
shale boulders of the Chicago clays, are similar to the macrospores of
_Salvinia_, except that they have a thicker wall and are a little less in
diameter, being about one-eightieth of an inch. The Brazilian sporocarps
are considerably larger than those of the modern _Salvinia_, and the
macrospores approach in size to those of the modern species, being one
seventy-fifth of an inch in diameter. They also seem, like the modern
species, to have thinner walls than those from Canada, Ohio, and Chicago.
No distinct indication has been observed in the fossil species of the
inner Sporangium of _Salvinia_. Possibly it was altogether absent, but
more probably it is not preserved as a distinct structure.
With reference to the microspores of _Salvinia_, it is to be observed
that the sporocarps, and the contained spores or antheridia, are very
delicate and destitute of the dense outer wall of the macrospores.
Hence such parts are little likely to have been preserved in a fossil
state; and in the Erian shales, if present, they probably appear merely
as flocculent carbonaceous matter not distinctly marked, or as minute
granules not well defined, of which there are great quantities in some of
the shales.
The vegetation appertaining to the Sporangites has not been distinctly
recognised. I have, however, found in one of the Brazilian specimens two
sporocarps attached to what seems a fragment of a cellular frond, and
numerous specimens of the supposed Algæ, named _Spirophyton_, are found
in the shales, but there is no evidence of any connection of this plant
with the _Protosalvinia_.
Modern Rhizocarps present considerable differences as to their vegetative
parts. Some, like _Pilularia_, have simple linear leaves; others, like
_Marsilea_, have leaves in whorls, and cuneate in form; while others,
like _Azolla_ and _Salvinia_, have frondose leaves, more or less pinnate
in their arrangement. If we inquire as to fossils representing these
forms of vegetation, we shall find that some of the plants to be noticed
in the immediate sequel may have been nearly allied to the Rhizocarps.
In the mean time I may state that I have proposed the generic name
_Protosalvinia_ for these curious macrospores and their coverings, and
have described in the paper in the "Bulletin of the Chicago Academy of
Sciences," already quoted, five species which may be referred to this
genus.
These facts lead to inquiries as to the origin of the bituminous matter
which naturally escapes from the rocks of the earth as petroleum and
inflammable gas, or which may be obtained from certain shales in
these forms by distillation. These products are compounds of carbon
and hydrogen, and may be procured from recent vegetable substances by
destructive distillation. Some vegetable matters, also, are much richer
in carbon and hydrogen than others, and it is a remarkable fact that
the spores of certain cryptogamous plants are of this kind, as we see in
the inflammable character of the dry spores of Lycopodium; and we know
that the slow putrefaction of such material underground effects chemical
changes by which bituminous matter can be produced. There is, therefore,
nothing unreasonable in the supposition advanced by Prof. Orton, that the
spores so abundantly contained in the Ohio black shales are important
or principal sources of the bituminous matter which they contain.
Microscopic sections of this shale show that much of its material
consists of the rich bituminous matter of these spores (Fig. 16). At the
same time, while we may trace the bitumen of these shales, and of some
beds of coal, to this cause, we must bear in mind that there are other
kinds of bituminous rocks which show no such structures, and may have
derived their combustible material from other kinds of vegetable matter,
whether of marine or of land plants. We shall better understand this when
we have considered the origin of coal.
The macrospores above referred to may have belonged to humble aquatic
plants mantling the surfaces of water or growing up from the bottom, and
presenting little aërial vegetation. But there are other Erian plants,
as already mentioned, which, while of higher structure, may be of
Rhizocarpean affinities.
One of these is the beautiful plant with whorls of wedge-shaped leaves,
to which the name _Sphenophyllum_ (see Fig. 20) has been given. Plants
referred to this genus have been described by Lesquereux from the upper
part of the Siluro-Cambrian,[AP] and a beautiful little species occurs
in the Erian shales of St. John, New Brunswick.[AQ] The genus is also
continued, and is still more abundant, in the Carboniferous. Many years
ago I observed, in a beautiful specimen collected by Sir W. E. Logan, in
New Brunswick, that the stem of this plant had an axis of reticulated and
scalariform vessels, and an outer bark.[AR] Renault and Williamson have
more recently obtained more perfect specimens, and the former has figured
a remarkably complex triangular axis, containing punctate and barred
vessels, and larger punctate vessels filling in its angles. Outside of
this there is a cellular inner bark, and this is surrounded by a thick
fibrous envelope. That a structure so complex should belong to a plant so
humble in its affinities is one of the strange anomalies presented by the
old world, and of which we shall find many similar instances. The fruit
of _Sphenophyllum_ was borne in spikes, with little whorls of bracts or
rudimentary leaves bearing round sporocarps.
[AP] "American Journal of Science."
[AQ] Dawson, "Report on Devonian Plants," 1870.
[AR] "Journal of the Geological Society," 1865.
[Illustration: Fig. 17.--_Ptilophyton plumosum_ (Lower Carboniferous,
Nova Scotia). Natural size and magnified.]
A second type of plant, which may have been Rhizocarpean in its
affinities, is that to which I have given the name _Ptilophyton_.[AS]
It consists of beautiful feathery fronds, apparently bearing on parts
of the main stem or petiole small rounded sporocarps. They are found
abundantly in the Middle Erian of the State of New York, and also occur
in Scotland, while one species appears to occur in Nova Scotia, as high
as the Lower Carboniferous (Figs. 17, 18).
[AS] _Plumalina_ of Hall.
[Illustration: Fig. 18.--_Ptilophyton Thomsoni_ (Scotland), _a_,
Impression of plant in vernation, _b_, Branches conjecturally restored,
_c_, Branches of _Lycopodites Milleri_, on same slab.]
These organisms have been variously referred to Lycopods, to Algæ, or to
Zoöphytes, but an extended comparison of American and Scottish specimens
has led me to the belief that they were aquatic plants, more likely to
have been allied to Rhizocarps than to any other group. Some evidence of
this will be given in a note appended to this chapter.
[Illustration: Fig. 19.--_Psilophyton princeps_, restored (Lower
Erian, Gaspé). _a_, Fruit, natural size. _b_, stem, natural size, _c_,
Scalariform tissue of the axis, highly magnified. In the restoration, one
side is represented in vernation and the other in fruit.]
Another genus, which I have named _Psilophyton_[AT] (Figs. 19, 21), may
be regarded as a connecting link between the Rhizocarps and the Lycopods.
It is so named from its resemblance, in some respects, to the curious
parasitic Lycopods placed in the modern genus _Psilotum_. Several species
have been described, and they are eminently characteristic of the Lower
Erian, in which they were first discovered in Gaspé. The typical species,
_Psilophyton princeps_, which fills many beds of shale and sandstone in
Gaspé Bay and the head of the neighbouring Bay des Chaleurs with its
slender stems and creeping, cord-like rhizomes, may be thus described:
[AT] "Journal of the Geological Society," vols, xv., xviii., and xix.,
"Report on Devonian Plants of Canada," 1871.
Stems branching dichotomously, and covered with interrupted ridges.
Leaves rudimentary, or short, rigid, and pointed; in barren stems,
numerous and spirally arranged; in fertile stems and branchlets, sparsely
scattered or absent; in decorticated specimens, represented by a minute
punctate scars. Young branches circinate; rhizomata cylindrical, covered
with hairs or ramenta, and having circular areoles irregularly disposed,
giving origin to slender cylindrical rootlets. Internal structure--an
axis of scalariform vessels, surrounded by a cylinder of parenchymatous
cells, and by an outer cylinder of elongated woody cells. Fructification
consisting of naked oval spore-cases, borne usually in pairs on slender,
curved pedicels, either lateral or terminal.
[Illustration: Fig. 20.--_Sphenophyllum antiquum_ (Erian, New
Brunswick). See pp. 61, 67.]
This species was fully described by me in the papers referred to above,
from specimens obtained from the rich exposures at Gaspé Bay, and which
enabled me to illustrate its parts more fully, perhaps, than those of any
other species of so great antiquity. In the specimens I had obtained I
was able to recognise the forms of the rhizomata, stems, branches, and
rudimentary leaves, and also the internal structure of the stems and
rhizomata, and to illustrate the remarkable resemblance of the forms and
structures to those of the modern _Psilotum_. The fructification was,
however, altogether peculiar, consisting of narrowly ovate sporangia,
borne usually in pairs, on curved and apparently rigid petioles. Under
the microscope these sporangia show indications of cellular structure,
and appear to have been membranous in character. In some specimens
dehiscence appears to have taken place by a slit in one side, and, clay
having entered into the interior, both walls of the spore-case can be
seen. In other instances, being flattened, they might be mistaken for
scales. No spores could be observed in any of the specimens, though in
some the surface was marked by slight, rounded prominences, possibly the
impressions of the spores within. This peculiar and very simple style
of spore-case is also characteristic of other species, and gives to
_Psilophyton_ a very distinct generic character. These naked spore-cases
may be compared to those of such lycopodiaceous plants as _Psilotum_,
in which the scales are rudimentary. They also bear some resemblance,
though on a much larger scale, to the spore-cases of some Erian ferns
(_Archæopteris_), to be mentioned in the sequel. On the whole, however,
they seem most nearly related to the sporocarps of the Rhizocarpeæ.
[Illustration: Fig. 21.--_Lepidodendron_ and _Psilophyton_ (Erian, New
Brunswick). A, _Lepidodendron Gaspianum_. B, C, _Psilophyton elegans_.]
_Arthrostigma_, which is found in the same beds with Psilophyton, was a
plant of more robust growth, with better-developed, narrow, and pointed
leaves, borne in a verticillate or spiral manner, and bearing at the
ends of its branches spikes of naked sporocarps, apparently similar to
those of _Psilophyton_ but more rounded in form. The two genera must
have been nearly related, and the slender branchlets of _Arthrostigma_
are, unless well preserved, scarcely distinguishable from the stems of
_Psilophyton_.[AU]
[AU] Reports of the author on "Devonian Plants," "Geological Survey of
Canada," which see for details as to Erian Flora of northeastern America.
If, now, we compare the vegetation of these and similar ancient plants
with that of modern Rhizocarps, we shall find that the latter still
present, though in a depauperated and diminished form, some of the
characteristics of their predecessors. Some, like _Pilularia_, have
simple linear leaves; others, like _Marsilea_, have leaves in verticils
and cuneate in form; while others, like Azolla and Salvinia, have
frondose leaves, more or less pinnate in their arrangement. The first
type presents little that is characteristic, but there are in the
Erian sandstones and shales great quantities of filamentous and linear
objects which it has been impossible to refer to any genus, and which
might have belonged to plants of the type of _Pilularia_. It is quite
possible, also, that such plants as _Psilophyton glabrum_ and _Cordaites
angustifolia_, of which the fructification is quite unknown, may have
been allied to Rhizocarps. With regard to the verticillate type, we are
at once reminded of _Sphenophyllum_ (Fig. 20), which many palæobotanists
have referred to the _Marsiliacæ_, though, like other Palæozoic Acrogens,
it presents complexities not seen in its modern representatives. _S.
primævum_ of Lesquereux is found in the Hudson River group, and my
_S. antiquum_ in the Middle Erian. Besides these, there are in the
Silurian and Erian beds plants with verticillate leaves which have
been placed with the Annulariæ, but which may have differed from them
in fructification. _Annularia laxa_, of the Erian, and _Protannularia
Harknessii_, of the Siluro-Cambrian, may be given as examples, and must
have been aquatic plants, probably allied to Rhizocarps. It is deserving
of notice, also, that the two best-known species of _Psilophyton_
(_P. princeps_ and _P. robustius_), while allied to Lycopods by the
structure of the stem and such rudimentary foliage as they possess, are
also allied, by the form of their fructification, to the Rhizocarps,
and not to ferns, as some palæobotanists have incorrectly supposed. A
similar remark applies to _Arthrostigma_; and the beautiful pinnately
leaved _Ptilophyton_ may be taken to represent that type of foliage as
seen in modern Rhizocarps, while the allied forms of the Carboniferous
which Lesquereux has named _Trochophyllum_, seem to have had sporocarps
attached to the stem in the manner of _Azolla_.
The whole of this evidence, I think, goes to show that in the Erian
period there were vast quantities of aquatic plants, allied to the modern
Rhizocarps, and that the so-called _Sporangites_ referred to in this
paper were probably the drifted sporocarps and macrospores of some of
these plants, or of plants allied to them in structure and habit, of
which the vegetative organs have perished. I have shown that in the Erian
period there were vast swampy flats covered with _Psilophyton_, and in
similar submerged tracts near to the sea the _Protosalvinia_ may have
filled the waters and have given off the vast multitudes of macrospores
which, drifted by currents, have settled in the mud of the black shales.
We have thus a remarkable example of a group of plants reduced in modern
times to a few insignificant forms, but which played a great role in the
ancient Palæozoic world.
Leaving the Rhizocarps, we may now turn to certain other families of
Erian plants. The first to attract our attention in this age would
naturally be the Lycopods, the club-mosses or ground-pines, which in
Canada and the Eastern States carpet the ground in many parts of our
woods, and are so available for the winter decoration of our houses
and public buildings. If we fancy one of these humble but graceful
plants enlarged to the dimensions of a tree, we shall have an idea of
a _Lepidodendron_, or of any of its allies (Figs. 15, 21). These large
lycopodiaceous trees, which in different specific and generic forms were
probably dominant in the Erian woods, resembled in general those of
modern times in their fruit and foliage, except that their cones were
large, and probably in most cases with two kinds of spores, and their
leaves were also often very long, thus bearing a due proportion to the
trees which they clothed. Their thick stems required, however, more
strength than is necessary in their diminutive successors, and to meet
this want some remarkable structures were introduced similar to those
now found only in the stems of plants of higher rank. The cells and
vessels of all plants consist of thin walls of woody matter, enclosing
the sap and other contents of these sacs and tubes, and when strength is
required it is obtained by lining their interior with successive coats
of the hardest form of woody matter, usually known as lignin. But while
the walls remain thin, they afford free passage to the sap to nourish
every part. If thickened all over, they would become impervious to sap,
and therefore unsuited to one of their most important functions. These
two ends of strength and permeability are secured by partial linings of
lignin, leaving portions of the original wall uncovered. But this may be
done in a great variety of ways.
The most ancient of these contrivances, and one still continued in the
world of plants, is that of the barred or scalariform vessel. This may
be either square or hexagonal, so as to admit of being packed without
leaving vacancies. It is strengthened by a thick bar of ligneous matter
up each angle, and these are connected by cross-bars so as to form a
framework resembling several ladders fastened together. Hence the name
_scalariform_, or ladder-like. Now, in a modern Lycopod there is a
central axis of such barred vessels associated with simpler fibres or
elongated cells. Even in _Sphenophyllum_ and _Psilophyton_, already
referred to as allied to Rhizocarps,[AV] there is such a central axis,
and in the former rigidity is given to this by the vascular and woody
elements being arranged in the form of a three-sided prism or three-rayed
star. But such arrangements would not suffice for a tree, and hence
in the arboreal Lycopods of the Erian age a more complex structure is
introduced. The barred vessels were expanded in the first instance into
a hollow cylinder filled in with pith or cellular tissue, and the outer
rind was strengthened with greatly thickened cells. But even this was not
sufficient, and in the older stems wedge-shaped bundles of barred tissue
were run out from the interior, forming an external woody cylinder, and
inside of the rind were placed bundles of tough bast fibres. Thus, a stem
was constructed having pith, wood, and bark, and capable of additions to
the exterior of the woody wedges by a true exogenous growth. The plan
is, in short, the same with that of the stems of the exogenous trees of
modern times, except that the tissues employed are less complicated.
The structures of these remarkable trees, and the manner in which
they anticipate those of the true exogens of modern times, have been
admirably illustrated by Dr. Williamson, of Manchester. His papers, it
is true, refer to these plants as existing in the Carboniferous age, but
there is every reason to believe that they were of the same character
in the Erian. The plan is the same with that now seen in the stems of
exogenous phænogams, and which has long ceased to be used in those of the
Lycopods. In this way, however, large and graceful lycopodiaceous trees
were constructed in the Erian period, and constituted the staple of its
forests.
[AV] First noticed by the author, "Journal of Geological Society," 1865;
but more completely by Renault, "Comptes Rendus," 1870.
The roots of these trees were equally remarkable with their stems, and
so dissimilar to any now existing that botanists were long disposed to
regard them as independent plants rather than roots. They were similar
in general structure to the stems to which they belonged, but are
remarkable for branching in a very regular manner by bifurcation like
the stems above, and for the fact that their long, cylindrical rootlets
were arranged in a spiral manner and distinctly articulated to the root
after the manner of leaves rather than of rootlets, and fitting them for
growing in homogeneous mud or vegetable muck. They are the so-called
_Stigmaria_ roots, which, though found in the Erian and belonging to
its lycopodiaceous plants, attained to far greater importance in the
Carboniferous period, where we shall meet with them again.
There were different types of lycopodiaceous plants in the Erian. In
addition to humble Lycopods like those of our modern woods and great
Lepidodendra, which were exaggerated Lycopods, there were thick-stemmed
and less graceful species with broad rhombic scars (_Leptophleum_), and
others with the leaf-scars in vertical rows (_Sigillaria_), and others,
again, with rounded leaf-scars, looking like the marks on Stigmaria, and
belonging to the genus _Cyclostigma_. Thus some variety was given to the
arboreal club-mosses of these early forests. (See Fig. 15.)
[Illustration: Fig. 22.--Erian ferns (New Brunswick), A, _Aneimites
obtusa_. C, _Neuropteris polymorpha_. F, _Sphenopteris pilosa_. N,
_Hymenophyllites subfurcatus_.]
Another group of plants which attained to great development in the Erian
age is that of the Ferns or Brackens. The oldest of these yet known
are found in the Middle Erian. The _Eopteris_ of Saporta, from the
Silurian, at one time supposed to carry this type much further back,
has unfortunately been found to be a mere imitative form, consisting of
films of pyrites of leaf-like shapes, and produced by crystallisation.
In the Middle Erian, however, more especially in North America, many
species have been found (Figs. 22 to 24).[AW] I have myself recorded
more than thirty species from the Middle Erian of Canada, and these
belong to several of the genera found in the Carboniferous, though
some are peculiar to the Erian. Of the latter, the best known are
perhaps those of the genus _Archæopteris_ (Fig. 24), so abundant in the
plant-beds of Kiltorcan in Ireland, as well as in North America. In this
genus the fronds are large and luxuriant, with broad obovate pinnules
decurrent, on the leaf-stalk, and with simple sac-like spore-cases
borne on modified pinnæ. Another very beautiful fern found with
_Archæopteris_ is that which I have named _Platyphyllum_, and which grew
on a creeping stem or parasitically on stems of other plants, and had
marginal fructification.[AX] Another very remarkable fern, which some
botanists have supposed may belong to a higher group than the ferns, is
Megalopteris (Fig. 26).
[AW] For descriptions of these ferns, see reports cited above.
[AX] "Reports on Fossil Plants of the Devonian and Upper Silurian of
Canada," 1871, &c.
[Illustration: Fig. 23.--Erian ferns (New Brunswick), B, _Cyclopteris
valida_, and pinnule enlarged, D, _Sphenopteris marginata_, and portion
enlarged. E, _Sphenopteris Harttii_. G, _Hymenophyllites curtilobus_. H,
_Hymenophyllites Gersdorffii_, and portion enlarged. I, _Alethopteris
discrepans_, K, _Pecopteris serrulata_, L, _Pecopteris preciosa_. M,
_Alethopteris Perleyi_.]
[Illustration: Fig. 24.--_Archæopteris Jacksoni_, Dawson (Maine). An
Upper Erian fern, _a_, _b_, Pinnules showing venation.]
[Illustration: Fig. 25.--An Erian tree-fern. _Caulopteris Lockwoodi_,
Dawson, reduced. (From a specimen from Gilboa, New York.)]
Some of the Erian ferns attained to the dimensions of tree-ferns. Large
stems of these, which must have floated out far from land, have been
found by Newberry in the marine limestone of Ohio (_Caulopteris antiqua_
and _C. peregrina_, Newberry),[AY] and Prof. Hall has found in the Upper
Devonian of Gilboa, New York, the remains of a forest of tree-ferns
standing _in situ_ with their great masses of aërial roots attached to
the soil in which they grew (_Caulopteris Lockwoodi_, Dn.).[AZ]
[AY] "Journal of the Geological Society," 1871.
[AZ] _Ibid._
[Illustration: Fig. 26.--_Megalopteris Dawsoni_, Hartt (Erian, New
Brunswick), _a_, Fragment of pinna. _b_, Point of pinnule, _c_, Venation,
(The midrib is not accurately given in this figure.)]
These aërial roots introduce us to a new contrivance for strengthening
the stems of plants by sending out into the soil multitudes of cord-like
cylindrical roots from various heights on the stem, and which form
a series of stays like the cordage of a ship. This method of support
still continues in the modern tree-ferns of the tropics and the southern
hemisphere. In one kind of tree-fern stem from the Erian of New York,
there is also a special arrangement for support, consisting of a
series of peculiarly arranged radiating plates of scalariform vessels,
not exactly like those of an exogenous stem, but doing duty for it
(_Asteropteris_)[BA] Similar plants have been described from the Erian of
Falkenberg, in Germany, and of Saalfeld, in Thuringia, by Goeppert and
Unger, and are referred to ferns by the former, but treated as doubtful
by the latter,[BB] This peculiar type of tree-fern is apparently a
precursor of the more exogenous type of _Heterangium_, recently described
and referred to ferns by Williamson. Here, again, we have a mechanical
contrivance now restricted to higher plants appropriated by these old
cryptogams.
[BA] "Journal of the Geological Society," London, 1881.
[BB] "Sphenopteris Refracta," Goeppert; "Flora des Uebergangsgebirges."
"Cladoxylon Mirabile," Unger; "Palæontologie des Thuringer Waldes."
[Illustration: Fig. 27.--_Calamites radiatus_ (Erian, New Brunswick).]
The history of the ferns in geological time is remarkably different
from that of the Lycopods; for while the latter have long ago descended
from their pristine eminence to a very humble place in nature, the
former still, in the southern hemisphere at least, retain their arboreal
dimensions and ancient dominance.
[Illustration: Fig. 28.--Asterophyllites (Erian, New Brunswick), A,
Asterophyllites latifolia. B, Do., apex of stem (?) fruit, C,
C^1, _A. scutigera_. D, _A. latifolia_, larger whorl of leaves. D^1,
Leaf.]
The family of the _Equisetaceæ_, or mare's-tails, was also represented
by large species of _Calamites_ and by _Asterophyllites_ in the Erian;
but, as its headquarters are in the Carboniferous, we may defer its
consideration till the next chapter. (Figs. 27, 28.)
Passing over these for the present, we find that the flowering plants are
represented in the Erian forests by at least two types of Gymnosperms,
that of _Taxineæ_ or yews, and an extinct family, that of the _Cordaites_
(Figs. 30, 31). The yew-trees are closely allied to the pines and
spruces, and are often included with them in the family of _Coniferæ_.
They differ, however, in the habit of producing berries or drupe-like
fruits instead of cones, and there is some reason to believe that this
was the habit of the Erian trees of this group, though their wood in
some instances resembles rather that of the Araucaria, or Norfolk
Island pine, than that of the modern yews. These trees are chiefly
known to us by their mineralised trunks, which are often found like
drift-wood on modern sand-banks embedded in the Erian sandstones or
limestones. It often shows its structure in the most perfect manner in
specimens penetrated by calcite or silica, or by pyrite, and in which
the original woody matter has been resolved into anthracite or even into
graphite. These trees have true woody tissues presenting that beautiful
arrangement of pores or thin parts enclosed in cup-like discs, which is
characteristic of the coniferous trees, and which is a great improvement
on the barred tissue already referred to, affording a far more strong,
tough, and durable wood, such as we have in our modern pines and yews
(Fig. 29).
[Illustration: Fig. 29.--_Dadoxylon Ouangondianum_, an Erian conifer, A,
Fragment showing Sternberg pith and wood; _a_, medullary sheath; _b_,
pith; _c_, wood; _d_, section of pith, B, Wood-cell; _a_, hexagonal
areole; _b_, pore, _c_, Longitudinal section of wood, showing, _a_,
areolation, and _b_, medullary rays, D, Transverse section, showing,
_a_, wood-cells, and _b_, limit of layer of growth, (B, C, D, highly
magnified.)]
These primitive pines make their appearance in the Middle Erian, in
various parts of America, as well as in Scotland and Germany, and they
are represented by wood indicating the presence of several species. I
have myself indicated and described five species from the Erian of Canada
and the United States. From the fact that these trees are represented
by drifted trunks embedded in sandstones and marine limestones, we may,
perhaps, infer that they grew on the rising grounds of the Erian land,
and that their trunks were carried by river-floods into the sea. No
instance has yet certainly occurred of the discovery of their foliage or
fruit, though there are some fan-shaped leaves usually regarded as ferns
which may have belonged to such trees. These in that case would have
resembled the modern _Gingko_ of China, and some of the fruits referred
to the genus _Cardiocarpum_ may have been produced by them. Various
names have been given to these trees. I have preferred that given by
Unger, _Dadoxylon_, as being more non-committal as to affinities than the
others.[BC] Many of these trees had very long internal pith-cylinders,
with curious transverse tubulæ, and which, when preserved separately,
have been named _Sternbergia_.
[BC] _Araucarites_, Goeppert; _Araucarioxylon_, Kraus.
Allied to these trees, and perhaps intermediate between them and the
_Cycads_, were those known as _Cordaites_ (Fig. 30), which had trunks
resembling those of _Dadoxylon_, but with still larger _Sternbergia_
piths and an internal axis of scalariform vessels, surrounded by a
comparatively thin woody cylinder. Some of them have leaves over a
foot in length, reminding one of the leaves of broad-leaved grasses or
iridaceous plants. Yet their flowers and fruit seem to have been more
nearly allied to the yews than to any other plants (Fig. 31). Their stems
were less woody and their piths larger than in the true pines, and some
of the larger-leaved species must have had thick, stiff branches. They
are regarded as constituting a separate family, intermediate between
pines and cycads, and, beginning in the Middle Devonian, they terminate
in the Permian, where, however, some of the most gigantic species occur.
In so far as the form and structure of the leaves, stems, and fruit are
concerned, there is marvellously little difference between the species
found in the Erian and the Permian. They culminated, however, in the
Carboniferous period, and the coal-fields of southern France have proved
so far the richest in their remains.
[Illustration: Fig. 30.--_Cordaites Robbii_ (Erian, New Brunswick),
_a_, Group of young leaves. _b_, Point of leaf, _c_, Base of leaf, _d_.
Venation, magnified.]
[Illustration: Fig. 31.--Erian fruits, &c., some gymnospermous, and
probably of _Cordaites_ and Taxine trees (St. John, New Brunswick), A,
_Cardiocarpum cornutum_. B, _Cardiocarpum acutum_. C, _Cardiocarpum
Crampii_. D, _Cardiocarpum Baileyi_. E, _Trigonocarpum racemosum_.
E^1, E^2, Fruits enlarged, F, _Antholithes Devonicus_. G, Annularia
acuminata, H, _Asterophyllites acicularis_. H^2, Fruit of the same, K,
_Cardiocarpum_ (? young of _A._), L, _Pinnularia dispalans_ (probably a
root).]
Lastly, a single specimen, collected by Prof. James Hall, of Albany,
at Eighteen-mile Creek, Lake Erie, has the structure of an ordinary
angiospermous exogen, and has been described by me as _Syringoxylon
mirabile_.[BD] This unique example is sufficient to establish the fact
of the existence of such plants at this early date, unless some accident
may have carried a specimen from a later formation to be mixed with
Erian fossils. It is to be observed, however, that the non-occurrence of
any similar wood in all the formations between the Upper Erian and the
Middle Cretaceous suggests very grave doubt as to the authenticity of the
specimen. I record the fact, waiting further discoveries to confirm it.
Of the character of the specimen which I have described I entertain no
doubt.
[BD] "Journal of the Geological Society," vol. xviii.
We shall be better able to realise the significance and relations of this
ancient flora when we have studied that of the succeeding Carboniferous.
We may merely remark here on the fact that, in these forests of the
Devonian and in the marshes on their margins, we find a wonderful
expansion of the now modest groups of Rhizocarps and Lycopods, and that
the flora as a whole belongs to the highest group of Cryptogams and the
lowest of Phænogams, so that it has about it a remarkable aspect of
mediocrity. Further, while there is evidence of some variety of station,
there is also evidence of much equality of climate, and of a condition
of things more resembling that of the insular climates of the temperate
portions of the southern hemisphere than that of North America or Europe
at present.
The only animal inhabitants of these Devonian woods, so far as known,
were a few species of insects, discovered by Hartt in New Brunswick, and
described by Dr. Scudder. Since, however, we now know that scorpions as
well as insects existed in the Silurian, it is probable that these also
occurred in the Erian, though their remains have not yet been discovered.
All the known insects of the Erian woods are allies of the shad-flies and
grasshoppers (_Neuroptera_ and _Orthoptera_), or intermediate between
the two. It is probable that the larvæ of most of them lived in water
and fed upon the abundant vegetable matter there, or on the numerous
minute crustaceans and worms. There were no land vertebrates, so far as
known, but there were fishes (_Dipterus_, etc.), allied to the modern
Barramunda or _Ceratodus_ of Australia, and with teeth suited for
grinding vegetable food. It is also possible that some of the smaller
plate-covered fishes (Placoganoids, like _Pterichthys_) might have fed
on vegetable matter, and, in any case, if they fed on lower animals, the
latter must have subsisted on plants. I mention these facts to show that
the superabundant vegetation of this age, whether aquatic or terrestrial,
was not wholly useless to animals. It is quite likely, also, that we have
yet much to learn of the animal life of the Erian swamps and woods.
NOTES TO CHAPTER III.
I.--Classification of Sporangites.
It is, of course, very unsatisfactory to give names to mere fragments of
plants, yet it seems very desirable to have some means of arranging them.
With respect to the organisms described above, which were originally
called by me _Sporangites_, under the supposition that they were
Sporangia rather than spores, this name has so far been vindicated by the
discovery of the spore-cases belonging to them, so that I think it may
still be retained as a provisional name; but I would designate the whole
as _Protosalviniæ_, meaning thereby plants with rhizocarpean affinities,
though possibly when better understood belonging to different genera. We
may under these names speak of their detached discs as macrospores and of
their cellular envelopes as sporocarps. The following may be recognized
as distinct forms:
1. _Protosalvinia Huronensis_, Dawson, _Syn._, _Sporangites Huronensis_,
"Report on Erian Flora of Canada," 1871.--Macrospores, in the form of
discs or globes, smooth and thick-walled, the walls penetrated by minute
radiating pores. Diameter about one one-hundredth of an inch, or a little
more, When in situ several macrospores are contained in a thin cellular
sporocarp, probably globular in form. From the Upper Erian, and perhaps
Lower Carboniferous shales of Kettle Point, Lake Huron, of various places
in the State of Ohio, and in the shale boulders of the boulder clay of
Chicago and vicinity. First collected at Kettle Point by Sir W. E. Logan,
and in Ohio by Prof. Edward Orton, and at Chicago by Dr. H. A. Johnson
and Mr. B. W. Thomas, also in New York by Prof. J. M. Clarke.
The macrospores collected by Mr. Thomas from the Chicago clays and shales
conform closely to those of Kettle Point, and probably belong to the
same species. Some of them are thicker in the outer wall, and show the
pores much more distinctly. These have been called by Mr. Thomas _S.
Chicagoensis_, and may be regarded as a varietal form. Specimens isolated
from the shale and mounted dry, show what seems to have been the hilum or
scar of attachment better than those in balsam.
Sections of the Kettle Point shale show, in addition to the macrospores,
wider and thinner shreds of vegetable matter, which I am inclined to
suppose to be remains of the sporocarps.
2. _Protosalvinia_ (_Sporangites_) _Braziliensis_, Dawson, "Canadian
Record of Science," 1883.--Macrospores, round, smooth, a little longer
than those of the last species, or about one seventy-fifth of an inch
in diameter, enclosed in round, oval, or slightly reniform sporocarps,
each containing from four to twenty-four macrospores. Longest diameter
of sporocarps three to six millimetres. Structure of wall of sporocarps
hexagonal cellular. Some sporocarps show no macrospores, and may possibly
contain microspores. The specimens are from the Erian of Brazil.
Discovered by Mr. Orville Derby. The formation, according to Mr. Derby,
consists of black shales below, about three hundred feet thick, and
containing the fucoid known as Spirophyton, and probably decomposed
vegetable matter. Above this is chocolate and reddish shale, in which
the well-preserved specimens of Protosalvinia occur. These beds are very
widely distributed, and abound in _Protosalvinia_ and _Spirophyton_.
3. _Protosalvinia_ (_Sporangites_) _bilobata_, Dawson, "Canadian
Record of Science," 1883.--Sporocarps, oval or reniform, three to six
millimetres in diameter, each showing two rounded prominences at the
ends, with a depression in the middle, and sometimes a raised neck or
isthmus at one side connecting the prominences. Structure of sporocarp
cellular. Some of the specimens indicate that each prominence or tubercle
contained several macrospores. At first sight it would be easy to mistake
these bodies for valves of _Beyrichia_.
Found in the same formations with the last species, though, in so far as
the specimens indicate, not precisely in the same beds. Collected by Mr.
Derby.
4. _Protosalvinia Clarkei_, Dawson, _P. bilobata_, Clarke, "American
Journal of Science."--Macrospores two-thirds to one millimetre in
diameter. One, two, or three contained in each sporocarp, which is
cellular. The macrospores have very thick walls with radiating tortuous
tubes. Unless this structure is a result of mineral crystallisation,
these macrospores must have had very thick walls and must have resembled
in structure the thickened cells of stone fruits and of the core of the
pear, or the tests of the Silurian and Erian seeds known as _Pachytheca_,
though on a smaller scale.
It is to be observed that bodies similar to these occur in the Boghead
earthy bitumen, and have been described by Credner.
I have found similar bodies in the so-called "Stellar coal" of the coal
district of Pictou, Nova Scotia, some layers of which are filled with
them. They occur in groups or patches, which seem to be enclosed in a
smooth and thin membrane or sporocarp. It is quite likely that these
bodies are generically distinct from _Protosalvinia_.
5. _Protosalvinia punctata_, Newton, "Geological Magazine," New Series,
December 2d, vol. ii.--Mr. Newton has named the discs found in the
white coal and Tasmanite, _Tasmanites_, the species being _Tasmanites
punctatus_, but as my name _Sporangites_ had priority, I do not think it
necessary to adopt this term, though there can be little doubt that these
organisms are of similar character. The same remark may be made with
reference to the bodies described by Huxley and Newton as occurring in
the Better-bed coal.
In Witham's "Internal Structure of Fossil Vegetables," 1833, Plate XI,
are figures of Lancashire cannel which shows _Sporangites_ of the type of
those in the Erian shales. Quekett, in his "Report on the Torbane Hill
Mineral," 1854, has very well figured similar structures from the Methel
coal and the Lesmahagow cannel coal. These are the earliest publications
on the subject known to me; and Quekett, though not understanding the
nature of the bodies he observed, holds that they are a usual ingredient
in cannel coals.
II.--The Nature and Affinities of Ptilophyton.
(_Lycopodites Vanuxemii_ of "Report on Devonian and Upper Silurian
Plants," Part I., page 35, _L. plumula_ of "Report on Lower Carboniferous
Plants," page 24, Plate I., Figs. 7, 8, 9.) In the reports above referred
to, these remarkable pinnate, frond-like objects were referred to the
genus _Lycopodites_, as had been done by Goeppert in his description
of the European species _Lycopodites pennæformis_, which is very near
to the American Erian form. Since 1871, however, there have been many
new specimens obtained, and very various opinions expressed as to their
affinities. While Hall has named some of them _Plumalina_, and has
regarded them as animal structures, allied to hydroids, Lesquereux
has described some of the Carboniferous forms under the generic name
_Trochophyllum_, which is, however, more appropriate to plants with
verticillate leaves which are included in this genus. Before I had seen
the publications of Hall and Lesquereux on the subject, I had in a paper
on "Scottish Devonian Plants"[BE] separated this group from the genus
_Lycopodites_, and formed for it the genus _Ptilophyton_, in allusion
to the feather-like aspect of the species. My reasons for this, and my
present information as to the nature of these plants, may be stated
as follows: Schimper, in his "Palæontologie Vegetale" (possibly from
inattention to the descriptions or want of access to specimens), doubts
the lycopodiaceous character of species of _Lycopodites_ described in my
published papers on plants of the Devonian of America and in my Report of
1871. Of these, _L. Richardsoni_ and _L. Matthewi_ are undoubtedly very
near to the modern genus _Lycopodium_. _L. Vanuxemii_ is, I admit, more
problematical; but Schimper could scarcely have supposed it to be a fern
or a fucoid allied to _Caulerpa_ had he observed that both in my species
and the allied _L. pennæformis_ of Goeppert, which he does not appear to
notice, the pinnules are articulated upon the stem, and leave scars where
they have fallen off. When in Belfast in 1870, my attention was again
directed to the affinities of these plants by finding in Prof. Thomson's
collection a specimen from Caithness, which shows a plant apparently
of this kind, with the same long narrow pinna? or leaflets, attached,
however, to thicker stems, and rolled up in a circinate manner. It seems
to be a plant in vernation, and the parts are too much crowded and
pressed together to admit of being accurately figured or described; but
I think I can scarcely be deceived as to its true nature. The circinate
arrangement in this case would favour a relationship to ferns; but some
lycopodiaceous plants also roll themselves in this way, and so do the
branches of the plants of the genus _Psilophyton_. (Fig. 17, _supra_.)
[BE] "Canadian Naturalist," 1878.
The specimen consists of a short, erect stem, on which are placed
somewhat stout alternate branches, extending obliquely outward and then
curving inward in a circinate manner. The lower ones appear to produce
on their inner sides short lateral branchlets, and upon these, and also
upon the curved extremities of the branches, are long, narrow, linear
leaves placed in a crowded manner. The specimen is thus not a spike of
fructification, but a young stem or branch in vernation, and which when
unrolled would be of the form of those peculiar pinnate _Lycopodites_
of which _L. Vanuxemii_ of the American Devonian and _L. pennæformis_ of
the European Lower Carboniferous are the types, and it shows, what might
have been anticipated from other specimens, that they were low, tufted
plants, circinate in vernation. The short stem of this plant is simply
furrowed, and bears no resemblance to a detached branch of Lycopodites
Milleri which lies at right angles to it on the same slab. As to the
affinities of the singular type of plants to which this specimen belongs,
I may quote from my "Report on the Lower Carboniferous Plants of Canada,"
in which I have described an allied species, _L. plumula_:
"The botanical relations of these plants must remain subject to doubt,
until either their internal structure or their fructification can be
discovered. In the mean time I follow Goeppert in placing them in what
we must regard as the provisional genus _Lycopodites_. On the one hand,
they are not unlike the slender twigs of _Taxodium_ and similar Conifers,
and the highly carbonaceous character of the stems gives some colour to
the supposition that they may have been woody plants. On the other hand,
they might, so far as form is concerned, be placed with Algæ of the type
of Brongniart's _Chondrites obtusus_, or the modern _Caulerpa plumaria_.
Again, in a plant of this type from the Devonian of Caithness to which
I have referred in a former memoir, the vernation seems to have been
circinate, and Schimper has conjectured that these plants may be ferns,
which seems also to have been the view of Shumard."
On the whole, these plants are allied to Lycopods rather than to ferns;
and as they constitute a small but distinct group, known only, so far
as I am aware, in the Lower Carboniferous and Erian or Devonian, they
deserve a generic name, and I proposed for them in my "Paper on Scottish
Devonian Plants," 1878, that of _Ptilophyton_, a name sufficiently
distinct in sound from Psilophyton, and expressing very well their
peculiar feather-like habit of growth. The genus was defined as follows:
"Branching plants, the branches bearing long, slender leaves in two or
more ranks, giving them a feathered appearance; vernation circinate.
Fruit unknown, but analogy would indicate that it was borne on the bases
of the leaves or on modified branches with shorter leaves."
The Scottish specimen above referred to was named _Pt. Thomsoni_, and was
characterised by its densely tufted form and thick branches. The other
species known are: _Pt. pennæformis_, Goeppert, L. Carboniferous; _Pt.
Vanuxemii_, Dawson, Devonian; _Pt. plumula_, Dawson, L. Carboniferous.
Shumard's _Filicites gracilis_, from the Devonian of Ohio, and Stur's
_Pinites antecedens_, from the Lower Carboniferous of Silesia, may
possibly belong to the same genus. The Scottish specimen referred to is
apparently the first appearance of this form in the Devonian of Europe.
I have at a still later date had opportunities of studying considerable
series of these plants collected by Prof. Williams, of Cornell
University, and prepared a note in reference to them for the American
Association, of which, however, only an abstract has been published.
I have also been favoured by Prof. Lesquereux and Mr. Lacoe, of
Pittston, with the opportunity of studying the specimens referred to
_Trochophyllum_.
Prof. Williams's specimens occur in a dark shale associated with
remains of land-plants of the genera _Psilophyton_. _Rhodea_, &c.,
and also marine shells, of which a small species of _Rhynchonella_ is
often attached to the stems of the _Ptilophyton_. Thus these organisms
have evidently been deposited in marine beds, but in association with
land-plants.
The study of the specimens collected by Prof. Williams develops the
following facts: (1) The plants are not continuous fronds, but slender
stems or petioles, with narrow, linear leaflets attached in a pinnate
manner. (2) The pinnules are so articulated that they break off, leaving
delicate transverse scars, and the lower parts of the stems are often
thus denuded of pinnæ for the length of one or more inches. (3) The
stems curve in such a manner as to indicate a circinate vernation. (4)
In a few instances the fronds were observed to divide dichotomously
toward the top; but this is rare. (5) There are no indications of cells
in the pinnules; but, on the other hand, there is no appearance of
fructification unless the minute granules which roughen some of the
sterns are of this nature. (6) The stems seem to have been lax and
flexuous, and in some instances they seem to have grown on the petioles
of ferns preserved with them in the same beds. (7) The frequency of the
attachment of small brachiopods to the specimens of _Ptilophyton_ would
seem to indicate that the plant stood erect in the water. (8) Some of
the specimens show so much carbonaceous matter as to indicate that the
pinnules were of considerable consistency. All these characters are those
rather of an aquatic plant than of an animal organism or of a land-plant.
The specimens communicated by Prof. Lesquereux and Mr. Lacoe are from the
Lower Carboniferous, and evidently represent a different species with
similar slender pitted stems, often partially denuded of pinnules below;
but the pinnules are much broader and more distant. They are attached
by very narrow bases, and apparently tend to lie on a plane, though they
may possibly have been spirally arranged. On the same slabs are rounded
sporangia or macrospores like those of _Lepidodendron_, but there is no
evidence that these belonged to _Trochophyllum_. On the stems of this
plant, however, there are small, rounded bodies apparently taking the
places of some of the pinnules. These may possibly be spore-cases; but
they may be merely imperfectly developed pinnules. Still the fact that
similar small granules appear on the stems of the Devonian species,
favours the idea that they may be organs of fructification.
The most interesting discovery, however, which results from the study
of Mr. Lacoe's specimens, is that the pinnules were cylindrical and
hollow, and probably served to float the plant. This would account for
many of the peculiarities in the appearance and mode of occurrence of the
Devonian _Ptilophyton_, which are readily explained if it is supposed to
be an aquatic plant, attaching itself to the stems of submerged vegetable
remains and standing erect in the water by virtue of its hollow leaves.
It may well, however, have been a plant of higher organisation than the
Algæ, though no doubt cryptogamous.
The species of _Ptilophyton_ will thus constitute a peculiar group
of aquatic plants, belonging to the Devonian and Lower Carboniferous
periods, and perhaps allied to Lycopods and Pillworts in their
organisation and fruit, but specially distinguished by their linear
leaves serving as floats and arranged pinnately on slender stems. The
only species yet found within the limits of Canada is _Pt. plumula_,
found by Dr. Honeyman in the Lower Carboniferous of Nova Scotia; but as
_Pt. Vanuxemii_ abounds in the Erian of New York, it will no doubt be
found in Canada also.
III.--Tree-Ferns of the Erian Period.
As the fact of the occurrence of true tree-ferns in rocks so old as the
Middle Erian or Devonian has been doubted in some quarters, the following
summary is given from descriptions published in the "Journal of the
Geological Society of London" (1871 and 1881), where figures of the
species will be found:
Of the numerous ferns now known in the Middle and Upper Devonian of
North America, a great number are small and delicate species, which were
probably herbaceous; but there are other species which may have been
tree-ferns. Little definite information, however, has, until recently,
been obtained with regard to their habit of growth.
The only species known to me in the Devonian of Europe is the
_Caulopteris Peachii_ of Salter, figured in the "Quarterly Journal of
the Geological Society" for 1858. The original specimen of this I had an
opportunity of seeing in London, through the kindness of Mr. Etheridge,
and have no doubt that it is the stem of a small arborescent fern, allied
to the genus _Caulopteris_, of the coal formation.
In my paper on the Devonian of Eastern America ("Quarterly Journal
of the Geological Society," 1862), I mentioned a plant found by Mr.
Richardson at Perry, as possibly a species of _Megaphyton_, using that
term to denote those stems of tree-ferns which have the leaf-scars in
two vertical series; but the specimen was obscure, and I have not yet
obtained any other.
More recently, in 1869, Prof. Hall placed in my hands an interesting
collection from Gilboa, New York, and Madison County, New York, including
two trunks surrounded by aërial roots, which I have described as
_Psaronius textilis_ and _P. Erianus_, in my "Revision of the Devonian
Flora," read before the Royal Society.[BF] In the same collection were
two very large petioles, _Rhachiopteris gigantea_ and _R. palmata_, which
I have suggested may have belonged to tree-ferns.
[BF] Abstract in "Proceedings of the Royal Society," May, 1870; also
"Report on Erian Plants of Canada," 1871.
My determination of the species of _Psaronius_, above mentioned, has
recently been completely confirmed by the discovery on the part of Mr.
Lockwood, of Gilboa, of the upper part of one of these stems, with its
leaf-scars preserved and petioles attached, and also by some remarkable
specimens obtained by Prof. Newberry, of New York, from the Corniferous
limestone of Ohio, which indicate the existence there of three species of
tree-ferns, one of them with aërial roots similar to those of the Gilboa
specimens. The whole of these specimens Dr. Newberry has kindly allowed
me to examine, and has permitted me to describe the Gilboa specimen,
as connected with those which I formerly studied in Prof. Hall's
collections. The specimens from Ohio he has himself named, but allows me
to notice them here by way of comparison with the others. I shall add
some notes on specimens found with the Gilboa ferns.
It may be further observed that the Gilboa specimens are from a bed
containing erect stumps of tree-ferns, in the Chemung group of the Upper
Devonian, while those from Ohio are from a marine limestone, belonging to
the lower part of the Middle Devonian.
1. _Caulopteris Lockwoodi_, Dawson.--Trunk from two to three inches in
diameter, rugose longitudinally. Leaf-scars broad, rounded above, and
radiatingly rugose, with an irregular scar below, arranged spirally in
about five ranks; vascular bundles not distinctly preserved. Petioles
slender, much expanded at the base, dividing at first in a pinnate
manner, and afterwards dichotomously. Ultimate pinnæ with remains of
numerous, apparently narrow pinnules.
This stem is probably the upper part of one or other of the species
of _Psaronius_ found in the same bed (_P. Erianus_, Dawson, and _P.
textilis_, Dawson).[BG] It appears to have been an erect stem embedded
in situ in sandstone, and preserved as a cast. The stem is small, being
only two inches, or a little more, in diameter. It is coarsely wrinkled
longitudinally, and covered with large leaf-scars, each an inch in
diameter, of a horseshoe-shape. The petioles, five of which remain,
separate from these scars with a distinct articulation, except at one
point near the base, where probably a bundle or bundles of vessels passed
into the petiole. They retain their form at the attachment to the stem,
but a little distance from it they are flattened. They are inflated
at the base, and somewhat rapidly diminish in size. The leaf-scars
vary in form, and are not very distinct, but they appear to present
a semicircular row of pits above, largest in the middle. From these
there proceed downward a series of irregular furrows, converging to a
second and more obscure semicircle of pits, within or below which is the
irregular scar or break above referred to. The attitude and form of the
petioles will be seen from Fig. 24, _supra_.
[BG] Memoir on Devonian Flora, "Proceedings of the Royal Society," May,
1870.
The petioles are broken off within a few inches of the stem; but other
fragments found in the same beds appear to show their continuation, and
some remains of their foliage. One specimen shows a series of processes
at the sides, which seem to be the remains of small pinnæ, or possibly
of spines on the margin of the petiole. Other fragments show the
division of the frond, at first in a pinnate manner, and subsequently by
bifurcation; and some fragments show remains of pinnules, possibly of
fertile pinnules. These are very indistinct, but would seem to show that
the plant approached, in the form of its fronds and the arrangement of
its fructification, to the Cyclopterids of the sub-genus _Aneimites_,
one of which (_Aneimites Acadica_), from the Lower Carboniferous of Nova
Scotia, I have elsewhere described as probably a tree-fern,[BH] The
fronds were evidently different from those of _Archæopteris_[BI] a genus
characteristic of the same beds, but of very different habit of growth.
This accords with the fact that there is in Prof. Hall's collection a
mass of fronds of _Cyclopteris_ (_Archæopteris_) Jacksoni, so arranged
as to make it probable that the plant was an herbaceous fern, producing
tufts of fronds on short stems in the ordinary way. The obscurity of
the leaf-scars may render it doubtful whether the plant above described
should be placed in the genus _Caulopteris_ or in _Stemmatopteris_; but
it appears most nearly allied to the former. The genus is at present,
of course, a provisional one; but I have thought it only justice to the
diligent labours of Mr. Lockwood to name this curious and interesting
fossil _Caulopteris Lockwoodi_.
[BH] "Quarterly Journal of the Geological Society," 1860.
[BI] The genus to which the well-known _Cyclopteris_ (_Adiantites_)
_Hibernicus_ of the Devonian of Ireland belongs.
I have elsewhere remarked on the fact that trunks, and petioles, and
pinnules of ferns are curiously dissociated in the Devonian beds--an
effect of water-sorting, characteristic of a period in which the
conditions of deposition were so varied. Another example of this is,
that in the sandstones of Gaspé Bay, which have not as yet afforded
any example of fronds of ferns, there are compressed trunks, which Mr.
Lockwood's specimens allow me at least to conjecture may have belonged
to tree-ferns, although none of them are sufficiently perfect for
description.
Mr. Lockwood's collection includes specimens of _Psaronius textilis_; and
in addition to these there are remains of erect stems somewhat different
in character, yet possibly belonging to the higher parts of the same
species of tree-fern. One of these is a stem crushed in such a manner
that it does not exhibit its form with any distinctness, but surrounded
by smooth, cylindrical roots, radiating from it in bundles, proceeding at
first horizontally, and then curving downward, and sometimes terminating
in rounded ends. They resemble in form and size the aërial roots of
_Psaronius Erianus_; and I believe them to be similar roots from a higher
part of the stem, and some of them young and not prolonged sufficiently
far to reach the ground. This specimen would thus represent the stem
of _P. Erianus_ at a higher level than those previously found. We can
thus in imagination restore the trunk and crown of this once graceful
tree-fern, though we have not the detail of its fronds. Mr. Lockwood's
collections also contain a specimen of the large fern-petiole which I
have named _Rhachiopteris punctata_. My original specimen was obtained by
Prof. Hall from the same horizon in New York. That of Mr. Lockwood is of
larger size, but retains no remains of the frond. It must have belonged
to a species quite distinct from _Caulopteris Lockwoodi_, but which may,
like it, have been a tree-fern.
2. _Caulopteris antiqua_, Newberry.--This is a flattened stem, on a slab
of limestone, containing Brachiopods, Trilobites, &c., of the Corniferous
limestone. It is about eighteen inches in length, and three and a half
inches in average breadth. The exposed side shows about twenty-two large
leaf-scars arranged spirally. Each leaf, where broken off, has left
a rough fracture; and above this is a semicircular impression of the
petiole against the stem, which, as well as the surface of the bases of
the petioles, is longitudinally striated or tuberculated. The structures
are not preserved, but merely the outer epidermis, as a coaly film. The
stem altogether much resembles _Caulopteris Peachii_, but is of larger
size. It differs from _C. Lockwoodi_ in the more elongated leaf-bases,
and in the leaves being more remotely placed; but it is evidently of the
same general character with that species.
3. _Caulopteris_ (_Protopteris_) _peregrina_, Newberry.--This is a much
more interesting species than the last, as belonging to a generic or
subgeneric form not hitherto recognised below the Carboniferous, and
having its minute structure in part preserved.
The specimens are, like the last, on slabs of marine limestone of the
Corniferous formation, and flattened. One represents an upper portion
of the stem with leaf-scars and remains of petioles; another a lower
portion, with aërial roots. The upper part is three inches in diameter,
and about a foot in length, and shows thirty leaf-scars which are about
three-fourths of an inch wide, and rather less in depth. The upper part
presents a distinct rounded and sometimes double marginal line, sometimes
with a slight depression in the middle. The lower part is irregular,
and when most perfect shows seven slender vascular bundles, passing
obliquely downward into the stem. The more perfect leaf-bases have the
structure preserved, and show a delicate, thin-walled, oval parenchyma,
while the vascular bundles show scalariform vessels with short bars in
several rows, in the manner of many modern ferns. Some of the scars show
traces of the hippocrepian mark characteristic of _Protopteris_; and the
arrangement of the vascular bundles at the base of the scars is the same
as in that genus, as are also the general form and arrangement of the
scars. On careful examination, the species is indeed very near to the
typical _P. Sternbergii_, as figured by Corda and Schimper.[BJ]
[BJ] Corda, "Beiträge," Pl. 48, copied by Schimper, Pl. 52.
The genus _Protopteris_ of Sternberg, though the original species (_P.
punctata_) appears as a _Lepidodendron_ in his earlier plate (Plate 4),
and as a _Sigillaria_ (_S. punctata_) in Brongniart's great work, is a
true tree-fern; and the structure of one species (_P. Cottai_) has been
beautifully figured by Corda. The species hitherto described are from the
Carboniferous and Permian.
The second specimen of this species represents a lower part of the stem.
It is thirteen inches long and about four inches in diameter, and is
covered with a mass of flattened aërial roots lying parallel to each
other, in the manner of the _Psaronites_ of the coal-formation and of _P.
Erianus_ of the Upper Erian or Devonian.
4. _Asteropteris noveboracensis_, gen. and sp. n.--The genus
_Asteropteris_ is established for stems of ferns having the axial portion
composed of vertical radiating plates of scalariform tissue embedded in
parenchyma, and having the outer cylinder composed of elongated cells
traversed by leaf-bundles of the type of those of _Zygopteris_.
The only species known to me is represented by a stem 2·5 centimetres in
diameter, slightly wrinkled and pitted externally, perhaps by traces of
aërial roots which have perished. The transverse section shows in the
centre four vertical plates of scalariform or imperfectly reticulated
tissue, placed at right angles to each other, and united in the middle
of the stem. At a short distance from the centre, each of these plates
divides into two or three, so as to form an axis of from ten to twelve
radiating plates, with remains of cellular tissue filling the angular
interspaces. The greatest diameter of this axis is about 1·5 centimetre.
Exterior to the axis the stem consists of elongated cells, with somewhat
thick walls, and more dense toward the circumference. The walls of
these cells present a curious reticulated appearance, apparently caused
by the cracking of the ligneous lining in consequence of contraction
in the process of carbonization. Embedded in this outer cylinder are
about twelve vascular bundles, each with a dumb-bell-shaped group of
scalariform vessels enclosed in a sheath of thick-walled fibres. Each
bundle is opposite to one of the rays of the central axis. The specimen
shows about two inches of the length of the stem, and is somewhat bent,
apparently by pressure, at one end.
This stem is evidently that of a small tree-fern of a type, so far as
known to me, not before described,[BK] and constituting a very complex
and symmetrical form of the group of Palæozoic ferns allied to the
genus _Zygopteris_ of Schimper. The central axis alone has a curious
resemblance to the peculiar stem described by Unger ("Devonian Flora
of Thuringia") under the name of _Cladoxylon mirabile_; and it is just
possible that this latter stem may be the axis of some allied plant. The
large aërial roots of some modern tree-ferns of the genus _Angiopteris_
have, however, an analogous radiating structure.
[BK] Prof. Williamson, to whom I have sent a tracing of the structure,
agrees with me that it is new.
The specimen is from the collection of Berlin H. Wright, Esq., of Penn
Yan, New York, and was found in the Portage group (Upper Erian) of Milo,
New York, where it was associated with large petioles of ferns and trunks
of _Lepidodendra_, probably _L. Chemungense_ and _L. primævum_.
The occurrence of this and other stems of tree-ferns in marine beds
has recently been illustrated by the observation of Prof. A. Agassiz
that considerable quantities of vegetable matter can be dredged from
great depths in the sea on the leeward side of the Caribbean Islands.
The occurrence of these trunks further connects itself with the great
abundance of large petioles (_Rhachiopteris_) in the same beds, while the
rarity of well-preserved fronds is explained by the coarseness of the
beds, and also by the probably long maceration of the plant-remains in
the sea-water.
In connection with this I may refer to the remarkable facts recently
stated by Williamson[BL] respecting the stems known as Heterangium and
_Lyginodendron_. It would seem that these, while having strong exogenous
peculiarities, are really stems of tree-ferns, thus placing this family
in the same position of advancement with the Lycopods and Equisetaceæ of
the Coal period.
[BL] "Proceedings of the Royal Society," January 6, 1887.
IV.--On Erian Trees of the Genus Dadoxylon, Unger. (_Araucarites_ of
Goeppert, _Araucarioxylon_ of Kraus.)
Large woody trunks, carbonised or silicified, and showing wood-cells with
hexagonal areoles having oval pores inscribed in them, occur abundantly
in some beds of the Middle Erian of America, and constitute the most
common kind of fossil wood all the way to the Trias. They have in the
older formations, generally, several rows of pores on each fibre, and
medullary rays composed of two or more series of cells, but become more
simple in these respects in the Permian and Triassic series. The names
_Araucarites_ and _Araucarioxylon_ are perhaps objectionable, inasmuch
as they suppose affinities to _Araucaria_ which may not exist. Unger's
name, which is non-committal, is therefore, I think, to be preferred. In
my "Acadian Geology," and in my "Report on the Geology of Prince Edward
Island," I have given reasons for believing that the foliage of some at
least of these trees was that known as _Walchia_, and that they may have
borne nutlets in the manner of Taxine trees (_Trigonocarpum_, &c). Grand
d'Eury has recently suggested that some of them may have belonged to
_Cordaites_, or to plants included in that somewhat varied and probably
artificial group.
The earliest discovery of trees of this kind in the Erian of America was
that of Matthew and Hartt, who found large trunks, which I afterwards
described as _Dadoxylon Ouangondianum_, in the Erian sandstone of St.
John, New Brunswick, hence named by those geologists the "Dadoxylon
sandstone." A little later, similar wood was found by Prof. Hall and
Prof. Newberry in the Hamilton group of New York and Ohio, and the allied
wood of the genus _Ormoxylon_ was obtained by Prof. Hall in the Portage
group of the former State. These woods proved to be specifically distinct
from that of St. John, and were named by me _D. Halli_, _D. Newberryi_,
and _Ormoxylon Erianum_. The three species of _Dadoxylon_ agreed in
having composite medullary rays, and would thus belong to the group
_Palæoxylon_ of Brongniart. In the case of _Ormoxylon_ this character
could not be very distinctly ascertained, but the medullary rays appeared
to be simple.
I am indebted to Prof. J. M. Clarke, of Amherst College, Massachusetts,
for some well-preserved specimens of another species from the Genesee
shale of Canandaigua, New York. They show small steins or branches, with
a cellular pith surrounded with wood of coniferous type, showing two
to three rows of slit-formed, bordered pores in hexagonal borders. The
medullary sheath consists of pseudo-scalariform and reticulated fibres;
but the most remarkable feature of this wood is the structure of the
medullary rays, which are very frequent, but short and simple, sometimes
having as few as four cells superimposed. This is a character not before
observed in coniferous trees of so great age, and allies this Middle
Erian form with some Carboniferous woods which have been supposed to
belong to _Cordaites_ or _Sigillaria_. In any case this structure is new,
and I have named the species _Dadoxylon Clarkii_, after its discoverer.
The specimens occur, according to Prof. Clarke, in a calcareous layer
which is filled with the minute shells of _Styliola fissurella_ of Hall,
believed to be a Pteropod; and containing also shells of _Goniatites_ and
_Gyroceras_. The stems found are only a few inches in diameter, but may
be branches of larger trees.
It thus appears that we already know five species of Coniferous trees
of the genus _Dadoxylon_ in the Middle Erian of America, an interesting
confirmation of the facts otherwise known as to the great richness and
variety of this ancient flora. The late Prof. Goeppert informed me that
he had recognised similar wood in the Devonian of Germany, and there
can be no doubt that the fossil wood discovered by Hugh Miller in the
Old Red Sandstone of Scotland, and described by Salter and McNab, is of
similar character, and probably belongs to the genus _Dadoxylon_. Thus
this type of Coniferous tree seems to have been as well established and
differentiated into species in the Middle Devonian as in the succeeding
Carboniferous.
I may here refer to the fact that the lower limit of the trees of
this group coincides, in America, with the upper limit of those
problematical trees which in the previous chapter I have named Protogens
(_Nematophyton_, _Celluloxlyon_,[BM] _Nematoxylon_[BN]), though
_Aporoxylon_ of Unger extends, in Thuringia, up to the Upper Devonian
(Cypridina schists).
[BM] "Journal of the Geological Society," May, 1881.
[BN] _Ibid._, vol. xix, 1863.
V.--Scottish Devonian Plants of Hugh Miller and others. (Edinburgh
Geological Society, 1877.)
Previously to the appearance of my descriptions of Devonian plants
from North America, Hugh Miller had described forms from the Devonian
of Scotland, similar to those for which I proposed the generic name
_Psilophyton_; and I referred to these in this connection in my earliest
description of that genus.[BO] He had also recognised what seemed to be
plants allied to Lycopods and Conifers. Mr. Peach and Mr. Duncan had made
additional discoveries of this kind, and Sir J. Hooker and Mr. Salter had
described some of these remains. More recently Messrs. Peach, Carruthers,
and McNab have worked in this field, and still later[BP] Messrs. Jack and
Etheridge have summed up the facts and have added some that are new.
[BO] "Journal of the Geological Society," London, 1859.
[BP] _Ibid._, 1877.
The first point to which I shall refer, and which will lead to the
other matters to be discussed, is the relation of the characteristic
_Lepidodendron_ of the Devonian of eastern America, _L. Gaspianum_, to
_L. nothum_ of Unger and of Salter. At the time when I described this
species I had not access to Scottish specimens of _Lepidodendron_ from
the Devonian, but these had been well figured and described by Salter,
and had been identified with _L. nothum_ of Unger, a species evidently
distinct from mine, as was also that figured and described by Salter,
whether identical or not with Unger's species. In 1870 I had for the
first time an opportunity to study Scottish specimens in the collection
of Mr. Peach; and on the evidence thus afforded I stated confidently
that these specimens represented a species distinct from _L. Gaspianum_,
perhaps even generically so.[BQ] It differs from _L. Gaspianum_ in
its habit of growth by developing small lateral branches instead of
bifurcating, and in its foliage by the absence or obsolete character of
the leaf-bases and the closely placed and somewhat appressed leaves. If
an appearance of swelling at the end of a lateral branch in one specimen
indicates a strobile of fructification, then its fruit was not dissimilar
from that of the Canadian species in its position and general form,
though it may have differed in details. On these grounds I declined to
identify the Scottish species with _L. Gaspianum_. The Lepidodendron
from the Devonian of Belgium described and figured by Crepin,[BR] has
a better claim to such identification, and would seem to prove that
this species existed in Europe as well as in America. I also saw in Mr.
Peach's collection in 1870 some fragments which seemed to me distinct
from Salter's species, and possibly belonging to _L. Gaspianum_.[BS]
[BQ] "Report on Devonian Plants of Canada," 1871.
[BR] "Observations sur quelques Plantes Fossiles des dépôts Devoniens."
[BS] "Proceedings of the Geological Society of London," March, 1871.
In the earliest description of _Psilophyton_ I recognised its probable
generic affinity with Miller's "dichotomous plants," with Salter's
"rootlets," and with Goeppert's _Haliserites Dechenianus_, and stated
that I had "little doubt that materials exist in the Old Red Sandstone of
Scotland for the reconstruction of at least one species of this genus."
Since, however, Miller's plants had been referred to coniferous roots,
and to fucoids, and Goeppert's _Haliserites_ was a name applicable only
to fucoids, and since the structure and fruit of my plants placed them
near to Lycopods, I was under the necessity of giving them a special
generic name, nor could I with certainty affirm their specific identity
with any European species. The comparison of the Scottish specimens with
woody rootlets, though incorrect, is in one respect creditable to the
acumen of Salter, as in almost any state of preservation an experienced
eye can readily perceive that branchlets of _Psilophyton_ must have been
woody rather than herbaceous, and their appearance is quite different
from that of any true Algæ.
The type of _Psilophyton_ is my _P. princeps_, of which the whole of the
parts and structures are well known, the entire plant being furnished in
abundance and in situ in the rich plant-beds of Gaspé. A second species,
_P. robustius_, has also afforded well-characterised fructification. _P.
elegans_, whose fruit appears as "oval scales," no doubt bore sac-like
spore-cases resembling those of the other species, but in a different
position, and perfectly flattened in the specimens procured. The only
other Canadian species, _P. glabrum_, being somewhat different in
appearance from the others, and not having afforded any fructification,
must be regarded as uncertain.
The generic characters of the first three species may be stated as
follows:
Stems dichotomous, with rudimentary subulate leaves, sometimes obsolete
in terminal branchlets and fertile branches; and in decorticated
specimens represented only by punctiform scars. Young branches circinate.
Rhizomata cylindrical, with circular root-areoles. Internal structure of
stem, an axis of scalariform vessels enclosed in a sheath of imperfect
woody tissue and covered with a cellular bark more dense externally.
Fruit, naked sac-like spore-cases, in pairs or clusters, terminal or
lateral.
The Scottish specimens conform to these characters in so far as they are
known, but not having as yet afforded fruit or internal structure, they
cannot be specifically determined with certainty. More complete specimens
should be carefully searched for, and will no doubt be found.
In Belgium, M. Crepin has described a new species from the Upper Devonian
of Condroz under the name _P. Condrusianum_ (1875). It wants, however,
some of the more important characters of the genus, and differs in having
a pinnate ramification, giving it the aspect of a fern. In a later paper
(1876) the author considers this species distinct from _Psilophyton_, and
proposes for it a new generic name _Rhacophyton_.
The characters given by Mr. Carruthers, in his paper of 1873, for the
species _P. Dechenianum_, are very few and general: "Lower branches short
and frequently branching, giving the plant an oblong circumscription."
Yet even these characters do not apply, so far as known, to Miller's
fucoids or Salter's rootlets or Goeppert's _Haliserites_. They merely
express the peculiar mode of branching already referred to in Salter's
_Lepidodendron nothum_. The identification of the former plants with
the _Lepidodendron_ and _Lycopodites_, indeed, rests only on mere
juxtaposition of fragments, and on the slight resemblance of the
decorticated ends of the branches of the latter plants to _Psilophyton_.
It is contradicted by the obtuse ends of the branches of the
_Lepidodendron_ and _Lycopodites_, and by the apparently strobilaceous
termination of some of them.
Salter's description of his _Lepidodendron nothum_ is quite definite, and
accords with specimens placed in my hands by Mr. Peach: "Stems half an
inch broad, tapering little, branches short; set on at an acute angle,
blunt at their terminations. Leaves in seven to ten rows, very short,
not a line long, and rather spreading than closely imbricate." These
characters, however, in so far as they go, are rather those of the genus
_Lycopodites_ than of _Lepidodendron_, from which this plant differs
in wanting any distinct leaf-bases, and in its short, crowded leaves.
It is to be observed that they apply also to Salter's _Lycopodites
Milleri_, and that the difference of the foliage of that species may be
a result merely of different state of preservation. For these reasons I
am disposed to place these two supposed species together, and to retain
for the species the name _Lycopodites Milleri_. It may be characterised
by the description above given, with merely the modification that the
leaves are sometimes nearly one-third of an inch long and secund (Fig.
17, _supra_, lower figure).
Decorticated branches of the above species may no doubt be mistaken
for _Psilophyton_, but are nevertheless quite distinct from it, and
the slender branching dichotomous stems, with terminations which, as
Miller graphically states, are "like the tendrils of a pea," are too
characteristic to be easily mistaken, even when neither fruit nor leaves
appear. With reference to fructification, the form of _L. Milleri_
renders it certain that it must have borne strobiles at the ends of its
branchlets, or some substitute for these, and not naked spore-cases like
those of _Psilophyton_.
The remarkable fragment communicated by Sir Philip Egerton to Mr.
Carruthers,[BT] belongs to a third group, and has, I think, been quite
misunderstood. I am enabled to make this statement with some confidence,
from the fact that the reverse or counterpart of Sir Philip's specimen
was in the collection of Sir Wyville Thomson, and was placed by him in
my hands in 1870. It was noticed in my paper on "New Devonian Plants,"
in the "Journal of the Geological Society of London," and referred to my
genus _Ptilophyton_, as stated above under Section II., page 86 _et seq._
[BT] "Journal of Botany," 1873.
Mr. Salter described, in 1857,[BU] fragments of fossil wood from the
Scottish Devonian, having the structure of Dadoxylon, though very
imperfectly preserved; and Prof. McNab has proposed[BV] the generic name
_Palæopitys_ for another specimen of coniferous wood collected by Hugh
Miller, and referred to by him in the "Testimony of the Rocks." From
Prof. McNab's description, I should infer that this wood may, after all,
be generically identical with the woods usually referred to Dadoxylon
of Unger (_Araucarioxylon_ of Kraus). The description, however, does
not mention the number and disposition of the rows of pores, nor the
structure of the medullary rays, and I have not been able to obtain
access to the specimens themselves. I have described five species of
Dadoxylon from the Middle and Upper Erian of America, all quite distinct
from the Lower Carboniferous species. There is also one species of an
allied genus, Ormoxylon. All these have been carefully figured, and it is
much to be desired that the Scottish specimens should be re-examined and
compared with them.
[BU] "Journal of the London Geological Society."
[BV] "Transactions of the Edinburgh Botanical Society," 1870.
Messrs. Jack and Etheridge have given an excellent summary of our present
knowledge of the Devonian flora of Scotland, in the Journal of the
London Geological Society (1877). From this it would appear that species
referable to the genera _Calamities_, _Lepidodendron_, _Lycopodites_,
_Psilophyton_, _Arthrostigma_, _Archæopteris_, _Caulopteris_,
_Palæopitys_, _Araucarioxylon_, and _Stigmaria_ have been recognised.
The plants described by these gentlemen from the Old Red Sandstone of
Callender, I should suppose, from their figures and descriptions, to
belong to the genus _Arthrostigma_, rather than to Psilophyton. I do
not attach any importance to the suggestions referred to by them, that
the apparent leaves may be leaf-bases. Long leaf-bases, like those
characteristic of _Lepidofloyos_, do not occur in these humbler plants
of the Devonian. The stems with delicate "horizontal processes" to which
they refer may belong to _Ptilophyton_ or to _Pinnularia_.
In conclusion, I need scarcely say that I do not share in the doubts
expressed by some British palæontologists as to the distinctness of
the Devonian and Carboniferous floras. In eastern America, where these
formations are mutually unconformable, there is, of course, less room
for doubt than in Ireland and in western America, where they are
stratigraphically continuous. Still, in passing from the one to the
other, the species are for the most part different, and new generic forms
are met with, and, as I have elsewhere shown, the physical conditions of
the two periods were essentially different.[BW]
[BW] "Reports on Devonian Plants and Lower Carboniferous Plants of
Canada."
It is, however, to be observed that since--as Stur and others have
shown--_Calamities radiatus_, and other forms distinctively Devonian in
America, occur in Europe in the Lower Carboniferous, it is not unlikely
that the Devonian flora, like that of the Tertiary, appeared earlier in
America. It is also probable, as I have shown in the "Reports" already
referred to, that it appeared earlier in the Arctic than in the temperate
zone. Hence an Arctic or American flora, really Devonian, may readily be
mistaken for Lower Carboniferous by a botanist basing his calculations on
the fossils of temperate Europe. Even in America itself, it would appear,
from recent discoveries in Virginia and Ohio, that certain Devonian forms
lingered longer in those regions than farther to the northeast;[BX] and
it would not be surprising if similar plants occurred in later beds in
Devonshire or in the south of Europe than in Scotland. Still, these
facts, properly understood, do not invalidate the evidence of fossil
plants as to geological age, though errors arising from the neglect of
them are still current.
[BX] Andrews, "Palæontology of Ohio," vol. ii.; Meek, "Fossil Plants from
Western Virginia," Philosophical Society, Washington, 1875.
VI.--Geological Relations of some Plant-bearing Beds of Eastern Canada.
("Report on Erian Plants," 1871.)
The Gaspé sandstones have been fully described by Sir W. E. Logan,
in his "Report on the Geology of Canada," 1863. He there assigns to
them a thickness of seven thousand and thirty-six feet, and shows that
they rest conformably on the Upper Silurian limestones of the Lower
Helderberg group (Ludlow), and are in their turn overlaid unconformably
by the conglomerates which form the base of the Carboniferous rocks of
New Brunswick. I shall add here merely a few remarks on points in their
physical character connected with the occurrence of plants in them.
_Prototaxites_ (_Nematophyton_) _Logani_ and other characteristic Lower
Erian plants occur in the base of the sandstones at Little Gaspé. This
fact, along with the occurrence, as stated in my paper of 1863, of
rhizomes of _Psilophyton_ preserving their scalariform structure, in
the upper part of the marine Upper Silurian limestones,[BY] proves the
flora of the Devonian rocks to have had its beginning at least in the
previous geological period, and to characterise the lower as well as the
upper beds of the Devonian series. In this connection I may state that,
from their marine fossils, as well as their stratigraphical arrangement,
Sir W. E. Logan and Mr. Billings regard the lower portions of the Gaspé
sandstones as the equivalents of the Oriskany sandstone of New York.
On the other hand, the great thickness of this formation, the absence
of Lower Devonian fossils from its upper part, and the resemblance of
the upper beds to those of the newer members of the Devonian elsewhere,
render it probable that the Gaspé sandstones, though deficient in the
calcareous members of the system, seen farther to the westward, represent
the whole of the Devonian period.
[BY] The marine fossils of these beds have been determined by Mr.
Billings. They are Upper Silurian, with an intermixture of Lower Devonian
in the upper part. Fragments of _Nematophyton_ occur in beds of the same
age in the Bay des Chaleurs, at Cape Bon Ami.
The Gaspé sandstones, as their name imports, are predominantly
arenaceous, and often coarsely so, the sandstones being frequently
composed of large grains and studded with quartz-pebbles. Grey and buff
are prevalent colours, but red beds also occur, more especially in the
upper portion. There are also interstratified shaly beds, sometimes
occurring in groups of considerable thickness, and associated with
fine-grained and laminated argillaceous sandstone, the whole having
in many places the lithological aspect of the coal-measures. At one
place, near the middle of the series, there is a bed of coal from one
inch to three inches in thickness, associated with highly bituminous
shales abounding in remains of plants, and also containing fragments
of crustaceans and fishes (_Pterygotus_, _Ctenacanthus ?_ &c). The
beds connected with this coal are grey sandstones and grey and dark
shales, much resembling those of the ordinary coal formation. The
coal is shining and laminated, and both its roof and floor consist of
laminated bituminous shale with fragments of _Psilophyton_. It has no
true under-clay, and has been, I believe, a peaty mass of rhizomes of
_Psilophyton_. It occurs near Tar Point, on the south side of Gaspé Bay,
a place so named from the occurrence of a thick dyke of trap holding
petroleum in its cavities. The coal is of considerable horizontal extent,
as in its line of strike a similar bed has been discovered on the Douglas
River, about four miles distant. It has not been recognised on the north
side of the bay, though we find there beds, probably on very nearly the
same horizon, holding _Psilophyton_ in situ.
As an illustration of one of the groups of shaly beds, and of the
occurrence of roots of _Psilophyton_, I may give the following sectional
list of beds seen near "Watering Brook," on the north shore of the bay.
The order is descending:
FT. IN.
1. Grey sandstones and reddish pebbly sandstone of great
thickness
2. Bright-red shale 8 0
3. Grey shales with stems of _Psilophyton_, very abundant
but badly preserved 0 5
4. Grey incoherent clay, slickensided, and with many
rhizomes and roots of _Psilophyton_ 0 3
5. Hard grey clay or shale, with fragments and roots of
_Psilophyton_ 4 0
6. Red shale 8 0
7. Grey and reddish crumbling sandstone
Groups of beds similar to the above, but frequently much more rich in
fossils, occur in many parts of the section, and evidently include fossil
soils of the nature of under-clays, on which little else appears to have
grown than a dense herbage of _Psilophyton_, along with plants of the
genus _Arthrostigma_.
In addition to these shaly groups, there are numerous examples of beds of
shale of small thickness included in coarse sandstones, and these beds
often occur in detached fragments, as if the remnants of more continuous
layers partially removed by currents of water. It is deserving of notice
that nearly all these patches of shale are interlaced with roots or
stems of _Psilophyton_, which sometimes project beyond their limits into
the sandstone, as if the vegetable fibres had preserved the clay from
removal. In short, these lines of patches of shale seem to be remnants
of soils on which _Psilophyton_ has flourished abundantly, and which
have been partially swept away by the currents which deposited the sand.
Some of the smaller patches may even be fragments of tough swamp soils
interwoven with roots, drifted by the agency of the waves or possibly by
ice; such masses are often moved in this way on the borders of modern
swamps on the sea-coast.
The only remaining point connected with local geology to which I shall
allude is the admirable facilities afforded by the Gaspé coast both for
ascertaining the true geological relations of the beds, and for studying
the Devonian plants, as distinctly exposed on large surfaces of rock.
On the coast of the river St. Lawrence, at Cape Rozier and its vicinity,
the Lower Silurian rocks of the Quebec group are well exposed, and are
overlaid unconformably by the massive Upper Silurian limestones of Cape
Gaspé, which rise into cliffs six hundred feet in height, and can be
seen filled with their characteristic fossils on both sides of the cape.
Resting upon these, and dipping at high angles toward Gaspé Bay, are the
Devonian sandstones, which are exposed in rugged cliffs slightly oblique
to their line of strike, along a coast-line of ten miles in length, to
the head of the bay. On the opposite side of the bay they reappear; and,
thrown into slight undulations by three anticlinal curves, occupy a
line of coast fifteen miles in length. The perfect manner in which the
plant-bearing beds are exposed in these fine natural sections may serve
to account for the completeness with which the forms and habits of growth
of the more abundant species can be described.
In the Bay des Chaleurs, similar rocks exist with some local variations.
In the vicinity of Campbellton are calcareous and magnesian breccia
or agglomerate, hard shales, conglomerates and sandstones of Lower
Devonian age. The agglomerate and lower shales contain abundant remains
of fishes of the genera _Cephalaspis_, _Coccosteus_, _Ctenacanthus_,
and _Homacanthus_, and also fragments of _Pterygotus_. The shales
and sandstones abound in remains of _Psilophyton_, with which are
_Nematophyton_, _Arthrostigma_, and _Leptophleum_ of the same species
found in the Lower Devonian of Gaspé Bay. These beds near Campbellton dip
to the northward, and the Restigouche River here occupies a synclinal,
for on the opposite side, at Bordeaux Quarry, there are thick beds of
grey sandstone dipping to the southward, and containing large silicified
trunks of Prototaxites, in addition to _Psilophyton_. These beds are
all undoubtedly Lower Erian, but farther to the eastward, on the north
side of the river, there are newer and overlying strata. These are best
seen at Scaumenac Bay, opposite Dalhousie, between Cape Florissant
and Maguacha Point, where they consist of laminated and fine-grained
sandstone, with shales of grey colours, but holding some reddish beds
at top, and overlaid unconformably by a great thickness of Lower
Carboniferous red conglomerate and sandstone. In these beds numerous
fossil fishes have been found, among which Mr. Whiteaves recognises
species of _Pterichthys_, _Glyptolepis_, _Cheirolepis_, &c. With these
are found somewhat plentifully four species of fossil ferns, all of Upper
Erian types, of which one is peculiar to this locality; but the others
are found in the Upper Erian of Perry, in Maine, or in the Cat skill
group of New York.
In order that distinct notions may be conveyed as to the geological
horizons of the species, I may state that the typical Devonian or Erian
series of Canada and New York may be divided in descending order into--1.
The Chemung group, including the Chemung and Portage sandstones and
shales. 2. The Hamilton group, including the Genesee, Hamilton, and
Marcellus shales. 3. The Corniferous limestone and its associated beds.
4 The Oriskany sandstone. As the Corniferous limestone, which is the
equivalent of the Lower Carboniferous limestone in the Carboniferous
period, is marine, and affords scarcely any plants, we may, as is usually
done for like purposes in the Carboniferous, group it with the Oriskany
under the name Lower Erian. The Hamilton rocks will then be Middle Erian,
and the Chemung group Upper Erian. In the present state of our knowledge,
the series may be co-ordinated with the rocks of Gaspé, New Brunswick,
and Maine, as in the following table:
| New York | Gaspé | Southern | Coast
Subdivisions. | and | and Bay des | New | of
| Western Canada.| Chaleurs. | Brunswick. | Maine.
--------------+----------------+--------------+--------------+-----------
Upper | Chemung | Upper |Mispec Group. | Perry
Devonian or | Group. | Sandstones. | Shale, |Sandstones.
Erian. | |Long Cove, &c.| Sandstone, |
| | Scauminac | and |
| | Beds. |Conglomerate. |
| | | |
Middle | Hamilton | Middle | Little R. |
Devonian or | Group. | Sandstones. | Group |
Erian. | | Bois Brulé, | (including |
| | Cape Oiseau, | Cordaite |
| | &c. | Shales and |
| | | Dadoxylon |
| | | Sandstone).|
| | | |
Lower | Corniferous | Lower | Lower |
Devonian or | and | Sandstones. |Conglomerates,|
Erian. | Oriskany | Gaspé Basin, | &c. |
| groups. | Little Gaspé,| |
| | &c. | |
| | Campbellton | |
| | Beds. | |
--------------+----------------+--------------+--------------+-----------
It may be proper, before closing this note, to state the reasons which
have induced me to suggest in the following pages the use of the term
"Erian," as equivalent to "Devonian," for the great system of formations
intervening between the Upper Silurian and the Lower Carboniferous in
America. I have been induced to adopt this course by the following
considerations: 1. The great area of undisturbed and unaltered rocks
of this age, including a thickness in some places of eighteen thousand
feet, and extending from east to west through the Northern States of the
Union and western Canada for nearly seven hundred miles, while it spreads
from north to south from the northern part of Michigan far into the
Middle States, is undoubtedly the most important Devonian area now known
to geologists. 2. This area has been taken by all American geologists
as their typical Devonian region. It is rich in fossils, and these have
been thoroughly studied and admirably illustrated by the New York and
Canadian Surveys. 3. The rocks of this area surround the basin of Lake
Erie, and were named, in the original reports of the New York Survey,
the "_Erie Division_" 4. Great difficulties have been experienced in
the classification of the European Devonian, and the uncertainties thus
arising have tended to throw doubt on the results obtained in America in
circumstances in which such difficulties do not occur.
These reasons are, I think, sufficient to warrant me in holding the great
_Erie Division_ of the New York geologists as the typical representative
of the rocks deposited between the close of the Upper Silurian and the
beginning of the Carboniferous period, and to use the term Erian as the
designation of this great series of deposits as developed in America,
in so far at least as their flora is concerned. In doing so, I do not
wish to introduce a new name merely for the sake of novelty; but I hope
to keep before the minds of geologists the caution that they should
not measure the Erian formations of America, or the fossils which they
contain, by the comparatively depauperated representatives of this
portion of the geological scale in the Devonian of western Europe.
VII.--On the Relations of the so-called "Ursa Stage" of Bear Island with
the Palæozoic Flora of North America.
The following note is a verbatim copy of that published by me in
1873, and the accuracy of which has now been vindicated by the recent
observations of Nathorst:
The plants catalogued by Dr. Heer, and characterising what he calls
the "Ursa Stage," are in part representatives of those of the American
flora which I have described as the "Lower Carboniferous Coal-Measures"
(Subcarboniferous of Dana), and whose characteristic species, as
developed in Nova Scotia, I noticed in the "Journal of the Geological
Society" in 1858 (vol. xv.). Dr. Heer's list, however, includes some
Upper Devonian forms; and I would suggest that either the plants of two
distinct beds, one Lower Carboniferous and the other Upper Devonian, have
been near to or in contact with each other and have been intermixed, or
else that in this high northern latitude, in which (for reasons stated
in my "Report on the Devonian Flora"[BZ]) I believe the Devonian plants
to have originated, there was an actual intermixture of the two floras.
In America, at the base of the Carboniferous of Ohio, a transition of
this kind seems to occur; but elsewhere in northeastern America the Lower
Carboniferous plants are usually unmixed with the Devonian.
[BZ] "Geological Survey of Canada," 1871.
Dr. Heer, however, proceeds to identify these plants with those of the
American Chemung, and even with those of the Middle Devonian of New
Brunswick, as described by me--a conclusion from which I must altogether
dissent, inasmuch as the latter belong to beds which were disturbed and
partially metamorphosed before the deposition of the lowest Carboniferous
or "Subcarboniferous" beds.
Dr. Heer's error seems to have arisen from want of acquaintance with the
rich flora of the Middle Devonian, which, while differing in species, has
much resemblance in its general facies, and especially in its richness in
ferns, to that of the coal-formation.
To geologists acquainted with the stratigraphy and the accompanying
animal fossils, Dr. Heer's conclusions will of course appear untenable;
but they may regard them as invalidating the evidence of fossil plants;
and for this reason it is, I think, desirable to give publicity to the
above statements.
I consider the British equivalent of the lower coal-measures of eastern
America to be the lower limestone shales, the _Tweedian group_ of Mr.
Tate (1858), but which have sometimes been called the "Calciferous
Sandstone" (a name preoccupied for a Cambrian group in America). This
group does not constitute "beds of passage" to the Devonian, more
especially in eastern America, where the lower coal-formation rests
unconformably on the Devonian, and is broadly distinguished by its
fossils.
The above notes would not have been extended to so great length, but
for the importance of the Erian flora as the precursor of that of the
Carboniferous, and the small amount of attention hitherto given to it by
geologists and botanists.
CHAPTER IV.
THE CARBONIFEROUS FLORA--CULMINATION OF THE
ACROGENS--FORMATION OF COAL.
Ascending from the Erian to the Carboniferous system, so called because
it contains the greatest deposits of anthracite and bituminous coal, we
are still within the limits of the Palæozoic period. We are still within
the reign of the gigantic club-mosses, cordaites, and taxine pines. At
the close of the Erian there had been over the whole northern hemisphere
great changes of level, accompanied by active volcanic phenomena, and
under these influences the land flora seems to have much diminished. At
length all the old Erian species had become extinct, and their place was
supplied by a meagre group of lycopods, ferns, and pines of different
species from those of the preceding Erian. This is the flora of the Lower
Carboniferous series, the Tweedian of England, the Horton series of Nova
Scotia, the lower coal-measures of Virginia, the culm of Germany. But
the land again subsided, and the period of the marine limestone of the
Lower Carboniferous was introduced. In this the older flora disappeared,
and when the land emerged we find it covered with the rich flora of the
coal-formation proper, in which the great tribes of the lycopods and
cordaites attained their maxima, and the ferns were continued as before,
though under new generic and specific forms.
[Illustration: Fig. 32.--Foliage from the coal-formation, _a_,
_Alethopteris lonchitica_, fern (Moose River). _b_, _Sphenophyllum
Schlotheimii_ (Pietou). _c_, _Lepidodendron binerve_ (Sydney), _d_,
_Asterophyllites foliosa_ (_?_) (Sydney). _e_, _Cordaites_ (Joggins).
_f_, _Neuropteris rarinervis_, fern (Sydney). _g_, _Odontopteris
subcuneata_, fern (Sydney).]
There is something very striking in this succession of a new plant
world without any material advance. It is like passing in the modern
world from one district to another, in which we see the same forms of
life, only represented by distinct though allied species. Thus, when
the voyager crosses the Atlantic from Europe to America, he meets with
pines, oaks, birches, poplars, and beeches of the same genera with those
he had left behind; but the species are distinct. It is something like
this that meets us in our ascent into the Carboniferous world of plants.
Yet we know that this is a succession in time, that all our old Erian
friends are dead and buried long ago, and that these are new forms lately
introduced (Fig. 32).
Conveying ourselves, then, in imagination forward to the time when our
greatest accumulations of coal were formed, and fancying that we are
introduced to the American or European continent of that period, we find
ourselves in a new and strange world. In the Devonian age, and even in
the succeeding Lower Carboniferous, there was in the interior of America
a wide inland sea, with forest belts clinging to its sides or clothing
its islands. But in the coal period this inland sea had given place
to vast swampy flats, and which, instead of the oil-bearing shales
of the Erian, were destined to produce those immense and wide-spread
accumulations of vegetable matter which constitute our present beds of
bituminous and anthracite coal. The atmosphere of these great swamps
is moist and warm. Their vegetation is most exuberant, but of forms
unfamiliar to modern eyes, and they swarm with insects, millipedes, and
scorpions, and with batrachian reptiles large and small, among which we
look in vain for representatives of the birds and beasts of the present
day.
[Illustration: Fig. 33.--_Sigillariæ_, restored. A, _Sigillaria Brownii_.
B, _Sigillaria elegans_.]
[Illustration: Fig. 34.--_Sigillaria Lorwayana_, Dawson. _a_, Zones of
fruit-scars. _b_, Leaf-scar enlarged, _c_, Fruit-scar enlarged. See
appended note.]
[Illustration: Fig. 35.--Stem of _Sigillaria Brownii_. reduced. Natural
size.]
[Illustration: Fig. 36.--Two ribs of _Sigillaria Brownii_.]
[Illustration: Fig. 37.--Portion of lower part of stem of _S. Brownii_.
Natural size.]
Prominent among the more gigantic trees of these swampy forests are
those known to us as _Sigillariæ_ (Fig. 33). They have tall, pillar-like
trunks, often several feet in diameter, ribbed like fluted columns, but
in the reverse way, and spreading at the top into a few thick branches,
which are clothed with long, grass-like leaves. They resemble in some
respects the Lepidodendra of the Erian age, but are more massive, with
ribbed instead of scaly trunks, and longer leaves. If we approach one
of them more closely, we are struck with the regular ribs of its trunk,
dotted with rows of scars of fallen leaves, from which it receives its
name _Sigillaria_, or seal-tree (Figs. 34-37). If we cut into its stem,
we find that, instead of the thin bark and firm wood with which we are
familiar in our modern trees, it has a hard external rind, then a great
thickness of cellular matter with rope-like bands of fibres, constituting
an inner bark, while in the centre is a firm, woody axis of comparatively
small diameter, and somewhat intermediate in its structures between that
of the Lepidodendra and those of the cycads and the taxine conifers.
Thus a great stem, five feet in diameter, may consist principally of
cellular and bast fibres with very little true woody matter. The roots
of this tree are perhaps its most singular feature. They usually start
from the stem in four main branches, then regularly bifurcate several
times, and then run out into great cylindrical cables, running for a
long distance, and evidently intended to anchor the plant firmly in a
soft and oozy soil. They were furnished with long, cylindrical rootlets
placed regularly in a spiral manner, and so articulated that when they
dropped off they left regular rounded scars. They are, in short, the
_Stigmariæ_, which we have already met with in the Erian (Figs. 38, 39).
In Fig. 33 I have endeavoured to restore these strange trees. It is not
wonderful that such plants have caused much botanical controversy. It was
long before botanists could be convinced that their roots are properly
roots at all, and not stems of some aquatic plant. Then the structure of
their stems is most puzzling, and their fruit is an enigma, for while
some have found connected with them cones supposed to resemble those of
lycopods, others attribute to them fruits like those of yew-trees. For
years I have been myself gathering materials from the rich coal-formation
deposits of Nova Scotia in aid of the solution of these questions, and
in the mean time Dr. Williamson, of Manchester, and Renault and other
botanists in France, have been amassing and studying stores of specimens,
and it is still uncertain who may finally be the fortunate discoverer
to set all controversies at rest. My present belief is, that the true
solution consists in the fact that there are many kinds of _Sigillariæ_.
While in the modern forests of America and Europe the species of any of
our ordinary trees, as oaks, birches, or maples, may almost be counted
on one's fingers, Schimper in his vegetable palæontology enumerates
about eighty species of Carboniferous _Sigillariæ_; and while on the one
hand many of these are so imperfectly known that they may be regarded
as uncertain, on the other hand many species must yet remain to be
discovered.[CA] Now, in so vast a number of species there must be a
great range of organisation, and, indeed, it has already been attempted
to subdivide them into several generic groups. The present state of the
question appears to me to be this, that in these _Sigillariæ_ we have
a group divisible into several forms, some of which will eventually be
classed with the Lepidodendra as lycopods, while others will be found
to be naked-seeded phænogams, allied to the pines and cycads, and to a
remarkable group of trees known as _Cordaites_, which we must shortly
notice.
[CA] In a recent memoir (Berlin, 1887) Stur has raised the number of
species in one subdivision of the _Sigillariæ_ (the _Favulariæ_) to
forty-seven!
[Illustration: Fig. 38.--_Stigmaria_ root, seen from above, showing its
regular divisions. From "Acadian Geology".]
[Illustration: Fig. 39.--Portion of bark of _Stigmaria_, showing scars of
attachment of rootlets.]
Before considering other forms of Carboniferous vegetation, let us
glance at the accumulation of coal, and the agency of the forests of
_Sigillariæ_ therein. Let us imagine, in the first instance, such trees
as those represented in the figures, growing thickly together over vast
swampy flats, with quantities of undergrowth of ferns and other plants
beneath their shade, and accumulating from age to age in a moist soil
and climate a vast thickness of vegetable mould and trunks of trees, and
spores and spore-cases, and we have the conditions necessary for the
growth of coal. Many years ago it was observed by Sir William Logan that
in the coal-field of South Wales it was the rule with rare exceptions
that, under every bed of coal, there is a bed of clay filled with roots
of the _Stigmaria_, already referred to as the root of _Sigillaria_. This
discovery has since been extended to all the coal-fields of Europe and
America, and it is a perfectly conclusive fact as regards the origin of
coal. Each of these "under-clays," as they are called, must, in fact,
have been a soil on which grew, in the first instance, Sigillariæ and
other trees having stigmaria-roots. Thus, the growth of a forest of
_Sigillariæ_ was the first step toward the accumulation of a bed of
coal. More than this, in some of the coarser and more impure coals,
where there has been sufficient earthy matter to separate and preserve
impressions of vegetable forms, we can see that the mass of the coal is
made up of flattened _Sigillariæ_, mixed with vegetable _débris_ of all
kinds, including sometimes vast quantities of lepidodendroid spores,
and the microscopic study of the coal gives similar results (Fig. 40).
Further, on the surfaces of many coals, and penetrating the shales or
sandstones which form their roofs, we find erect stumps of sigillaria
and other trees, showing that the accumulation of the coal terminated as
it had begun, by a forest-growth. I introduce here a section of a few of
the numerous beds of coal exposed in the cliffs of the South Joggins, in
Nova Scotia, in illustration of these facts. We can thus see how in the
slowly subsiding areas of the coal-swamps successive beds of coal were
accumulated, alternating with beds of sandstone and shale (Figs. 41, 42).
For other details of this kind I must refer to papers mentioned in the
sequel.
[Illustration: Fig. 40.--Vegetable tissues from coal. _a_, _Sigillaria_
and _Cordaites_. _Calamodendron_.]
Returning to the more special subject of this work, I may remark that the
lepidodendroid trees and the ferns, both the arborescent and herbaceous
kinds, are even more richly represented in the Carboniferous than in the
preceding Erian, I must, however, content myself with merely introducing
a few representatives of some of the more common kinds, in an appended
note, and here give a figure of a well-known Lower Carboniferous
lepidodendron, with its various forms of leaf-bases, and its foliage and
fruit (Fig. 43), and a similar illustration of an allied generic form,
that known as _Lepidophloios_[CB] (Fig. 44).
[CB] For full descriptions of these, see "Acadian Geology."
[Illustration: Fig. 41.--Beds associated with the main coal (S. Joggins,
Nova Scotia). 1, Shale and sandstone--plants with _Spirorbis_ attached;
rain-marks (?). (2, Sandstone and shale, eight feet--erect _Calamites_;
3, Gray sandstone, seven feet; 4, Gray shale, four feet--an erect
coniferous (?) tree, rooted on the shale, passes up through fifteen feet
of the sandstones and shale.) 5, Gray sandstone, four feet. 6, Gray
shale, six inches--prostrate and erect trees, with rootlets, leaves,
_Naiadites_, and _Spirorbis_ on the plants. 7, Main coal-seam, five feet
of coal in two seams. 8, Underclay, with rootlets.]
Another group which claims our attention is that of the _Calamites_.
These are tall, cylindrical, branchless stems, with whorls of branchlets,
bearing needle-like leaves and spreading in stools from the base, so
as to form dense thickets, like Southern cane-brakes (Fig. 46). They
bear, in habit of growth and fructification, a close relation to our
modern equisetums, or mare's-tails, but, as in other cases we have met
with, are of gigantic size and comparatively complex structure. Their
stems, in cross-section, show radiating bundles of fibres, like those
of exogenous woods, yet the whole plan of structure presents some
curious resemblances to the stems of their humble successors, the modern
mare's-tails. It would seem, from the manner in which dense brakes of
these _Calamites_ have been preserved in the coal-formation of Nova
Scotia, that they spread over low and occasionally inundated flats, and
formed fringes on the seaward sides of the great Sigillaria forests.
In this way they no doubt contributed to prevent the invasion of the
areas of coal accumulation by the muddy waters of inundations, and thus,
though they may not have furnished much of the material of coal, they
no doubt contributed to its purity. Many beautiful plants of the genera
Asterophyllites and _Annularia_ are supposed to have been allied to the
_Calamites_, or to have connected them with the _Rhizocarps_. The stems
and fruit of these plants have strong points of resemblance to those of
_Sphenophyllum_, and the leaves are broad, and not narrow and angular
like those of the true _Calamites_ (Fig. 45).
[Illustration: Fig. 42.--Erect _Sigillaria_, standing on a coal-seam (S.
Joggins, Nova Scotia).]
[Illustration: Fig. 43.--_Lepidodendron corrugatum_, Dawson, a tree
characteristic of the Lower Carboniferous, A, Restoration. B, Leaf,
natural size, C, Cone and branch, D, Branch and leaves, E. Various forms
of leaf-areoles. F, _Sporangium_, I, L, M, Bark, with leaf-scars, N,
Bark, with leaf-scars of old stem, O, Decorticated stem (_Knorria_).]
[Illustration: Fig. 44.--_Lepidophloios Acadianus_, Dawson, a
lepidodendroid tree of the coal-formation, A, Restoration. B, Portion
of bark (two thirds natural size), C, Ligneous surface of the same, F,
Cone (two thirds natural size). G, Leaf (natural, size), K, Portion of
woody cylinder, showing outer and inner series of vessels magnified, L,
Scalariform vessels (highly magnified), M, Various forms of leaf-scars
and leaf-bases (natural size).]
[Illustration: Fig. 45.--_Asterophyllites_, _Sphenophyllum_, and
_Annularia_. A, _Asterophyllites trinerne_. A^1, Leaf enlarged, B,
_Annularia sphenophylloides_. B^1, Leaf enlarged, C, _Sphenophyllum
erosum_. C^1, Leaflet enlarged. C^2, Scalariform vessel of
_Sphenophyllum_. D, _Pinnularia ramosissima_, probably a root.]
No one has done more than my friend Dr. Williamson, of Manchester, to
illustrate the structure of Calamites, and he has shown that these
plants, like other cryptogams of the Carboniferous, had mostly stems
with regular fibrous wedges, like those of exogens. The structure of
the stem is, indeed, so complex, and differs so much in different
stages of growth, and different states of preservation, that we are
in danger of falling into the greatest confusion in classifying these
plants. Sometimes what we call a Calamite is a mere cast of its pith
showing longitudinal striæ and constrictions at the nodes. Sometimes
we have the form of the outer surface of the woody cylinder, showing
longitudinal ribs, nodes, and marks of the emission of the branchlets.
Sometimes we have the outer surface of the plant covered with a smooth
bark showing flat ribs, or almost smooth, and having at the nodes regular
articulations with the bases of the verticillate branchlets, or on the
lower part of the stem the marks of the attachment of the roots. The
Calamites grew in dense clumps, budding off from one another, sometimes
at different levels, as the mud or sand accumulated about their stems,
and in some species there were creeping rhizomata or root-stocks (Figs.
46 to 49).
[Illustration: Fig. 46.--_Calamites_. A, _C. Suckovii_. B, _C. Cistii_.
(From "Acadian Geology.")]
[Illustration: Fig. 47.--Erect _Calamites_, with roots attached (Nova
Scotia).]
[Illustration: Fig. 48.--Node of _C. Cistii_, with long leaves (Nova
Scotia).]
But all Calamites were not alike in structure. In a recent paper[CC]
Dr. Williamson describes three distinct structural types. What he
regards as typical Calamites has in its woody zone wedges of barred
vessels, with thick bands of cellular tissue separating them. A second
type, which he refers to _Calamopitus_, has woody bundles composed of
reticulated or multiporous fibres, with their porous sides parallel to
the medullary rays, which are better developed than in the previous form.
The intervening cellular masses are composed of elongated cells. This is
a decided advance in structure, and is of the type of those forms having
the most woody and largest stems, which Brongniart named _Calamodendron_
(Fig. 50). A third form, to which Dr. Williamson seems to prefer to
assign this last name, has the tissue of the woody wedges barred, as
in the first, but the medullary rays are better developed than in the
second. In this third form the intermediate tissue, or primary medullary
rays, is truly fibrous, and with secondary medullary rays traversing
it. My own observations lead me to infer that there was a fourth type
of calamitean stem, less endowed with woody matter, and having a
larger fistulous or cellular cavity than any of those described by Dr.
Williamson.
[CC] "Memoirs of the Philosophical Society," Manchester, 1886-'87.
[Illustration: Fig. 49.--Erect _Calamites_ (_C. Suckovii_), showing the
mode of growth of new stems (_b_), and different forms of the ribs (_a_,
_c_). (Pictou, Nova Scotia.) Half natural size.]
There is every reason to believe that all these various and complicated
stems belonged to higher and nobler types of mare's-tails than those of
the modern world, and that their fructification was equisetaceous and of
the form known as _Calamostachys_.
We have already seen that noble tree-ferns existed in the Erian period,
and these were continued, and their number and variety greatly extended,
in the Carboniferous. In regard to the structure of their stems, and the
method of supporting these by aërial roots, the tree-ferns of all ages
have been nearly alike, and the form and structure of the leaves, except
in some comparatively rare and exceptional types, has also been much the
same. Any ordinary observer examining a collection of coal-formation
ferns recognises at once their kinship to the familiar brackens of our
own time. Their fructification is, unfortunately, rarely preserved, so
that we are not able, in the case of many species, to speak confidently
of their affinities with modern forms; but the knowledge of this subject
has been constantly extending, and a sufficient amount of information
has been obtained to enable us to say something as to their probable
relationships. (Figs. 51 to 55.)
[Illustration: Fig. 50.--Stems of _Calamodendron_ and tissues magnified
(Nova Scotia), _a_, _b_, Casts of axis in sandstone, with woody envelope
(reduced). _c_, _d_, Woody tissue (highly magnified).]
The families into which modern ferns are divided are, it must be
confessed, somewhat artificial, and in the case of fossil ferns, in
which the fructification is for the most part wanting, it is still more
so, depending in great part on the form and venation of the divisions
of the fronds. Of about eight families into which modern ferns are
divided, seven are found in a fossil state, and of these, four at least,
the _Cyathaceæ_, the _Ophioglosseæ_, the _Hymenophyllaceæ_, and the
_Marattiaceæ_, go back to the coal-formation.[CD]
[CD] Mr. R. Kidston has recently described very interesting forms of fern
fructification from the coal-formation of Great Britain, and much has
been done by European palæobotanists, and also by Lesquereux and Fontaine
in America.
[Illustration: Fig. 51.--Group of coal-formation ferns, A, _Odontopteris
subcuneata_ (Bunbury), B, _Neuropteris cordata_ (Brongniart). C,
_Alethopteris lonchitica_ (Brongniart). D, _Dictyopteris obliqua_
(Bunbury). E, _Phyllopteris antiqua_ (Dawson), magnified; E^1, Natural
size, F, _Neuropteris cyclopteroides_ (Dawson).]
[Illustration: Fig. 52.--_Alethopteris grandis_ (Dawson). Middle
coal-formation of Nova Scotia.]
[Illustration: Fig. 53.--_Cyclopteris_ (_Aneimites_) _Acadica_ (Dawson),
a tree-fern of the Lower Carboniferous. _a_, Pinnules. _b_, Fragment of
petiole. _c_, Remains of fertile pinnules.]
[Illustration: Fig. 54.--_Sphenopteris latior_, Dawson. Coal-formation,
_a_, Pinnule magnified, with traces of fructification.]
[Illustration: Fig. 55.--Fructification of Palæozoic ferns, _a_, Thecæ of
_Archæopteris_ (Erian). _b_, Theca of _Senftenbergia_ (Carboniferous).
_c_, Thecæ of _Asterotheca_ (Carboniferous).]
[Illustration: Fig. 56.--Tree-ferns of the Carboniferous. A, _Megaphyton
magnificum_, Dawson, restored. B, Leaf-scar of the same, two thirds
natural size. B^1, Row of leaf-scars, reduced. C, _Palæopteris Harttii_,
scars half natural size. D, _Acadica_, scars half natural size.]
Some of these ferns have the more complex kind of spore-case, with a
jointed, elastic ring. It is to be observed, however, that those forms
which have a simple spore-case, either netted or membranous, and without
annulus, are most common in the Devonian and lowest Carboniferous.
Some of the forms in these old rocks are somewhat difficult to place
in the system. Of these, the species of _Archæopteris_, of the Upper
and Middle Erian, are eminent as examples. This type, however, scarcely
extends as high as the coal-formation.[CE] Some of the tree-ferns of the
Carboniferous present very remarkable features. One of these, of the
genus _Megaphyton_, seems to have two rows of great leaves, one at each
side of the stem, which was probably sustained by large bundles of aërial
roots (Fig. 56).
[CE] The pretty little ferns of the genus _Botrychium_ (moonwort), so
common in American and European woods, seem to be their nearest modern
allies.
In the Carboniferous, as in the Erian, there are leaves which have been
referred to ferns, but are subject to doubt, as possibly belonging to
broad-leaved taxine trees allied to the gingko-tree of China. One of
these, represented in Fig. 57, has been found in the coal-formation of
Nova Scotia, and referred to the doubtful genus _Noeggerathia_. Fontaine
has proposed for similar leaves found in Virginia the new generic name
_Saportea_.
[Illustration: Fig. 57.--_Noeggerathia disbar_ (half natural size).]
Ferns, as might be inferred from their great age, are at the present time
dispersed over the whole world; but their headquarters, and the regions
to which tree-ferns are confined, are the more moist climates of the
tropics and of the southern hemisphere. The coal-swamps of the northern
hemisphere seem to have excelled even these favoured regions of the
present world as a paradise for ferns.
I have already stated that the Carboniferous constitutes the headquarters
of the _Cordaites_ (Fig. 58), of which a large number of species have
been described, both in Europe and America. We sometimes, though
rarely, find their stems showing structure. In this case we have a
large cellular pith, often divided by horizontal partitions into flat
chambers, and constituting the objects which, when detached, are called
_Sternbergiæ_ (Fig. 62). These Sternbergia piths, however, occur in true
conifers as well, as they do in the modern world in some trees, like our
common butternut, of higher type; and I showed many years ago that the
Sternbergia type may be detected in the young twigs of the balsam-fir
(_Abies balsamifera_). The pith was surrounded by a ring of scalariform
or barred tissue, often of considerable thickness, and in young stems so
important as to have suggested lycopodiaceous affinities. But as the stem
grew in size, a regular ring of woody wedges, with tissue having rounded
or hexagonal pores or discs, like those of pines, was developed. Outside
this was a bark, often apparently of some thickness. This structure in
many important points resembles that of cycads, and also approaches to
the structure of Sigillaria, while in its more highly developed forms it
approximates to that of the conifers.
[Illustration: Fig. 58.--_Cordaites_ (_Dorycordaites_), Grand d'Eury,
reduced.]
[Illustration: Fig. 59.--Fruits of _Cordaites_ and Taxine Conifers
(coal-formation. Nova Scotia.) A, _Antholithes squamosus_ (two thirds).
B, _A. rhabdocarpi_. (two thirds). B^1, Carpel restored. C, _A. spinosus_
(natural size). D, _Trigonocarpum intermedium_. E, _T. Noeggerathii_.
F, _T. avellanum_. G, _Rhabdocarpus insignis_, reduced. H, _Antholithes
pygmæus_. I, _Cardiocarpum fluitans_. K, _Cardiocarpum bisectum_. L,
_Sporangites papillata_, lycopodiaceous macrospores (natural size and
magnified).]
On the stems so constructed were placed long and often broad many-nerved
leaves, with rows of stomata or breathing-pores, and attached by somewhat
broad bases to the stem and branches. The fruit consisted of racemes, or
clusters of nutlets, which seem to have been provided with broad lateral
wings for flotation in the air, or in some cases with a pulpy envelope,
which flattens into a film. There seem to have been structures of both
these kinds, though in the state of preservation of these curious seeds
it is extremely difficult to distinguish them. In the first case they
must have been intended for dissemination by the wind, like the seeds
of spruces. In the latter case they may have been disseminated like the
fruits of taxine trees by the agency of animals, though what these were
it would be difficult to guess. These trees had very great reproductive
power, since they produced numerous seeds, not singly or a few together,
as in modern yews, but in long spikes or catkins bearing many seeds (Fig.
59).
It is to be observed that the Cordaites, or the _Cordaitinæ_, as
they have been called, as a family,[CF] constitute another of those
intermediate groups with which we have already become familiar. On
the one hand they approach closely to the broader-leaved yews like
Gingko, Phyllocladus, and Podocarpus, and, on the other hand, they
have affinities with Cycadaceæ, and even with Sigillariæ. They were
beautiful and symmetrical trees, adding something to the variety of the
rather monotonous Palæozoic forests. They contributed also somewhat to
the accumulation of coal. I have found that some thin beds are almost
entirely composed of their leaves, and the tissues of their wood are not
infrequent in the mineral charcoal of the larger coal-seams. There is
no evidence that their roots were of the stigmaroid type, though they
evidently grew in the same swampy flats with the Sigillariæ and Calamites.
[CF] Engler; Cordaitées of Renault.
It may, perhaps, be well to say here that I believe there was a
considerably wide range of organisation in the Cordaitinæ as well as in
the Calamites and Sigillariæ, and that it will eventually be found that
there were three lines of connection between the higher cryptogams and
the phænogams, one leading from the lycopods by the Sigillariæ, another
leading by the Cordaites, and the third leading from the Equisetums by
the Calamites. Still further back the characters afterward separated in
the club-mosses, mare's-tails, and ferns, were united in the Rhizocarps,
or, as some now, but I think somewhat unreasonably, prefer to call
them, the "heterosporous Filicinæ." In the more modern world, all the
connecting links have become extinct and the phænogams stand widely
separated from the higher cryptogams. I do not make these remarks in
a Darwinian sense, but merely to state what appear to be the lines of
natural affinity and the links wanting to give unity to the system of
nature.
Of all the trees of the modern world, none are perhaps so widely
distributed as the pines and their allies. On mountain-tops and
within the Arctic zone, the last trees that can struggle against the
unfavourable conditions of existence are the spruces and firs, and in
the warm and moist islands of the tropics they seem equally at home
with the tree-ferns and the palms. We have already seen that they are
a very ancient family, and in the sandstones of the coal-formation
their great trunks are frequently found, infiltrated with calcareous or
silicious matter, and still retaining their structure in the greatest
perfection (Fig. 60). So far as we know, the foliage of some of them
which constitutes the genera _Walchia_ and _Araucarites_ of some authors
(Figs. 60, 63) was not dissimilar from that of modern yews and spruces,
though there is reason to believe that some others had broad, fern-like
leaves like those of the gingko. None of them, so far as yet certainly
known, were cone-bearing trees, their fruit having probably been similar
to that of the yews (Fig. 61). The minute structures of their stems are
nearer to those of the conifers of the islands of the southern hemisphere
than to that of those in our northern climes--a correlation, no doubt, to
the equable climate of the period. There is not much evidence that they
grew with the Sigillariæ in the true coal-swamps, though some specimens
have been found in this association. It is more likely that they were in
the main inland and upland trees, and that in consequence they are mostly
known to us by drifted trunks borne by river inundations into the seas
and estuaries.
[Illustration: Fig. 60.--Coniferous wood and foliage (Carboniferous). A,
_Araucarites gracilis_, reduced, b, _Dadoxylon Acadianum_ (radial), 90
diams.; B^1 (tangential), 90 diams; B^2, cell showing areolation, 250
diams. C, _Dadoxylon materiarium_ (radial), 90 diams.; C^1 (tangential),
90 diams. C^2, cell showing areolation, 250 diams. D, _Dadoxylon
antiquius_ (radial), 90 diams.; D^1 (tangential), 90 diams.; D^2, cell
showing areolation, 250 diams.]
[Illustration: Fig. 61.--__Trigonocarpum Hookeri_, Dawson from the
coal-measures of Cape Breton. Probably the fruit of a Taxine tree. A,
Broken specimen magnified twice natural size, B, Section magnified: _a_,
the testa; _b_, the tegmen; _c_, the nucleus; _d_, the embryo, _c_,
Portion of the surface of the inner coat more highly magnified.]
A remarkable fact in connection with them, and showing also the manner in
which the most durable vegetable structures may perish by decay, is that,
like the Cordaites, they had large piths with transverse partitions, a
structure which, as I have already mentioned, appears on a minute scale
in the twigs of the fir-tree, and that sometimes casts of these piths in
sandstone appear in a separate form, constituting what have been named
_Sternbergiæ_ or _Artisiæ_. As Renault well remarks with reference to
Cordaites, the existence of this chambered form of pith implies rapid
elongation of the stem, so that the Cordaites and conifers of the
coal-formation were probably quickly growing trees (Fig. 62).
[Illustration: Fig. 62.--_Sternbergia_ pith of _Dadoxylon_. A, Specimen
(natural size), showing remains of wood at _a_, _a_. B, Junction of
wood and pith, magnified. C, Cells of the wood of do., _a_, _a_; _b_,
medullary ray; _c_, areolation.]
The same general statements may be made as to the coal-vegetation as in
relation to that of the Erian. In the coal period we have found none of
the higher exogens, and there are only obscure and uncertain indications
of the presence of endogens, which we may reserve for a future chapter;
but gymnosperms abound and are highly characteristic. On the other hand,
we have no mosses or lichens, and very few Algæ, but a great number of
ferns and Lycopodiaceæ or club-mosses (Fig. 63). Thus, the coal-formation
period is botanically a meeting-place of the lower phænogams and the
higher cryptogams, and presents many forms which, when imperfectly known,
have puzzled botanists in regard to their position in one or other
series. In the present world, the flora most akin to that of the coal
period is that of warm, temperate regions in the southern hemisphere. It
is not properly a tropical flora, nor is it the flora of a cold region,
but rather indicative of a moist and equable climate. Still, we must bear
in mind that we may often be mistaken in reasoning as to the temperature
required by extinct species of plants, differing from those now in
existence. Further, we must not assume that the climatal conditions of
the northern hemisphere were in the coal period at all similar to those
which now prevail. As Sir Charles Lyell has shown, a less amount of land
in the higher latitudes would greatly modify climates, and there is
every reason to believe that in the coal period there was less land than
now. Further, it has been shown by Tyndall that a very small additional
amount of carbonic acid in the atmosphere would, by obstructing the
radiation of heat from the earth, produce almost the effect of a glass
roof or conservatory, extending over the whole world. Again, there is
much in the structure of the leaves of the coal-plants, as well as in
the vast amount of carbon which they accumulated in the form of coal,
and the characteristics of the animal life of the period, to indicate,
on independent grounds, that the carboniferous atmosphere differed
from that of the present world in this way, or in the presence of more
carbonic acid--a substance now existing in the very minute proportion
of one thousandth of the whole--a quantity adapted to the present
requirements of vegetable and animal life, but probably not to those of
the coal period.
[Illustration: Fig. 63.--_Walchia imbricatula_, S. N., Permian, Prince
Edward Island.]
Thus, if we inquire as to any analogous distribution of plants in the
modern world, we find this only in the warmer insular climates of the
southern hemisphere, where ferns, lycopods, and pines appear under forms
somewhat akin to those of the Carboniferous, but mixed with other types,
some of which are modern, others allied to those of the next succeeding
geological ages of the Mesozoic and Tertiary; and under these periods it
will be more convenient to make comparisons.
The readers of recent English popular works on geology will have observed
the statement reiterated that a large proportion of the material of
the great beds of bituminous coal is composed of the spore-cases of
lycopodiaceous plants--a statement quite contrary to that resulting
from my microscopical examinations of the coal of more than eighty
coal-beds in Nova Scotia and Cape Breton, as stated in "Acadian Geology"
(page 463), and more fully in my memoir of 1858 on the "Structures
in Coal,"[CG] and that of 1866, on the "Conditions of Accumulation
of Coal."[CH] The reason of this mistake is, that an eminent English
naturalist, happening to find in certain specimens of English coal a
great quantity of remains of spores and spore-cases, though even in his
specimens they constitute only a small portion of the mass, and being
apparently unacquainted with what others had done in this field, wrote a
popular article for the "Contemporary Review," in which he extended an
isolated and exceptional fact to all coals, and placed this supposed
origin of coal in a light so brilliant and attractive that he has been
followed by many recent writers. The fact is, as stated in "Acadian
Geology," that trunks of _Sigillariæ_ and similar trees constitute a
great part of the denser portion of the coal, and that the cortical
tissues of these rather than the wood remain as coal. But cortical or
epidermal tissues in general, whether those of spore-cases or other
parts of plants, are those which from their resistance to water-soakage
and to decay, and from their highly carbonaceous character, are best
suited to the production of coal. In point of fact, spore-cases, though
often abundantly present, constitute only an infinitesimal part of the
matter of the great coal-beds. In an article in "The American Journal
of Science," which appeared shortly after that above referred to, I
endeavoured to correct this error, though apparently without effect in
so far as the majority of British geological writers are concerned. From
this article I have taken with little change the following passages, as
it is of importance in theoretical geology that such mistakes, involving
as they do the whole theory of coal accumulation, should not continue to
pass current. The early part of the paper is occupied with facts as to
the occurrence of spores and spore-cases as partial ingredients in coal.
Its conclusions are as follows: It is not improbable that sporangites,
or bodies resembling them, may be found in most coals; but it is most
likely that their occurrence is accidental rather than essential to coal
accumulation, and that they are more likely to have been abundant in
shales and cannel coals, deposited in ponds or in shallow waters in the
vicinity of lycopodiaceous forests, than in the swampy or peaty deposits
which constitute the ordinary coals. It is to be observed, however,
that the conspicuous appearance which these bodies, and also the strips
and fragments of epidermal tissue, which resemble them in texture,
present in slices of coal, may incline an observer, not having large
experience in the examination of coals, to overrate their importance; and
this I think has been done by most microscopists, especially those who
have confined their attention to slices prepared by the lapidary. One
must also bear in mind the danger arising from mistaking concretionary
accumulations of bituminous matter for sporangia. In sections of the
bituminous shales accompanying the Devonian coal above mentioned, there
are many rounded yellow spots, which on examination prove to be the
spaces in the epidermis of _Psilophyton_ through which the vessels
passing to the leaves were emitted. To these considerations I would add
the following, condensed from the paper above referred to (p. 139), in
which the whole question of the origin of coal is fully discussed:[CI]
[CG] "Journal of the Geological Society," vol. xv.
[CH] _Ibid._, vol. xxii.
[CI] See also "Acadian Geology," 2d ed., pp. 138, 461, 493.
1. The mineral charcoal or 'mother coal' is obviously woody tissue
and fibres of bark, the structure of the varieties of which, and the
plants to which it probably belongs, I have discussed in the paper above
mentioned.
2. The coarser layers of coal show under the microscope a confused mass
of fragments of vegetable matter belonging to various descriptions of
plants, and including, but not usually in large quantities, sporangites.
3. The more brilliant layers of the coal are seen, when separated by thin
laminæ of clay, to have on their surfaces the markings of _Sigillariæ_
and other trees, of which they evidently represent flattened specimens,
or rather the bark of such specimens. Under the microscope, when their
structures are preserved, these layers show cortical tissues more
abundantly than any others.
4. Some thin layers of coal consist mainly of flattened layers of leaves
of _Cordaites_ or _Pychnophyllum_.
5. The _Stigmaria_ under-clays and the stumps of _Sigillaria_ in the
coal-roofs equally testify to the accumulation of coal by the growth of
successive forests, more especially of _Sigillariæ_. There is, on the
other hand, no necessary connection of sporangite-beds with Stigmarian
soils. Such beds are more likely to be accumulated in water, and
consequently to constitute bituminous shales and cannels.
6. _Lepidodendron_ and its allies, to which the spore-cases in question
appear to belong, are evidently much less important to coal accumulation
than _Sigillaria_, which cannot be affirmed to have produced spore-cases
similar to those in question, even though the observation of Goldenberg
as to their fruit can be relied on; the accuracy of which, however, I am
inclined to doubt.
On the whole, then, while giving due credit to those who have advocated
the spore-theory of coal, for directing attention to this curious and no
doubt important constituent of mineral fuel, and admitting that I may
possibly have given too little attention to it, I must maintain that
sporangite-beds are exceptional among coals, and that cortical and woody
matters are the most abundant ingredients in all the ordinary kinds; and
to this I cannot think that the coals of England constitute an exception.
It is to be observed, in conclusion, that the spore-cases of plants,
in their indestructibility and richly carbonaceous character, only
partake of qualities common to most suberous and epidermal matters, as I
have explained in the publications already referred to. Such epidermal
and cortical substances are extremely rich in carbon and hydrogen,
in this resembling bituminous coal. They are also very little liable
to decay, and they resist more than other vegetable matters aqueous
infiltration--properties which have caused them to remain unchanged,
and to continue free from mineral additions more than other vegetable
tissues. These qualities are well seen in the bark of our American white
birch. It is no wonder that materials of this kind should constitute
considerable portions of such vegetable accumulations as the beds of
coal, and that when present in large proportion they should afford richly
bituminous beds. All this agrees with the fact, apparent on examination
of the common coal, that the greater number of its purest layers consist
of the flattened bark of _Sigillariæ_ and similar trees, just as any
single flattened trunk embedded in shale becomes a layer of pure coal.
It also agrees with the fact that other layers of coal, and also the
cannels and earthy bitumens, appear under the microscope to consist of
finely comminuted particles, principally of epidermal tissues, not only
from the fruits and spore-cases of plants, but also from their leaves and
stems. These considerations impress us, just as much as the abundance of
spore-cases, with the immense amount of the vegetable matter which has
perished during the accumulation of coal, in comparison with that which
has been preserved.
I am indebted to Dr. T. Sterry Hunt for the following very valuable
information, which at once places in a clear and precise light the
chemical relations of epidermal tissue and spores with coal. Dr. Hunt
says: "The outer bark of the cork-tree, and the cuticle of many if not
all other plants, consists of a highly carbonaceous matter, to which
the name of _suberin_ has been given. The spores of _Lycopodium_ also
approach to this substance in composition, as will be seen by the
following, one of two analyses by Duconi,[CJ] along with which I give
the theoretical composition of pure cellulose or woody fibre, according
to Payen and Mitscherlich, and an analysis of the suberin of cork, from
_Quercus suber_, from which the ash and 2·5 per cent of cellulose have
been deducted.[CK]
[CJ] Liebig and Kopp, "Jahresbuch," 1847-'48.
[CK] Gmelin, "Handbook," xv., 145.
---------+-----------+--------+------------
| Cellulose.| Cork. | Lycopodium.
---------+-----------+--------+------------
Carbon | 44·44 | 65·73 | 64·80
Hydrogen | 6·17 | 8·33 | 8·73
Nitrogen | .... | 1·50 | 6·18
Oxygen | 49·39 | 24·44 | 20·29
+-----------+--------+------------
Total | 100·00 | 100·00 | 100·00
---------+-----------+--------+------------
"This difference is not less striking when we reduce the above centesimal
analyses to correspond with the formula of cellulose, C{24}H{20}O{20},
and represent cork and _Lycopodium_ as containing twenty-four equivalents
of carbon. For comparison I give the composition of specimens of peat,
brown coal, lignite, and bituminous coal:[CL]
[CL] "Canadian Naturalist," vi., 253.
Cellulose C{24}H{20}O{20}
Cork C{24}H{18-2/10}O{6-7/10}
Lycopodium C{24}H{19-4/10}NO{5-6/10}
Peat (Vaux) C{24}H{14-4/10}O{10}
Brown coal (Schröther) C{24}H{14-3/10}O{10-6/10}
Lignite (Vaux) C{24}H{11-3/10}O{6-4/10}
Bituminous coal (Regnault) C{24}H{10}O{3-3/10}
"It will be seen from this comparison that, in ultimate composition, cork
and _Lycopodium_ are nearer to lignite than to woody fibre, and may be
converted into coal with far less loss of carbon and hydrogen than the
latter. They in fact approach closer in composition to resins and fats
than to wood, and, moreover, like those substances repel water, with
which they are not easily moistened, and thus are able to resist those
atmospheric influences which effect the decay of woody tissue."
I would add to this only one further consideration. The nitrogen
present in the _Lycopodium_ spores, no doubt, belongs to the protoplasm
contained in them, a substance which would soon perish by decay; and
subtracting this, the cell-walls of the spores and the walls of the
spore-cases would be most suitable material for the production of
bituminous coal. But this suitableness they share with the epidermal
tissue of the scales of strobiles, and of the stems and leaves of ferns
and lycopods, and, above all, with the thick, corky envelope of the
stems of _Sigillariæ_ and similar trees, which, as I have elsewhere
shown,[CM] from its condition in the prostrate and erect trunks contained
in the beds associated with coal, must have been highly carbonaceous
and extremely enduring and impermeable to water. In short, if, instead
of "spore-cases," we read "epidermal tissues in general, including
spore-cases," all that has been affirmed regarding the latter will
be strictly and literally true, and in accordance with the chemical
composition, microscopical characters, and mode of occurrence of coal. It
will also be in accordance with the following statement, from my paper on
the "Structures in Coal," published in 1859:
[CM] "Vegetable Structures in Coal," "Journal of Geological Society,"
xv., 626. "Conditions of Accumulation of Coal," _ibid._, xxii., 95.
"Acadian Geology," 197, 464.
"A single trunk of _Sigillaria_ in an erect forest presents an epitome of
a coal-seam. Its roots represent the _Stigmaria_ under-clay; its bark the
compact coal; its woody axis the mineral charcoal; its fallen leaves (and
fruits), with remains of herbaceous plants growing in its shade, mixed
with a little earthy matter, the layers of coarse coal. The condition of
the durable outer bark of erect trees concurs with the chemical theory
of coal, in showing the especial suitableness of this kind of tissue for
the production of the purer compact coals. It is also probable that the
comparative impermeability of the bark to mineral infiltration is of
importance in this respect, enabling this material to remain unaffected
by causes which have filled those layers, consisting of herbaceous
materials and decayed wood, with pyrites and other mineral substances."
We need not go far in search of the uses of the coal vegetation, when .we
consider the fact that the greatest civilised nations are dependent on
it for their fuel. Without the coal of the Carboniferous period and the
iron-ore which is one of the secondary consequences of coal accumulation,
just as bog-ores of iron occur in the subsoils of modern peats, it would
have been impossible either to sustain great nations in comfort in the
colder climates of the northern hemisphere or to carry on our arts and
manufactures. The coal-formation yields to Great Britain alone about one
hundred and sixty million tons of coal annually, and the miners of the
United States extract mainly from the same formation nearly a hundred
million tons, while the British colonies and dependencies produce about
five million tons; and it is a remarkable fact that it is to the English
race that the greatest supply of this buried power and heat and light has
been given.
The great forests of the coal period, while purifying the atmosphere of
its excess of unwholesome carbonic acid, were storing up the light and
heat of Palæozoic summers in a form in which they could be recovered in
our human age, so that, independently of their uses to the animals which
were their contemporaries, they are indispensable to the existence of
civilised man.
Nor can we hope soon to be able to dispense with the services of
this accumulated store of fuel. The forests of to-day are altogether
insufficient for the supply of our wants, and though we are beginning
to apply water-power to the production of electricity, and though some
promising plans have been devised for the utilisation of the direct heat
and light of the sun, we are still quite as dependent as any of our
predecessors on what has been done for us in the Palæozoic age.
In the previous pages I have said little respecting the physical
geography of the Carboniferous age; but, as may be inferred from the
vegetation, this in the northern hemisphere presented a greater expanse
of swampy flats little elevated above the sea than we find in any other
period. As to the southern hemisphere, less is known, but the conditions
of vegetation would seem to have been essentially the same.
Taking the southern hemisphere as a whole, I have not seen any evidence
of a Lower Devonian or Upper Silurian flora; but in South Africa and
Australia there are remains of Upper Devonian or Lower Carboniferous
plants. These were succeeded by a remarkable Upper Carboniferous or
Permian group, which spread itself all over India, Australia, and South
Africa,[CN] and contains some forms (_Vertebraria_, _Phyllotheca_,
_Glossopteris_, &c. ) not found in rocks of similar age in the northern
hemisphere, so that, if the age of these beds has been correctly
determined, the southern hemisphere was in advance in relation to some
genera of plants. This, however, is to be expected when we consider that
the Triassic and Jurassic flora of the north contains or consists of
intruders from more southern sites. These beds are succeeded in India by
others holding cycads, &c., of Upper Jurassic or Lower Cretaceous types
(Rajmahal and Jabalpur groups).
[CN] Wyley, "Journal Geol. Society," vol. xxiii., p. 172; Daintree,
_ibid._, vol. xxviii.; also Clarke and McCoy.
Blanford has shown that there is a very great similarity in this series
all over the Australian and Indian region.[CO] Hartt and Darby have in
like manner distinguished Devonian and Carboniferous forms in Brazil
akin to those of the northern hemisphere. Thus the southern hemisphere
would seem to have kept pace with the northern, and according to Blanford
there is evidence there of cold conditions in the Permian, separating
the Palæozoic flora from that of the Mesozoic, in the same manner that
Ramsay has supposed a similar period of cold to have done north of the
equator. This would imply a very great change of climate, since we have
evidence of the extension of the Lower Carboniferous flora at least as
far north as Spitzbergen. The upper coal-formation we cannot, however,
trace nearly so far north; so that a gradual refrigeration may have been
going on before the Permian. Thus in both hemispheres there was a general
similarity in the later Palæozoic flora, and perhaps similar conditions
leading to its extinction and to its replacement by that to be described
in the next chapter.
[CO] "Journal Geol. Society," vol. xxxi.
* * * * *
NOTES TO CHAPTER IV.
I. Characters and Classification of Palæozoic Plants.
In the space available in this work it would be impossible to enter fully
into the classification of Palæozoic plants; but it may be well to notice
some important points for the guidance of those who may desire to collect
specimens; more especially as much uncertainty exists as to affinities
and very contradictory statements are made. The statements below may
be regarded as the results of actual observation and of the study of
specimens _in situ_ in the rocks, as well as in the cabinet and under the
microscope.
Gymnospermeæ.
_Family_ Coniferæ; _Genus_ Dadoxylon, Endlicher; Araucarites, Goeppert;
Araucarioxylon, Kraus.
The trunks of this genus occur from the Middle Devonian to the Permian
inclusive, as drift-logs calcified, silicified, or pyritised. The
only foliage associated with them is of the type of _Walchia_ and
_Araucarites_--viz., slender branches with numerous small spiral acicular
leaves. Two of the coal-formation species, _D. materiarum_ and another,
had foliage of this type. That of the others is unknown. They are all
distinct from the wood of _Cordaites_, for which see under that genus.
The following are North American species:
_Trunks._
_Dadoxylon Ouangondianum_, Dn. M. Erian Report, 1871.[CP]
_D. Halli_, Dn. " "
_D. Newberryi_, Dn. " "
_D. Clarkii_, Dn. (Cordæoxylon ?) " Report, 1882.
_D. Acadianum_, Dn. Coal-formation Acadian Geology.
and millstone
grit.
_D. Materiarum_, Dn. Do. and Permo- "
Carb.
_D._ (_Palæoxylon_) _antiquius_, Dn. L. Carboniferous. "
_D. annulatum_, Dn. Coal-formation. "
_Ormoxylon Erianum_, Dn. Erian Report, 1871.
_Foliage._
_Araucarites gracilis_, Dn. N. Coal-formation "
and Permian.
{ Report on
_Walchia robusta_, Dn. Permian. { Prince Edward
_W. imbricatula_, Dn. " { Island.
[CP] "Geological Survey of Canada: Fossil Plants of Erian and Upper
Silurian Formations," by J. W. Dawson.
All of the above can be vouched for as good species based upon
microscopic examination of a very large number of trunks from different
parts of North America. The three Erian species of _Dadoxylon_ and _D.
antiquius_ from the Lower Carboniferous have two or more rows of cells
in the medullary rays. The last named has several rows, and is a true
_Palæoxylon_ allied to _D. Withami_ of Great Britain. _D. materiarium_
is specially characteristic of the upper coal-formation and Permian,
and to it must belong one or both of the species of foliage indicated
above. _D. Clarkii_ has very short, simple medullary rays of only a few
cells superimposed, and has an inner cylinder of scalariform vessels,
approaching in these points to _Cordaites_. _Ormoxylon_ has a very
peculiar articulated pith and simple medullary rays.
Witham in 1833 described several Carboniferous species of pine-wood,
under the generic name Pinites, separating under the name _Pitus_ species
which appeared to have the discs on the cell-walls separate and in
transverse lines. Witham's name was changed by Goeppert to _Araucarites_,
to indicate the similarity of these woods to Araucaria, _Pinites_ being
reserved for trees more closely allied to the ordinary pines. Endlicher,
restricting Araucarites to foliage, etc., of Araucaria-like trees, gave
the name _Dadoxylon_ to the wood; and this, through Unger's "Genera and
Species," has gained somewhat general acceptance. Endlicher also gave the
name _Pissadendron_ to the species which Witham had called _Pitus_; but
Brongniart proposed the name _Palæoxylon_ to include all the species with
thick and complex medullary rays, whatever the arrangement of the discs.
In Schimper's new work Kraus substitutes _Araucarioxylon_ for Endlicher's
_Dadoxylon_, and includes under _Pissadendron_ all the species placed by
Brongniart in _Palæoxylon_.
To understand all this confusion, it may be observed that the characters
available in the determination of Palæozoic coniferous wood are chiefly
the form and arrangement of the wood-cells, the character of the bordered
pores or discs of their walls, and the form and composition of the
medullary rays.
The character on which Witham separated his genus _Pitus_ from _Pinites_
is, as I have ascertained by examination of slices of one of his original
specimens kindly presented to me by Mr. Sanderson, of Edinburgh,
dependent on state of preservation, the imperfectly preserved discs
or areolations of the walls of the fibre presenting the appearance of
separate and distinct circles, while in other parts of the same specimens
these discs are seen to be contiguous and to assume hexagonal forms, so
that in this respect they do not really differ from the ordinary species
of _Dadoxylon_. The true character for subdividing those species which
are especially characteristic of the Carboniferous, is the composite
structure of the medullary rays, which are thick and composed of several
radial piles of cells placed side by side. This was the character
employed by Brongniart in separating the genus _Palæoxylon_, though he
might with convenience have retained Witham's name, merely transferring
to the genus the species of Witham's _Pinites_ which have complex
medullary rays. The Erian rocks present the greatest variety of types,
and _Palæoxylon_ is especially characteristic of the Lower Carboniferous,
while species of _Dadoxylon_ with two rows of bordered pores and simple
medullary rays are especially plentiful in the upper coal-formation and
Permo-Carboniferous.
The following table will clearly show the distinctive characters and
relations of the genera in question, as held by the several authors above
referred to:
_Wood of Palæozoic Conifers._
Woody | Medullary rays | Generic | Geological
fibres. | and pith. | names. | age.
-------------+--------------------+---------------------+---------------
No discs. | One or two series | _Aporoxylon_, | Devonian
| of cells. | Unger. | (Erian).
-------------+--------------------+---------------------+---------------
| Complex, or of two | { _Pitus_, |
| or more series of | { Witham. | Middle and
| or more series of | { _Palæoxylon_, | Lower
| cells. | { Brongniart. | Carboniferous
| Pith Sternbergian. | { _Pissadendron_, | and Devonian.
| | { Endlicher. |
+--------------------+---------------------+---------------
Discs in one | Simple, or of one | { _Araucarites_, |
series | row of cells. | { Goeppert | Upper
contiguous, | Pith Sternbergian. | { _Dadoxylon_, | Carboniferous
or in | | { Endlicher. | and Permian.
several | | { _Araucarioxylon_, |
series | | { Schimper. |
spirally +--------------------+---------------------+---------------
arranged. | Pith in spherical | _Ormoxylon_,[CQ] Dn. | Devonian.
| chambers. | |
+--------------------+---------------------+---------------
| Medullary sheath | | Devonian.
| scalariform. | _Dadoxylon_ |
| Medullary rays | (Cordæoxylon),[CR] |
| frequent, simple, | Dn. |
| short. | |
-------------+--------------------+---------------------+---------------
[CQ] Type _O. Erianum_, Dn., "Report on Canadian Plants," 1871.
[CR] Type _D. Clarkii_, Dn., "Report on Canadian Plants," 1882. This may
be wood of Cordaites, to which it approaches very closely.
_Family_ Cordaites, _Genus_ Cordaites, Brongniart.
Trunks marked by transverse scars of attachment of bases of leaves;
leaves broad, with many parallel veins, and attached by a broad
base; pistillate and staminate catkins of the nature of Antholithes.
Fruit winged or pulpy, of the kind known as _Cardiocarpum_. Stem
with a Sternbergia pith, usually large, surrounded by a ring of
pseudo-scalariform vessels, and with a cylinder usually narrow, of woody
wedges, with bordered pores in one or more series, and with simple
medullary rays.
From specimens kindly presented to me by Prof. Renault, I have been
able to ascertain that the stems of some at least of these plants
(Eucordaites) are distinct in structure from all the species of
_Dadoxylon_, above mentioned, except _D. Clarkii_, of the Erian. They may
be regarded as intermediate between those of conifers and cycads, which
is indeed the probable position of these remarkable plants.
Grand d'Eury has divided the _Cordaites_ into sub-genera, as follows:
1. _Eucordaites._--Leaves spatulate, obovate, elliptical, or lanceolate,
sessile, entire, with rounded apices and of leathery consistency. The
leaves are from twenty to ninety centimetres in length. The nerves are
either equally or unequally strong.
2. _Dorycordaites._--Leaves lanceolate, with sharp points; nerves
numerous, fine, and equal in strength. The leaves attain a length of from
forty to fifty centimetres.
3. _Poacordaites._--Leaves narrow, linear, entire, blunt at the point,
with nerves nearly equally strong. The leaves are as much as forty
centimetres in length.
To these Renault and Zeiller have added a fourth group, _Scutocordaites_.
_Genus_ Sternbergia.
This is merely a provisional genus intended to receive casts of the pith
cylinders of various fossil trees. Their special peculiarity is that,
as in the modern _Cecropia peltata_, and some species of _Ficus_, the
pith consists of transverse dense partitions which, on the elongation
of the internodes, become separated from each other, so as to produce a
chambered pith cavity, the cast of which shows transverse furrows. The
young twigs of the modern _Abies balsamifera_ present a similar structure
on a minute scale. I have ascertained and described such pith-cylinders
in large stems of _Dadoxylon Ouangondianum_, and _D. materiarium_. They
occur also in the stems of _Cordaites_ and probably of _Sigillariæ_. I
have discussed these curious fossils at length in "Acadian Geology" and
in the "Journal of the Geological Society of London," 1860. The following
summary is from the last-mentioned paper:
_a._ As Prof. Williamson and the writer have shown, many of the
_Sternbergia_ piths belong to coniferous trees of the genus _Dadoxylon_.
_b._ A few specimens present multiporous tissue, of the type of
_Dictyoxylon_, a plant of unknown affinities, and which, according to
Williamson, has a _Sternbergia_ pith.
_c._ Other examples show a true scalariform tissue, comparable with that
of _Lepidodendron_ or _Sigillaria_, but of finer texture. Corda has shown
that plants of the type of the former genus (his _Lomatophloios_) had
_Sternbergia_ piths. Some plants of this group are by external characters
loosely reckoned by botanists as ribless _Sigillariæ_ (_Clathraria_); but
I believe that they are not related even ordinally to that genus.
_d._ Many Carboniferous _Sternbergiæ_ show structures identical with
those described above as occurring in _Cordaites_, and also in some of
the trees ordinarily reckoned as _Sigillariæ_.
_Genus_ Cardiocarpum.
I have found at least eight species of these fruits in the Erian and
Carboniferous of New Brunswick and Nova Scotia, all of which are
evidently fruits of gymnospermous trees. They agree in having a dense
coaly nucleus of appreciable thickness, even in the flattened specimens,
and surrounded by a thin and veinless wing or margin. They have thus
precisely the appearance of samaras of many existing forest-trees, some
of which they also resemble in the outline of the margin, except that the
wings of samaras are usually veiny. The character of the nucleus, and the
occasional appearance in it of marks possibly representing cotyledons or
embryos, forbids the supposition that they are spore-cases. They must
have been fruits of phænogams. Whether they were winged fruits or seeds,
or fruits with a pulpy envelope like those of cycads and some conifers,
may be considered less certain. The not infrequent distortion of the
margin is an argument in favour of the latter view, though this may also
be supposed to have occurred in samaras partially decayed. On the other
hand, their being always apparently flattened in one plane, and the
nucleus being seldom, if ever, found denuded of its margin, are arguments
in favour of their having been winged nutlets or seeds. Until recently I
had regarded the latter view as more probable, and so stated the matter
in the second edition of "Acadian Geology." I have, however, lately
arrived at the conclusion that the _Cardiocarpa_ of the type of _C.
cornutum_ were gymnospermous seeds, having two cotyledons embedded in an
albumen and covered with a strong membranous or woody tegmen surrounded
by a fleshy outer coat, and that the notch at the apex represents the
foramen or micropyle of the ovule. The structure was indeed very similar
to that of the seeds of _Taxus_ and of _Salisburia_. With respect to some
of the other species, however, especially those with very broad margins,
it still appears likely that they were winged.
The _Cardiocarpa_ were borne in racemes or groups, and it seems certain
that some of them at least are the seeds of _Cordaites_. The association
of some of them and of those of the next genus with _Sigillariæ_ is so
constant that I cannot doubt that some of them belong to plants of that
genus, or possibly to taxine conifers. The great number of distinct
species of these seeds, as compared with that of known trees which could
have produced them, is very remarkable.
_Genus_ Trigonocarpum.
These are large angled nuts contained in a thick envelope, and showing
internal structures resembling those of the seeds of modern _Taxineæ_.
There are numerous species, as well as allied seeds referred to the
provisional genera _Rhabdocarpus_ and _Carpolithes_. In _Trigonocarpum
Hookeri_ I have described the internal structure of one of those seeds,
and many fine examples from the coal-field of St. Etienne, in France,
have been described by Brongniart, so that their internal structure is
very well known.
_Genus_ Antholithes.
This is also a provisional genus, to include spikes of floral organs,
some of which are known to have belonged to _Cordaites_, others probably
to _Sigillariæ_.
Of Uncertain Affinities.
_Family_ Sigillariaceæ.
Under this name palæobotanists have included a great number of trees
of the Carboniferous system, all of which are characterised by broad
leaf-sears, with three vascular scars, and usually arranged in
vertical rows, and by elongated three-nerved leaves, and roots of the
stigmaria type--that is, with rounded pits, marking the attachment of
rootlets spirally arranged. These trees, however, collected in the
genus Sigillaria by arbitrary characters, which pass into those of
the Lepidodendroid trees, have been involved in almost inextricable
confusion, to disentangle which it will be necessary to consider: 1. The
external characters of _Sigillariæ_, and trees confounded with them. 2.
Subdivision of _Sigillariæ_ by external markings. 3. The microscopic
character of their stems. 4. What is known of their foliage and fruit.
1. _Characters of Sigillaroid and Lepidodendroid Trunks_.
It may be premised that the modes of determination in fossil botany
are necessarily different from those employed in recent botany. The
palæobotanist must have recourse to characters derived from the leaves,
the scars left by their fall, and the internal structures of the stem.
These parts, held in little esteem by botanists in describing modern
plants, and much neglected by them, must hold the first place in the
regard of the fossil botanist, whereas the fructification, seldom
preserved, and generally obscure, is of comparatively little service. It
is to be remarked also that in such generalised plants as those of the
Palæozoic, remarkable rather for the development of the vegetative than
of the reproductive organs, the former rise in importance as compared
with their value in the study of modern plants.
In _Sigillariæ_, _Lepidodendra_, &c., the following surfaces of the stem
may be presented to our inspection:
1. The outer surface of the epidermis without its leaves, but with
the leaf-bases and leaf-scars more or less perfectly preserved. On
this surface we may recognise: (1) Cellular swellings or projections
of the bark to which the leaves are attached. These may be called
leaf-bases, and they are sometimes very prominent. (2) The actual mark
of the attachment of the leaf situated in the most prominent part of
the leaf-base. This is the _leaf-scar_. (3) In the leaf-scar when well
preserved we can see one or more minute punctures or prominences which
are the points where the vascular bundles passing to the leaf found exit.
These are the vascular scars.
When the leaves are attached, the leaf-scars and vascular scars cannot
be seen, but the leaf-bases can be made out. Hence it is important, if
possible, to secure specimens with and without the leaves. In flattened
specimens the leaf-bases are often distorted by pressure and marked with
furrows which must not be mistaken for true structural characters. The
leaf-bases, which are in relief on the outer surface of the stem, of
course appear as depressions on the mould in the containing rock, in
which the markings often appear much more distinctly than on the plant
itself.
2. The outer surface of the epidermis may have been removed or may be
destroyed by the coarseness of the containing rock. In this case the
leaf-bases are usually preserved on the surface of the outer or corky
bark, but the leaf-scars and vascular scars have disappeared. This gives
that condition of Lepidodendroid trees to which the name _Knorria_ has
been applied. When plants are in this state careful inspection may
sometimes discover traces of the leaf-scars on portions of the stem, and
thus enable the _Knorria_ to be connected with the species to which it
belongs.
3. The outer or corky bark may be removed, exposing the surface of the
inner or fibrous and cellular bark, which in the plants in question is
usually of great thickness. In this case neither the leaf-bases nor the
scars are seen, but punctures or little furrows or ridges appear where
the vascular bundles entered the inner bark. Specimens in this state are
usually said to be decorticated, though only the outer bark is removed.
It is often difficult to determine plants in this condition, unless some
portion of the stem can be found still retaining the bark; but when care
is taken in collecting, it will not infrequently be found that the true
outer surface can be recovered from the containing rock, especially if
a coaly layer representing the outer bark intervenes between this and
the inner impression. Specimens of this kind, taken alone, have been
referred to the genera _Knorria_, _Bothrodendron_, and _Halonia_.
4. In some cases, though not frequently, the outer surface of the
ligneous cylinder is preserved. It almost invariably presents a regularly
striated or irregularly wrinkled appearance, depending upon the vertical
woody wedges, or the positions of the medullary rays or vascular bundles.
Specimens of this kind constituted some of the _Endogenites_ of the
older botanists, and the genus _Schizodendron_ of Eichwald appears to
include some of them. Many of them have also been incorrectly referred to
Calamites.
5. In some cases the cast of the medullary cylinder or pith may alone
be preserved. This may be nearly smooch or slightly marked by vertical
striæ, but more usually presents a transverse striation, and not
infrequently the transverse constrictions and septa characteristic of the
genus Sternbergia. Loose _Sternbergiæ_ afford little means of connecting
them with the species to which they belong, except by the microscopic
examination of the shreds of the ligneous cylinder which often cling to
them.[CS]
[CS] See my paper, "Journal of Geological Society," vol. xxvii.
These facts being premised, the following general statements may be made
respecting some of the more common Palæozoic genera, referring, however,
principally to the perfect markings as seen on the epidermis:
_Sigillaria._--Leaf-bases hexagonal or elongated, or confluent on a
vertical ridge. Leaf-scars hexagonal or shield-shaped. Vascular scars
three, the two lateral larger than the central. This last character
is constant, depending on the fact that the leaves of Sigillaria have
two or more vascular bundles. All so-called _Sigillariæ_ having the
central vascular scar largest, or only one vascular bundle, should be
rejected from this genus. In young branches of branching _Sigillariæ_ the
leaf-scars sometimes appear to be spiral, but in the older stems they
form vertical rows; interrupted, however, by transverse rows or bands
of _fruit-scars_, each with a single large central vascular scar, and
which have borne the organs of fructification. _Arthrocaulis_ of McCoy is
founded on this peculiarity.
_Syringodendron._--Differs from Sigillaria in the leaf-scars, which are
circular and with a single vascular bundle. It is a matter of doubt
whether these plants were of higher rank than Sigillaria tending toward
the pines, or of lower rank tending toward Cyclostigma. Their leaf-bases
form vertical ridges.
_Lepidodendron._--Leaf-bases rhombic, oval, or lanceolate, moderately
prominent. Leaf-scars rhombic or sometimes shield-shaped or heart-shaped,
in the middle or upper part of the leaf-base. Vascular scars three--the
middle one always largest and corresponding to the single nerve of the
leaf; the lateral ones sometimes obsolete.
In older stems three modes of growth are observed. In some species
the expansion of the bark obliterates the leaf-bases and causes the
leaf-scars to appear separated by wide spaces of more or less wrinkled
bark, which at length becomes longitudinally furrowed and simulates the
ribbed character of Sigillaria. In others the leaf-bases grow in size
as the trunk expands, so that even in large trunks they are contiguous
though much larger than those on the branches. In others the outer bark,
hardening at an early age, is incapable of either of the above changes,
and merely becomes cleft into deep furrows in the old trunks.
_Lepidophloios._--Leaf-bases transverse and prominent--often very much
so. Leaf-scars transversely rhombic or oval with three vascular scars,
the central largest. Leaves very long and one-nerved. Large strobiles
or branchlets borne in two ranks or spirally on the sides of the stem,
and leaving large, round scars (_cone-scars_), often with radiating
impressions of the basal row of scales.
Species with long or drooping leaf-bases have been included in
_Lepidophloios_ and _Lomatophloios_, Species with short leaf-bases and
cone-scars in two rows have been called _Ulodendron_, and some of them
have been included in Sigillaria (sub-genus _Clathraria_). Decorticated
stems are Bothrodendron and _Halonia_. Some of the species approach near
to the last genus, especially to the Lepidodendra with rhombic leaf-bases
like _L. tetragonum_.
_Cyclostigma._--Leaf-bases undeveloped. Leaf-scars circular or
horseshoe-shaped, small, with a central vascular scar. In old trunks of
Cyclostigma the leaf-scars become widely separated, and sometimes appear
in vertical rows. Young branches of Lepidodendron sometimes have the
leaf-scars similar to those of Cyclostigma.
_Leptophleum._--Leaf-bases flat, rhombic; leaf-scars obsolete; vascular
scar single, central. The last two genera are characteristically Devonian.
In contradistinction from the trees above mentioned, the following
general statements may be made respecting other groups:
In conifers the leaf-bases are usually elongated vertically, often scaly
in appearance, and with the leaf-scar terminal and round, oval, or
rhombic, and with a single well-marked vascular scar.
In Calamites, Calamodendron, and Asterophyllites the scars of the
branchlets or leaves are circular or oval, with only a single vascular
scar, and situated in verticils at the top of well-marked nodes of the
stem.
In tree-ferns the leaf-bases are large and usually without a distinct
articulating surface. The vascular bundles are numerous. Protopteris
has rounded leaf-scars with a large horseshoe-shaped bundle of vessels
above and small bundles below. Caulopteris has large elliptic or oval
leaf-scars with vascular scars disposed concentrically. Palæopteris,[CT]
of Geinitz, has the leaf-scars transversely oval and the vascular bundles
confluent in a transverse band with an appendage or outlying bundle
below. Stemmatopteris has leaf-scars similar to those of Caulopteris, but
the vascular bundles united into a horseshoe-shaped band.
[CT] This name, preoccupied by Geinitz, has been inadvertently misapplied
to the Devonian ferns of the genus _Archæopteris_.
2. _Subdivision of Sigillariæ in Accordance with their Markings_.
The following groups may be defined in this way; but, being based on one
character only, they are of course in all probability far from natural:
1. _Sigillaria_, Brongniart. Type, _Sigillaria reniformis_, Brongniart,
or _S. Brounii_, Dawson.--Stem with broad ribs, usually much broader than
the usually oval or elliptical tripunctate areoles, but disappearing at
base, owing to expansion of the stem. Leaves narrow, long, three-nerved.
2. _Rhytidolepis_, Sternberg. Type, _S. scutellata_, Brongniart.--Ribs
narrow, and often transversely striate. Areoles large, hexagonal or
shield-shaped, tripunctate. Leaves as in last group. Kings of rounded
scars on the stems and branches mark attachment of fruit. It is possible
that some of the smaller stems of this group may be branches of trees of
group first.
3. _Syringodendron_, Sternberg. Type, _S. organum_, L. and H., _S.
oculata_, Brongniart.--Stems ribbed; areoles small and round, and
apparently with a single scar, or three closely approximated. These are
rare, and liable to be confounded with decorticated examples of other
groups; but I have some specimens which unquestionably represent the
external surface.
4. _Favularia_, Sternberg. Type, _Sigillaria elegans_ of
Brongniart.--Leaf-bases hexagonal, or in young branches elliptical, in
vertical rows, but without distinct ribs, except in old or decorticated
stems. Fruit borne in verticils on the branches bearing transverse rows
of rounded scars. Leaves somewhat broad and longitudinally striate.
5. _Leioderma_, Goldenberg. Type, _S. Sydnensis_, Dawson.--Ribs obsolete.
Cortical and ligneous surfaces striate. Vascular scars double, elongate
longitudinally, and alike on cortical and inner surfaces. Areoles in rows
and distinct; stigmaria-roots striate, with small and distinct areoles.
6. _Clathraria_, Brongniart. Type, _S. Menardi_, Brongniart.--Areoles
hexagonal, not in distinct rows, but having a spiral appearance. Some
of the plants usually referred to this group are probably branches of
_Favularia_. Others are evidently fragments of plants of the genus
_Lepidophloios_.
3. _Internal Structures of Sigillaria-Stems_.
I long ago pointed out, on the evidence of the external markings and
mode of growth, that the stems of _Sigillariæ_ must have been exogenous,
and this conclusion has now been fully confirmed by the microscopic
researches of Williamson, not only in the case of _Sigillariæ_, but
of _Lepidodendra_ and _Calamodendra_ as well. Confining myself to my
own observations, three types of _Sigillariæ_ are known to me by their
internal structures, though I cannot certainly correlate all of these
with the external markings referred to above.
1. _Diploxylon_, in which the stem consists of a small internal axis
surrounded by a very thick inner bark and a dense outer cortex. A fine
example from the South Joggins is thus described:[CU]
[CU] "Journal of the Geological Society of London," November, 1877.
"The axis of the stem is about six centimetres in its greatest diameter,
and consists of a central pith-cylinder and two concentric coats of
scalariform tissue. The pith-cylinder is replaced by sandstone, and is
about one centimetre in diameter. The inner cylinder of scalariform
tissue is perfectly continuous, not radiated, and about one millimetre
in thickness. Its vessels are somewhat crushed, but have been of large
diameter. Its outer surface, which readily separates from that of the
outer cylinder, is striated longitudinally. The outer cylinder, which
constitutes by much the largest part of the whole, is also composed of
scalariform tissue; but this is radially arranged, with the individual
cells quadrangular in cross-section. The cross-bars are similar on all
the sides and usually simple and straight, but sometimes branching or
slightly reticulated. The wall intervening between the bars has extremely
delicate longitudinal waving lines of ligneous lining, in the manner
first described by Williamson as occurring in the scalariform tissue
of certain _Lepidodendra_. A few small radiating spaces, partially
occupied with pyrites, obscurely represent the medullary rays, which must
have been very feebly developed. The radiating bundles passing to the
leaves run nearly horizontally; but their structure is very imperfectly
preserved. The stem being old and probably long deprived of its leaves,
they may have been partially disorganised before it was fossilised. The
outer surface of the axis is striated longitudinally, and in some places
marked with impressions of tortuous fibres, apparently those of the inner
bark. In the cross-section, where weathered, it shows concentric rings;
but under the microscope these appear rather as bands of compressed
tissue than as proper lines of growth. They are about twenty in number.
This tree has an erect, ribbed trunk, twelve feet in height and fifteen
inches in diameter, swelling to about two feet at the base."
2. _Favularia Type._--This has been well described by Brongniart and
by Renault,[CV] and differs from the above chiefly in the fact that
the outer exogenous woody zone is composed of reticulated instead of
scalariform tissue, and the inner zone is of the peculiar form which I
have characterised as pseudo-scalariform.
[CV] "Botanique Fossile," Paris, 1881.
3. _Sigillaria Proper._--This I have illustrated in my paper in the
"Journal of the Geological Society" for May, 1871, and it appears
to represent the highest and most perfect type of the larger ribbed
_Sigillaria_. This structure I have described as follows, basing my
description on a very fine axis found in an erect stem, and on the
fragments of the woody axis found in the bases of other erect stems:
_a._ A dense cellular outer bark, usually in the state of compact
coal--but when its structure is preserved, showing a tissue of thickened
parenchymatous cells.
_b._ A very thick inner bark, which has usually in great part perished,
or been converted into coal, but which, in old trunks, contained a large
quantity of prosenchymatous tissue, very tough and of great durability.
This "bast-tissue" is comparable with that of the inner bark of modern
conifers, and constitutes much of the mineral charcoal of the coal-seams.
_c._ An outer ligneous cylinder, composed of wood-cells, either with
a single row of large bordered pores,[CW] in the manner of pines
and cycads, or with two, three, or four rows of such pores sometimes
inscribed in hexagonal areoles in the manner of Dadoxylon. This woody
cylinder is traversed by medullary rays, which are short, and composed
of few rows of cells superimposed. It is also traversed by oblique
radiating bundles of pseudo-scalariform tissue proceeding to the leaves.
In some _Sigillariæ_ this outer cylinder was itself in part composed of
pseudo-scalariform tissue, as in Brongniart's specimen of _S. elegans_;
and in others its place may have been taken by multiporous tissue, as in
a case above referred to; but I have no reason to believe that either of
these variations occurred in the typical ribbed species now in question.
The woody fibres of the outer cylinder may be distinguished most readily
from those of conifers, as already mentioned, by the thinness of their
walls, and the more irregular distribution of the pores. Additional
characters are furnished by the medullary rays and the radiating bundles
of scalariform tissue when these can be observed.
[CW] These are the same with the wood-cells elsewhere called discigerous
tissue, and to which I have applied the terms uniporous and multiporous.
The markings on the walls are caused by an unlined portion of the
cell-wall placed in a disk or depression, and this often surrounded by an
hexagonal rim of thickened wall; but in all cases these structures are
less pronounced than in _Dadoxylon_, and less regular in the walls of the
same cell, as well as in different layers of the tissues of the axis.
_d._ An inner cylinder of pseudo-scalariform tissue. I have adopted the
term pseudo-scalariform for this tissue, from the conviction that it is
not homologous with the scalariform ducts of ferns and other acrogens,
but that it is merely a modification of the discigerous wood-cells, with
pores elongated transversely, and sometimes separated by thickened bars,
corresponding to the hexagonal areolation of the ordinary wood-cells.
A similar tissue exists in cycads, and is a substitute for the spiral
vessels existing in ordinary exogens.
_e._ A large medulla, or pith, consisting of a hollow cylinder of
cellular tissue, from which proceed numerous thin diaphragms towards the
centre of the stem.
These structures of the highest type of _Sigillaria_ are on the one
hand scarcely advanced beyond those of Calamopitus, as described by
Williamson, and on the other approach to those of _Cordaites_, as seen in
specimens presented to me by Renault.
Finally, as to the fruit of _Sigillariæ_, I have no new facts to offer.
The strobiles or spikes associated with these trees have been variously
described as gymnospermous (Renault) or cryptogamous (Groldenberg and
Williamson). 1 have never seen them in place. Two considerations,
however, have always weighed with me in reference to this subject. One
is the constant abundance of Trigonocarpa and Cardiocarpa in the soil
of the Sigiliaria forests, as I have studied this at the South Joggins.
The other is that the rings of fruit-scars on the branches of Sigiliaria
are homologous with leaf-scars, not with branches, and therefore should
have borne single carpels and not cones or spikes of inflorescence. These
are merely suggestions, but I have no doubt they will be vindicated
by future discoveries, which will, I have no doubt, show that in the
family _Sigillariaceæ_ we have really two families, one possibly of
gymnospermous rank, or at least approaching to this, the other allied to
the Lepidodendra.
Cryptogamia.
(_Acrogenes._)
_Family_ Lepidodendreæ; _Genus_ Lepidodendron, Sternberg.
These are arboreal Lycopods having linear one-nerved leaves, stems
branching dichotomously, and with ovate or rhombic leaf-bases bearing
rhombic leaf-scars, often very prominent. The fruit is in scaly
strobiles, terminal or lateral, and there are usually, if not always,
macrospores and microspores in each strobile. The young branches and
stems have a central pith, a cylinder of scalariform tubes sending out
ascending bundles to the leaves through a thick cellular and fibrous
inner bark, and externally a dense cortex confluent with or consisting of
the leaf-bases. Older stems have a second or outer layer of scalariform
fibres in wedges with medullary rays, and strengthening the stem by a
true exogenous growth, much as in the Diploxylon type of Sigiliaria. The
development of this exogenous cylinder is different in amount and rate
in different species.[CX] This different development of the exogenous
axis is accompanied with appropriate external appearances in the stems,
and the changes which take place in their markings. These are of three
kinds. In some species the areoles, at first close together, become, in
the process of the expansion of the stem, separated by intervening spaces
of bark in a perfectly regular manner; so that in old stems, while widely
separated, they still retain their arrangement, while in young stems they
are quite close to one another. This is the case in _L. corrugatum_. In
other species the leaf-scars or bases increase in size in the old stems,
still retaining their forms and their contiguity to each other. This
is the case in _L. undulatum_, and generally in those _Lepidodendra_
which have large leaf-bases. In these species the continued vitality
of the bark is shown by the occasional production of lateral strobiles
on large branches, in the manner of the modern red pine of America. In
other species the areoles neither increase in size nor become regularly
separated by growth of the intervening bark; but in old stems the bark
splits into deep furrows, between which may be seen portions of bark
still retaining the areoles in their original dimensions and arrangement.
This is the case with _L. Pictoense_. This cracking of the bark no doubt
occurs in very old trunks of the first two types, but not at all to the
same extent.
[CX] See "Memoirs of Dr. Williamson," in "Philosophical Transactions,"
for ample details.
As a type of Lepidodendron, I may describe one of the oldest
Carboniferous species characteristic of the Lower Carboniferous in
America, and corresponding to _L. Veltheimianum_ of Europe.
Lepidodendron Corrugatum, Dawson.--(See Fig. 43, _supra_.) "Quarterly
Journal of Geological Society," vol. xv.; "Acadian Geology," page 451.
_Habit of Growth._--Somewhat slender, with long branches and long,
slender leaves having a tendency to become horizontal or drooping.
_Markings of Stem._--Leaf-bases disposed in quincunx or spirally,
elongate, ovate, acute at both ends, but more acute and slightly
oblique at the lower end; most prominent in the upper third, and with
a slight vertical ridge. Leaf-scars small, rounded, and showing only
a single punctiform vascular scar. The leaf-scar on the outer surface
is in the upper third of the base; but the obliquity of the vascular
bundle causes it to be nearly central on the inside of the epidermis.
In young succulent shoots the leaf-scars are contiguous and round as
in Cyclostigma, without distinct leaf-bases. In this state it closely
resembles _L. Olivieri_, Eichwald.[CY]
[CY] Lethæa Rossica, Plate Y, Figs. 12, 13.
In the ordinary young branches the leaf-scars are contiguous, and closely
resemble those of _L. elegans_, Brongt. (Fig. 43 C). As the branches
increase in diameter the leaf-scars slightly enlarge and sometimes assume
a verticillate appearance (Fig. 43 D). As they still further enlarge they
become separated by gradually increasing spaces of bark, marked with many
waving striæ or wrinkles (Fig. 43 I, N). At the base of old stems the
bark assumes a generally wrinkled appearance without distinct scars.
_Knorria or Decorticated States._--Of these there is a great variety,
depending on the state of preservation, and the particular longitudinal
ridges. Fig. 43 D shows a form in which the vascular bundles appear
as cylindrical truncate projections. Other forms show the leaf-bases
prominent, or have an appearance of longitudinal ribbing produced by the
expansion of the bark.
_Structure of Stem._--This is not perfectly preserved in any of my
specimens, but one flattened specimen shows a central medulla with a
narrow ring of scalariform vessels surrounding it, and constituting the
woody axis. The structure is thus similar to that of _L. Harcourtii_,
which I regard as probably the same with the closely allied European
species L. Veltheimianum.
_Leaves._--These are narrow, one-nerved, curving somewhat rapidly outward
(Figs. 43, B, C, D). They vary from one to two inches in length.
_Roots._--I have not seen these actually attached, but they occur very
abundantly in the under-clays of some erect forests of these plants at
Horton Bluff, and are of the character of Stigmariæ (Figs. 30, 31). In
some of the under-clays the long, flattened rootlets are excessively
abundant, and show the mark of a central vascular bundle.
_Fructification._--Cones terminal, short, with many small, acute
imbricate scales. Spore-cases globular, smooth (Fig. 43 C). On the
surface of some shales and sandstones at Horton there are innumerable
round spore-cases of this tree about the size of mustard-seed (Fig. 43
F). Large slabs are sometimes covered with these, and thin layers of
shale are filled with flattened specimens.
This is the characteristic species of the Lower Carboniferous
coal-measures, occurring in great profusion at Horton Bluff and its
vicinity, also at Sneid's Mills near Windsor, Noel and Five-Mile River,
at Norton Creek and elsewhere in New Brunswick (Matthew's collection),
and at Antigonish (Honeyman's collection).
I have received from the lowest Carboniferous beds of Ohio specimens
of this species.[CZ] According to Rogers and Lesquereux similar forms
occur in the Vespertine of Pennsylvania and in the Lower Carboniferous
of Illinois. _L. Veltheimianum_ of western Europe and _L. glincanum_ of
Russia are closely allied Lower Carboniferous species.[DA]
[CZ] "Journal of Geological Society," November, 1862, p. 313.
[DA] For comparisons of these see "Report on Plants of Lower
Carboniferous of Canada," p. 21.
A very different type is furnished by a new species from the middle
coal-formation of Clifton, New Brunswick.
Lepidodendron Cliftonense, Dawson.--Habit of Growth.--Robust, with thick
branches, and leaves several inches in length. Terminal branches becoming
slender, with shorter leaves.
_Markings of Stem._--Leaf-bases long oval, pointed at ends, enlarging
with growth of stem. Leaf-scars central, rhombic, transverse.
_Leaves._--One-nerved, acutely pointed, from four inches in length on the
larger branches to one inch or less on the branchlets.
_Fructification._--Cones large, cylindrical or long oval, with large
scales of trigonal form, and not elongated but lying close to the
surface. Borne on lateral, slender branchlets, with short leaves.
_Genus_ Lepidophloios, Sternberg; Ulodendron, L. and H.; Lomatophloios,
Corda.
_Lepidophloios._--Under this generic name, established by Sternberg,
I include those lycopodiaceous trees of the coal-measures which have
thick branches, transversely elongated leaf-scars, each with three
vascular points and placed on elevated or scale-like protuberances,
long one-nerved leaves, and large lateral strobiles in vertical rows or
spirally disposed. Their structure resembles that of _Lepidodendron_,
consisting of a _Sternbergia_ pith, a slender axis of large scalariform
vessels, giving off from its surface bundles of smaller vessels to the
leaves, a very thick cellular bark, and a thin dense outer bark, having
some elongated cells or bast-tissue on its inner side. In these trees the
exogenous outer cylinder is less developed than in the Lepidodendra, and
is sometimes wanting in stems or branches of some thickness.
Regarding _L. laricinum_ of Sternberg as the type of the genus, and
taking in connection with this the species described by Goldenberg,
and my own observations on numerous specimens found in Nova Scotia, 1
have no doubt that _Lomatophloios crassicaulis_ of Corda, and other
species of that genus described by Goldenberg, _Ulodendron_ and
_Bothrodendron_ of Lindley, _Lepidodendron ornatissimum_ of Brongniart,
and _Halonia punctata_ of Geinitz, all belong to this genus, and differ
from each other only in conditions of growth and preservation. Several
of the species of _Lepidostrobus_ and _Lepidophyllum_ also belong to
_Lepidophloios_.
The species of _Lepidophloios_ are readily distinguished from
_Lepidodendron_ by the form of the areoles, and by the round scars on the
stem, which usually mark the insertion of the large strobiles, though
in barren stems they may also have produced branches; still, the fact
of my finding the strobiles _in situ_ in one instance, the accurate
resemblance which the scars bear to those left by the cones of the red
pine when borne on thick branches, and the actual impressions of the
radiating scales in some specimens, leave no doubt in my mind that
they are usually the marks of cones; and the great size of the cones of
_Lepidophloios_ accords with this conclusion.
The species of _Lepidophloios_ are numerous, and individuals are quite
abundant in the coal formation, especially toward its upper part. Their
flattened bark is frequent in the coal-beds and their roofs, affording
a thin layer of pure coal, which sometimes shows the peculiar laminated
or scaly character of the bark when other characters are almost entirely
obliterated. The leaves also are nearly as abundant as those of
Sigillaria in the coal-shales. They can readily be distinguished by their
strong, angular midrib.
The markings of _Lepidophloios_ may easily be mistaken for those of the
_Clathraria_ type of _Sigillaria_. When the stem only is seen, they can
be distinguished by the length of the leaf-bases in _Lepidophloios_, and
by the dominant central vascular scar; also by the one-nerved and ribbed
leaves. Where the large, round marks of the cones are present, these are
an infallible guide, never being present in _Sigillaria_. As the cones
grew on the upper sides of the branches, the impression of the lower side
often shows no cone-scars, or only two lateral rows, whereas on the upper
side of the same branch they appear spirally arranged. I may describe as
an example--
_Lepidophloios Acadianus_, Dawson. Leaf-bases broadly rhombic, or in old
stems regularly rhombic, prominent, ascending, terminated by very broad
rhombic scars having a central point and two lateral obscure points.
Outer bark laminated or scaly. Surface of inner bark with single points
or depressions. Leaves long, linear, with a strong keel on one side,
five inches or more in length. Cone-scars sparsely scattered on thick
branches, either in two rows or spirally, both modes being sometimes seen
on the same branch. Scalariform axis scarcely an inch in diameter in a
stem five inches thick. Fruit, an ovate strobile with numerous acute
scales covering small globular spore-cases. This species is closely
allied to _Ulodendron majus_ and _Lepidophloios laricinus_, and presents
numerous varieties of marking. Coal-formation, Nova Scotia.
_Family_ Calamiteæ; _Genus_ Calamites, Suckow.
The plants of this genus are unquestionably allied to the modern
_Equisetaceæ_, but excel these so much in variety of form and structure,
and are so capricious in their states of preservation, and so liable to
be mistaken for parts of plants generically different, that they have
given rise to much controversy. The following considerations will enable
us to arrive at some certainty.
The genus _Calamites_ was originally founded in the longitudinally
ribbed and jointed stems so frequent in the coal-formation, and of which
the common _C. Suckovii_ is a typical form. The most perfect of these
stems represent the outer surface immediately within the epidermis, in
which case transverse lines or constrictions mark the nodes, and at the
nodes there are rounded spots, sometimes indicating radial processes of
the pith, first described by Williamson; in other cases, the attachment
of branchlets, or in some specimens both. But some specimens show the
outer surface of the epidermis, in which case the transverse nodal
lines are usually invisible, though the scars of branchlets may appear.
In still other examples the whole of the outer tissues have perished,
and the so-called Calamite is a cast of the interior of the stem,
showing merely longitudinal ribbing and transverse nodal constrictions.
In studying these plants _in situ_ in the erect Calamite brakes of
the coal-formation of Nova Scotia, one soon becomes familiar with
these appearances, but they are evidently unknown to the majority of
palæobotanists, though described in detail more than twenty years ago.
When the outer surface is preserved it is sometimes seen to bear
verticils of long needle-like leaves (_C. Cistii_), or of branchlets with
secondary whorls of similar leaves (_C. Suckovii_ and _C. undulatus_).
No Calamite known to me bears broad one-nerved leaves like those of
_Asterophyllites_ and _Annularia_, though the larger stems of these
plants have been described as Calamites, and the term _Calamocladus_ has
been used to include both groups. The base of the Calamite stem usually
terminates in a blunt point, and may be attached to a rhizome, or several
stems may bud out from each other in a group or stool. The roots are long
and cylindrical, sometimes branching. The fruit consists of spikes of
spore-cases, borne in whorls and subtended by linear floral leaves. To
these strobiles the name Calamostachys has been given.
Williamson has shown that the stem of Calamites consists of a central
pith or cavity of large size surrounded by a cylinder consisting of
alternate wedges of woody and cellular matter, with vertical canals at
the inner sides of the wedges, and slender medullary rays. The thick
cellular wedges intervening between the woody wedges he calls primary
medullary rays; the smaller medullary rays in the wedges, secondary
medullary rays. There is thus a highly complex exogenous stem based on
the same principle with the stem of a common _Equisetum_, but with much
greater strength and complexity.
Williamson has also shown that there are different sub-types of these
stems. More especially he refers to the three following:
(_a_) _Calamites_ proper, which has the woody wedges of scalariform or
barred tissue with thin medullary rays, and the thick primary medullary
rays are cellular.
(_b_) _Calamopitus_ has reticulated or multiporous tissue in the woody
wedges with medullary rays, and the primary medullary wedges are composed
of elongated cells.
(_c_) _Calamodendron_ has the woody wedges of barred tissue as in _a_,
with medullary rays, but has the intervening medullary wedges of an
elongated tissue approaching to woody fibre, and also with medullary rays.
To these I would add a fourth type, which I have described, from the
coal-formation of Nova Scotia.[DB]
[DB] "Quarterly Journal of the Geological Society," 1871.
(_d_) _Eucalamodendron_ differs from _Calamodendron_ in having true
bordered pores or pseudo-scalariform slit-pored tissue, and corresponds
to the highest type of calamitean stem.
I would also add that under _a_ and _b_ there are some species in which
the woody cylinder is very thin in comparison to the size of the stem. In
_c_ and _d_ the woody cylinder is thick and massive, and the stems are
often large and nodose.
As an example of an ordinary Calamite in which the external surface and
foliage are preserved, I may quote the following from my report on the
"Flora of the Lower Carboniferous and Millstone Grit," 1873:
Calamites Undulatus, Brongniart.--This species is stated by Brongniart
to be distinguished from the _C. Suckovii_, the characteristic Calamite
of the middle coal-formation, by its undulated ribs marked with peculiar
cellular reticulation. He suggests that it may be merely a variety of
_C. Suckovii_, an opinion in which Schimper coincides; but since I have
received large additional collections from Mr. Elder, containing not
only the stems and branches, but also the leaves and rhizomes, I am
constrained to regard it as a distinct though closely allied species.
The rhizomata are slender, being from one to two inches in diameter,
and perfectly flattened. They are beautifully covered with a cellular
reticulation on the thin bark, and show occasional round areoles marking
the points of exit of the rootlets. I have long been familiar with
irregular flattened stems thus reticulate, but have only recently been
able to connect them with this species of Calamite.
The main stems present a very thin carbonaceous bark reticulated like
the rhizomes. They have flat, broad ribs separated by deep and narrow
furrows, and undulated in a remarkable manner even when the stems are
flattened. This undulation is, however, perhaps an indication of vertical
pressure while the plant was living, as it seems to have had an unusually
thin and feeble cortical layer, and the undulations are apparently best
developed in the lower part of the stem. At the nodes the ribs are often
narrowed and gathered together, especially in the vicinity of the rounded
radiating marks which appear to indicate the points of insertion of
the branches. At the top of each rib we have the usual rounded areole,
probably marking the insertion of a primary branchlet.
The branches have slender ribs and distant nodes, from which spring
secondary branchlet s in whorls, these bearing in turn small whorls of
acicular leaflets much curved upward, and which are apparently round in
cross section and delicately striate. They are much shorter than the
leaves of _Calamites Suckovii_, and are less dense and less curved than
those of _C. nodosus_, which I believe to be the two most closely allied
species.
Lesquereux notices this species as characteristic of the lower part of
the Carboniferous in Arkansas.
It will be observed that I regard the striated and ribbed stems not as
internal axes, but as representing the outer surface of the plants. This
was certainly the case with the present species and with _C. Suckovii_
and _C. nodosus_. Other species, and especially those which belonged to
Calamodendron, no doubt had a smooth or irregularly wrinkled external
bark; but this gives no good ground for the manner in which some writers
on this subject confound Calamites with Calamodendra, and both with
Asterophyllites and Sphenophyllum. With this no one who has studied these
plants, rooted in their native soils, and with their appendages still
attached, can for a moment sympathise. One of the earliest geological
studies of the writer was a bed of these erect Calamites, which he
showed to Sir C. Lyell in 1844, and described in the "Proceedings of
the Geological Society" in 1851, illustrating the habit of growth as
actually seen well exposed in a sandstone cliff. Abundant opportunities
of verifying the conclusions formed at that time have since occurred,
the results of which have been summed up in the figures in Acadian
Geology, which, though they have been treated by some botanists as merely
restorations, are in reality representations of facts actually observed.
On these subjects, without entering into details, and referring for these
to the elaborate discussions of Schimper, Williamson, and McNab, and to
my paper on the subject, "Journal of the Geological Society," vol. xxvii,
p. 54, I may remark:
1. That the aërial stems of ordinary Calamites had a thin cortical layer,
with lacunæ and fibrous bundles and multiporous vessels--the whole not
differing much from the structure of modern Equiseta.
2. Certain arborescent forms, perhaps allied to the true Calamites,
as well as possibly the old underground stems of ordinary species[DC]
assumed a thick-walled character in which the tissues resembled the
wedges of an exogen, and abundance of pseudo-scalariform fibres were
developed, while the ribbing of the external surface became obsolete or
was replaced by a mere irregular wrinkling.
[DC] Williamson, "Transactions of the Royal Society." McNab, in
"Proceedings of the Edinburgh Botanical Society."
3. Sufficient discrimination has not been exercised in separating casts
of the internal cavities of Calamites and Calamodendron from those
representing other surfaces and the proper external surface.
4. There is no excuse for attributing to Calamites the foliage of
Annularia, Asterophyllites, and Sphenophyllum, since these leaves have
not been found attached to true Calamite stems, and since the structure
of the stems of Asterophyllites as described by Williamson, and that of
Sphenophyllum as described by the writer,[DD] are essentially different
from those of Calamites.
[DD] "Journal of the Geological Society," 1866.
5. As the species above described indicates, good external characters can
be found for establishing species of this genus, and these species are of
value as marks of geological age.
_Genus_ Archæocalamites, Sternberg.
This genus has been established to include certain Calamites of the
Devonian and Lower Carboniferous, in which the furrows on the stem do not
alternate at the nodes or joints, and the leaves in one species at least
bifurcate. _C. radiatus_, Brongniart, is the typical species. In North
America it occurs in the Erian, probably as low as the Middle Erian. In
Europe it has so far been recognised in the Lower Carboniferous only. I
have, however, seen stems from alleged Devonian beds in Devonshire which
may have belonged to this species.
_Family_ Asterophylliteæ; _Genus_ Asterophyllites, Brongniart.
Stems ribbed and jointed like the _Calamites_, but with inflated nodes
and a stout internal woody cylinder, which has been described by
Williamson. From the joints proceeded whorls of leaves or of branchlets,
bearing leaves which differed from those of _Calamites_ in their having
a distinct middle rib or vein. The fructification consisted of long
slender cones or spikes, having whorls of scales bearing the spore-cases.
Some authors speak of _Asterophyllites_ as only branches and leaves of
_Calamites_; but though at first sight the resemblance is great, a close
inspection shows that the leaves of Asterophyllites have a true midrib,
which is wanting in _Calamites_.
_Genus_ Annularia.--It is perhaps questionable whether these plants
should be separated from _Asterophyllites_, The distinction is that
they produce branches in pairs, and that their whorls of leaves are
one-sided and usually broader than those of _Asterophyllites_, and united
into a ring at their insertion on the stem. One little species, _A.
sphenophylloides_, is very widely distributed.
Pinnularia--a provisional genus---includes slender roots or stems
branching in a pinnate manner, and somewhat irregularly. They are very
abundant in the coal shales, and were probably not independent plants,
but aquatic roots belonging to some of the plants last mentioned.
The probability of this is farther increased by their resemblance in
miniature to the roots of _Calamites_. They are always flattened, but
seem originally to have been round, with a slender thread-like axis of
scalariform vessels, enclosed in a soft, smooth, cellular bark.
_Family_ Rhizocarpeæ; _Genus_ Sphenophyllum.
Leaves in whorls, wedge-shaped, with forking veins. Fructification on
spikes, with verticils of sporocarps. These plants are by some regarded
as allied to the _Calamiteæ_ and _Asterophylliteæ_, by others as a high
grade of Rhizocarps of the type of Marsilia. The stem had a star-shaped
central bundle of scalariform or reticulato-scalariform vessels.
_Genus_ Sporangites. (_Sporocarpon_, Williamson.)
Under this name we may provisionally include those rounded spherical
bodies found in the coal and its accompanying beds, and also in
the Erian, which may be regarded as Macrospores or Sporocarps of
Protosalvinia, or other Rhizocarpean plants akin to those described above
in Chapter III, which see for description.
_Genus_ Protosalvinia.--Under this we include sporocarps allied to those
of _Salvinia_, as described in Chapter III.
_Family_ Filices.
Under this head I shall merely refer to a few groups of special interest,
and to the provisional arrangement adopted for the fronds of ferns when
destitute of fructification.
The external appearances of trunks of tree-ferns have been already
referred to.
With respect to tree ferns, the oldest known examples are those from
the Middle Devonian of New York and Ohio, which I have described in the
"Journal of the Geological Society," 1871 and 1881. As these are of some
interest, I have reproduced their descriptions in a note appended to
Chapter III, which see.
The other forms most frequently occurring in the Carboniferous are
_Caulopteris_, _Palæopteris_, and _Megaphyton_[DE] Stems showing merely
masses of aërial roots are known by the name _Psaronius_.
[DE] See my "Acadian Geology," also below.
With reference to the classification of Palæozoic ferns, this has
hitherto been quite arbitrary, being based on mere form and venation of
fronds, but much advance has recently been made in the knowledge of their
fructification, warranting a more definite attempt at classification. The
following are provisional genera usually adopted:
1. _Cyclopteris_, Brongniart.--Leaflets more or less rounded or
wedge-shaped, without midrib, the nerves spreading from the point of
attachment. This group includes a great variety of fronds evidently of
different genera, were their fructification known; and some of them
probably portions of fronds, the other parts of which may be in the next
genus.
2. _Neuropteris_, Brongniart.--Fronds pinnate, and with the leaflets
narrowed at the base; midrib often not distinct, and disappearing toward
the apex. Nervures equal, and rising at an acute angle. Ferns of this
type are among the most abundant in the coal-formation.
3. _Odontopteris_, Brongniart.--In these the frond is pinnate, and
the leaflets are attached by their whole base, with the nerves either
proceeding wholly from the base, or in part from an indistinct midrib,
which soon divides into nervures.
4. _Dictyopteris_, Gutbier.--This is a beautiful style of fern, with
leaflets resembling those of _Neuropteris_, but the veins arranged
in a network of oval spaces. Only a few species are known in the
coal-formation.
5. _Lonchopteris_, Brongniart.--Ferns with netted veins like the above,
but with a distinct midrib, and the leaflets attached by the whole base.
Of this, also, we can boast but few species.
6. _Sphenopteris_, Brongniart.--These are elegant ferns, very numerous
in species, and most difficult to discriminate. Their most distinctive
characters are leaflets narrowed at the base, often lobed, and with
nervures dividing in a pinnate manner from the base.
7. _Phyllopteris_, Brongniart.--These are pinnate, with long lanceolate
pinnules, having a strong and well-defined midrib, and nerves proceeding
from it very obliquely, and dividing as they proceed toward the margin.
The ferns of this genus are for the most part found in formations more
recent than the Carboniferous; but I have referred to it, with some
doubt, one of our species.
8. _Alethopteris_, Brongniart.--This genus includes many of the most
common coal-formation ferns, especially the ubiquitous _A. lonchitica_,
which seems to have been the common brake of the coal-formation,
corresponding to _Pteris aquilina_ in modern Europe and America. These
are brake-like ferns, pinnate, with leaflets often long and narrow,
decurrent on the petiole, adherent by their whole base, and united at
base to each other. The midrib is continuous to the point, and the
nervures run off from it nearly at right angles. In some of these ferns
the fructification is known to have been marginal, as in _Pteris_.
9. _Pecopteris_, Brongniart.--This genus is intermediate between the
last and _Neuropteris_. The leaflets are attached by the whole base,
but not usually attached to each other; the midrib, though slender,
attains to the summit; the nervures are given off less obliquely than
in _Neuropteris_. This genus includes a large number of our most common
fossil ferns.
10. _Beinertia_, Goeppert.--A genus established by Goeppert for a curious
Pecopteris-like fern, with flexuous branching oblique nervures becoming
parallel to the edge of the frond.
11. _Hymenophyllites_, Goeppert.--These are ferns similar to
Sphenopteris, but divided at the margin into one-nerved lobes, in the
manner of the modern genus _Hymenophyllum_.
12. _Palæopteris_, Geinitz.--This is a genus formed to include certain
trunks of tree-ferns with oval transverse scars of leaves.
13. _Caulopteris_, Lindley and Hutton.--Is another genus of fossil trunks
of tree-ferns, but with elongate scars of leaves.
14. _Psaronius_, Cotta.--Includes other trunks of tree-ferns with
alternate scars or thick scales, and ordinarily with many aërial roots
grouped round them, as in some modern tree-ferns.
15. _Megaphyton_, Artis.--Includes trunks of tree-ferns which bore their
fronds, which were of great size, in two rows, one on each side of the
stem. These were very peculiar trees, less like modern ferns than any
of the others. My reasons for regarding them as ferns are stated in the
following extract from a recent paper:
"Their thick stems, marked with linear scars and having two rows of
large depressed areoles on the sides, suggest no affinities to any known
plants. They are usually ranked with _Lepidodendron_ and _Ulodendron_,
but sometimes, and probably with greater reason, are regarded as allied
to tree-ferns. At the Joggins a very fine species (_M. magnificum_) has
been found, and at Sydney a smaller species (_M. humile_); but both
are rare and not well preserved. If the large scars bore cones and the
smaller bore leaves, then, as Brongniart remarks, the plant would much
resemble _Lepidophloios_, in which the cone-scars are thus sometimes
distichous. But the scars are not round and marked with radiating scales
as in _Lepidophloios_; they are reniform or oval, and resemble those
of tree-ferns, for which reason they may be regarded as more probably
leaf-scars; and in that case the smaller linear scars would indicate
ramenta, or small aërial roots. Further, the plant described by Corda
as _Zippea disticha_ is evidently a _Megaphyton_, and the structure
of that species is plainly that of a tree-fern of somewhat peculiar
type. On these grounds I incline to the opinion of Geinitz that these
curious trees were allied to ferns, and bore two rows of large fronds,
the trunks being covered with coarse hairs or small aërial roots. At
one time I was disposed to suspect that they may have crept along the
ground; but a specimen from Sydney shows the leaf-stalks proceeding from
the stem at an angle so acute that the stem must, I think, have been
erect. From the appearance of the scars it is probable that only a pair
of fronds were borne at one time at the top of the stem; and, if these
were broad and spreading, it would be a very graceful plant. To what
extent plants of this type contributed to the accumulation of coal I have
no means of ascertaining, their tissues in the state of coal not being
distinguishable from those of ferns and _Lycopodiaceæ_."
16. For descriptions of the genus _Archæopteris_ and other Erian ferns,
see Chapter III.
CHAPTER V.
THE FLORA OF THE EARLY MESOZOIC.
Great physical changes occurred at the close of the Carboniferous age.
The thick beds of sediment that had been accumulating in long lines
along the primitive continents had weighed down the earth's crust. Slow
subsidence had been proceeding from this cause in the coal-formation
period, and at its close vast wrinklings occurred, only surpassed by
those of the old Laurentian time. Hence in the Appalachian region of
America we have the Carboniferous beds thrown into abrupt folds, their
shales converted into hard slates, their sandstones into quartzite and
their coals into anthracite, and all this before the deposition of the
Triassic Red Sandstones which constitute the earliest deposit of the
great succeeding Mesozoic period. In like manner the coal-fields of Wales
and elsewhere in western Europe have suffered similar treatment, and
apparently at the same time.
This folding is, however, on both sides of the Atlantic limited to a
band on the margin of the continents, and to certain interior lines of
pressure, while in the middle, as in Ohio and Illinois in America, and
in the great interior plains of Europe, the coal-beds are undisturbed
and unaltered. In connection with this we have an entire change in the
physical character of the deposits, a great elevation of the borders
of the continents, and probably a considerable deepening of the seas,
leading to the establishment of general geographical conditions which
still remain, though they have been temporarily modified by subsequent
subsidences and re-elevations.
Along with this a great change was in progress in vegetable and animal
life. The flora and fauna of the Palæozoic gradually die out in the
Permian and are replaced in the succeeding Trias by those of the Mesozoic
time. Throughout the Permian, however, the remains of the coal-formation
flora continue to exist, and some forms, as the _Calamites_, even seem
to gain in importance, as do also certain types of coniferous trees. The
Triassic, as well as the Permian, was marked by physical disturbances,
more especially by great volcanic eruptions discharging vast beds and
dykes of lava and layers of volcanic ash and agglomerate. This was the
case more especially along the margins of the Atlantic, and probably
also on those of the Pacific. The volcanic sheets and dykes associated
with the Red Sandstones of Nova Scotia, Connecticut, and New Jersey are
evidences of this.
At the close of the Permian and beginning of the Trias, in the midst of
this transition time of physical disturbance, appear the great reptilian
forms characteristic of the age of reptiles, and the earliest precursors
of the mammals, and at this time the old Carboniferous forms of plants
finally pass away, to be replaced by a flora scarcely more advanced,
though different, and consisting of pines, cycads, and ferns, with
gigantic equiseta, which are the successors of the genus _Calamites_,
a genus which still survives in the early Trias. Of these groups the
conifers, the ferns, and the equiseta are already familiar to us, and, in
so far as they are concerned, a botanist who had studied the flora of the
Carboniferous would have found himself at home in the succeeding period.
The cycads are a new introduction. The whole, however, come within the
limits of the cryptogams and the gymnosperms, so that here we have no
advance.[DF]
[DF] Fontaine's "Early Mesozoic Flora of Virginia" gives a very good
summary of this flora in America.
[Illustration: Fig. 64.--Jurassic vegetation. Cycads and pines. (After
Saporta.)]
As we ascend, however, in the Mesozoic, we find new and higher types.
Even within the Jurassic epoch, the next in succession to the Trias,
there are clear indications of the presence of the endogens, in species
allied to the screw-pines and grasses; and the palms appear a little
later, while a few exogenous trees have left their remains in the Lower
Cretaceous, and in the Middle and Upper Cretaceous these higher plants
come in abundantly and in generic forms still extant, so that the dawn
of the modern flora belongs to the Middle and Upper Cretaceous. It will
thus be convenient to confine ourselves in this chapter to the flora of
the earlier Mesozoic.
Passing over for the present the cryptogamous plants already familiar
in older deposits, we may notice the new features of gymnospermous and
phænogamous life, as they present themselves in this earlier part of the
great reptilian age, and as they extended themselves with remarkable
uniformity in this period over all parts of the world. For it is a
remarkable fact that, if we place together in our collections fossil
plants of this period from Australia, India, China, Siberia, Europe,
or even from Greenland, we find wonderfully little difference in their
aspect. This uniformity we have already seen prevailed in the Palæozoic
flora; and it is perhaps equally marked in that of the Mesozoic. Still we
must bear in mind that some of the plants of these periods, as the ferns
and pines, for example, are still world-wide in their distribution; but
this does not apply to others, more especially the cycads (Fig. 65).
[Illustration: Fig. 65.--_Podozamites lanceolatus_, Sternb. L.
Cretaceous.]
The cycads constitute a singular and exceptional type in the modern
world, and are limited at present to the warmer climates, though very
generally distributed in these, as they occur in Africa, India, Japan,
Australia, Mexico, Florida, and the West Indies. In the Mesozoic age,
however, they were world-wide in their distribution, and are found as far
north as Greenland, though most of the species found in the Cretaceous
of that country are of small size/ and may have been of low growth, so
that they may have been protected by the snows of winter. The cycads
have usually simple or unbranching stems, pinnate leaves borne in a
crown at top, and fruits which, though somewhat various in structure and
arrangement, are all of the simpler form of gymnospermous type. The stems
are exogenous in structure, but with slender wood and thick bark, and
barred tissue, or properly as tissue intermediate between this and the
disc-bearing fibres of the pines.
Though the cycads have a considerable range of organisation and of
fructification, and though some points in reference to the latter might
assign them a higher place, on the whole they seem to occupy a lower
position than the conifers or the cordaiteæ of the Carboniferous.
In the Carboniferous some of the fern-like leaves assigned to the
genus _Noeggerathia_ have been shown by Stur and Weiss to have been
gymnosperms, probably allied to cycads, of which they may be regarded at
least as precursors. Thus the cycadean type does not really constitute an
advance in grade of organisation in the Mesozoic, any further than that,
in the period now in question, it becomes much more developed in number
and variety of forms. But the conifers would seem to have had precedence
of it for a long time in the Palæozoic, and it replaces in the Mesozoic
the _Cordaites_, which in many respects excelled it in complexity.
The greater part of the cycads of the Mesozoic age would seem to have had
short stems and to have constituted the undergrowth of woods in which
conifers attained to greater height. An interesting case of this is the
celebrated dirt-bed of the quarries of the Isle of Portland, long ago
described by Dean Buckland. In this fossil soil trunks of pines, which
must have attained to great height, are interspersed with the short,
thick stems of cycads, of the genus named _Cycadoidea_ by Buckland,
and which from their appearance are called "fossil birds' nests" by the
quarrymen. Some, however, must have attained a considerable height so as
to resemble palms.
The cycads, with their simple, thick trunks, usually marked with rhombic
scars, and bearing broad spreading crowns of large, elegantly formed
pinnate leaves, must have formed a prominent part of the vegetation
of the northern hemisphere during the whole of the Mesozoic period. A
botanist, had there been such a person at the time, would have found
this to be the case everywhere from the equator to Spitzbergen, and
probably in the southern hemisphere as well, and this throughout all
the long periods from the Early Trias to the Middle Cretaceous. In a
paper published in the "Linnæan Transactions" for 1868, Dr. Carruthers
enumerates twenty species of British Mesozoic cycads, and the number
might now be considerably increased.
[Illustration: Fig. 66.--_Salisburia_ (Gingko) _Sibirica_, Heer. L.
Cretaceous, Siberia and North America.]
The pines present some features of interest. We have already seen their
connection with the broad-leaved _Cordaites_, and in the Permian there
are some additional types of broad-leaved coniferæ. In the Mesozoic we
have great numbers of beautiful trees, with those elegant fan-shaped
leaves characteristic of but one living species, the Salisburia, or
gingko-tree of China. It is curious that this tree, though now limited
to eastern Asia, will grow, though it rarely fruits, in most parts of
temperate Europe, and in America as far north as Montreal, and that in
the Mesozoic period it occupied all these regions, and even Siberia and
Greenland, and with many and diversified species (Fig. 66).
_Salisburia_ belongs to the yews, but an equally curious fact applies to
the cypresses. The genus _Sequoia_, limited at present to two species,
both Californian, and one of them the so-called "big tree," celebrated
for the gigantic size to which it attains, is represented by species
found as far back at least as the Lower Cretaceous, and in every part
of the northern hemisphere.[DG] It seems to have thriven in all these
regions throughout the Mesozoic and early Kainozoic, and then to have
disappeared, leaving only a small remnant to represent it in modern days.
A number of species have been described from the Mesozoic and Tertiary,
all of them closely related to those now existing (Fig. 67).
[DG] In the Eocene of Australia.
[Illustration: Fig. 67.--_Sequoia Smithiana_, Heer. L. Cretaceous.]
The following notice of these trees is for the most part translated, with
some modifications and abridgment, from a paper read by the late Prof.
Heer before the Botanical Section of the Swiss Natural History Society:
The name itself deserves consideration. It is that of an Indian of the
Cherokee tribe, Sequo Yah, who invented an alphabet without any aid from
the outside world of culture, and taught it to his tribe by writing it
upon leaves. This came into general use among the Cherokees, before the
white man had any knowledge of it; and afterward, in 1828, a periodical
was published in this character by the missionaries. Sequo Yah was
banished from his home in Alabama, with the rest of his tribe, and
settled in New Mexico, where he died in 1843.
When Endlicher was preparing his synopsis of the conifers, in 1846, and
had established a number of new genera, Dr. Jacbon Tschudi, then living
with Endlicher, brought before his notice this remarkable man, and asked
him to dedicate this red-wooded tree to the memory of a literary genius
so conspicuous among the red men of America. Endlicher consented to do
so, and only endeavored to make the name pronounceable by changing two of
its letters.
Endlicher founded the genus on the redwood of the Americans, _Taxodium
sempervirens_ of Lamb; and named the species _Sequoia sempervirens_.
These trees form large forests in California, which extend along the
coast as far as Oregon. Trees are there met with of 300 feet in height
and 20 feet in diameter. The seeds have been brought to Europe a number
of years ago, and we already see in upper Italy and around the Lake of
Geneva, and in England, high trees; but, on the other hand, they have not
proved successful around Zurich.
In 1852, a second species of Sequoia was discovered in California, which,
under the name of big tree, soon attained a considerable celebrity.
Lindley described it, in 1853, as _Wellingtonia gigantea_; and, in the
following year, Decaisne and Torrey proved that it belonged to Sequoia,
and that it accordingly should be called _Sequoia gigantea_.
While the _Sequoia sempervirens_, in spite of the destructiveness of
the American lumbermen, still forms large forests along the coast, the
_Sequoia gigantea_ is confined to the isolated clumps which are met with
inland at a height of 5,000 to 7,000 feet above sea-level, and are much
sought after by tourists as one of the wonders of the country. Reports
came to Europe concerning the largest of them which were quite fabulous,
but we have received accurate accounts of them from Prof. Whitney. The
tallest tree measured by him has a height of 325 feet, and in the case of
one of the trees the number of the rings of growth indicated an age of
about 1,300 years. It had a girth of 50 to 60 feet.
We know only two living species of _Sequoia_, both of which are confined
to California. The one (_S. sempervirens_) is clothed with erect leaves,
arranged in two rows, very much like our yew-tree, and bears small, round
cones; the other (_S. gigantea_) has smaller leaves, set closely against
the branches, giving the tree more the appearance of the cypress. The
cones are egg-shaped, and much larger. These two types are therefore
sharply defined.
Both of these trees have an interesting history. If we go back into the
Tertiary, this same genus meets us with a long array of species. Two of
these species correspond to those living at present: the _S. Langsdorfii_
to the _S. sempervirens_, and the _S. Couttsiæ_ to the _S. gigantea_.[DH]
But, while the living species are confined to California, in the Tertiary
they are spread over several quarters of the globe.
[DH] _S. Couttsiæ_ has leaves like _S. gigantea_, and cones like those of
_8. sempervirens_.
Let us first consider the _Sequoia Langsdorfii_. This was first
discovered in the lignite of Wetterau, and was described as _Taxites
langsdorfii_. Heer found it in the upper Rhone district, and there lay
beside the twigs the remains of a cone, which showed that the _Taxites
Langsdorfii_ of Brongniart belonged to the Californian genus Sequoia
established by Endlicher. He afterward found much better preserved
cones, together with seeds, along with the plants of east Greenland,
which fully confirmed the determination. At Atanekerdluk in Greenland
(about 70° north latitude) this tree is very common. The leaves, and
also the flowers and numerous cones, leave no doubt that it stands very
near to the modern redwood. It differs from it, however, in having
a much larger number of scales in the cone. The tree is also found
in Spitzbergen at nearly 78° north latitude, where Nordenskiöld has
collected, at Cape Lyell, wonderfully preserved branches. From this high
latitude the species can be followed down through the whole of Europe
as far as the middle of Italy (at Senegaglia, Gulf of Spezia). In Asia,
also,, we can follow it to the steppes of Kirghisen, to Possiet, and to
the coast of the Sea of Japan, and across to Alaska and Sitka. It is
recognized by Mr. Starkie Gardner as one of the species found in the
Eocene of Mull in the Hebrides.[DI] It is thus known in Europe, Asia, and
America, from 43° to 78° north latitude, while its most nearly related
living species, perhaps even descended from it, is now confined to
California.
[DI] It is _Fareites Campbelli_ of Forbes.
With this _S. Langsdorfii_, three other Tertiary species are
nearly related (_S. brevifolia_, Hr., _S. disticha_, Hr., and _S.
Nordenskiöldi_, Hr.). These have been met with in Greenland and
Spitzbergen, and one of them has lately been found in the United States.
Three other species, in addition to these, have been described by
Lesquereux, which appear to belong to the group of the _S. Langsdorfii_,
viz., _S. longifolia_, Lesq., _S. angustifolia_, and _S. acuminata_,
Lesq. Several species also occur in the Cretaceous and Eocene of Canada.
These species thus answer to the living _Sequoia sempervirens_; but we
can also point to Tertiary representatives of the _S. gigantea_. Their
leases are stiff and sharp-pointed, are thinly set round the branches,
and lie forward in the same way: the egg-shaped cones are in some cases
similar.
There are, however, in the early Tertiary six species, which fill up
the gap between _S. sempervirens_ and _S. gigantea_. They are the _S.
Couttsiæ_, _S. affinis_, Lesq., _S. imbricata_, Hr., _S. sibirica_, Hr.,
_S. Heerii_, Lesq., and _S. biformis_, Lesq. Of these, _S. Couttsiæ_,
Hr., is the most common and most important species. It has short leaves,
lying along the branch, like _S. gigantea_, and small, round cones, like
_S. Langsdorfii_ and _sempervirens_. Bovey Tracey in Devonshire has
afforded splendid specimens of cones, seeds, and twigs, which have been
described in the "Philosophical Transactions." More lately, Count Saporta
has described specimens of cones and twigs from Armissan. Specimens of
this species have also been found in the older Tertiary of Greenland, so
that it must have had a wide range. It is very like to the American _S.
affinis_, Lesq.
In the Tertiary there have been already found fourteen well-marked
species, which thus include representatives of the two living types, _S.
sempervirens_ and _S. gigantea_.
We can follow this genus still further back. If we go back to the
Cretaceous age, we find ten species, of which five occur in the Urgon
of the Lower Cretaceous, two in the Middle, and three in the Upper
Cretaceous. Among these, the Lower Cretaceous exhibits the two types
of the Sequoia sempervirens and _S. gigantea_. To the former the _S.
Smithiana_ answers, and to the latter, the _Reichenbachii_, Gein. The
_S. Smithiana_ stands indeed uncommonly near the _S. Langsdorfii_, both
in the appearance of the leaves on the twigs and in the shape of the
cones. These are, however, smaller, and the leaves do not become narrower
toward the base. The _S. pectina_, Hr., of the Upper Cretaceous, has its
leaves arranged in two rows, and presents a similar appearance. The _S.
Reichenbachii_ is a type more distinct from those now living and those
in the Tertiary. It has indeed stiff, pointed leaves, lying forward, but
they are arcuate, and the cones are smaller. This tree has been known
for a long time, and it serves in the Cretaceous as a guiding star,
which we can follow from the Urgonian of the Lower Cretaceous up to the
Cenomanian. It is known in France, Belgium, Bohemia, Saxony, Greenland,
and Spitzbergen (also in Canada and the United States). It has been
placed in another genus--Geinitzia--but we can recognise, by the help of
the cones, that it belongs to Sequoia.
Below this, there is found in Greenland a nearly related species, the _S.
ambigua_, Hr., of which the leaves are shorter and broader, and the cones
round and somewhat smaller.
The connecting link between _S. Smithiana_ and _Reichenbachii_ is formed
by _S. subulata_, Hr., and _S. rigida_, Hr., and three species (_S.
gracilis_, Hr., _S. fastigiata_ and _S. Gardneriana_, Carr.), with leaves
lying closely along the branch, and which come very near to the Tertiary
species _S. Couttsiæ_. We have therefore in the Cretaceous quite an
array of species, which fill up the gap between the _S. sempervirens_
and _gigantea_, and show us that the genus Sequoia had already attained
a great development in the Cretaceous. This was still greater in
the Tertiary, in which it also reached its maximum of geographical
distribution. Into the present world the two extremes of the genus have
alone continued; the numerous species forming its main body have fallen
out in the Tertiary.
If we look still further back, we find in the Jura a great number of
conifers, and, among them, we meet in the genus Pinus with a type which
is highly developed, and which still survives; but for Sequoia we have
till now looked in vain, so that for the present we can not place the
rise of the genus lower than the Urgonian of the Cretaceous, however
remarkable we may think it that in that period it should have developed
into so many species; and it is still more surprising that two species
already make their appearance which approach so near to the living
_Sequoia sempervirens_ and _S. gigantea_.
Altogether, we have become acquainted, up to the present time, with
twenty-six species of Sequoia. Fourteen of these species are found in the
Arctic zone, and have been described and figured in the "Fossil Flora of
the Arctic Regions." Sequoia has been recognised by Ettingshausen even in
Australia, but there in the Eocene.
This is, perhaps, the most remarkable record in the whole history of
vegetation. The Sequoias are the giants of the conifers, the grandest
representatives of the family, and the fact that, after spreading over
the whole northern hemisphere and attaining to more than twenty specific
forms, their decaying remnant should now be confined to one limited
region in western America and to two species constitutes a sad memento
of departed greatness.[DJ] The small remnant of _S. gigantea_ still,
however, towers above all competitors, as eminently the "big trees ";
but, had they and the allied species failed to escape the Tertiary
continental submergences and the disasters of the glacial period, this
grand genus would have been to us an extinct type. In like manner the
survival of the single gingko of eastern Asia alone enables us to
understand that great series of taxine trees with fern-like leaves of
which it is the sole representative.
[DJ] The writer has shown that much of the material of the great lignite
beds of the Canadian Northwest consists of wood of Sequoia of both the
modern types.
Besides these peculiar and now rare forms, we have in the Mesozoic many
others related closely to existing yews, cypresses, pines, and spruces,
so that the conifers were probably in greater abundance and variety than
they are at this day.
In this period, also, we find the earliest representatives of
the endogenous plants. It is true that some plants found in the
coal-formation have been doubtfully referred to these, but the earliest
certain examples would seem to be some bamboo-like and screw-pine-like
plants occurring in the Jurassic rocks. Some of these are, it is true,
doubtful forms, but of others there seems to be no question. The modern
_Pandanus_ or screw-pine of the tropical regions, which is not a pine,
however, but a humble relation of the palms, is a stiffly branching
tree, of a candelabra-like form, and with tufts of long leaves on its
branches, and nuts or great hard berries for fruit, borne sometimes in
large masses, and so protected as to admit of their drifting uninjured
on the sea. The stems are supported by masses of aërial roots like those
which strengthen the stems of tree-ferns. These structures and habits
of growth fit the Pandanus for its especial habitat on the shores of
tropical islands, to which its masses of nuts are drifted by the winds
and currents, and on whose shores it can establish itself by the aid of
its aërial roots.
Some plants referred to the cycads have proved veritable botanical
puzzles. One of these, the _Williamsonia gigas_ of the English oölite,
originally discovered by my friend Dr. Williamson, and named by him
_Zamia gigas_, a very tall and beautiful species, found in rocks of this
age in various parts of Europe, has been claimed by Saporta for the
Endogens, as a plant allied to _Pandanus_. Some other botanists have
supposed the flowers and fruits to be parasites on other plants, like the
modern _Rafflesia_ of Sumatra, but it is possible that after all it may
prove to have been an aberrant cycad.
The tree-palms are not found earlier than the Middle Cretaceous, where
we shall notice them in the next chapter. In like manner, though a
few Angiosperms occur in rocks believed to be Lower or Lower Middle
Cretaceous in Greenland and the northwest territory of Canada, and in
Virginia, these are merely precursors of those of the Upper Cretaceous,
and are not sufficient to redeem the earlier Cretaceous from being a
period of pines and cycads.
On the whole, this early Mesozoic flora, so far as known to us, has a
monotonous and mean appearance. It no doubt formed vast forests of tall
pines, perhaps resembling the giant Sequoias of California; but they must
for the most part have been dark and dismal woods, probably tenanted by
few forms of life, for the great reptiles of this age must have preferred
the open and sunny coasts, and many of them dwelt in the waters. Still
we must not be too sure of this. The berries and nuts of the numerous
yews and cycads were capable of affording much food. We know that in
this age there were many great herbivorous reptiles, like _Iguanodon_
and _Hadrosaurus_, some of them fitted by their structure to feed upon
the leaves and fruits of trees. There were also several kinds of small
herbivorous mammals, and much insect life, and it is likely that few of
the inhabitants of the Mesozoic woods have been preserved as fossils.
We may yet have much to learn of the inhabitants of these forests of
ferns, cycads, and pines. We must not forget in this connection that in
the present day there are large islands, like New Zealand, destitute of
mammalia, and having a flora comparable with that of the Mesozoic in the
northern hemisphere, though more varied. We have also the remarkable
example of Australia, with a much richer flora than that of the early
Mesozoic, yet inhabited only by non-placental mammals, like those of the
Mesozoic.
The principal legacy that the Mesozoic woods have handed down to our
time is in some beds of coal, locally important, but of far less extent
than those of the Carboniferous period. Still, in America, the Richmond
coal-field in Virginia is of this age, and so are the anthracite beds of
the Queen Charlotte Islands, on the west coast of Canada, and the coal
of Brora in Sutherlandshire. Valuable beds of coal, probably of this
age, also exist in China, India, and South Africa; and jet, which is so
extensively used for ornament, is principally derived from the carbonised
remains of the old Mesozoic pines.
In the next chapter we have to study a revolution in vegetable life most
striking and unique, in the advent of the forest-trees of strictly modern
types.
NOTE TO CHAPTER V.
I append to this chapter a table showing the plant-bearing series of the
Cretaceous and Laramie of North America, from a paper in "Trans. R. S.
C," 1885, which see for further details:
(In Descending Order.)
Periods. Floras and sub-floras. References.
--------------+---------------------------+-----------------------------
Transition Upper Laramie or Porcupine { Platanus beds of Souris
Eocene to Hill. Fort Union { River and Calgary. Report
Cretaceous. group, U. S. territory. { of Geol. Survey of Canada
{ for 1879, and Memoir of
{ 1885.
--------------+---------------------------+-----------------------------
Middle Laramie or Willow
Creek beds.
{ Lemna and Pistia beds of
Lower Laramie or St. { bad lands of 49th parallel,
Mary River. { Red Deer River, &c., with
Upper { lignites. Report 49th
Cretaceous { Parallel and Memoir of
{ 1885.
(Danian and
Senonian). Fox Hill series Marine.
Fort Pierre series Marine.
{ Sequoia and Brasenia beds
Belly River { of S. Saskatchewan, Belly
{ River, &c. with lignites.
{ Memoir of 1885.
Coal measures of Nanaimo, { Memoir of 1883. Many
B.C., probably here. { dicotyledons, palms, &c.
--------------+---------------------------+-----------------------------
Middle Dunvegan series of Peace }
Cretaceous River. Dakota group, } Memoir of 1883. Many
(Turonian and U. S. Amboy clays, } dicotyledons, cycads, &c.
Cenomanian). U. S. }
Mill Creek beds of Rocky { Dicotyledonous leaves,
Mountains. { similar to Dakota group of
{ the U. S. Memoir of 1885.
--------------+---------------------------+-----------------------------
Lower Suskwa River beds and }
Cretaceous Queen Charlotte Island } Cycads, pines, a few
(Neocomian. coal series. Intermediate } dicotyledons. Report Geol.
&c). beds of Rocky } Survey. Memoir of 1885.
Mountains. Potomac }
series of Virginia. }
Kootanie series of Rocky { Cycads, pines, and ferns.
Mountains. { Memoir of 1885.
--------------+---------------------------+------------------------------
CHAPTER VI.
THE REIGN OF ANGIOSPERMS IN THE LATER CRETACEOUS
AND KAINOZOIC.
[Illustration: Fig. 68.--_Populus primæva_, Heer. Cretaceous, of
Greenland. One of the oldest known Angiosperms.]
It is a remarkable fact in geological chronology that the culmination
of the vegetable kingdom antedates that of the animal. The placental
mammals, the highest group of the animal kingdom, are not known till the
beginning of the Eocene Tertiary. The dicotyledonous Angiosperms, which
correspond to them in the vegetable kingdom, occur far earlier--in the
beginning of the Upper Cretaceous or close of the Lower Cretaceous. The
reign of cycads and pines holds throughout the Lower Cretaceous, but at
the close of that age there is a sudden incoming of the higher plants,
and a proportionate decrease, more especially of the cycads.
I have already referred to the angiospermous wood supposed to be
Devonian, but I fear to rest any conclusion on this isolated fact.
Beyond this, the earliest indications of plants of this class have
been found in the Lower Cretaceous. Many years ago Heer described and
figured the leaves of a poplar (_Populus primæva_) from the supposed
Lower Cretaceous of Komé, in Greenland (Fig. 68). Two species, a
_Sterculia_ and a _Laurus_ or _Salix_, occur among fossils described by
me in the upper part of the Kootanie series of the Rocky Mountains, and
Fontaine has recently found in the Potomac group of Virginia--believed
to be of Neocomian age--several angiospermous species (_Sassafras_,
_Menispermites_, _Sapindus_, _Aralia_, _Populus_, &c.) mixed with a rich
flora of cycads and pines. These are the early forerunners of the modern
angiospermous flora; but so far as known they do not occur below the
Cretaceous, and in its lower portions only very rarely. When, however, we
ascend into the Upper Cretaceous, whether of Europe or America, there is
a remarkable incoming of the higher plants, under generic forms similar
to those now existing. This is, in truth, the advent of the modern flora
of the temperate regions of the earth. A very interesting tabular view
of its early distribution is given by Ward, in the "American Journal of
Science" for 1884, of which the following is a synopsis, with slight
emendations. I may add that the new discoveries made since 1884 would
probably tend to increase the proportionate number of dicotyledons in the
newer groups.
Dicotyledonous Trees in the Cretaceous.
_Upper Senonian_ 179 species.
(Fox Hill group of America.)
_Lower Senonian_ 81 species.
Upper white chalk of Europe; Fort Pierre
group of America; coal-measures of Nanaimo?
_Turonian_ 20 species.
Lower white chalk; New Jersey marls;
Belly R. group.
_Cenomanian._ 357 species.
(Chalk-marl, greensand, and Gault, Niobrara
and Dakota groups of America); Dunvegan
group of Canada; Amboy clays of New Jersey.
_Neocomian_ 20 species.[DK]
(Lower greensand and Speeton clay, Wealden
and Hastings sands, Kootanie and Queen
Charlotte groups of Canada.)
[DK] Including an estimate of Fontaine's undescribed species.
Thus we have a great and sudden inswarming of the higher plants of
modern types at the close of the Lower Cretaceous. In relation to this,
Saporta, one of the most enthusiastic of evolutionists, is struck by
this phenomenon of the sudden appearance of so many forms, and some of
them the most highly differentiated of dicotyledonous plants. The early
stages of their evolution may, he thinks, have been obscure and as yet
unobserved, or they may have taken place in some separate region, or
mother country as yet undiscovered, or they may have been produced by
a rapid and unusual multiplication of flower-haunting insects! Or it
is even conceivable that the apparently sudden elevation of plants may
have been due to causes still unknown. This last seems, indeed, the only
certain inference in the case, since, as Saporta proceeds to say in
conclusion: "Whatever hypothesis one may prefer, the fact of the rapid
multiplication of dicotyledons, and of their simultaneous appearance in a
great number of places in the northern hemisphere at the beginning of the
Cenomanian epoch, cannot be disputed."[DL]
[DL] "Monde des Plantes," p. 197.
The leaves described by Heer, from the Middle Cretaceous of Greenland,
are those of a poplar (_P. primæva_). Those which I have described from
a corresponding horizon in the Rocky Mountains are a _Sterculites_ (_S.
vetustula_), probably allied to the mallows, and an elongated leaf,
_Laurophyllum_ (_L. crassinerve_) (Fig. 69), which may, however, have
belonged to a willow rather than a laurel. These are certainly older than
the Dakota group of the United States and the corresponding formations
in Canada. On the eastern side of the American continent, in Virginia,
the Potomac series is supposed to be of Lower Cretaceous age, and here
Fontaine, as already stated, has found an abundant flora of cycads,
conifers, and ferns, with a few angiospermous leaves, which have not yet
been described.
[Illustration: Fig. 69.--_Stercalia_ and _Laurophyllum_ or Salix, the
oldest Angiosperms Known in the Cretaceous of Canada.]
In the Canadian Rocky Mountains, a few hundreds of feet above the beds
holding the beforementioned species, are the shales of the Mill Creek
series, rich in many species of dicotyledonous leaves, and corresponding
in age with the Dakota group, whose fossils have been so well described,
first by Heer and Capellini, and afterward by Lesquereux. We may take
this Dakota group and the quader-sand stone of Germany as types of the
plant-bearing Cenomanian, and may notice the forms occurring in them.
In the first place, we recognise here the successors of our old friends,
the ferns and the pines, the latter represented by such genera as
_Taxites_, _Sequoia_, _Glyptostrobus_, _Gingko_, and even _Pinus_ itself.
We also have a few cycads, but not so dominant as in the previous
ages. The fan-palms are well represented, both in America and in the
corresponding series in Europe, especially by the genus _Sabal_, which
is the characteristic American type of fan-palm, and there is one genus
which Saporta regards as intermediate between the fan-palms and the
pinnately leaved species. There are also many fragments of stems and
leaves of carices and grasses, so that these plants, now so important
to the nourishment of man and his companion animals, were already
represented.
[Illustration: Fig. 70.--Vegetation of Later Cretaceous. Exogens and
palms. (After Saporta.)]
But the great feature of the time was its dicotyledonous forests, and I
have only to enumerate the genera supposed to be represented in order
to show the richness of the time in plants of this type. It may be
necessary to explain here that the generic names used are mostly based
on leaves, and consequently cannot be held as being absolutely certain,
since we know that at present one genus may have considerable variety
in its leaves, and, on the other hand, that plants of different genera
may be very much alike in their foliage. There is, however, undoubtedly
a likeness in plan or type of structure in leaves of closely allied
plants, and, therefore, if judiciously studied, they can be determined
with at least approximate certainty.[DM] More especially we can attain
to much certainty when the fruits as well as the leaves are found,
and when we can obtain specimens of the wood, showing its structure.
Such corroboration is not wanting, though unfortunately the leaves
of trees are generally found drifted away from the other organs once
connected with them. In my own experience, however, I have often found
determinations of the leaves of trees confirmed by the discovery of their
fruits or of the structure of their stems. Thus, in the rich cretaceous
plant-beds of the Dunvegan series we have beech-nuts associated in
the same beds with leaves referred to _Fagus_. In the Laramie beds I
determined many years ago nuts of the _Trapa_ or water-chestnut, and
subsequently Lesquereux found, in beds in the United States, leaves which
he referred to the same genus. Later, I found in collections made on the
Red Deer River of Canada my fruits and Lesquereux's leaves on the same
slab. The presence of trees of the genera _Carya_ and _Juglans_ in the
same formation was inferred from their leaves, and specimens have since
been obtained of silicified wood, with the microscopic structure of the
modern butternut. Still we are willing to admit that determinations from
leaves alone are liable to doubt.
[DM] Great allowance has to be made for the variability of leaves of the
same species. The modern hazel (_C. rostrata_) is a case in point. Its
leaves, from different parts of the same plant, are so dissimilar in form
and size that they might readily be regarded as of different species.
In the matter of names of fossil leaves, I sympathise very strongly
with Dr. Nathorst, of Stockholm, in his objection to the use of modern
generic names for mere leaves, and would be quite content to adopt some
non-committal termination, as that of "_phyllum_" or "_ites_" suggested
by him. I feel, however, that almost as much is taken for granted if a
plant is called Corylophyllum or _Corylites_, as if called _Corylus_. In
either case a judgment is expressed as to its affinities, which if wrong
under the one term is wrong under the other; and after so much has been
done by so many eminent botanists, it seems inexpedient to change the
whole nomenclature for so small and questionable an advantage. I wish
it, however, to be distinctly understood that plants catalogued on the
evidence of leaves alone are for the most part referred to certain genera
on grounds necessarily imperfect, and their names are therefore subject
to correction, as new facts may be obtained.
The more noteworthy modern genera included in the Dakota flora, as
catalogued by Lesquereux, are the following: _Liquidambar_, the
sweet-gum, is represented both in America and Europe, the leaves
resembling those of the modern species, but with entire edges, which
seems to be a common peculiarity of Cretaceous foliage.[DN] _Populus_
(poplar), as already stated, appears very early in Greenland, and
continues with increasing number of species throughout the Cretaceous and
Tertiary. _Salix_ (willow) appears only a little later and continues. Of
the family _Cupuliferæ_ we have _Fagus_ (beech), _Quercus_ (oak), and
_Castanea_ (chestnut), which appear together in the Dakota group and
its equivalents. Fruits of some of the species are known, and also wood
showing structure. _Betula_ (birch) is represented by a few species,
and specimens of its peculiar bark are also common. _Alnus_ (alder)
appears in one species at least. The genus Plat anus (Fig. 71), that of
the plane-trees, represented at present by one European and one American
species, has several species in the Cretaceous, though the plane-trees
seem to culminate in the early part of the succeeding Eocene, where
there are several species with immense leaves. The large leaves,
known as _Credneria_, found in the Cenomanian of Europe, and those
called _Protophyllum_ (Fig. 72) in America, appear to be nearer to the
plane-trees than to any others, though representing an extinct type. The
laurels are represented in this age, and the American genus Sassafras,
which has now only one species, has not one merely but several species in
the Cretaceous. _Diospyros_, the persimmon-tree, was also a Cretaceous
genus.
[DN] With reference to this, something may be learned from the leaves
of modern trees. In these, young shoots have leaves often less toothed
and serrated than those of the adult tree. A remarkable instance is the
_Populus grandidentatus_ of America, the young shoots of which have
entire leaves, quite unlike except in venation those of the parent tree,
and having an aspect very similar to that of the Cretaceous poplars.
[Illustration: Fig. 71.--_Platanus nobilis_, Newberry, variety
_basilobata_. Laramie. Much reduced.]
[Illustration: Fig. 72.--_Protophyllum boreale_, Dawson, reduced. Upper
Cretaceous, Canada.]
The single species of the beautiful _Liriodendron_, or tulip-tree, is
a remnant of a genus which had several Cretaceous species (Figs. 74,
75). The magnolias, still well represented in the American flora, were
equally plentiful in the Cretaceous (Fig. 73). The walnut family were
well represented by species of _Juglans_ (butternut) and _Carya_, or
hickory. In all, no less than forty-eight genera are present belonging
to at least twenty-five families, running through the whole range of the
dicotyledonous exogens. This is a remarkable result, indicating a sudden
profusion of forms of these plants of a very striking character. It is
further to be observed that some of the genera have many species in the
Cretaceous and dwindle toward the modern. In others the reverse is the
case--they have expanded in modern times. In a number there seems to have
been little change.
[Illustration: Fig. 73.--_Magnolia magnifica_, Dawson, reduced. Upper
Cretaceous, Canada.]
Dr. Newberry has given, in the "Bulletin of the Torrey Botanical Club" an
interesting _résumé_ of the history of the beautiful _Liriodendron_, or
tulip-tree, which may be taken as an example of a genus which has gone
down in importance in the course of its geological history.
"The genus _Liriodendron_, as all botanists know, is represented in the
present flora by a single species, 'the tulip-tree' which is confined
to eastern America, but grows over all the area lying between the Lakes
and the Gulf, the Mississippi and the Atlantic. It is a magnificent
tree, on the whole, the finest in our forests. Its cylindrical trunk,
sometimes ten feet in diameter, carries it beyond all its associates in
size, while the beauty of its glossy, lyre-shaped leaves and tulip-like
flowers is only surpassed by the flowers and foliage of its first cousin,
_Magnolia grandiflora_. That a plant so splendid should stand quite alone
in the vegetation of the present day excited the wonder of the earlier
botanists, but the sassafras, the sweet-gum, and the great Sequoias of
the far West afford similar examples of isolation, and the latter are
still more striking illustrations of solitary grandeur." (Figs. 74 and
75.)
[Illustration: Fig. 74.--_Liriodendron Meekii_, Heer. (After Lesquereux.)]
[Illustration: Fig. 75.--_Liriodendron primævum_, Newberry. (After
Newberry.)]
"Three species of _Liriodendron_ are indicated by leaves found in the
Amboy clays--Middle Cretaceous--of New Jersey, and others have been
obtained from the Dakota group in the West, and from the Upper Cretaceous
strata of Greenland. Though differing considerably among themselves in
size and form, all these have the deep sinus of the upper extremity
so characteristic of the genus, and the nervation is also essentially
the same. Hence, we must conclude that the genus _Liriodendron_, now
represented by a single species, was in the Cretaceous age much more
largely developed, having many species, and those scattered throughout
many lands. In the Tertiary age the genus continued to exist, but
the species seem to have been reduced to one, which is hardly to be
distinguished from that now living. In many parts of Europe leaves of
the tulip-tree have been found, and it extended as far south as Italy.
Its presence there was first made known by Unger, in his 'Synopsis,'
page 232, and in his 'Genera et Species,' page 443, where he describes
it under the name of _Liriodendron procaccinii_. The genus has also been
noticed in Europe by Massalongo, Heer, and Ettingshausen, and three
species have been distinguished. All these are, however, so much like the
living species that they should probably be united with it. We here have
a striking illustration of the wide distribution of a species which has
retained its characters both of fruit and leaf quite unchanged through
long migrations and an enormous lapse of time.
"In Europe the tulip-tree, like many of its American associates, seems
to have been destroyed by the cold of the Ice period, the Mediterranean
cutting off its retreat, but in America it migrated southward over the
southern extension of the continent and returned northward again with the
amelioration of the climate."
Leaves of _Liriodendron_ have been recognised in the Cretaceous of
Greenland, though it is now a tree of the warm temperate region, and
Lesquereux describes several species from the Dakota group. But the genus
has not yet been recognised in the Laramie or in the Upper Cretaceous of
British Columbia. In the paper above quoted, Newberry describes three new
species from the Amboy clays, one of which he considers identical with a
Greenland form referred by Heer to _L. Meekii_ of the Dakota group. Thus,
if all Lesquereux's species are to be accepted, the genus begins in the
Middle Cretaceous with at least nine American species.
In New Jersey the Amboy clays are referred to the same age with the
Dakota beds of the West. In these Dr. Newberry has found a rich flora,
including many angiosperms. The following is condensed from a preliminary
notice in the "Bulletin of the Torrey Botanical Club":[DO]
[DO] March, 1886.
"The flora of the Amboy clays is closely related to that of the Dakota
group--most of the genera and some of the species being identical--so
that we may conclude they were nearly contemporaneous, though the absence
in New Jersey of the Fort Benton and Niobrara groups of the upper
Missouri and the apparent synchronism of the New Jersey marls and the
Pierre group indicate that the Dakota is a little the older.
"At least one-third of the species of the Amboy clays seem to be
identical with leaves found in the Upper Cretaceous clays of Greenland
and Aachen (Aix la Chapelle), which not only indicates a chronological
parallelism, but shows a remarkable and unexpected similarity in the
vegetation of these widely separated countries in the middle and last
half of the Cretaceous age. The botanical character of the flora of the
Amboy clays will be seen from the following brief synopsis:
"_Algæ._--A small and delicate form, allied to Chondrites.
"_Ferns._--Twelve species, generally similar and in part identical with
those described by Heer from the Cretaceous beds of Greenland, and
referred to the genera _Dicksonia_, _Gleichenia_, and _Aspidium_.
"_Cycads._--Two species, probably identical with the forms from Greenland
described by Heer under the names of _Podozamites marginatus_ and _P.
tenuinervis_.
"_Conifers._--Fourteen species, belonging to the genera _Moriconia_,
_Brachyphyllum_, _Cunninghamites_, _Pinus_, _Sequoia_, and others
referred by Heer to _Juniperus_, _Libocedrus_, _Frenelopsis_, _Thuya_,
and _Dammara_. Of these, the most abundant and most interesting
are _Moriconia cyclotoxon_--the most beautiful of conifers--and
_Cunninghamites elegans_, both of which occur in the Cretaceous clays of
Aachen, Prussia, and Patoot, Greenland. The _Brachyphyllum_ was a large
and strong species, with imbricated cones, eight inches in length.
"The angiosperms form about seventy species, which include three of
_Magnolia_, four of _Liriodendron_, three or four of _Salix_, three of
_Celastrophyllum_ (of which one is identical with a Greenland species),
one _Celastrus_ (also found in Greenland), four or five _Aralias_,
two _Sassafras_, one _Cinnamomum_, one _Hedera_; with leaves that
are apparently identical with those described by Heer as belonging
to _Andromeda_, _Cissites_, _Cornus_, _Dewalquea_, _Diospyros_,
_Eucalyptus_, _Ficus_, _Ilex_, _Juglans_, _Laurus_, _Menispermites_,
_Myrica_, _Myrsine_, _Prunus_, _Rhamnus_, and others not yet determined.
"Some of the Aralias had palmately-lobed leaves, nearly a foot in
diameter, and two of the tulip-trees (_Liriodendron_) had leaves quite
as large as those of the living species. One of these had deeply lobed
leaves, like those of the white oak. Of the other, the leaves resembled
those of the recent tulip-tree, but were larger. Both had the peculiar
emargination and the nervation of _Liriodendron_.
"Among the most interesting plants of the collection are fine species of
_Bauhinia_ and _Hymenæa_. Of these, the first is represented by a large
number of leaves, some of which are six or seven inches in diameter.
They are deeply bilobed, and have the peculiar and characteristic form
and nervation of the leaves of this genus. _Bauhinia_ is a leguminous
genus allied to _Cercis_, and now inhabits tropical and warm temperate
climates in both hemispheres. Only one species occurs in the United
States, _Bauhinia lunarioides_, Gray, found by Dr. Bigelow on the Rio
Grande.
"_Hymenæa_ is another of the leguminosæ, and inhabits tropical America. A
species of this genus has been found in the Upper Cretaceous of France,
but quite different from the one before us, in which the leaves are much
larger, and the leaflets are united in a common petiole, which is winged;
this is a modification not found in the living species, and one which
brings it nearer to _Bauhinia_.
"But the most surprising discovery yet made is that of a number of quite
large helianthoid flowers, which I have called _Palæanthus_. These
are three to four inches in diameter, and exhibit a scaly involucre,
enclosing what much resembles a fleshy receptacle with achenia. From
the border of this radiate a number of ray florets, one to two inches
in length, which are persistent and must have been scarious, like
those of _Helichrysum_. Though these flowers so much resemble those
of the compositæ, we are not yet warranted in asserting that such is
certainly their character. In the Jurassic rocks of Europe and India
some flowers not very unlike these have been found, which have been
named _Williamsonia_, and referred to cycads by Carruthers. A similar
fossil has been found in the Cretaceous rocks of Greenland, and named
by Heer _Williamsonia cretacea_, but he questions the reference of the
genus to the Cycadeæ, and agrees with Nathorst in considering all the
species of _Williamsonia_ as parasitic flowers, allied to _Brugmansia_
or _Rafflesia_. The Marquis of Saporta regards them as monocotyledons,
similar to _Pandanus_. More specimens of the flowers now exhibited
will perhaps prove--what we can now only regard as probable--that the
Compositæ, like the _Leguminosæ_, _Magnoliaceæ_, _Celastraceæ_, and
other highly organised plants, formed part of the Cretaceous flora. No
composite flowers have before been found in the fossil state, and, as
these are among the most complex and specialised forms of florescence, it
has been supposed that they belonged only to the recent epoch, where they
were the result of a long series of formative changes."
The above presents some interesting new types not heretofore found in the
Middle Cretaceous. More especially the occurrence of large flowers of the
composite type presents a startling illustration of the early appearance
of a very elevated and complex form. Great interest also attaches to
these Amboy beds, as serving, with those of Aix and Greenland, to show
that the margins of the Atlantic were occupied with a flora similar to
that occurring at the same time in the interior plateau of North America
and on the Pacific slope.
The beds at Aix-la-Chapelle are, however, probably somewhat newer than
the Dakota or Amboy beds, and correspond more nearly in age with those
of the Cretaceous coal-field of Vancouver Island, where there is a
very rich Upper Cretaceous flora, which I have noticed in detail in
the "Transactions of the Royal Society of Canada."[DP] In these Upper
Cretaceous beds there are fan-palms as far north at least as the latitude
of 49°, indicating a very mild climate at this period. This inference
is corroborated by the Upper Cretaceous flora of Atané and Patoot in
Greenland, as described by Heer.
[DP] Vol. ii., 1884.
The dicotyledonous plants above referred to are trees and shrubs. Of the
herbaceous exogens of the period we know less. Obviously their leaves
are less likely to find their way into aqueous deposits than the leaves
of trees. They are, besides, more perishable, and in densely wooded
countries there are comparatively few herbaceous plants. I have examined
the beds of mud deposited at the mouth of a woodland streamlet, and have
found them stored with the fallen leaves of trees, but it was in vain to
search for the leaves of herbaceous plants.
[Illustration: Fig. 76.--_Brasenia antiqua_. Upper Cretaceous, South
Saskatchewan River. Natural size, _a_, _b_, Diagrams of venation,
slightly enlarged.]
The climate of North America and Europe, represented by the Cenomanian
vegetation, is not tropical but warm temperate; but the flora was more
uniform than at present, indicating a very equable climate and the
possibility of temperate genera existing within the Arctic circle, and it
would seem to have become warmer toward the close of the period.
The flora of the Cenomanian is separated in most countries from that of
the Senonian, or uppermost Cretaceous, by a marine formation holding few
plants. This depends on great movements of elevation and depression, to
which we must refer in the sequel. In a few regions, however, as in the
vicinity of the Peace River in Canada, there are plant-bearing beds which
serve to bridge over the interval between the Early Cenomanian and the
later Cretaceous.[DQ]
[DQ] See paper by the author in the "Transactions of the Royal Society of
Canada," 1882.
To this interval also would seem to belong the Belly River series of
western Canada, which contains important beds of Coal, but is Closely
associated with the marine Fort Pierre series. A very curious herbaceous
plant of this group, which I have named _Brasenia antiqua_, occurs in the
beds associated with one of the coals. It is a close ally of the modern
_B. peltata_, an aquatic plant which occurs in British Columbia and in
eastern America, and is also said to be found in Japan, Australia, and
India, a width of distribution appropriate to so old a type (Fig. 76).
In so far as vegetable life is concerned, the transition from the Upper
Cretaceous to the Tertiary or Kainozoic is easy, though in many parts
of the world, and more especially in western Europe, there is a great
gap in the deposits between the upper Chalk and the lowest Eocene.
With reference to fossil plants, Schimper recognises in the Kainozoic,
beginning with the oldest, five formations--Palæocene, Eocene, Oligocene,
Miocene, and Pliocene. Throughout these a flora, similar to that of the
Cretaceous on the one hand and the modern on the other, though with
important local peculiarities, extends. There is evidence, however,
of a gradual refrigeration, so that in the Pliocene the climates of
the northern hemisphere were not markedly different from their present
character.
In the first instance an important error was committed by palæobotanists,
in referring to the Miocene many deposits really belonging to the Eocene.
This arose from the early study of the rich plant-bearing Miocene beds of
Switzerland, and from the similarity of the flora all the way from the
Middle Cretaceous to the later Tertiary. The differences are now being
worked out, and we owe to Mr. Starkie Gardner the credit of pointing
these out in England, and to the Geological Survey of Canada that of
collecting the material for exhibiting them in the more northern part of
America.
In the great interior plain of America there rests on the Cretaceous
a series of clays and sandstones with beds of lignite, some of them
eighteen feet in thickness. This was formerly known as the lignitic or
lignite Tertiary, but more recently as the Laramie series. These beds
were deposited in fresh or brackish water, in an internal sea or group of
lakes and swamps, when the continent was lower than at present. They have
been studied both in the United States[DR] and Canada; and, though their
flora was originally referred by mistake to the Miocene, it is now known
to be Eocene or Palæocene, or even in part a transition group between
the latter and the Cretaceous. The following remarks, taken chiefly from
recent papers by the author,[DS] will serve to illustrate this:
[DR] See more especially the elaborate and valuable reports by Lesquereux
and Newberry, and a recent memoir by Ward on "Types of the Laramie
Flora," "Bulletins of the United States Geological Survey," 1887.
[DS] "Transactions of the Royal Society of Canada," 1886-'87.
On the geological map of Canada the Laramie series, formerly known as
the lignitic or lignite Tertiary, occurs, with the exception of a few
outliers, in two large areas west of the 100th meridian, and separated
from each other by a tract of older Cretaceous rocks, over which the
Laramie beds may have extended, before the later denudation of the region.
The most eastern of these areas, that of the Souris River and Wood
Mountain, extends for some distance along the United States boundary,
between the 102d and 109th meridians, and reaches northward to about
thirty miles south of the "elbow" of the South Saskatchewan River, which
is on the parallel of 51° north. In this area the lowest beds of the
Laramie are seen to rest on those of the Fox Hill group of the Upper
Cretaceous, and at one point on the west they are overlaid by beds of
Miocene Tertiary age, observed by Mr. McConnell, of the Geological
Survey, in the Cypress Hills, and referred by Cope, on the evidence of
mammalian remains, to the White River division of the United States
geologists, which is regarded by them as Lower Miocene.[DT] The age of
the Laramie beds is thus stratigraphically determined to be between the
Fox Hill Cretaceous and the Lower Miocene. They are also undoubtedly
continuous with the Fort Union group of the United States geologists on
the other side of the international boundary, and they contain similar
fossil plants. They are divisible into two groups--a lower, mostly
argillaceous, and to which the name of "Bad Lands beds" may be given,
from the "bad lands" of Wood Mountain, where they are well exposed, and
an upper, partly arenaceous member, which may be named the Souris River
or Porcupine Creek division. In the lower division are found reptilian
remains of Upper Cretaceous type, with some fish remains more nearly
akin to those of the Eocene.[DU] Neither division has as yet afforded
mammalian remains.
[DT] "Report of the Geological Survey of Canada," 1885.
[DU] Cope, in Dr. G. M. Dawson's "Report on the 49th Parallel."
The western area is of still larger dimensions, and extends along the
eastern base of the Rocky Mountains from the United States boundary
to about the 55th parallel of latitude, and stretches eastward to the
111th meridian. In this area, and more especially in its southern part,
the officers of the Geological Survey of Canada have recognised three
divisions, as follows: (1) The Lower Laramie or St. Mary River series,
corresponding in its character and fossils to the Lower or Bad Lands
division of the other area. (2) A middle division, the Willow Creek beds,
consisting of clays, mostly reddish, and not recognised in the other
area. (3) The Upper Laramie or Porcupine Hills division, corresponding
in fossils, and to some extent in mineral character, to the Souris River
beds of the eastern area.
The fossil plants collected by Dr. G. M. Dawson in the eastern area were
noticed by the author in an appendix to Dr. Dawson's report on the 49th
parallel, in 1875, and a collection subsequently made by Dr. Selwyn
was described in the "Report of the Geological Survey of Canada" for
1879-'80. Those of the western area, and especially collections made by
myself near Calgary in 1883, and by officers of the Geological Survey in
1884, have been described in the "Transactions of the Royal Society of
Canada" vols. iii. and iv.
In studying these fossil plants, I have found that there is a close
correspondence between those of the Lower and Upper Laramie in the two
areas above referred to respectively, and that the flora of the Lower
Laramie is somewhat distinct from that of the Upper, the former being
especially rich in certain aquatic plants, and the latter much more
copious on the whole, and much more rich in remains of forest-trees. This
is, however, possibly an effect rather of local conditions than of any
considerable change in the flora, since some Upper Laramie forms recur
as low as the Belly River series of the Cretaceous, which is believed on
stratigraphical grounds to be considerably older than the Lower Laramie.
With reference to the correlation of these beds with those of the United
States, some difficulty has arisen from the tendency of palæobotanists
to refer the plants of the Upper Laramie to the Miocene age, although
in the reports of Mr. Clarence King, the late director of the United
States Geological Survey, these beds are classed, on the evidence of
stratigraphy and animal fossils, as Upper Cretaceous. More recently,
however, and partly perhaps in consequence of the views maintained by the
writer since 1875, some change of opinion has occurred, and Dr. Newberry
and Mr. Lesquereux seem now inclined to admit that what in Canada we
recognise as Upper Laramie is really Eocene, and the Lower Laramie
either Cretaceous or a transition group between this and the Eocene. In
a recent paper [DV] Dr. Newberry gives a comparative table, in which he
correlates the Lower Laramie with the Upper Cretaceous of Vancouver
Island and the Faxoe and Maestricht beds of Europe, while he regards the
Upper Laramie as equivalent to European Eocene. Except in so far as the
equivalence of the Lower Laramie and Vancouver Island beds is concerned,
this corresponds very nearly with the conclusions of the writer in a
paper published last year[DW]--namely, that we must either regard the
Laramie as a transition Cretaceo-Eocene group, or must institute our
line of separation in the Willow Creek or Middle Laramie division, which
has, however, as yet afforded no fossil plants. I doubt, however, the
equivalence of the Vancouver beds and the Lower Laramie, except perhaps
in so far as the upper member of the former is concerned. I have also to
observe that in the latest report of Mr. Lesquereux he still seems to
retain in the Miocene certain formations in the West, which from their
fossil plants I should be inclined to regard as Eocene.[DX]
[DV] Newberry, "Transactions of the New Fork Academy," February, 1886.
[DW] "Transactions of the Royal Society of Canada," vol. ii.
[DX] While these sheets were going through the press I received a very
valuable report of Mr. Lester F. Ward upon the Laramie of the United
States. I have merely had time to glance at this report, but can see that
the views of the author agree closely with those above expressed.
Two ferns occurring in these beds are remarkable as evidence of the
persistence of species, and of the peculiarities of their ancient and
modern distribution. _Onoclea sensibilis_, the very common sensitive
fern of eastern America, is extremely abundant in the Laramie beds over
a great area in the West. Mr. Starkie Gardner and Dr. Newberry have also
shown that it is identical with the _Filicites Hebridicus_ of Forbes,
from the early Eocene beds of the Island of Mull, in Scotland. Thus we
have a species once common to Europe and America, but now restricted
to the latter, and which has continued to exist over all the vast ages
between the Cretaceous and the present day. In the Laramie beds I have
found associated with this species another and more delicate fern,
the modern _Davallia_ (_Stenloma_) _tenuifolia_, but this, unlike its
companion, no longer occurs in America, but is found in the mountains of
Asia. This is a curious illustration of the fact that frail and delicate
plants may be more ancient than the mountains or plains on which they
live.
There are also some very interesting and curious facts in connection with
the conifers of the Laramie. One of the most common of these is a _Thuja_
or arbor vitæ (the so-called "cedar" of Canada). The Laramie species has
been named _T. interrupta_ by Newberry, but it approaches very closely
in its foliage to _T. occidentalis_, of eastern Canada, while its fruit
resembles that of the western species, _T. gigantea_.
Still more remarkable are the Sequoias to which we have already referred,
but which in the Laramie age seem to have been spread over nearly
all North America. The fossil species are of two types, representing
respectively the modern _S. gigantea_ and _S. sempervirens_, and their
wood, as well as that of Thuja, is found in great abundance in the
lignites, and also in the form of silicified trunks, and corresponds
with that of the recent species. The Laramie contains also conifers
of the genera _Glyptostrobus_, _Taxodium_, and _Taxus_; and the
genus _Salisburia_ or gingko--so characteristic of the Jurassic and
Cretaceous--is still represented in America as well as in Europe in the
early Eocene.
We have no palms in the Canadian or Scottish Palæocene, though I
believe they are found further south. The dicotyledonous trees are
richly represented. Perhaps the most conspicuous were three species of
_Platanus_, the leaves of which sometimes fill the sandstones, and one
of which, _P. nobilis_, Newberry, sometimes attains the gigantic size of
a foot or more in diameter of its blade. The hazels are represented by a
large-leaved species, _C. Macquarrii_, and by leaves not distinguishable
from those of the modern American species, _C. Americana_ and _C.
rostrata_. There are also chestnuts and oaks. But the poplars and willows
are specially abundant, being represented by no less than six species,
and it would seem that all the modern types of poplar, as indicated by
the forms and venation of the leaves, existed already in the Laramie, and
most of them even in the Upper Cretaceous. _Sassafras_ is represented by
two species, and the beautiful group of _Viburnum_,, to which the modern
tree-cranberry belongs, has several fine species, of some of which both
leaves and berries have been found. The hickories and butternuts are
also present, the horse-chestnut, the _Catalpa_ and _Sapindus_, and some
curious leaves which seem to indicate the presence of the modern genus
_Symphorocarpus_, the snow-berry tribe.
The above may suffice to give an idea of the flora of the older Eocene
in North America, and I may refer for details to the works of Newberry,
Lesquereux, and Ward, already cited. I must now add that the so-called
Miocene of Atanekerdluk, Greenland, is really of the same age, as also
the "Miocene" of Mull, in Scotland, of Antrim, in Ireland, and of Bovey
Tracey, in the south of England, and the Gelinden, or "Heersian" beds,
of Belgium, described by Saporta. In comparing the American specimens
with the descriptions given by Gardner of the leaf-beds at Ardtown, in
Mull, we find, as already stated, _Onoclea sensibilis_, common to both.
The species of _Sequoia_, _Gingko_, _Taxus_, and _Glyptostrobus_ are
also identical or closely allied, and so are many of the dicotyledonous
leaves. For example, _Platanoides Hebridicus_ is very near to _P.
nobilis_, and _Corylus Macquarrii_ is common to both formations, as well
as _Populus Arctica_ and _P. Richardsoni_. I may add that ever since
1875-'76, when I first studied the Laramie plants, I have maintained
their identity with those of the Fort Union group of the United States,
and of the so-called Miocene of McKenzie River and Greenland, and that
the whole are Paleocene; and this conclusion has now been confirmed by
the researches of Gardner in England, and by the discovery of true Lower
Miocene beds in the Canadian northwest, overlying the Laramie or lignite
series.
In a bulletin of the United States Geological Survey (1886), Dr. White
has established in the West the continuous stratigraphical succession of
the Laramie and the Wahsatch Eocene, thus placing the Laramie conformably
below the Lower Eocene of that region. Cope has also described as the
Puerta group a series of beds holding vertebrate fossils, and forming a
transition from the Laramie to the Wahsatch. White also testifies that
a number of fresh-water mollusks are common to the Wahsatch and the
Laramie. This finally settles the position of the Laramie so far as the
United States geologists are concerned, and shows that the flora is to be
regarded as Eocene if not Upper Cretaceous, in harmony with what has been
all along maintained in Canada. An important _résumé_ of the flora has
just been issued by Ward in the bulletins of the United States Geological
Survey (1887).
Before leaving this part of the subject, I would deprecate the remark,
which I see occasionally made, that fossil plants are of little value in
determining geological horizons in the Cretaceous and Tertiary. I admit
that in these periods some allowance must be made for local differences
of station, and also that there is a generic sameness in the flora of the
northern hemisphere, from the Cenomanian to the modern, yet these local
differences and general similarity are not of a nature to invalidate
inferences as to age. No doubt, so long as palæobotanists seemed obliged,
in deference to authority, and to the results of investigations limited
to a few European localities, to group together, without distinction, all
the floras of the later Cretaceous and earlier Tertiary, irrespective
of stratigraphical considerations, the subject lost its geological
importance. But, when a good series has been obtained in any one region
of some extent, the case becomes different. Though there is still much
imperfection in our knowledge of the Cretaceous and Tertiary floras
of Canada, I think the work already done is sufficient to enable any
competent observer to distinguish by their fossil plants the Lower,
Middle, and Upper Cretaceous, and the latter from the Tertiary; and, with
the aid of the work already done by Lesquereux and Newberry in the United
States, to refer approximately to its true geological position any group
of plants from beds of unknown age in the West.
An important consequence arising from the above statements is that
the period of warm climate which enabled a temperate flora to exist
in Greenland was that of the later Cretaceous and early Eocene rather
than, as usually stated, the Miocene. It is also a question admitting of
discussion whether the Eocene flora of latitudes so different as those of
Greenland, Mackenzie River, northwest Canada, and the United States, were
strictly contemporaneous, or successive within a long geological period
in which climatal changes were gradually proceeding. The latter statement
must apply at least to the beginning and close of the period; but the
plants themselves have something to say in favour of contemporaneity. The
flora of the Laramie is not a tropical but a temperate flora, showing no
doubt that a much more equable climate prevailed in the more northern
parts of America than at present. But this equability of climate implies
the possibility of a great geographical range on the part of plants.
Thus it is quite possible and indeed highly probable that in the Laramie
age a somewhat uniform flora extended from the Arctic seas through the
great central plateau of America far to the south, and in like manner
along the western coast of Europe. It is also to be observed that, as
Gardner points out, there are some differences indicating a diversity
of climate between Greenland and England, and even between Scotland
and Ireland and the south of England, and we have similar differences,
though not strongly-marked, between the Laramie of northern Canada and
that of the United States. When all our beds of this age from the Arctic
sea to the 49th parallel have been ransacked for plants, and when the
palæobotanists of the United States shall have succeeded in unravelling
the confusion which now exists between their Laramie and the Middle
Tertiary, the geologist of the future will be able to restore with much
certainty the distribution of the vast forests which in the early Eocene
covered the now bare plains of interior America. Further, since the
break which in western Europe separates the flora of the Cretaceous from
that of the Eocene does not exist in America, it will then be possible
to trace the succession from the Mesozoic flora of the Trias and of the
Queen Charlotte Islands and Kootanie series of the Lower Cretaceous up
to the close of the Eocene; and to determine, for America at least, the
manner and conditions under which the angiospermous flora of the later
Cretaceous succeeded to the pines and cycads which characterised the
beginning of the Cretaceous period. In so far as Europe is concerned,
this may be more difficult, since the want of continuity of land from
north to south seems there to have been fatal to the continuance of some
plants during changes of climate, and there were also apparently in the
Kainozoic period invasions at certain times of species from the south and
east, which did not occur to the same extent in America.
In recent reports on the Tertiary floras of Australia and New
Zealand,[DY] Ettingshausen holds that the flora of the Tertiary, as a
whole, was of a generalised character; forms now confined to the southern
and northern hemispheres respectively being then common to both. It would
thus seem that the present geographical diversities must have largely
arisen from the great changes in climate and distribution of land and
water in the later Tertiary.
[DY] "Geological Magazine," August, 1887.
The length of our discussion of the early angiospermous flora does not
permit us to trace it in detail through the Miocene and Pliocene, but
we may notice the connection through these in the next chapter, and may
refer to the magnificent publications of Heer and Lesquereux on the
Tertiary floras of Europe and America respectively.
CHAPTER VII.
PLANTS FROM THE TERTIARY TO THE MODERN PERIOD.
It may be well to begin this chapter with a sketch of the general
physical and geological conditions of the period which was characterised
by the advent and culmination of the dicotyledonous trees.
In the Jurassic and earliest Cretaceous periods the prevalence, over the
whole of the northern hemisphere and for a long time, of a monotonous
assemblage of gymnospermous and acrogenous plants, implies a uniform and
mild climate, and facility for intercommunication in the north. Toward
the end of the Jurassic and beginning of the Cretaceous, the land of the
northern hemisphere was assuming greater dimensions, and the climate
probably becoming a little less uniform. Before the close of the Lower
Cretaceous period the dicotyledonous flora seems to have been introduced,
under geographical conditions which permitted a warm temperate climate to
extend as far north as Greenland.
In the Cenomanian or Middle Cretaceous age we find the northern
hemisphere tenanted with dicotyledonous trees closely allied to those of
modern times, though still indicating a climate much warmer than that
which at present prevails. In this age, extensive but gradual submergence
of land is indicated by the prevalence of chalk and marine limestones
over the surface of both continents; but a circumpolar belt seems to
have been maintained, protecting the Atlantic and Pacific basins from
floating ice, and permitting a temperate flora of great richness to
prevail far to the north, and especially along the southern margins and
extensions of the circumpolar land. These seem to have been the physical
conditions which terminated the existence of the old Mesozoic flora and
introduced that of the Middle Cretaceous.
As time advanced the quantity of land gradually increased, and the
extension of new plains along the older ridges of land was coincident
with the deposition of the great Laramie series, and with the origination
of its peculiar flora, which indicates a mild climate and considerable
variety of station in mountain, plain, and swamp, as well as in great
sheets of shallow and weedy fresh water.
In the Eocene and Miocene periods, the continents gradually assumed their
present form, and the vegetation became still more modern in aspect.
In that period of the Eocene, however, in which the great nummulitic
limestones were deposited, a submergence of land occurred on the eastern
continent which must have assimilated its physical conditions to those of
the Middle Cretaceous. This great change, affecting materially the flora
of Europe, was not equally great in America, which also by the north and
south extension of its mountain-chains permitted movements of migration
not possible in the Old World. From the Eocene downward, the remains of
land-animals and plants are found chiefly in lake-basins occupying the
existing depressions of the land, though more extensive than those now
remaining. It must also be borne in mind that the great foldings and
fractures of the crust of the earth which occurred at the close of the
Eocene, and to which the final elevation of such ranges as the Alps and
the Rocky Mountains belongs, permanently modified and moulded the forms
of the continents.
These statements raise, however, questions as to the precise equivalence
in time of similar floras found in different latitudes. However
equable the climate, there must have been some appreciable difference
in proceeding from north to south. If, therefore, as seems in every
way probable, the new species of plants originated on the Arctic land
and spread themselves southward, this latter process would occur most
naturally in times of gradual refrigeration or of the access of a more
extreme climate--that is, in times of the elevation of land in the
temperate latitudes, or, conversely, of local depression of land in the
Arctic, leading to invasions of northern ice. Hence, the times of the
prevalence of particular types of plants in the far north would precede
those of their extension to the south, and a flora found fossil in
Greenland might be supposed to be somewhat older than a similar flora
when found farther south. It would seem, however, that the time required
for the extension of a new flora to its extreme geographical limit is so
small, in comparison with the duration of an entire geological period,
that, practically, this difference is of little moment, or at least does
not amount to antedating the Arctic flora of a particular type by a whole
period, but only by a fraction of such period.
It does not appear that, during the whole of the Cretaceous and Eocene
periods, there is any evidence of such refrigeration as seriously to
interfere with the flora, but perhaps the times of most considerable
warmth are those of the Dunvegan group in the Middle Cretaceous, and
those of the later Laramie and oldest Eocene.
It would appear that no cause for the mild temperature of the Cretaceous
needs to be invoked, other than those mutations of land and water which
the geological deposits themselves indicate. A condition, for example, of
the Atlantic basin in which the high land of Greenland should be reduced
in elevation, and at the same time the northern inlets of the Atlantic
closed against the invasion of Arctic ice, would at once restore climatic
conditions allowing of the growth of a temperate flora in Greenland.
As Dr. Brown has shown,[DZ] and as I have elsewhere argued, the absence
of light in the Arctic winter is no disadvantage, since, during the
winter, the growth of deciduous trees is in any case suspended; while the
constant continuance of light in the summer is, on the contrary, a very
great stimulus and advantage.
[DZ] "Florula Discoana."
It is a remarkable phenomenon in the history of genera of plants in
the later Mesozoic and Tertiary, that the older genera appear at once
in a great number of specific types, which become reduced as well as
limited in range down to the modern. This is, no doubt, connected with
the greater differentiation of local conditions in the modern; but it
indicates also a law of rapid multiplication of species in the early life
of genera. The distribution of the species of _Salisburia_, _Sequoia_,
_Platanus_, _Sassafras_, _Liriodendron_, _Magnolia_, and many other
genera, affords remarkable proofs of this.
Gray, Saporta, Heer, Newberry, Lesquereux, and Starkie Gardner have all
ably discussed these points; but the continual increase of our knowledge
of the several floras, and the removal of error as to the dates of their
appearance, must greatly conduce to clearer and more definite ideas. In
particular, the prevailing opinion that the Miocene was the period of the
greatest extension of warmth and of a temperate flora into the Arctic,
must be abandoned in favour of the later Cretaceous and Eocene; and, if
I mistake not, this will be found to accord better with the evidence of
general geology and of animal fossils.
In these various revolutions of the later Cretaceous and Kainozoic
periods, America, as Dr. Gray has well pointed out, has had the advantage
of a continuous stretch of high land from north to south, affording a
more sure refuge to plants in times of submergence, and means of escape
to the south in times of refrigeration. Hence, the greater continuity
of American vegetation and the survival of genera like _Sequoia_ and
_Liriodendron_, which have perished in the Old World. Still, there are
some exceptions to this, for the gingko-tree is a case of survival in
Asia of a type once plentiful in America, but now extinct there. Eastern
Asia has had, however, some considerable share of the same advantage
possessed by America, with the addition, referred to by Gray, of a better
and more insular climate.
But our survey of these physical conditions can not be considered
complete till we shall have considered the great Glacial age of the
Pleistocene. It is certain that throughout the later Miocene and Pliocene
the area of land in the northern hemisphere was increasing, and the large
and varied continents were tenanted by the noblest vegetation and the
grandest forms of mammalian life that the earth has ever witnessed. As
the Pliocene drew to a close, a gradual diminution of warmth came on,
and more especially a less equable climate, and this was accompanied
with a subsidence of the land in the temperate regions and with changes
of the warm ocean-currents. Thus gradually the summers became cooler
and the winters longer and more severe, the hill-tops became covered
with permanent snows, glaciers ploughed their way downward into the
plains, and masses and fields of floating ice cooled the seas. In these
circumstances the richer and more delicate forms of vegetation must have
been chilled to death or obliged to remove farther south, and in many
extensive regions, hemmed in by the advance of the sea on the one hand
and land-ice on the other, they must have altogether perished.
Yet even in this time vegetation was not altogether extinct. Along the
Gulf of Mexico in America, and in the Mediterranean basin in Europe,
there were still some remains of a moderate climate and certain boreal
and arctic forms moving southward continued to exist here and there in
somewhat high latitudes, just as similar plants now thrive in Grinnell
Land within sight of the snows of the Greenland mountains. A remarkable
summary of some of these facts as they relate to England was given by an
eminent English botanist, Mr. Carruthers, in his address as President of
the Biological Section of the British Association at Birmingham in 1886.
At Cromer, on the coast of Norfolk, the celebrated forest-bed of newer
Pliocene age, and containing the remains of a copious mammalian fauna,
holds also remains of plants in a state admitting of determination. These
have been collected by Mr. Reid, of the Geological Survey, and were
reported on by Carruthers, who states that they represent a somewhat
colder temperature than that of the present day. I quote the following
details from the address.
With reference to the plants of the forest-bed or newer Pliocene he
remarks as follows:
"Only one species (_Trapa natans_, Willd.) has disappeared from our
islands. Its fruits, which Mr. Reid found abundantly in one locality,
agree with those of the plants found until recently in the lakes of
Sweden. Four species (_Prunus speciosa_, L., _[OE]nanthe Tichenalii_,
Sm., _Potamogeton pterophyllus_, Sch., and _Pinus abies_, L.) are found
at present only in Europe, and a fifth (_Potamogeton trichoides_, Cham.)
extends also to North America; two species (_Peucedanum palustre_,
Moench, and _Pinus sylvestris_, L.) are found also in Siberia, while six
more (_Sanguisorba officinalis_, L., _Rubus fruticosus_, L., 5 _Cornus
sanguinea_, L., _Euphorbia amygdaloides_, L., _Quercus robur_, L., and
_Potamogeton crispus_, L.) extend into western Asia, and two (_Fagus
sylvatica_, L., and _Alnus glutinosa_, L.) are included in the Japanese
flora. Seven species, while found with the others, enter also into the
Mediterranean flora, extending to North Africa: these are _Thalictrum
minus_, L., _Thalictrum flavum_, L., _Ranunculus repens_, L., _Stellaria
aquatica_, Scop., _Corylus avellana_, L., _Yannichellia palustris_, L.,
and _Cladium mariscus_, Br. With a similar distribution in the Old World,
eight species (_Bidens tripartita_, L., _Myosotis cæspitosa_, Schultz,
_Suæda maritima_, Dum., _Ceratophyllum demersum_,, L., _Sparganium
ramosum_, Huds., _Potamogeton pectinatus_, L., _Carex paludosa_, Good.,
and _Osmunda regalis_, L.) are found also in North America. Of the
remainder, ten species (_Nuphar luteum_, Sm., _Menyanthes trifoliata_,
L., _Stachys palustris_, L., _Rumex maritimus_, L., _Rumex acetosella_,
L., _Betula alba_, L., _Scirpus pauciflorus_, Lightf., _Taxus baccata_,
L., and _Isoetes lacustris_, L.) extend round the north temperate
zone, while three (_Lycopus europæus_, L., _Alisma plantago_, L., and
_Phragmites communis_, Trim), having the same distribution in the north,
are found also in Australia, and one (_Hippuris vulgaris_, L.) in the
south of South America. The list is completed by _Ranunculus aquatilis_,
L., distributed over all the temperate regions of the globe, and _Scirpus
lacustris_, L., which is found in many tropical regions as well."
He remarks that these plants, while including species now very widely
scattered, present no appreciable change of characters.
Above this bed are glacial clays, which hold other species indicating an
extremely cold climate. They are few in number, only _Salix polaris_, a
thoroughly arctic species, and its ally, _S. cinerea_, L., and a moss,
_Hypnum turgescens_, Schimp., no longer found in Britain, but an Alpine
and arctic species. This bed belongs to the beginning of the Glacial
period, the deposits of which have as yet afforded no plants in England.
But plants occur in post-glacial and upper-glacial beds in different
parts of England, to which Carruthers thus refers:
"The period of great cold, during which arctic ice extended far into
temperate regions, was not favorable to vegetable life. But in some
localities we have stratified clays with plant-remains later than the
Glacial epoch, yet indicating that the great cold had not then entirely
disappeared. In the lacustrine beds at Holderness is found a small
birch (_Betula nana_, L.), now limited in Great Britain to some of the
mountains of Scotland, but found in the arctic regions of the Old and
New World and on Alpine districts in Europe, and with it _Prunus padus_,
L., _Quercus robur_, L., _Corylus avellana_, L., _Alnus glutinosa_, L.,
and _Pinus sylvestris_, L. In the white clay-beds at Bovey Tracey of
the same age there occur the leaves of _Arctostaphylos uva-ursi_, L.,
three species of willow, viz., _Salix cinerea_, L., _S. myrtilloides_,
L., and _S. polaris_, Wahl., and in addition to our Alpine _Betula
nana_, L., the more familiar _B. alba_, L. Two of these plants have
been lost to our flora from the change of climate that has taken place,
viz., _Salix myrtilloides_, L., and _S. polaris_, Wahl.; and _Betula
nana_, L., has retreated to the mountains of Scotland. Three others
(_Dryas octopetala_, L., _Arctostaphylos uva-ursi_, L., and _Salix
herbacea_, L.) have withdrawn to the mountains of northern England,
Wales, and Scotland, while the remainder are still found scattered over
the country. Notwithstanding the diverse physical conditions to which
these plants have been subjected, the remains preserved in these beds
present no characters by which they can be distinguished from the living
representatives of the species."
One of the instances referred to is very striking. At Bovey Tracey the
arctic beds rest directly on those holding the rich, warm temperate flora
of the Eocene; so that here we have the evidence of fossil plants to show
the change from the climate of the Eocene to that of arctic lands, and
the modern vegetation to indicate the return of a warm temperature.
In Canada, in the Pleistocene beds known as the Leda clays, intervening
between the lower boulder clay and the Saxicava sand, which also holds
boulders, there are beds holding fossil plants, in some places intermixed
with sea-shells and bones of marine fishes, showing that they were
drifted into the sea at a time of submergence. These remains are boreal
rather than arctic in character, and with the remains of drift-wood often
found in the boulder deposits serve to indicate that there were at all
times oases of hardy life in the glacial deserts, just as we find these
in polar lands at the present day. I condense from a paper on these
plants[EA] the following facts, with a few additional notes:
[EA] "Canadian Naturalist," 1866.
The importance of all information bearing on the temperature of the
Post-pliocene period invests with much interest the study of the
land-plants preserved in deposits of this age. Unfortunately, these
are few in number, and often not well preserved. In Canada, though
fragments of the woody parts of plants occasionally occur in the marine
clays and sands, there is only one locality which has afforded any
considerable quantity of remains of their more perishable parts. This
is the well-known deposit of Leda clay at Green's Creek, on the Ottawa,
celebrated for the perfection in which the skeletons of the capelin
and other fishes are preserved in the calcareous nodules imbedded in
the clay. In similar nodules, contained apparently in a layer somewhat
lower than that holding the ichthyolites, remains of land-plants are
somewhat abundant, and, from their association with shells of _Leda
glacialis_, seem to have been washed down from the land into deep water.
The circumstances would seem to have been not dissimilar from those
at present existing in the northeast arm of Gaspé Basin, where I have
dredged from mud now being deposited in deep water, living specimens of
_Leda limatula_, mixed with remains of land-plants.
The following are the species of plants recognised in these nodules:
1. _Drosera rotundifolia_, Linn. In a calcareous nodule from Green's
Creek, the leaf only preserved. This plant is common in bogs in Canada,
Nova Scotia, and Newfoundland, and thence, according to Hooker, to the
Arctic circle. It is also European.
[Illustration: Fig. 77.--_Gaylussaccia resinosa_. Pleistocene, Canada.]
2. _Acer spicatum_, Lamx. (_Acer montanum_, Aiton.) Leaf in a nodule from
Green's Creek. Found in Nova Scotia and Canada, also at Lake Winnipeg,
according to Richardson.
3. _Potentilla Canadensis_, Linn. In nodules from Green's Creek; leaves
only preserved. I have had some difficulty in determining these, but
believe they must be referred to the species above named, or to _P.
simplex_, Michx., supposed by Hooker and Gray to be a variety. It occurs
in Canada and New England, but I have no information as to its range
northward.
4. _Gaylussaccia resinosa_, Torrey and Gray. Leaf in nodule at Green's
Creek. Abundant in New England and in Canada, also on Lake Huron and the
Saskatchewan, according to Richardson (Fig. 77).
5. _Populus balsamifera_, Linn. Leaves and branches in nodules at Green's
Creek. This is by much the most common species, and its leaves are of
small size, as if from trees growing in cold and exposed situations. The
species is North American and Asiatic, and abounds in New England and
Canada. It extends to the Arctic circle, and is abundant on the shores
of the Great Slave Lake and on the McKenzie River, and according to
Richardson constitutes much of the drift timber of the Arctic coast (Fig.
78).
[Illustration: Fig. 78.--_Populus balsamifera_. Pleistocene, Canada.]
6. _Thuja occidentalism_ Linn. Trunks and branches in the Leda clay
at Montreal. This tree occurs in New England and Canada, and extends
northward into the Hudson Bay territories. It is a northern though
not arctic species in its geographical range. According to Lyell it
occurs associated with the bones of Mastodon in New Jersey. From the
great durability of its wood, it is one of the trees most likely to be
preserved in aqueous deposits.
7. _Potamogeton perfoliatus_, Linn. Leaves and seeds in nodules at
Green's Creek. Inhabits streams of the Northern States and Canada, and
according to Richardson extends to Great Slave Lake.
8. _Potamogeton pusillus._ Quantities of fragments which I refer to this
species occur in nodules at Green's Creek. They may possibly belong to a
variety of _P. hybridus_ which, together with _P. natans_, now grows in
the river Ottawa, where it flows over the beds containing these fossils.
9. _Cariceæ and Gramineæ._ Fragments in nodules from Green's Creek appear
to belong to plants of these groups, but I cannot venture to determine
their species.
10. _Equisetum scirpoides_, Michx. Fragments in nodules, Green's Creek.
This is a widely distributed species, occurring in the Northern States
and Canada.
11. _Fontinalis._ In nodules at Green's Creek there occur, somewhat
plentifully, branches of a moss apparently of the genus _Fontinalis_.
[Illustration: Fig. 79.--Frond of _Fucus_. Pleistocene, Canada.]
12. _Algæ._ With the plants above mentioned, both at Green's Creek and at
Montreal, there occur remains of sea-weeds (Fig. 79). They seem to belong
to the genera _Fucus_ and _Ulva_, but I cannot determine the species.
A thick stem in one of the nodules would seem to indicate a large
_Laminaria_. With the above there are found at Green's Creek a number of
fragments of leaves, stems, and fruits, which I have not been able to
refer to their species, principally on account of their defective state
of preservation.
None of the plants above mentioned is properly arctic in its
distribution, and the assemblage may be characterised as a selection from
the present Canadian flora of some of the more hardy species having the
most northern range. Green's Creek is in the central part of Canada, near
to the parallel of 46°, and an accidental selection from its present
flora, though it might contain the same species found in the nodules,
would certainly include with these, or instead of some of them, more
southern forms. More especially the balsam poplar, though that tree
occurs plentifully on the Ottawa, would not be so predominant. But such
an assemblage of drift-plants might be furnished by any American stream
flowing in the latitude of 50° to 55° north. If a stream flowing to the
north, it might deposit these plants in still more northern latitudes, as
the McKenzie River does now. If flowing to the south, it might deposit
them to the south of 50°. In the case of the Ottawa, the plants could
not have been derived from a more southern locality, nor probably from
one very far to the north. We may therefore safely assume that the
refrigeration indicated by these plants would place the region bordering
the Ottawa in nearly the same position with that of the south coast of
Labrador fronting on the Gulf of St. Lawrence at present. The absence of
all the more arctic species occurring in Labrador should perhaps induce
us to infer a somewhat milder climate than this.
The moderate amount of refrigeration thus required would in my opinion
accord very well with the probable conditions of climate deducible from
the circumstances in which the fossil plants in question occur. At the
time when they were deposited the sea flowed up the Ottawa valley to a
height of 200 to 400 feet above its present level, and the valley of the
St. Lawrence was a wide arm of the sea, open to the arctic current. Under
these conditions the immense quantities of drift-ice from the northward,
and the removal of the great heating surface now presented by the low
lands of Canada and New England, must have given for the Ottawa coast
of that period a summer temperature very similar to that at present
experienced on the Labrador coast, and with this conclusion the marine
remains of the Leda clay, as well as the few land molluscs whose shells
have been found in the beds containing the plants, and which are species
still occurring in Canada, perfectly coincide.
The climate of that portion of Canada above water at the time when these
plants were imbedded may safely be assumed to have been colder in summer
than at present, to an extent equal to about 5° of latitude, and this
refrigeration may be assumed to correspond with the requirements of
the actual geographical changes implied. In other words, if Canada was
submerged until the Ottawa valley was converted into an estuary inhabited
by species of _Leda_, and frequented by capelin, the diminution of the
summer heat consequent on such depression would be precisely suitable to
the plants occurring in these deposits, without assuming any other cause
of change of climate.
I have arranged elsewhere the Post-pliocene deposits of the central
part of Canada, as consisting of, in ascending order: (1) The boulder
clay; (2) a deep-water deposit, the Leda clay; and (3) a shallow-water
deposit, the Saxicava sand. But, although I have placed the boulder clay
in the lowest position, it must be observed that I do not regard this as
a continuous layer of equal age in all places. On the contrary, though
locally, as at Montreal, under the Leda clay, it is in other places and
at other levels contemporaneous with or newer than that deposit, which
itself also locally contains boulders.
At Green's Creek the plant-bearing nodules occur in the lower part of the
Leda clay, which contains a few boulders, and is apparently in places
overlaid by large boulders, while no distinct boulder clay underlies
it. The circumstances which accumulated the thick bed of boulder clay
near Montreal were probably absent in the Ottawa valley. In any case we
must regard the deposits of Green's Creek as coeval with the Leda clay
of Montreal, and with the period of the greatest abundance of _Leda
glacialis_, the most exclusively arctic shell of these deposits. In other
words, I regard the plants above mentioned as probably belonging to the
period of greatest refrigeration of which we have any evidence, of course
not including that mythical period of universal incasement in ice, of
which, as I have elsewhere endeavoured to show, in so far as Canada is
concerned, there is no evidence whatever.[EB]
[EB] Notes on Post-Pliocene of Canada, "Canadian Naturalist," 1872.
The facts above stated in reference to Post-pliocene plants concur, with
all the other evidence I have been able to obtain, in the conclusion that
the refrigeration of Canada in the Post-pliocene period consisted of a
diminution of the summer heat, and was of no greater amount than that
fairly attributable to the great depression of the land and the different
distribution of the ice-bearing arctic current.
In connection with the plants above noticed, it is interesting to
observe that at Green's Creek, at Pakenham Mills, at Montreal, and at
Clarenceville on Lake Champlain, species of Canadian _Pulmonata_ have
been found in deposits of the same age with those containing the plants.
The species which have been noticed belong to the genera _Lymnea_ and
_Planorbis_.
The Glacial age was, fortunately, not of very long duration, though its
length has been much exaggerated by certain schools of geologists,[EC]
It passed away, and a returning cosmic spring gladdened the earth, and
was ushered in by a time of great rainfall and consequent denudation
and deposit, which has been styled the "Pluvial Period" The remains of
the Pliocene forests then returned--with somewhat diminished numbers of
species--from the south and again occupied the land, though they have
not been able, in their decimated condition, to restore the exuberance
of the flora of the earlier Tertiary. In point of fact, as we shall
see in the next chapter, it is the floras originating within the polar
circle and coming down from the north that are rich and copious. Those
that, after periods of cold or submergence, return from the south, are
comparatively poor. Hence the modern flora is far inferior to that of the
Middle Kainozoic. In America, however, and in eastern Asia, for reasons
already stated, the return was more abundant than in Europe.
[EC] This I have long maintained on grounds connected with Pleistocene
fossils, amount of denudation and deposit, &c., and I am glad to see that
Prestwich, the best English authority on such subjects, has recently
announced similar conclusions, based on independent reasons.
Simultaneously with the return of the old temperate flora, the arctic
plants that had overspread the land retreated to mountain-tops, now bared
of ice and snow, and back to the polar lands whence they came; and so it
happens that, on the White Mountains, the Alps, and the Himalayas, we
have insular patches of the same groups of plants that exist around the
pole.
These changes need not have required a very long time, for the
multiplication and migration of plants are very rapid, especially when
aided by the agency of migratory animals. Many parts of the land must,
indeed, have been stocked with plants from various sources, and by
agencies--as that of the sea--which might at first sight seem adverse to
their distribution. The British Islands, for example, have no indigenous
plants. Their flora consists mainly of Germanic plants, which must have
migrated to Britain in that very late period of the Post-glacial when
the space now occupied by the North Sea was mostly dry land. Other
portions of it are Scandinavian plants, perhaps survivors of the Glacial
age, or carried by migratory birds; and still another element consists
of Spanish plants, brought north by spring migrants, and establishing
themselves in warm and sheltered spots, just as the arctic plants do on
the bleak hill-tops. The Bermudas, altogether recent islands, have one
hundred and fifty species of native plants, all of which are West Indian
and American, and must have been introduced by the sea-currents or by
migratory birds.
And so the earth became fitted for the residence of modern man. Yet it is
not so good or Edenic a world as it once was, or as it may yet become,
were another revolution to restore a mild climate to the arctic regions,
and to send down a new swarm of migratory species to renew the face of
the earth and restore it to its pristine fertility of vegetable life.
Thus closes this long history of the succession of plants, reaching
from the far back Laurentian to the present day. It has, no doubt, many
breaks, and much remains to be discovered. Yet it may lead us to some
positive conclusions regarding the laws of the introduction of plants.
One of these, and perhaps the most remarkable of all, is that certain
principles were settled very far back, and have remained ever since.
We have seen that in the earliest geological periods all that pertains
to the structure, powers, and laws of the vegetable cell was already
fixed and settled. When we consider how much this implies of mechanical
structure and chemical and vital property, the profound significance
of this statement becomes apparent. The relations in these respects
between the living cell and the soil, the atmosphere and the sunshine,
were apparently as perfect in the early Palæozoic as in any subsequent
time. The same may be said of the structures of the leaf and of the
stem. In such old forms as Nematophyton these were, it is true, peculiar
and rudimentary, but in the Devonian and Carboniferous the structure
of leaves and stems embodied all the parts and principles that we find
at present. In regard to fructification there has been more progress,
for, so far as we know, the highest and most complex forms of flowery,
fruits, and seeds belong to the more recent periods, and simpler forms
were at least dominant in the older times. Yet even in this respect the
great leading laws and structures of bisexual reproduction were perfected
in the early Palæozoic, and the improvements introduced in the gymnosperm
and the angiosperm of later periods have consisted mainly in additions of
accessory parts, and in modifications and refinements suited to the wants
of the higher and more complex types.
CHAPTER VIII.
GENERAL LAWS OF ORIGIN AND MIGRATIONS OF PLANTS.--RELATIONS OF RECENT
AND FOSSIL FLORAS.
The origination of the successive floras which have occupied the northern
hemisphere in geological time, not, as one might at first sight suppose,
in the sunny climes of the south, but under the arctic skies, is a fact
long known or suspected. It is proved by the occurrence of fossil plants
in Greenland, in Spitzbergen, and in Grinnell Land, under circumstances
which show that these were their primal homes. The fact bristles with
physical difficulties, yet is fertile of the most interesting theoretical
deductions, to reach which we may well be content to wade through some
intricate questions. Though not at all a new fact, its full significance
seems only recently to have dawned on the minds of geologists, and within
the last few years it has produced a number of memoirs and addresses to
learned societies, besides many less formal notices.[ED]
[ED] Saporta, "Ancienne Végétation Polaire"; Hooker, "Presidential
Address to Royal Society," 1878; Thistleton Dyer, "Lecture on Plant
Distribution"; Mr. Starkie Gardner, "Letters in 'Nature,'" 1878, &c. The
basis of most of these brochures is to be found in Heer's "Flora Fossilis
Arctica."
The earliest suggestion on the subject known to the writer is that of
Prof. Asa Gray, in 1867, with reference to the probable northern source
of the related floras of North America and eastern Asia. With the aid
of the new facts disclosed by Heer and Lesquereux, Gray returned to
the subject in 1872, and more fully developed this conclusion with
reference to the Tertiary floras,[EE] and he has recently still further
discussed these questions in an able lecture on "Forest Geography and
Archæology."[EF] In this he puts the case so well and tersely that we may
quote the following sentences as a text for what follows:
[EE] Address to American Association.
[EF] "American Journal of Science," xvi., 1818.
"I can only say, at large, that the same species (of Tertiary fossil
plants) have been found all round the world; that the richest and
most extensive finds are in Greenland; that they comprise most of
the sorts which I have spoken of, as American trees which once lived
in Europe--magnolias, sassafras, hickories, gum-trees, our identical
southern cypress (for all we can see of difference), and especially
_Sequoias_, not only the two which obviously answer to the two
big-trees now peculiar to California, but several others; that they
equally comprise trees now peculiar to Japan and China, three kinds of
gingko-trees, for instance, one of them not evidently distinguishable
from the Japan species which alone survives; that we have evidence, not
merely of pines and maples, poplars, birches, lindens, and whatever
else characterise the temperate zone forests of our era, but also of
particular species of these, so like those of our own time and country
that we may fairly reckon them as the ancestors of several of ours. Long
genealogies always deal more or less in conjecture; but we appear to
be within the limits of scientific inference when we announce that our
existing temperate trees came from the north, and within the bounds of
nigh probability when we claim not a few of them as the originals of
present species. Remains of the same plants have been found fossil in our
temperate region as well as in Europe."
Between 1860 and 1870 the writer was engaged in working out all that
could be learned of the Devonian plants of eastern America, the oldest
known flora of any richness, and which consists almost exclusively of
gigantic, and to us grotesque, representatives of the club-mosses,
ferns, and mares'-tails, with some trees allied to the cycads and pines.
In this pursuit nearly all the more important localities were visited,
and access was had to the large collections of Prof. Hall and Prof.
Newberry, in New York and Ohio, and to those made in the remarkable
plant-bearing beds of New Brunswick by Messrs. Matthew and Hartt. In
the progress of these researches, which developed an unexpectedly rich
assemblage of species, the northern origin of this old flora seemed to be
established by its earlier culmination in the northeast, in connection
with the growth of the American land to the southward, which took place
after the great Upper Silurian subsidence, by elevations beginning in
the north while those portions of the continent to the southwest still
remained under the sea. The same result was indicated by the persistence
in the Carboniferous of the south and west of old Erian forms, like
_Megalopteris_.
When, in 1870, the labours of those ten years were brought before the
Royal Society of London, in the Bakerian lecture of that year, and in a
memoir illustrating no less than one hundred and twenty-five species of
plants older than the great Carboniferous system, these deductions were
stated in connection with the conclusions of Hall, Logan, and Dana, as
to the distribution of sediment along the northeast side of the American
continent, and the anticipation was hazarded that the oldest Palæozoic
floras would be discovered to the north of Newfoundland. Mention was also
made of the apparent earlier and more copious birth of the Devonian flora
in America than in Europe, a fact which is itself connected with the
greater northward extension of this continent.
The memoir containing these results was not published by the Royal
Society, but its publication was secured in a less complete form in the
reports of the "Geological Survey of Canada." The part of the memoir
relating to Canadian fossil plants, with a portion of the theoretical
deductions, was published in a report issued in 1871.[EG] In this report
the following language was used:
[EG] "Fossil Plants of the Devonian and Upper Silurian Formations of
Canada," pp. 92, twenty plates, Montreal, 1871.
"In eastern America, from the Carboniferous period onward, the centre
of plant distribution has been the Appalachian chain. From this the
plants and sediments extended westward in times of elevation, and to this
they receded in times of depression. But this centre was nonexistent
before the Devonian period, and the centre for this must have been to
the northeast, whence the great mass of older Appalachian sediment
was derived. In the Carboniferous period there was also an eastward
distribution from the Appalachians, and links of connection in the
Atlantic bed between the floras of Europe and America. In the Devonian
such connection can have been only far to the northeast. It is therefore
in Newfoundland, Labrador, and Greenland that we are to look for the
oldest American flora, and in like manner on the border of the old
Scandinavian nucleus for that of Europe.
"Again, it must have been the wide extension of the sea of the
corniferous limestone that gave the last blow to the remaining flora of
the Lower Devonian; and the re-elevation in the middle of that epoch
brought in the Appalachian ridges as a new centre, and established
a connection with Europe which introduced the Upper Devonian and
Carboniferous floras. Lastly, from the comparative richness of the later
Erian[EH] flora in eastern America, especially in the St. John beds, it
might be a fair inference that the northeastern end of the Appalachian
ridge was the original birthplace or centre of creation of what we may
call the later Palæozoic flora, or of a large part of that flora."
[EH] See pages 107 and 108.
When my paper was written I had not seen the account published by
the able Swiss palæobotanist Heer, of the remarkable Devonian flora
of Bear Island, near Spitzbergen.[EI] From want of acquaintance with
the older floras of America and western Europe, Heer fell into the
unfortunate error of regarding the whole of Bear Island plants as
Lower Carboniferous, a mistake which his great authority has tended to
perpetuate, and which has even led to the still graver error of some
European geologists, who do not hesitate to regard as Carboniferous the
fossil plants of the American deposits from the Hamilton to the Chemung
groups inclusive, though these belong to formations underlying the oldest
Carboniferous, and characterised by animal remains of unquestioned
Devonian age. In 1872 I addressed a note to the Geological Society of
London on the subject of the so-called "Ursa stage" of Heer, showing
that, though it contained some forms not known at so early a date in
temperate Europe, it was clearly, in part at least, Devonian when tested
by North American standards; but that in this high latitude, in which,
for reasons stated in the report above referred to, I believed the
Devonian plants to have originated, there might be an intermixture of the
two floras. But such a mixed group should in that latitude be referred
to a lower horizon than if found in temperate regions. Dr. Nathorst,
as already stated, has recently obtained new facts which go to show
that plants of two distinct horizons may have been intermixed in the
collections submitted to Heer.
[EI] "Transactions of the Swedish Academy" 1871; "Journal of the London
Geological Society," vol. xxviii.
Between 1870 and 1873 my attention was turned to the two sub-floras
intermediate between those of the Devonian and the coal-formation, the
floras of the Lower Carboniferous (Subcarboniferous of some American
geologists) and the Millstone Grit, and in a report upon these[EJ]
similar deductions were expressed. It was stated that in Newfoundland
the coal-beds seem to belong to the Millstone Grit series, and as we
proceed southward they belong to progressively newer portions of the
Carboniferous system. The same fact is observed in the coal-beds of
Scotland, as compared with those of England, and it indicates that the
coal-formation flora, like that of the Devonian, spread itself from the
north, and this accords with the somewhat extensive occurrence of Lower
Carboniferous rocks and fossils in the Parry Islands and elsewhere in the
arctic regions.
[EJ] "Fossil Plants of Lower Carboniferous and Millstone Grit Formations
of Canada," pp. 47, ten plates, Montreal, 1873.
Passing over the comparatively poor flora of the earlier Mesozoic,
consisting largely of cycads, pines, and ferns, and as yet little
known in the arctic, and which may have originated in the south,
though represented, according to Heer, by the supposed Jurassic flora
of Siberia, we find, especially at Komé and Atané in Greenland, an
interesting occurrence of those earliest precursors of the truly modern
forms of plants which appear in the Cretaceous, the period of the English
chalk and of the New Jersey greensands. There are two plant-groups of
this age in Greenland; one, that of Komé, consists almost entirely of
ferns, cycads, and pines, and is of decidedly Mesozoic aspect. This is
called Lower Cretaceous. The other, that of Atané, holds remains of many
modern temperate genera, as _Populus_, _Myrica_, _Ficus_, _Sassafras_,
and _Magnolia_. This is regarded as Upper Cretaceous. Resting upon
these Upper Cretaceous beds, without the intervention of any other
formation,[EK] are beds rich in plants of much more modern appearance,
and referred by Heer to the Miocene period, a reference, as we have
seen, not warranted by comparison with the Tertiary plants of Europe or
of America. Still farther north this so-called Miocene assemblage of
plants appears in Spitzbergen and Grinnell Land; but there, owing to the
predominance of trees allied to the spruces, it has a decidedly more
boreal character than in Greenland, as might be anticipated from its
nearer approach to the pole.[EL]
[EK] Nordenskiöld, "Expedition to Greenland," "Geological Magazine," 1872.
[EL] Yet even here the bald cypress (_Taxodium distichum_), or a tree
nearly allied to it, is found, though this species is now limited to
the Southern States. Fielden and De Ranee, "Journal of the Geological
Society," 1878.
If now we turn to the Cretaceous and Tertiary floras of western America,
as described by Lesquereux, Newberry, and others, we find in the lowest
Cretaceous rocks there known--those of the Dakota group--which may be
in the lower part of the Middle Cretaceous, a series of plants[EM]
essentially similar to those of the so-called Upper Cretaceous of
Greenland. They occur in beds indicating land and fresh-water conditions
as prevalent at the time over great areas of the interior of America.
But overlying this plant-bearing formation we have an oceanic limestone
(the Niobrara), corresponding in many respects to the European chalk,
and extending far north into the British territory,[EN] indicating that
the land of the Lower Cretaceous was replaced by a vast Mediterranean
Sea, filled with warm water from the equatorial currents, and not
invaded by cold waters from the north. This is succeeded by thick Upper
Cretaceous deposits of clay and sandstone, with marine remains, though
very sparsely distributed; and these show that further subsidence or
denudation in the north had opened a way for the arctic currents, killing
out the warm-water animals of the Niobrara group, and rilling up the
Mediterranean of that period. Of the flora of these Upper Cretaceous
periods, which must have been very long, we know something in the
interior regions, from the discovery of a somewhat rich flora in the
Dunvegan beds of the Peace River district, on the northern shore of the
great Cretaceous Mediterranean;[EO] and on the coast of British Columbia
we have the remarkable Cretaceous coal-field of Vancouver Island, which
holds the remains of plants of modern genera, and, indeed, of almost
as modern aspect as those of the so-called Miocene of Greenland. They
indicate, however, a warmer climate as then prevalent on the Pacific
coast, and in this respect correspond with a peculiar transition flora,
intermediate between the Cretaceous and Eocene or earliest Tertiary of
the interior regions, and which is described by Lesquereux as the Lower
Lignitic.
[EM] Lesquereux, "Report on Cretaceous Flora."
[EN] G. M. Dawson, "Report on Forty-ninth Parallel."
[EO] "Reports of Dr. G. M. Dawson, Geological Survey of Canada." Also,
"Transactions of the Royal Society of Canada," vol. i.
Immediately above these Upper Cretaceous beds we have the great
Lignite Tertiary of the West--the Laramie group of recent American
reports--abounding in fossil plants, at one time regarded as Miocene,
but now known to be Lower Eocene, though farther south extending upward
toward the Miocene age.[EP] These beds, with their characteristic plants,
have been traced into the British territory north of the forty-ninth
parallel, and it has been shown that their fossils are identical with
those of the McKenzie River valley, described by Heer as Miocene, and
probably also with those of Alaska, referred to the same age.[EQ] Now
this truly Eocene flora of the temperate and northern parts of America
has so many species in common with that called Miocene in Greenland that
its identity can scarcely be doubted. These facts have led to scepticism
as to the Miocene age of the upper plant-bearing beds of Greenland, and
more especially Mr. J. Starkie Gardner has ably argued, from comparison
with the Eocene flora of England and other considerations, that they are
really of that earlier date.[ER]
[EP] Lesquereux's "Tertiary Flora"; "White on the Laramie Group";
Stevenson, "Geological Relations of Lignitic Groups," American
Philosophical Society, June, 1875; Dawson, "Transactions of the Royal
Society of Canada," vol. iv.; Ward, "Bulletin of United States Geological
Survey."
[EQ] G. M. Dawson, "Report on the Geology of the Forty-ninth Parallel,"
where full details on these points may be found. "Transactions of the
Royal Society of Canada," vol. iv.
[ER] "Nature," December 12, 1878.
In looking at this question, we may fairly assume that no climate,
however equable, could permit the vegetation of the neighbourhood of
Disco in Greenland to be exactly identical with that of Colorado and
Missouri, at a time when little difference of level existed in the two
regions. Either the southern flora migrated north in consequence of a
greater amelioration of climate, or the northern flora moved southward as
the climate became colder. The same argument, as Gardner has ably shown,
applies to the similarity of the Tertiary plants of temperate Europe to
those of Greenland. If Greenland required a temperature of about 50°,
as Heer calculates, to maintain its Eocene flora, the temperature of
England and that of the Southwestern States must have been higher, though
probably more equable, than at present.
We cannot certainly affirm anything respecting the migrations of these
floras, but there are some probabilities which deserve attention. The
ferns and cycads of the so-called Lower Cretaceous of Greenland are
nothing but a continuation of the previous Jurassic flora. Now this was
established at an equally early date in the Queen Charlotte Islands,[ES]
and still earlier in Virginia,[ET] The presumption is, therefore,
that it came from the south. It has, indeed, the facies of a southern
hemisphere and insular flora, and probably spread itself northward as
far as Greenland, at a time when our northern continents were groups of
islands, and when the ocean currents were carrying warm water far toward
the arctic regions. The flora which succeeds this in the sections at
Atané has no special affinities with the southern hemisphere, and is of
a more temperate and continental character.[EU] It is not necessarily
Upper Cretaceous, since it is similar to that of the Dakota group farther
south, and this is at least Middle Cretaceous. This flora must have
originated either somewhere in temperate America or within the Arctic
circle, and it must have replaced the older one by virtue of increasing
coolness and continental character of climate. It must, therefore, have
been connected with that elevation of the land which took place at the
beginning of the Cretaceous. During this elevation it spread over all
western America at one time or another, and, as the land again subsided
under the sea of the Niobrara chalk, it assumed an aspect more suited
to a warm climate, but still held its place on such islands as remained
above water along the Pacific coast and in the north, and it continued
to exist on these islands till the colder seas of the Upper Cretaceous
had again given place to the warm plains and land-locked brackish seas
or fresh-water lakes of the Laramie period (Eocene). Thus the true Upper
Cretaceous marks a cool period intervening between the so-called Upper
Cretaceous (really Middle Cretaceous) and the so-called Miocene (really
Lower Eocene) floras of Greenland.
[ES] "Reports of the Geological Survey of Canada."
[ET] Fontaine has well described the Mesozoic flora of Virginia,
"American Journal of Science," January, 1879, and "Report on Early
Mesozoic Floras."
[EU] In the "Proceedings of the Royal Society of Tasmania," 1887, Mr. R.
M. Johnston, F. L. S., states that in the Miocene beds of Tasmania trees
of European genera abound. The Mesozoic flora of that island is of the
usual conifero-cycadean type. Ettingshausen makes a similar statement in
the "Geological Magazine" respecting the Tertiary flora of Australia and
New Zealand, stating that, like the Tertiary floras of Europe, they have
a mixed character, being partly of types now belonging to the northern
hemisphere.
This latter established itself in Greenland, and probably all around the
Arctic circle, in the warm period of the earliest Eocene, and, as the
climate of the northern hemisphere became gradually reduced from that
time till the end of the Pliocene, it marched on over both continents to
the southward, chased behind by the modern arctic flora, and eventually
by the frost and snow of the Glacial age. This history may admit of
correction in details; but, so far as present knowledge extends, it is in
the main not far from the truth.
Perhaps the first great question which it raises is that as to the causes
of the alternations of warm and cold climates in the north, apparently
demanded by the vicissitudes of the vegetable kingdom. Here we may set
aside the idea that in former times plants were suited to endure greater
cold than at present. It is true that some of the fossil Greenland
plants are of unknown genera, and many are species new to us; but we are
on the whole safe in affirming that they must have required conditions
similar to those necessary to their modern representatives, except within
such limits as we now find to hold in similar cases among existing
plants. Still we know that at the present time many species found in
the equable climate of England will not live in Canada, though species
to all appearance similar in structure are native here. There is also
some reason to suppose that species when new may have greater hardiness
and adaptability than when in old age and verging toward extinction. In
any case these facts can account for but a small part of the phenomena,
which require to be explained by physical changes affecting the earth as
a whole, or at least the northern hemisphere. Many theoretical views have
been suggested on this subject, and perhaps the most practical way of
disposing of these will be first to set aside a number which are either
precluded by the known facts, incapable of producing the effects, or
altogether uncertain as to their possible occurrence.
1. In this class we may place the theory that the poles of the earth have
changed their position. Independently of astronomical objections, there
is good geological evidence that the poles of the earth must have been
nearly in their present places from the dawn of life until now. From the
Laurentian upward, those organic limestones which mark the areas where
warm and shallow equatorial water was spreading over submerged continents
are so disposed as to prove the permanence of the poles. In like manner
all the great foldings of the crust of the earth have followed lines
which are parts of great circles tangent to the existing polar circles.
So, also, from the Cambrian age the great drift of sediment from the
north has followed the line of the existing Arctic currents from the
northeast to the southwest, throwing itself, for example, along the line
of the Appalachian uplifts in eastern America, and against the ridge of
the Cordilleras in the west.
2. Some of the above considerations, along with astronomical evidence,
prevent us from assuming any considerable change in the obliquity of the
axis of the earth during geological time.
3. That the earth and the sun have diminished in heat during geological
time seems probable; but physical and geological facts alike render it
certain that this influence could have produced no appreciable effect,
even in the times of the earliest floras, and certainly not in the case
of Tertiary vegetation.
4. It has been supposed that the earth may have at different times
traversed more or less heated zones of space, giving alternations of warm
and cold temperature. No such differences in space are, however, known,
nor does there seem any good ground for imagining their existence.
5. The heat of the sun is known to be variable, and the eleven years'
period of sun-spots has recently attracted much attention as producing
appreciable effects on the seasons. There may possibly be longer cycles
of solar energy, or the sun may be liable, like some variable stars, to
paroxysms of increased energy. Such changes are possible, and may fairly
be taken into the account, provided that we fail to find known causes
sufficient to account for the phenomena.
Of well-known causes there seem to be but three. These are: First, that
urged by Lyell--viz., the varying distribution of land and water along
with that of marine currents; secondly, the varying eccentricity of
the earth's orbit, along with the precession of the equinoxes, and the
effects of this on oceanic circulation, as illustrated by Croll; thirdly,
the different conditions of the earth's atmosphere with reference to
radiation, as argued by Tyndall and Hunt. As these causes are all founded
on known facts, and not exclusive of each other, we may consider them
together. I shall take the Lyellian theory first, regarding it as the
most important, and the best supported by geological facts.
We know that the present distribution of land and water greatly
influences climate, more especially by affecting that of the ocean
currents and of the winds, and by the different action of land as
compared with water in the reception and radiation of heat. The
present distribution of land gives a large predominance to the arctic
and sub-arctic regions, as compared with the equatorial and with the
antarctic; and we might readily imagine other distributions that would
give very different results. But this is not an imaginary case. We
know that, while the forms and positions of the great continents have
been fixed from a very early date, they have experienced many great
submergences and re-elevations, and that these have occurred in somewhat
regular sequence, as evidenced by the cyclical alternations of organic
limestones and earthy sediments in successive geological formations.
An example bearing on our present subject may serve to illustrate this.
In the latter part of the Upper Silurian period (the Lower Helderberg
age), vast areas of the American continent[EV] were covered with an
ocean in which were deposited organic limestones whose fossils show
that this great interior sea was pervaded by equatorial waters bringing
food and warmth, while the incipient ranges of the Appalachians on the
east, and the Cordilleras on the west, and the Laurentian axis on the
north, fenced off from it the colder arctic waters. How different must
the climate of America and of the region north of it have been in these
circumstances from that which prevails at present, or from that which
prevailed in certain other periods, when it was open to the incursions
of the arctic ice-laden currents, bearing loads of fine sediment![EW] It
was in these circumstances, and in the similar circumstances in which the
great Corniferous limestone of the Devonian was deposited--a limestone
showing in its rich coral fauna even warmer waters than those of the
Lower Helderberg--that the Devonian flora took its origin in the north
and advanced southward over new lands in process of emergence from the
sea. The somewhat similar condition evidenced by the Lower Carboniferous
limestone preceded the advent of the great and rich flora of the
coal-formation.
[EV] See a memoir and map by Prof. Hall, "Reports of the Regents of New
York," 1874-'75.
[EW] It seems certain that the faunæ of the old limestones, like the
Trenton, Niagara, Lower Helderberg, and Corniferous, belong to warm and
sheltered sea areas, and that those rich in graptolites and trilobites,
enclosed in muddy sediments, belong to the colder arctic waters. Such
arctic faunæ are those of the Quebec group and of the Utica shale, and to
some extent that of the Hamilton group.
Lyell's theory on this subject has, I think, in some recent publications,
been somewhat misapprehended. It is true that he stated hypothetically
two contrasted conditions of distribution, in one of which all the land
was equatorial, in another all polar; but he did not suppose that these
conditions had actually occurred; and even in his earlier editions,
before the recent discoveries and discussions as to ocean currents,
he was always careful to attach due value to these in connection with
subsidences and elevations.[EX] In his later editions he introduced more
full references to current action, and also stated Croll's theory, but
still maintained the validity of his original conclusions.
[EX] See "Principles of Geology," edition of 1840, chapter vii.
The sufficiency of this Lyellian theory to account for the facts, in so
far as plants are concerned, may, I think, be inferred from the course
of the isothermal lines at present. The south end of Greenland is on
the latitude of Christiania in Norway on the one hand, and of Fort
Liard in the Peace River region on the other; and while Greenland is
clad in ice and snow, wheat and other grains, and the ordinary trees
of temperate climates, grow at the latter places,[EY] It is evident,
therefore, that only exceptionally unfavourable circumstances prevent
the Greenland area from still possessing a temperate flora, and these
unfavourable circumstances possibly tell even on the localities with
which we have compared it. Further, the mouth of the McKenzie River is in
the same latitude with Disco, near which are some of the most celebrated
localities of fossil Cretaceous and Tertiary plants. Yet the mouth of
the McKenzie River enjoys a much more favourable climate and has a much
more abundant flora than Disco. If north Greenland were submerged, and
low land reaching to the south terminated at Disco, and if from any cause
either the cold currents of Baffin's Bay were arrested, or additional
warm water thrown into the North Atlantic by the Gulf Stream, there is
nothing to prevent a mean temperature of 45° Fahr. from prevailing at
Disco; and the estimate ordinarily formed of the requirements of its
extinct floras is 50°,[EZ] which is probably above rather than below the
actual temperature required.
[EY] See "Macoun's Report," "Geological Survey of Canada," and
Richardson's "Boat Voyage."
[EZ] Heer. See, also, papers by Prof. Haughton and by Gardner in "Nature"
for 1878.
Since, then, geological facts assure us of mutations of the continents
much greater than those apparently required to account for the changes of
climate implied in the existence of the ancient arctic floras, it does
not seem absolutely necessary to invoke any others.[FA] If, however,
there are other true causes which might either aid or counteract those
above referred to, it may be well to consider them.
[FA] Sir William Thomson, "Transactions of the Geological Society of
Glasgow," February 22, 1878.
Mr. Croll has, in his valuable work "Climate and Time" and in various
memoirs, brought forward an ingenious astronomical theory to account
for changes of climate. This theory, as stated by himself in a recent
paper,[FB] is that when the eccentricity of the earth's orbit is at a
high value, and the northern winter solstice is in perihelion, agencies
are brought into operation which make the southeast trade-winds stronger
than the northeast, and compel them to blow over upon the northern
hemisphere as far as the Tropic of Cancer. The result is that all the
great equatorial currents of the ocean are impelled into the northern
hemisphere, which thus, in consequence of the immense accumulation of
warm water, has its temperature raised, so that ice and snow must to a
great extent disappear from the arctic regions. In the prevalence of
the converse conditions, the arctic zone becomes clad in ice, and the
southern has its temperature raised.
[FB] "Cataclysmic Theories of Geological Climate," "Geological Magazine,"
May, 1878.
At the same time, according to Croll's calculations, the accumulation
of ice on either pole would tend, by shifting the earth's centre of
gravity, to raise the level of the ocean and submerge the land on the
colder hemisphere. Thus a submergence of land would coincide with a cold
condition, and emergence with increasing warmth. Facts already referred
to, however, show that this has not always been the case, but that in
many cases submergence was accompanied with the influx of warm equatorial
waters and a raised temperature, this apparently depending on the
question of local distribution of land and water; and this in its turn
being regulated not always by mere shifting of the centre of gravity,
but by foldings occasioned by contraction, by equatorial subsidences
resulting from the retardation of the earth's rotation, and by the excess
of material abstracted by ice and frost from the arctic regions, and
drifted southward along the lines of arctic currents. This drifting must
in all geological times have greatly exceeded, as it certainly does at
present, the denudation caused by atmospheric action at the equator,
and must have tended to increase the disposition to equatorial collapse
occasioned by retardation of rotation.[FC]
[FC] Croll, in "Climate and Time," and in a note read before the British
Association in 1876, takes an opposite view; but this is clearly contrary
to the facts of sedimentation, which show a steady movement of _débris_
toward the south and southwest.
While such considerations as those above referred to tend to reduce
the practical importance of Mr. Croll's theory., on the other hand
they tend to remove one of the greatest objections against it--namely,
that founded on the necessity of supposing that glacial periods recur
with astronomical regularity in geological time. They cannot do so if
dependent on other causes inherent in the earth itself, and producing
important movements of its crust.
The third great cause of warmer climates in the past is the larger
proportion of carbon dioxide, or carbonic-acid gas, in the atmosphere in
early geological times, as proved by the immense amount of carbon now
sealed up in limestone and coal, and which must at one time have been in
the air. It has been shown that a very small additional quantity of this
substance would so obstruct radiation of heat from the earth as to act
almost like a glass roof. If, however, the quantity of carbonic acid,
great at first, was slowly and regularly removed, even if, as suggested
by Hunt, small additional supplies were gradually added from space, this
cause could have affected only the very oldest floras. But it is known
that some comets and meteorites contain carbonaceous matter, and this
allows us to suppose that accessions of carbon may have been communicated
at irregular intervals. If so, there may have been cycles of greater
and less abundance of this substance, and an atmosphere rich in carbon
dioxide might at one and the same time afford warmth and abundance of
food to plants.
It thus appears that the causes of ancient vicissitudes of climate are
somewhat complex, and when two or more of them happened to coincide very
extreme changes might result, having most important bearings on the
distribution of plants.
This may help us to deal with the peculiarities of the great Glacial age,
which may have been rendered exceptionally severe by the combination of
several of the causes of refrigeration. We must not suppose, however,
that the views of those extreme glacialists who suppose continental
ice-caps reaching half way to the equator are borne out by facts. In
truth, the ice accumulating round the pole must have been surrounded by
water, and there must have been tree-clad islands in the midst of the icy
seas, even in the time of greatest refrigeration. This is proved by the
fact that, in the Leda clay of eastern Canada, which belongs to the time
of greatest submergence, and whose fossil shells show sea-water almost
at the freezing-point, there are leaves of poplars and other plants
which must have been drifted from neighbouring shores. Similar remains
occur in clays of like origin in the basin of the great lakes and in the
West. These have been called "interglacial," but there is no evidence to
prove that they are not truly glacial. Thus, while we need not suppose
that plants existed within the Arctic circle in the Glacial age, we have
evidence that those of the cold temperate and sub-arctic zones continued
to exist pretty far north. At the same time the warm temperate flora
would be driven to the south, except where sustained in insular spots
warmed by the equatorial currents. It would return northward on the
re-elevation of the land and the renewal of warmth.
If, however, our modern flora is thus one that has returned from the
south, this would account for its poverty in species as compared with
those of the early Tertiary. Groups of plants descending from the north
have been rich and varied. Returning from the south they are like the
shattered remains of a beaten army. This, at least, has been the case
with such retreating floras as those of the Lower Carboniferous, the
Permian, and the Jurassic, and possibly that of the Lower Eocene of
Europe.
The question of the supply of light to an arctic flora is much less
difficult than some have imagined. The long summer day is in this respect
a good substitute for a longer season of growth, while a copious covering
of winter snow not only protects evergreen plants from those sudden
alternations of temperature which are more destructive than intense
frost, and prevents the frost from penetrating to their roots, but, by
the ammonia which it absorbs, preserves their greenness. According to
Dr. Brown, the Danish ladies of Disco long ago solved this problem.[FD]
He informs us that they cultivate in their houses most of our garden
flowers--as roses, fuchsias, and geraniums--showing that it is merely
warmth and not light that is required to enable a sub-tropical flora to
thrive in Greenland. Even in Canada, which has a flora richer in some
respects than that of temperate Europe, growth is effectually arrested
by cold for nearly six months, and though there is ample sunlight there
is no vegetation. It is, indeed, not impossible that in the plans of the
Creator the continuous summer sun of the arctic regions may have been
made the means for the introduction, or at least for the rapid growth and
multiplication, of new and more varied types of plants.
[FD] "Florula Discoana," Botanical Society of Edinburgh, 1868.
Much, of course, remains to be known of the history of the old floras,
whose fortunes I have endeavoured to sketch, and which seem to have been
driven like shuttle-cocks from north to south, and from south to north,
especially on the American continent, whose meridional extension seems to
have given a field specially suited for such operations.
This great stretch of the western continent, from north to south, is also
connected with the interesting fact that, when new floras are entering
from the arctic regions, they appear earlier in America than in Europe,
and that in times when old floras are retreating from the south old
genera and species linger longer in America. Thus, in the Devonian and
Cretaceous new forms of those periods appear in America long before they
are recognized in Europe, and in the modern epoch forms that would be
regarded in Europe as Miocene still exist. Much confusion in reasoning
as to the geological ages of the fossil floras has arisen from want of
attention to this circumstance.
What we have learned respecting this wonderful history has served
strangely to change some of our preconceived ideas. We must now be
prepared to admit that an Eden can be planted even in Spitzbergen, that
there are possibilities in this old earth of ours which its present
condition does not reveal to us; that the present state of the world is
by no means the best possible in relation to climate and vegetation; that
there have been and might be again conditions which could convert the
ice-clad arctic regions into blooming paradises, and which at the same
time would moderate the fervent heat of the tropics. We are accustomed to
say that nothing is impossible with God; but how little have we known of
the gigantic possibilities which lie hidden under some of the most common
of his natural laws!
These facts have naturally been made the occasion of speculations as
to the spontaneous development of plants by processes of varietal
derivation. It would, from this point of view, be a nice question to
calculate how many revolutions of climate would suffice to evolve the
first land-plant; what are the chances that such plant would be so
dealt with by physical changes as to be preserved and nursed into a
meagre flora like that of the Upper Silurian or the Jurassic; how many
transportations to Greenland would suffice to promote such meagre flora
into the rich and abundant forests of the Upper Cretaceous, and to
people the earth with the exuberant vegetation of the early Tertiary.
Such problems we may never be able to solve. Probably they admit of no
solution, unless we invoke the action of an Almighty mind, operating
through long ages, and correlating with boundless power and wisdom
all the energies inherent in inorganic and organic nature. Even then
we shall perhaps be able to comprehend only the means by which, after
specific types have been created, they may, by the culture of their
Maker, be "sported" into new varieties or subspecies, and thus fitted
to exist under different conditions or to occupy higher places in the
economy of nature.
Before venturing on such extreme speculations as some now current on
questions of this kind, we would require to know the successive extinct
floras as perfectly as those of the modern world, and to be able to
ascertain to what extent each species can change either spontaneously
or under the influence of struggle for existence or expansion under
favourable conditions, and under arctic semi-annual days and nights, or
the shorter days of the tropics. Such knowledge, if ever acquired, it may
take ages of investigation to accumulate.
As to the origin and mode of introduction of successive floras, I am, for
the reasons above stated, not disposed to dogmatise, or to adopt as final
any existing theory of the development of the vegetable kingdom. Still,
some laws regulating the progress of vegetable life may be recognised,
and I propose to state these in connection with the Palæozoic floras, to
which my own studies have chiefly related.
Fossil plants are almost proverbially uncertain with reference to their
accurate determination, and have been regarded as of comparatively little
utility in the decision of general questions of palæontology. This
results principally from the fragmentary condition in which they have
been studied, and from the fact that fragments of animal structures are
more definite and instructive than corresponding portions of plants.
It is to be observed, however, that our knowledge of fossil plants
becomes accurate in proportion to the extent to which we can carry the
study of specimens in the beds in which they are preserved, so as to
examine more perfect examples than those usually to be found in museums.
When structures are taken into the account, as well as external forms, we
can also depend more confidently on our results. Further, the abundance
of specimens to be obtained in particular beds often goes far to make up
for their individual imperfection. The writer of these pages has been
enabled to avail himself very fully of these advantages; and on this
account, if on no other, feels entitled to speak with some authority on
theoretical questions.
It is an additional encouragement to pursue the subject, that, when
we can obtain definite information as to the successive floras of any
region, we thereby learn much as to climate and vicissitudes in regard to
the extent of land and water; and that, with reference to such points,
the evidence of fossil plants, when properly studied, is, from the close
relation of plants to those stations and climates, even more valuable
than that of animal fossils.
It is necessary, however, that in pursuing such inquiries we should have
some definite views as to the nature and permanence of specific forms,
whether with reference to a single geological period or to successive
periods; and I may be excused for stating here some general principles,
which I think important for our guidance.
1. Botanists proceed on the assumption, vindicated by experience, that,
within the period of human observation, species have not materially
varied or passed into each other. We may make, for practical purposes,
the same assumption with regard to any given geological period, and may
hold that for each such period there are specific types which, for the
time at least, are invariable.
2. When we inquire what constitutes a good species for any given period,
we have reason to believe that many names in our lists represent merely
varietal forms or erroneous determinations. This is the case even in the
modern flora; and in fossil floras, through the poverty of specimens,
their fragmentary condition, and various states of preservation, it
is still more likely to occur. Every revision of any group of fossils
detects numerous synonyms, and of these many are incapable of detection
without the comparison of large suites of specimens.
3. We may select from the flora of any geological period certain
forms, which I shall call _specific types_, which may for such period
be regarded as unchanging. Having settled such types, we may compare
them with similar forms in other periods, and such comparisons will
not be vitiated by the uncertainty which arises from the comparison of
so-called species which may, in many cases, be mere varietal forms, as
distinguished from specific types. Our types may be founded on mere
fragments, provided that these are of such a nature as to prove that they
belong to distinct forms which cannot pass into each other, at least
within the limits of one geological period.
4. When we compare the specific types of one period with those of another
immediately precedent or subsequent, we shall find that some continue
unchanged through long intervals of geological time, that others are
represented by allied forms regarded either as varietal or specific, and
as derived or otherwise, according to the view which we may entertain as
to the permanence of species. On the other hand, we also find new types
not rationally deducible on any theory of derivation from those known
in other periods. Further, in comparing the types of a poor period with
those of one rich in species, we may account for the appearance of new
types in the latter by the deficiency of information as to the former;
where many new types appear in the poorer period this conclusion seems
less probable. For example, new types appearing in poor formations, like
the Lower Erian and Lower Carboniferous, have greater significance than
if they appeared in the Middle Erian or in the Coal Measures.
5. When specific types disappear without any known successors, under
circumstances in which it seems unlikely that we should have failed to
discover their continuance, we may fairly assume that they have become
extinct, at least locally; and where the field of observation is very
extensive, as in the great coal-fields of Europe and America, we may
esteem such extinction as practically general, at least for the northern
hemisphere. When many specific types become extinct together, or in close
succession, we may suppose that such extinction resulted from physical
changes; but where single types disappear, under circumstances in which
others of similar habit continue, we may not unreasonably conjecture
that, as Pictet has argued in the case of animals, such types may have
been in their own nature limited in duration, and may have died out
without any external cause.
6. With regard to the _introduction_ of specific types we have not as
yet a sufficient amount of information. Even if we freely admit that
ordinary specific forms, as well as mere varieties, may result from
derivation, this by no means excludes the idea of primitive specific
types originating in some other way. Just as the chemist, after analysing
all compounds and ascertaining all allotropic forms, arrives at length at
certain elements not mutually transmutable or derivable, so the botanist
and zoölogist must expect sooner or later to arrive at elementary
specific types, which, if to be accounted for at all, must be explained
on some principle distinct from that of derivation. The position of
many modern biologists, in presence of this question, may be logically
the same with that of the ancient alchemists with reference to the
chemical elements, though the fallacy in the case of fossils may be of
more difficult detection. Our business at present, in the prosecution of
palæobotany, is to discover, if possible, what are elementary or original
types, and, having found these, to enquire as to the law of their
creation.
7. In prosecuting such questions geographical relations must be carefully
considered. When the floras of two successive periods have existed in
the same region, and under circumstances that render it probable that
plants have continued to grow on the same or adjoining areas throughout
these periods, the comparison becomes direct, and this is the case with
the Erian and Carboniferous floras in northeastern America. But, when
the areas of the two formations are widely separated in space as well
as in time, any resemblances of facies that we may observe may have no
connection whatever with an unbroken continuity of specific types.
I desire, however, under this head, to affirm my conviction that,
with reference to the Erian and Carboniferous floras of North America
and of Europe, the doctrine of "homotaxis," as distinct from actual
contemporaneity, has no place. The succession of formations in the
Palæozoic period evidences a similar series of physical phenomena on the
grandest scale throughout the northern hemisphere. The succession of
marine animals implies the continuity of the sea-bottoms on which they
lived. The headquarters of the Erian flora in America and Europe must
have been in connected or adjoining areas in the North Atlantic. The
similarity of the Carboniferous flora on the two sides of the Atlantic,
and the great number of identical species, proves a still closer
connection in that period. These coincidences are too extensive and too
frequently repeated to be the result of any accident of similar sequence
at different times, and this more especially as they extend to the more
minute differences in the features of each period, as, for instance, the
floras of the Lower and Upper Devonian, and of the Lower, Middle, and
Upper Carboniferous.
8. Another geographical question is that which relates to centres of
dispersion. In times of slow subsidence of extensive areas, the plants
inhabiting such areas must be narrowed in their range and often separated
from one another in detached spots, while, at the same time, important
climatal changes must also occur. On the re-emergence of the land such
of these species as remained would again extend themselves over their
former areas of distribution, in so far as the new climatal and other
conditions would permit. We would naturally suppose that the first of
the above processes would tend to the elimination of varieties, the
second, to their increase; but, on the other I hand, the breaking up of a
continental flora into that of distinct islets, and the crowding together
of many forms, might be a process fertile in the production of some
varieties if fatal to others.
Further, it is possible that these changes of subsidence may have some
connection with the introduction, as well as with the extinction, even of
specific types. It is certain, at least, in the case of land-plants, that
such types come in most plentifully immediately after elevation, though
they are most abundantly preserved in periods of slow subsidence. I do
not mean, however, that this connection is one of cause and effect; there
are, indeed, indications that it is not so. One of these is, that in some
cases the enlargement of the area of the land seems to be as injurious to
terrestrial species as its diminution.
9. Another point on which I have already insisted, and which has been
found to apply to the Tertiary as well as to the Palæozoic floras, is the
appearance of new types within the arctic and boreal areas, and their
migration southward. Periods in which the existence of northern land
coincided with a general warm temperature of the northern hemisphere seem
to have been those most favourable to the introduction of new forms of
land-plants. Hence, there has been throughout geological time a general
movement of new floras from the Palæarctic and Nearctic regions to the
southward.
Applying the above considerations to the Erian and Carboniferous floras
of North America, we obtain some data which may guide us in arriving at
general conclusions. The Erian flora is comparatively poor, and its
types are in the main similar to those of the Carboniferous. Of these
types a few only reappear in the middle coal-formation under identical
forms; a great number appear under allied forms; some altogether
disappear. The Erian flora of New Brunswick and Maine occurs side by
side with the Carboniferous of the same region; so does the Erian of New
York and Pennsylvania with the Carboniferous of those States. Thus we
have data for the comparison of successive floras in the same region. In
the Canadian region we have, indeed, in direct sequence, the floras of
the Upper Silurian, the Lower, Middle, and Upper Erian, and the Lower,
Middle, and Upper Carboniferous, all more or less distinct from each
other, and affording an admirable series for comparison in a region whose
geographical features are very broadly marked. All these floras are
composed in great part of similar types, and probably do not indicate
very dissimilar general physical conditions, but they are separated from
each other by the great subsidences of the Corniferous limestone and the
Lower Carboniferous limestone, and by the local but intense subterranean
action which has altered and disturbed the Erian beds toward the close
of that period. Still, these changes were not universal. The Corniferous
limestone is absent in Gaspé, and probably in New Brunswick, where,
consequently, the Erian flora could continue undisturbed during that long
period. The Carboniferous limestone is absent from the slopes of the
Appalachians in Pennsylvania, where a retreat may have been afforded to
the Upper Erian and Lower Carboniferous floras. The disturbances at the
close of the Erian were limited to those eastern regions where the great
limestone-producing subsidences were unfelt, and, on the other hand, are
absent in Ohio, where the subsidences and marine conditions were almost
at a maximum.
Bearing in mind these peculiarities of the area in question, we may now
group in a tabular form the distinct specific types recognised in the
Erian system, indicating, at the same time, those which are represented
by identical species in the Carboniferous, those represented by similar
species of the same general type, and those not represented at all.
For example, _Calamites cannæformis_ extends as a species into the
Carboniferous; _Asterophyllites latifolia_ does not so extend, but is
represented by closely allied species of the same type; _Nematophyton_
disappears altogether before we reach the Carboniferous.
_Table of Erian and Carboniferous Specific Types._
Erian types. Represented in By identical By related
Carboniferous-- types. forms.
1. Syringoxylon mirabile ?
2. Nematoxylon
3. Nematophyton
4. Aporoxylon
5. Ormoxylon
6. Dadoxylon *
7. Sigillaria Vanuxemii *
8. S. palpebra *
9. Didymophyllum
10. Calamodendron *
11. Calamites transitionis *
12. C. cannæformis *
13. Asterophyllites scutigera *
14. A. latifolia
15. Annularia laxa
16. Sphenophyllum antiquum *
17. Cyclostigma
18. Arthrostigma
19. Lepidodendron Gaspianum *
20. L. corrugatum *
21. Lycopodites Matthewi *
22. L. Richardsoni
23. Ptilophyton Vanuxemii
24. Lepidophloios antiquus *
25. Psilophyton princeps
26. P. robustius
27. Cordaites Robbii *
28. C. angustifolia
29. Archæopteris Jacksoni
30. Aneimites obtusa *
31. Platyphyllum Brownii
32. Cyclopteris varia *
33. C. obtusa
34. Neuropteris polymorpha *
35. N. serrulata *
36. N. retorquata *
37. N. resecta
38. Megalopteris Dawsoni.
39. Sphenopteris Hoeninghausi *
40. S. Harttii *
41. Hymenophyllites curtilobus
42. H. obtusilobus *
43. Alethopteris discrepans *
44. Pecopteris serrulata *
45. P. preciosa
46. Trichomanites *
47. Callipteris *
48. Cardiocarpum *
49. C. Crampii
50. Antholithes *
51. Trigonocarpum *
Of the above forms, fifty-one in all, found in the Erian of eastern
America, all, except the last four, are certainly distinct specific
types. Of these only four reappear in the Carboniferous under identical
species, but no less than twenty-six reappear under representative or
allied forms, some at least of which a derivationist might claim as
modified descendants. On the other hand, nearly one half of the Devonian
types are unknown in the Carboniferous, while there remain a very large
number of Carboniferous types not accounted for by anything known in
the Devonian. Further, a very poor flora, including only two or three
types, is the predecessor of the Erian flora in the Upper Silurian,
and the flora again becomes poor in the Upper Devonian and Lower
Carboniferous. Every new species discovered must more or less modify the
above statements, and the whole Erian flora of America, as well as the
Carboniferous, requires a thorough comparison with that of Europe before
general conclusions can be safely drawn. In the mean time I may indicate
the direction in which the facts seem to point by the following general
statements:
1. Some of the forms reckoned as specific in the Devonian and
Carboniferous may be really derivative races. There are indications that
such races may have originated in one or more of the following ways: (1)
By a natural tendency in synthetic types to become specialised in the
direction of one or other of their constituent elements. In this way
such plants as _Arthrostigma_ and _Psilophyton_ may have assumed new
varietal forms. (2) By embryonic retardation or acceleration,[FE] whereby
certain species may have had their maturity advanced or postponed, thus
giving them various grades of perfection in reproduction and complexity
of structure. The fact that so many Erian and Carboniferous plants seem
to be on the confines of the groups of Acrogens and Gymnosperms may be
supposed favourable to such exchanges. (3) The contraction and breaking
up of floras, as occurred in the Middle Erian and Lower Carboniferous,
may have been eminently favourable to the production of such varietal
forms as would result from what has been called the "struggle for
existence." (4) The elevation of a great expanse of new land at the
close of the Middle Erian and the beginning of the coal period would, by
permitting the extension of species over wide areas and fertile soils,
and by removing the pressure previously existing, be eminently favourable
to the production of new, and especially of improved, varieties.
[FE] In the manner illustrated by Hyatt and Cope.
2. Whatever importance we may attach to the above supposed causes of
change, we still require to account for the origin of our specific
types. This may forever elude our observation, but we may at least hope
to ascertain the external conditions favourable to their production. In
order to attain even to this it will be necessary to inquire critically,
with reference to every acknowledged species, what its claims to
distinctness are, so that we may be enabled to distinguish specific types
from mere varieties. Having attained to some certainty in this, we may be
prepared to inquire whether the conditions favourable to the appearance
of new varieties were also those favourable to the creation of new types,
or the reverse--whether these conditions were those of compression
or expansion, or to what extent the appearance of new types may be
independent of any external conditions, other than those absolutely
necessary for their existence. I am not without hope that the further
study of fossil plants may enable us thus to approach to a comprehension
of the laws of the creation, as distinguished from those of the continued
existence of species.
3. In the present state of our knowledge we have no good ground either
to limit the number of specific types beyond what a fair study of
our material may warrant, or to infer that such primitive types must
necessarily have been of low grade, or that progress in varietal
forms has always been upward. The occurrence of such an advanced and
specialised type as that of _Dadoxylon_ in the Middle Devonian should
guard us against these errors. The creative process may have been
applicable to the highest as well as to the lowest forms, and subsequent
deviations must have included degradation as well as elevation. I can
conceive nothing more unreasonable than the statement sometimes made that
it is illogical or even absurd to suppose that highly organised beings
could have been produced except by derivation from previously existing
organisms. This is begging the whole question at issue, depriving science
of a noble department of inquiry on which it has as yet barely entered,
and anticipating by unwarranted assertions conclusions which may perhaps
suddenly dawn upon us through the inspiration of some great intellect,
or may for generations to come baffle the united exertions of all the
earnest promoters of natural science. Our present attitude should not be
that of dogmatists, but that of patient workers content to labour for a
harvest of grand generalisations which may not come till we have passed
away, but which, if we are earnest and true to Nature and its Creator,
may reward even some of us.
Within the human period great changes of distribution of plants have
occurred, chiefly through the agency of man himself, and we have
had ample evidence that plants are able to establish themselves and
prosper in climates and conditions to which unaided they could not have
transported themselves, as, for instance, in the case of European weeds
naturalised in Australia and New Zealand. There is, however, no reason
to believe that any specific change has occurred to any plant within the
Pleistocene or modern period.
In a recent address, delivered to the biological section of the British
Association, Mr. Carruthers has discussed this question, and has shown
that the earliest vegetable specimens described by Dr. Schweinfurth from
the Egyptian tombs present no appearance of change. This fact appears
also in the leaves and other organs of plants preserved in the nodules
in the Pleistocene clays of the Ottawa, and in specimens of similar age
found in various places in Britain and the continent of Europe.[FF]
[FF] "Proceedings British Association," 1886, "Pleistocene Plants of
Canada," Canadian Naturalist, 1866.
The difficulties attending the ordinary theories of evolution as applied
to plants have been well set forth by the same able botanist in his
"Presidential Address to the Geological Association in 1877," a paper
which deserves careful study. One of his illustrations is that ancient
willow, _Salix polaris_, referred to in a previous chapter, which now
lives in the arctic regions, and is found fossil in the Pleistocene beds
at Cromer and at Bovey Tracey.
He notes the fact that the genus _Salix_ is a very variable one,
including 19 subgeneric groups and 160 species, with no less than 222
varieties and 70 hybrids. _Salix polaris_ belongs to a subgeneric group
containing 29 species, which are arranged in four sections, that to which
_S. polaris_ belongs containing six species. Now it is easy to construct
a theoretical phylogeny of the derivation of the willows from a supposed
ancestral source, but when we take our little _S. polaris_ we find
that this one twig of our ancestral tree takes us back without change
to the Glacial period. The six species would take us still farther,
and the sections, sub-genera, and genus at the same rate would require
an incalculable amount of past time. He concludes the inquiry in the
following terms:
"But when we have reached the branch representing the generic form we
have made but little progress in the phylogenesis of _Salix_. With
_Populus_ this genus forms a small order, Salicineæ, The two genera are
closely allied, yet separated by well-marked characters; it is not,
however, difficult to conceive of both having sprung from a generalised
form. But there is no record of such a form. The two genera appear
together among the earliest known dicotyledons, the willows being
represented by six and the poplars by nine species. The ordinal form, if
it ever existed, must necessarily be much older than the period of the
Upper Cretaceous rocks, that is, than the period to which the earliest
known dicotyledons belong.
"The Salicineæ are related to five other natural orders, in all of which
the apetalous flowers are arranged in catkins. These different though
allied orders must be led up by small modifications to a generalised
amentiferous type, and thereafter the various groups of apetalous
plants by innumerable eliminations of differentiating characters until
the primitive form of the apetalous plant is reached. Beyond this the
uncurbed imagination will have more active work in bridging over the gap
between Angiosperms and Gymnosperms, in finding the intermediate forms
that led up to the vascular cryptogams, and on through the cellular
plants to the primordial germ. Every step in this phylogenetic tree
must be imagined. The earliest dicotyledon takes us not a step farther
back in the phylogenetic history of _Salix_ than that supplied by
existing vegetation. All beyond the testimony of our living willows is
pure imagination, unsupported by a single fact. So that here, also, the
evidence is against evolution, and there is none in favour of it."
It is easy to see that similar difficulties beset every attempt to trace
the development of plants on the principle of slow and gradual evolution,
and we are driven back on the theory of periods of rapid origin, as we
have already seen suggested by Saporta in the case of the Cretaceous
dicotyledons. Such abrupt and plentiful introduction of species over
large areas at the same time, by whatever cause effected--and we are
at present quite ignorant of any secondary causes--becomes in effect
something not unlike the old and familiar idea of creation. Science
must indeed always be baffled by questions of ultimate origin, and,
however far it may be able to trace the chain of secondary causation
and development, must at length find itself in the presence of the
great Creative Mind, who is "before all things and in whom all things
consist."
APPENDIX.
I.--COMPARATIVE VIEW OF THE SUCCESSIVE PALÆOZOIC
FLORAS OF NORTHEASTERN AMERICA AND
GREAT BRITAIN.
In eastern Canada there is a very complete series of fossil plants,
extending from the Silurian to the Permian, and intermediate in its
species between the floras of interior America and of Europe. I may
use this succession, mainly worked out by myself,[FG] to summarise the
various Palæozoic floras and sub-floras, in order to give a condensed
view of this portion of the history of the vegetable kingdom, and to
direct attention to the important fact, too often overlooked, that there
is a definite succession of fossil plants as well as of animals, and
that this is important as a means of determining geological horizons.
A British list for comparison has been kindly prepared for me by Mr.
R. Kidston, F. Gr. S. For lists referring to the western and southern
portions of America, I may refer to the reports of Lesquereux and
Fontaine and White.[FH]
[FG] "Acadian Geology," "Reports on Fossil Plants of Canada," Geological
Survey of Canada.
[FH] "Geological Surveys of Pennsylvania, Ohio, and Illinois."
In this connection I am reminded, by an excellent little paper of M.
Zeiller,[FI] on Carboniferous plants from the region of the Zambesi,
in Africa, that the flora which in the Carboniferous period extended
over the temperate portions of the northern hemisphere and far into the
arctic, also passed across the equator and prevailed in the southern
hemisphere. Of eleven species brought from the Zambesi by M. Lapierre
and examined by M. Zeiller, all were identical with European species
of the upper coal-formation, and the same fact has been observed in the
coal flora of the Cape Colony.[FJ] These facts bear testimony to the
remarkable uniformity of climate and vegetation in the coal period, and
I perfectly agree with Zeiller that they show, when taken in connection
with other parallelisms in fossils, an actual contemporaneousness of the
coal flora over the whole world.
[FI] Paris, 1883.
[FJ] Grey, "Journal of the Geological Society," vol. xxvii.
1. Carboniferous Flora.
(1) _Permo-Carboniferous Sub-Flora_:
This occurs in the upper member of the Carboniferous system of Nova
Scotia and Prince Edward Island, originally named by the writer the
Newer Coal-formation, and more recently the Permo-Carboniferous, and
the upper beds of which may not improbably be contemporaneous with the
Lower Permian or Lower Dyas of Europe. In this formation there is a
predominance of red sandstones and shales, and it contains no productive
beds of coal. Its fossil plants are for the most part of species found
in the Middle or Productive Coal-formation, but are less numerous, and
there are a few new forms akin to those of the European Permian. The most
characteristic species of the upper portion of the formation, which has
the most decidedly Permian aspect, are the following:
_Dadoxylon materiarium_, Dawson.
* _Walchia_ (_Araucarites_) _robusta_, Dn.
* _W._ (_A._) _gracilis_, Dn.
* _W. imbricatula_, Dn.
_Calamites Suckovii_, Brongt.
_C. Cistii_, Brongt.
* _C. gigas_, Brongt.
_Neuropteris rarinervis_, Bunbury.
_Alethopteris nervosa_, Brongt.
_Pecopteris arborescens_, Brongt.
* _P. rigida_, Dn.
_P. oreopteroides_, Brongt.
* _Cordaites simplex_, Dn.
Of these species, those marked with an asterisk have not yet been found
in the middle or lower members of the Carboniferous system. They will
be found described, and several of them figured, in my "Report on the
Geology of Prince Edward Island."[FK] The others are common and widely
diffused Carboniferous species, some of which have extended to the
Permian period in Europe as well. From the upper beds, characterised by
these and a few other species, there is a gradual passage downward into
the productive coal-measures, and a gradually increasing number of true
coal-formation species.
[FK] 1871.
It is worthy of remark here that the association in the
Permo-Carboniferous of numerous trunks of _Dadoxylon_ with the branches
of _Walchia_ and with fruits of the character of _Trigonocarpa_, seems to
show that these were parts of one and the same plant.
This formation represents the Upper Barren Measures of West Virginia,
which are well described by Fontaine and White,[FL] and the reasons
which these authors adduce for considering the latter equivalent to the
European Permian will apply to the more northern and eastern deposits
as well, though these have afforded fewer species of plants, and are
apparently less fully developed.
[FL] "Report on the Permian Flora of Western Virginia and South
Pennsylvania," 1880.
(2) _Coal-formation Sub-Flora_:
The Middle or Productive Coal-formation, containing all the beds of coal
which are mined in Nova Scotia and Cape Breton, is the headquarters of
the Carboniferous flora. From this formation I have catalogued[FM] one
hundred and thirty-five species of plants; but, as several of these are
founded on imperfect specimens, the number of actual species may be
estimated at one hundred and twenty. Of these more than one half are
species common to Europe and America. No less than nineteen species
are _Sigillariæ_, and about the same number are _Lepidodendra_. About
fifty are ferns and thirteen are _Calamites_, _Asterophyllites_, and
_Sphenophylla_. The great abundance and number of species of Sigillariæ,
Lepidodendra, and ferns are characteristic of this sub-flora; and among
the ferns certain species of _Neuropteris_, _Pecopteris_, _Alethopteris_,
and _Sphenopteris_ greatly preponderate.
[FM] "Acadian Geology," and "Report on Flora of Lower Carboniferous,"
1873.
These beds are the equivalents of the Middle Coal-measures, or Productive
Coal-measures of Pennsylvania, Ohio, &c., and of the coal-formation
proper of various European countries. Very many of the species are common
to Nova Scotia and Pennsylvania; but in proceeding westward the number of
identical species seems to diminish.
(3) _The Millstone Grit Sub-Flora_:
In this formation the abundance of plants and the number of species
are greatly diminished.[FN] Trunks of coniferous trees of the species
_Dadoxylon Acadianum_, having wide wood-cells with three or more series
of discs and complex medullary rays, become characteristic. _Calamites
undulatum_ is abundant and seems to replace _C. Suckovii_, though _C.
cannæformis_ and _C. cistii_ continue. _Sigillariæ_ become very rare,
and the species of Lepidodendron are few, and mostly those with large
leaf-bases. _Lepidophloios_ still continues, and _Cordaites_ abounds in
some beds. The ferns are greatly reduced, though a few characteristic
coal-formation species occur, and the genus _Cardiopteris_ appears. Beds
of coal are rare in this formation; but where they occur there is in
connection with them a remarkable anticipation of the rich coal-formation
flora, which would thus seem to have existed locally in the Millstone
Grit period, but to have found itself limited by generally unfavorable
conditions. In America, as in Europe, it is in the north that this
earlier development of the coal-flora occurs, while in the south there
is a lingering of old forms in the newer beds. In Newfoundland and Cape
Breton, for instance, as well as in Scotland, productive coal-beds and a
greater variety of species of plants occur in this formation.
[FN] "Report on Fossil Plants of the Lower Carboniferous and Millstone
Grit of Canada," 1873.
The following would appear to be the equivalents of this formation, in
flora and geological position:
1. The Seral Conglomerate of Rogers in Pennsylvania, &c.
2. The Lower Coal-formation Conglomerate and Chester groups of Illinois
(Worthen).
3. The Lower Carboniferous Sandstone of Kentucky, Alabama, and Virginia.
4. The Millstone Grit and Yoredale rocks of northern England, and the
Culmiferous of Devonshire.
5. The Moor rock and Lower Coal-measures of Scotland.
6. Flagstones and Lower Shales of the south of Ireland, and Millstone
Grit of the north of Ireland.
7. The Jüngste Grauwacke of the Hartz, Saxony, and Silesia.
(4) _The Carboniferous Limestone Series_:
This affords few fossil plants in eastern America, and in so far as known
they are similar to those of the next group. In Scotland it is richer
in plants, but, according to Mr. Kidston, these are largely similar
to those of the underlying beds, though with some species which extend
upward into the Millstone Grit. In Scotland the alga named _Spirophyton_
and _Archæocalamites radiatus_--which in America are Erian--appear in
this formation.
(5) _The Lower Carboniferous Sub-Flora_:
This group of plants is best seen in the shales of the Horton series,
under the Lower Carboniferous marine limestones. It is small and
peculiar. The most characteristic species are the following:
_Dadoxylon_ (_Palæoxylon_) _antiquius_, Dn.--A species with large
medullary rays of three or more series of cells.
_Lepidodendron corrugatum_, Dn.--A species closely allied to _L.
Veltheimianum_ of Europe, and which is its American representative.
This is perhaps the most characteristic plant of the formation. It is
very abundant, and presents very protean appearances, in its old stems,
branches, twigs, and _Knorria_ forms. It had well-characterised stigmaria
roots, and constitutes the oldest erect forest known in Nova Scotia.
_Lepidodendron tetragonum_, Sternberg.
_L. obovatum_, Sternb.
_L. aculeatum_, Sternb.
_L. dichotomum_, Sternb.
The four species last mentioned are comparatively rare, and the specimens
are usually too imperfect to render their identification certain, but
Lepidodendra are especially characteristic trees of this horizon.
_Cyclopteris_ (_Aneimites_) _Acadica_, Dn.--A very characteristic
fern, allied in the form of its fronds to _C. tenuifolia_ of Goeppert,
to _C. nana_ of Eichwald, and to _Adiantites antiquus_ of Stur. Its
fructification, however, is nearer to that of _Aneimia_ than to that of
_Adiantum_.
Ferns of the genera Cardiopteris and _Hymenophyllites_ also occur, though
rarely.
_Ptilophyton plumula_, Dn.--This is the latest appearance of this Erian
genus, which also occurs in the Lower Carboniferous of Europe and of the
United States.
_Cordaites borassifolia_, Brongt.
On the whole, this small flora is markedly distinct from that of the
Millstone Grit and true coal-formation, from which it is separated by the
great length of time required for the deposition of the marine limestones
and their associated beds, in which no land-plants have been found; nor
is this gap filled up by the conglomerates and coarse arenaceous beds
which, as I have explained in "Acadian Geology," in some localities take
the place of the limestones, as they do also in the Appalachian region
farther south.
The palæobotanical and stratigraphical equivalents of this series abroad
would seem to be the following:
1. The Vespertine group of Rogers in Pennsylvania.
2. The Kinderhook group of Worthen in Illinois.
3. The Marshall group of Winchell in Michigan.
4. The Waverley sandstone (in part) of Ohio.
5. The Lower or False Coal-measures of Virginia.
6. The Calciferous sandstones of McLaren, or Tweedian group of Tate in
Scotland.
7. The Lower Carboniferous slate and Coomhala grits of Jukes in Ireland.
8. The Culm and Culm Grauwacke of Germany.
9. The Graywacke or Lower Coal-measures of the Vosges, as described by
Schimper.
10. The Older Coal-formation of the Ural, as described by Eichwald.
11. The so-called "Ursa Stage" of Heer includes this, but he has united
it with Devonian beds, so that the name cannot be used except for
the local development of these beds at Bear Island, Spitsbergen. The
Carboniferous plants of arctic America, Melville Island, &c., as well as
those of Spitzbergen, appear all to be Lower Carboniferous.[FO]
[FO] "Notes on Geological Map of the Northern Portion of the Dominion of
Canada," by Dr. G. M. Dawson, 1887.
All of the above groups of rocks are characterised by the prevalence of
_Lepidodendra_ of the type of _L. corrugatum_, _L. Veltheimianum_, and
_L. Glincanum_; pines of the sub-genus _Pitus_ of Witham, _Palæoxylon_
of Brongniart, and peculiar ferns of the genera _Cyclopteris_,
_Cardiopteris_, _Triphyllopteris_, and _Sphenopteris_. In all the regions
above referred to they form the natural base of the great Carboniferous
system.
In Virginia, according to Fontaine and White, types, such as
Archæopteris, which in the north are Upper Erian, occur in this group.
Unless there have been some errors in fixing the lower limit of the
Vespertine, this would indicate a longer continuance of old forms in the
south.
2. Erian Flora.
(1) _Upper Erian Sub-Flora_:
This corresponds to the Catskill and Chemung of the New York series, and
to the Upper Devonian of Europe.
The flora of this formation, which consists mostly of sandstones, is not
rich. Its most distinctive species on both sides of the Atlantic seem to
be the ferns of the genus _Archæopteris_, along with species referred to
the genus _Cyclopteris_, but which, in so far as their barren fronds are
concerned, for the most part resemble _Archæopteris_.
The characteristic American species are _Archæopteris Jacksoni_,
_A. Rogersi_, and _A. Gaspiensis_. _Cyclopteris obtusa_ and _C._
(_Platyphyllum_) _Brownii_ are also very characteristic species. In
Europe, _Archæopteris Hibernica_ is a prevalent species.
_Leptophleum rhombicum_ and fragments of _Psilophyton_ are also found in
the Upper Erian. There is evidence of the existence of vast numbers of
_Rhizocarps_ in this period, in the deposits of spore-cases (_Sporangites
Huronensis_) in the shales of Kettle Point, Lake Huron; and in deposits
of similar character in Ohio and elsewhere in the West.
The Upper Erian flora is thus very distinct from that of the Lower
Carboniferous, and the unconformable relation of the beds in the
Northeast may perhaps indicate a considerable lapse of time. Still,
even in localities where there appears to be a transition from the
Carboniferous into the Devonian, as in the Western States and in Ireland,
the characteristic flora of each formation may be distinguished, though,
as already stated, there is apparently some mixture in the South.
(2) _Middle Erian Sub-Flora_:
Both in Canada and the United States that part of the great Erian system
which may be regarded as its middle division, the Hamilton and Marcellus
shales of New York, the Cordaites shales of St. John, New Brunswick, and
the middle shales and sandstones of the Gaspé series, presents conditions
more favourable to the abundant growth of land-plants than either the
upper or lower member. In the St. John beds, in particular, there is a
rich fern flora, comparable with that of the coal-formation, and numerous
stipes of ferns and trunks of tree-ferns have been found in the Hamilton
and Corniferous series in the West, as well as trunks of _Dadoxylon_. It
is, however, distinguished by a prevalence of small and delicate species,
and by such forms as _Hymenophyllites_ and the smaller Sphenopterids, and
also by some peculiar ferns, as _Archæopteris_ and _Megalopteris_. In
addition to ferns, it has small _Lepidodendra_, of which _L. Gaspianum_
is the chief. _Calamiteæ_ occur, _Archæocalamites radiatus_ being the
dominant species. This plant, which in Europe appears to reach up into
the Lower Carboniferous, is so far strictly Erian in northeast America.
_Sigillariæ_ scarcely appear, but _Cordaites_ is abundant, and the
earliest known species of _Dadoxylon_ appear, while the Psilophyton, so
characteristic of the Lower Erian, still continues, and the remarkable
aquatic plants of the genus _Ptilophyton_ are locally abundant.
(3) _Lower Erian Sub-Flora_:
This belongs to the Lower Devonian sandstones and shales, and is
best seen in that formation at Gaspé and the Bay des Chaleurs. It is
equivalent to the Oriskany sandstone, so far as its animal fossils and
mineral character are concerned. It is characterised by the absence
of true ferns, _Calamites_ and _Sigillariæ_, and by the presence
of such forms as _Psilophyton_, _Arthrostigma_, _Leptophleum_, and
_Nematophyton_. _Lepidodendron Gaspianum_ and _Leptophleum_ already
occur, though not nearly so abundant as Psilophyton.
The Lower Erian plants have an antique and generalised aspect which would
lead us to infer that they are near the beginning of the land-flora, or
perhaps in part belong to the close of an earlier flora still in great
part unknown and few indications of land-plants have been found earlier.
At Campbellton and Scaumenac Bay, on the Bay des Chaleurs, fossil fishes
of genera characteristic of the Lower and Upper Devonian horizons
respectively, occur in association with fossil plants of these horizons,
and have been described by Mr. Whiteaves.[FP]
[FP] "Transactions of the Royal Society of Canada."
It is interesting to note that, as Fontaine and White have observed,
certain forms which are Erian in the northeast are found in the Lower
members of the Carboniferous in West Virginia, indicating the southward
march of species in these periods.
3. The Silurian Flora and still Earlier Indications of Plants.
In the upper beds of the Silurian, those of the Helderberg series, we
still find _Psilophyton_ and _Nematophyton_; but below these we know no
land-plants in Canada. In the United States, Lesquereux and Claypole have
described remains which may indicate the existence of lycopodiaceous and
annularian types as far back as the beginning of the Upper Silurian, or
even as low as the Hudson River group, and Hicks has found _Nematophyton_
and _Psilophyton_ in beds about as old in Wales, along with the uncertain
stems named _Berwynia_. In the Lower Silurian the _Protannularia_ of
the Skiddaw series in England may represent a land-plant, but this is
uncertain, and no similar species has been found in Canada.
The Cambrian rocks are so far barren of land-plants; the so-called
_Eophyton_ being evidently nothing but markings, probably produced by
crustaceans and other aquatic animals. In the still older Laurentian the
abundant beds of graphite probably indicate the existence of plants, but
whether aquatic or terrestrial it is impossible to decide at present.
It would thus appear that our certain knowledge of land-vegetation begins
with the Upper Silurian or the Silurio-Cambrian, and that its earliest
forms were Acrogens allied to Lycopods, and prototypal trees, forerunners
of the Acrogens or the gymnosperms. In the Lower Devonian little advance
is made. In the Middle Devonian this meagre flora had been replaced by
one rivalling that of the Carboniferous, and including pines, tree-ferns,
and arboreal forms of Lycopods and of equisetaceous plants, as well as
numerous herbaceous plants. At the close of the Erian the flora again
became meagre, and continued so in the Lower Carboniferous. It again
became rich and varied in the Middle Carboniferous, to decay in the
succeeding Permian.
II.--HEER'S LATEST RESULTS IN THE GREENLAND FLORA.
A very valuable report of Prof. Steenstrup, published in Copenhagen in
1883, the year in which Heer died, contains the results of his last
work on the Greenland plants, and is so important that a summary of its
contents will be interesting to all students of fossil botany or of the
vicissitudes of climate which the earth has undergone.[FQ]
[FQ] Meddelelser om Gronland, Hefte V., Copenhagen, 1883.
The plant-bearing beds of Greenland are as follows, in ascending order:
1. Cretaceous.
1. The _Komé_ series, of black shales resting on the Laurentian gneiss.
These beds are found at various other localities, but the name above
given is that by which they are generally known. Their flora is limited
to ferns, cycads, conifers, and a few endogens, with only _Populus
primæva_ to represent the dicotyledons. These beds are regarded as Lower
Cretaceous (Urgonian), but the animal fossils would seem to give them
a rather higher position. They may be regarded as equivalent to the
Kootanie and Queen Charlotte beds in Canada, and the Potomac series in
Virginia.
2. The _Atané_ series. These also are black shales with dark-coloured
sandstones. They are best exposed at Upernavik and Waigat. Here
dicotyledonous leaves abound, amounting to ninety species, or more than
half the whole number of species found. The fossil plants resemble those
of the Dakota series of the United States and the Dunvegan series of
Canada, and the animal fossils indicate the horizon of the Fort Pierre or
its lower part. They may be regarded as representing the lower part of
the Upper Cretaceous. The genera _Populus_, _Myrica_, _Quercus_, _Ficus_,
_Platanus_, _Sassafras_, _Laurus_, _Magnolia_, and _Liriodendron_
are among those represented in these beds, and the peculiar genera
_Macclintockia_ and _Credneria_ are characteristic. The genus _Pinus_ is
represented by five species, _Sequoia_ by five, and _Salisburia_ by two,
with three of the allied genus _Baiera_. There are many ferns and cycads.
3. The _Patoot_ series. These are yellow and red shales, which seem to
owe their colour to the spontaneous combustion of pyritous lignite, in
the manner observed on the South Saskatchewan and the Mackenzie rivers.
Their age is probably about that of the Fox-Hill group or Senonian,
and the Upper Cretaceous of Vancouver Island, and they afford a large
proportion of dicotyledonous leaves. The genera of dicotyledons are
not dissimilar from those of Atané, but we now recognise _Betula_ and
_Alnus_, _Comptonia_, _Planera_, _Sapotacites_, _Fraxinus_, _Viburnum_,
_Cornus_, _Acer_, _Celastrus_, _Paliurus_, _Ceanothus_, _Zizyphus_, and
_Cratægus_ as new genera of modern aspect.
On the whole there have been found in all these beds 335 species,
belonging to 60 families, of which 36 are dicotyledonous, and represent
all the leading types of arborescent dicotyledons of the temperate
latitudes. The flora is a warm temperate one, with some remarkable
mixtures of sub-tropical forms, among which perhaps the most remarkable
are _Kaidocarpum_ referred to the _Pandaneæ_, and such exogens as _Ficus_
and _Cinnamomum_.
2. Tertiary.
4. The _Unartok_ series. This is believed to be Eocene. It consists of
sandstone, which appears on the shores of Disco Island, and possibly at
some other places on the coast. The beds rest directly and apparently
conformably on the Upper Cretaceous, and have afforded only eleven
species of plants. _Magnolia_ is represented by two species, _Laurus_
by two, _Platanus_ by two, and one of these said to be identical with a
species found by Lesquereux in the Laramie,[FR] _Viburnum_, _Juglans_,
_Quercus_, each by one species; the ubiquitous _Sequoias_ by _S.
Langsdorfii_. This is pretty clearly a Lower Laramie flora.
[FR] _Viburnum marginatum_ of Lesquereux.
5. The _Atanekerdluk_ series, consisting of shaly beds, with limestone
intercalated between great sheets of basalt, much like the Eocene of
Antrim and the Hebrides. These beds have yielded 187 species, principally
in bands and concretions of siderite, and often in a good state of
preservation. They are referred to the Lower Miocene, but, as explained
in the text, the flora is more nearly akin to that of the Eocene of
Europe and the Laramie of America. The animal fossils are chiefly
fresh-water shells. _Onoclea sensibilis_, several conifers, as _Taxites_
_Olriki_, _Taxodium distichum_, _Glyptostrobus Europæus_, and _Sequoia
Langsdorfii_, and 42 of the dicotyledons are recognised as found also
in American localities. Of these, a large proportion of the more common
species occur in the Laramie of the Mackenzie River and elsewhere in
northwest Canada, and in the western United States. It is quite likely
also that several species regarded as distinct may prove to be identical.
It would seem that throughout the whole thickness of these Tertiary beds
the flora is similar, so that it is probable it belongs altogether to the
Eocene rather than to the Miocene.
No indication has been observed of any period of cold intervening between
the Lower Cretaceous and the top of the Tertiary deposits, so that, in
all the vast period which these formations represent, the climate of
Greenland would seem to have been temperate. There is, however, as is
the case farther south, evidence of a gradual diminution of temperature.
In the Lower Cretaceous the probable mean annual temperature in latitude
71° north is stated as 21° to 22° centigrade, while in the early Tertiary
it is estimated at 12° centigrade. Such temperatures, ranging from 71°
to 53° of Fahrenheit, represent a marvellously warm climate for so high
a latitude. In point of fact, however, the evidence of warm climates in
the arctic regions, in the Palæozoic as well as in the Mesozoic and early
Tertiary, should perhaps lead us to conclude that, relatively to the
whole of geological time, the present arctic climate is unusually severe,
and that a temperate climate in the arctic regions has throughout
geological time been the rule rather than the exception.
III.--MINERALISATION OF FOSSIL PLANTS.
The state of preservation of fossil plants has been referred to
incidentally in several places in the text; but the following more
definite statements may be of service to the reader.
I. Organic remains imbedded in aqueous deposits may occur in an unchanged
condition, or only more or less altered by decay. This is often the case
with such enduring substances as bark and wood, and even with leaves,
which appear as thin carbonaceous films when the layers containing
them are split open. In the more recent deposits such remains occur
little modified, or perhaps only slightly changed by partial decay of
their more perishable parts. In the older formations, however, they
are usually found in a more or less altered condition, in which their
original substance has been wholly or in part changed into coaly, or
bituminous, or anthracitic or graphitic matter, so that leaves are
sometimes represented by stains of graphite, as if drawn on stone with a
lead-pencil. Yet even in this case some portion of the original substance
remains, and without any introduction of foreign material.
II. On the other hand, such remains are often mineralised by the filling
of their pores or the replacement of their tissues with mineral matter,
so that they become hard and stony, and sometimes retain little or
nothing of their original substance. The more important of these changes,
in so far as they affect fossil plants, may be arranged under the
following heads:
(_a_) _Infiltration_ of mineral matter which has penetrated the pores
of the fossil in a state of solution. Thus the pores of fossil wood are
often filled with calcite, quartz, oxide of iron, or sulphide of iron,
while the woody walls of the cells and vessels remain in a carbonised
state, or converted into coaly matter. When wood is preserved in this
way it has a hard and stony aspect; but we can sometimes dissolve away
the mineral matter, and restore the vegetable tissue to a condition
resembling that before mineralisation. This is especially the case when
calcite is the mineralising substance. We sometimes find, on microscopic
examination, that even cavities so small as those of vegetable cells and
vessels have been filled with successive coats of different kinds of
mineral matter.
(_b_) Organic matters may be entirely _replaced_ by mineral
substances. In this case the cavities and pores have been first
filled, and then--the walls or solid parts being removed by decay or
solution--mineral matter, either similar to that filling the cavities,
or differing in colour or composition, has been introduced. Silicified
wood often occurs in this condition. In the case of silicified wood, it
sometimes happens that the cavities of the fibers have been filled with
silica, and the wood has been afterward removed by decay, leaving the
casts of the tubular fibers as a loose filamentous substance. Some of
the Tertiary coniferous woods of California are in this state, and look
like asbestus, though they show the minute markings of the tissue under
the microscope. In the case of silicified or agatized woods, it would
seem that the production of carbon dioxide from the decaying wood has
caused the deposition of silica in its place, from alkaline solutions
of that substance, and thus the carbon has been replaced, atom by atom,
by silicon, until the whole mass has been silicified, yet retaining
perfectly its structure.
(_c_) The cavities left by fossils which have decayed may be filled with
clay, sand, or other foreign matter, and this, becoming subsequently
hardened into stone, may constitute a _cast_ of the fossils. Trunks of
trees, roots, &c., are often preserved in this way, appearing as stony
casts, often with the outer bark of the plant forming a carbonaceous
coating on their surfaces. In connection with this state may be
mentioned that in which, the wood having decayed, an entire trunk has
been flattened so as to appear merely as a compressed film of bark, yet
retaining its markings; and that in which the whole of the vegetable
matter having been removed, a mere impression of the form remains.
Fossils preserved in either of the modes, (_a_) or (_b_), usually show
more or less of their minute structures under the microscope. These may
be observed:--(1) By breaking off small splinters or flakes and examining
them, either as opaque or as transparent objects. (2) By treating the
material with acids, so as to dissolve out the mineral matters, or
portions of them. This method is especially applicable to fossil woods
mineralised with calcite or pyrite. (3) By grinding thin sections. These
are first polished on one face on a coarse stone or emery hone, and then
on a fine hone, then attached by the polished face to glass slips with
a transparent cement or Canada balsam, and ground on the opposite face
until they become so thin as to be translucent. In most cities there are
lapidaries who prepare slices of this kind; but the amateur can readily
acquire the art by a little practice, and the necessary appliances can
be obtained through dealers in minerals or in microscopic materials.
Very convenient cutting and polishing machines, some of them quite small
and portable, are now made for the use of amateurs. In the case of
exogenous woods, three sections are necessary to exhibit the whole of the
structures. One of these should be transverse and two longitudinal, the
latter in radial and tangential planes.
IV.--GENERAL WORKS ON PALÆOBOTANY.
In the text frequent reference has been made to special memoirs and
reports on the fossil plants of particular regions or formations. There
are, however, some general books, useful to students, which may be
mentioned here. Perhaps the most important is Schimper's "Traité de
Paléontologie Végétale." Very useful information is also contained in
Renault's "Cours de Botanique Fossile," and in Balfour's "Introduction to
Palæontological Botany," and Nicholson's "Palæontology." Unger's "Genera
et Species," Brongniart's "Histoire des Végétaux Fossiles," and Lindley
and Button's "Fossil Flora," are older though very valuable works.
Williamson's "Memoirs," in the "Philosophical Transactions," have greatly
advanced our knowledge of the structures of Palæozoic plants. Lastly,
the "Palæophytology" of Schenk, now in course of publication in German
and French, in connection with Zittel's "Palæontology," is an important
addition to manuals of the subject.
INDEX.
Acer, 228.
Acrogens, 6.
Agassiz, Prof., 16.
Alaska, Flora of, 245.
Algæ, real and spurious, 26, 230.
Amboy clays, Flora of, 203.
America, Cretaceous of, 190.
Angiosperms, 6.
Annularia, 122.
Anogens, 6.
Antholithes, 132.
Aporoxylon, 25.
Araucarioxylon, 148.
Araucarites, 134.
Archæocalamites, 170.
Archæopteris, 77, 85.
Arctic origin of plants, 221, 238.
Arthrophycus, 30.
Arthrostigma, 67.
Asterophyllites, 78, 122, 170.
Asteropteris, 77, 85.
Astropolithon, 30.
Atané, Plants of, 242, 281.
Atanekerdluk, Plants of, 283.
Australia, Palæozoic flora of, 147.
Tertiary flora of, 217.
Bauhinia, 204.
Bear Island, 241.
Betula, 198.
Bilobites, 28.
Bovey Tracey, Plants of, 226.
Brasenia, 207.
Buckland, Dr., 179.
Buthotrephis, 37.
Calamites, 77, 123, 166.
Calamodendron, 125.
Cambrian flora, 20.
Canada, Erian of, 103.
Carboniferous of, 110.
Laramie of, 209.
Pleistocene of, 227.
Carbon in Laurentian, 9.
Carboniferous flora, 110.
Carboniferous, Climate of, 138.
of Southern Hemisphere, 147.
Cardiocarpum, 82, 153.
Carruthers, Mr., 24, 98, 180.
On modifications of modern plants, 225, 269.
Carya, 196.
Cauda-galli fucoid, 105.
Caulerpites, 29.
Caulopteris, 75, 94.
Clarke, Prof., 51.
Climate, Causes of, 247.
Climate and plants, 216, 220, 232.
of Carboniferous, 138.
of Cretaceous and Eocene, 216.
of Devonian, 47.
of Early Mesozoic, 178.
Climate and plants of Laurentian, 17.
of Pleistocene, 227, 230.
of Pliocene, 223.
Coal, origin of, 117, 139.
Comparison of floras, 272.
Composite, 266.
Cone-in-cone, 36.
Coniferæ, Erian, 78, 96.
Carboniferous, 134, 148.
Mesozoic, etc., 181.
Cope, Mr., 215.
Cordaites, 78, 130, 151.
Corylus, 213.
Crepin, M., 99.
Cretaceous, Flora of, 190.
Climate of, 216.
Croll on climate, 252.
Cromer, Plants of, 224.
Cycads, Mesozoic, 178.
Cyclostigma, 157.
Dadoxylon, 96, 134, 148.
Dawson, Dr. G. M., 52, 210.
Delgado, Prof., 26.
Dendrophycus, 33.
Derby, Orville, 53.
Devonian flora, 45.
Devonian or Erian, 107, 279.
Climate of, 47.
Dicotyledons, Cretaceous, 192.
Table of, 192.
Dictyolites, 33.
Dictyospongia, 39.
Disco, Exotic plants at, 256.
Flora of, 245, 282.
Drepanophycus, 39.
Drosera, 228.
Dunvegan beds, 244.
Eocene, Flora of, 208, 214.
Climate of, 216.
Eophyton, 31.
Eopteris, 72.
Eozoon of Laurentian, 9.
Equisetum, 176, 230.
Erian flora, 45, 279.
Climate of, 47.
Erian or Devonian, 107.
Ettingshausen, Dr., 187, 215.
Exogens, Cretaceous, 192.
Tertiary, 213, 224.
Fagus, 196, 197.
Ferns, Erian, 72.
Carboniferous, 126, 171.
Fructification of, 128.
Stems of, 90, 129.
Tertiary, 212.
Filices, 72, 126, 171.
Flora of Cambrian, 26.
of Carboniferous, 110, 274.
of Cretaceous, 190.
of Early Mesozoic, 175.
of Erian, 45, 279.
of Jurassic, 177, 186.
of Laramie, 209.
of Laurentian, 8.
of Miocene, 220, 223.
of Modern, 219.
of Permian, 274.
of Pleistocene, 223, 227.
of Tertiary, 191, 208, 214, 219.
Fontaine, Prof., 130, 176.
Fontinalis, 230.
Fort Union beds, 210.
Fucoids, 27.
Gardner, Mr. Starkie, 212.
Geinitz, Dr., 174.
Geological formations, Table of, 4.
Glossopteris, 147.
Glyptodendron, 25.
Glyptostrobus, 194.
Goeppert, Dr., 99.
Grant, Col., 36.
Graphite from plants, 8.
Gray, Dr., Origin of floras, 223, 237.
Greenland, Climate of, 216.
Fossil flora of, 247.
Gulielmites, 35.
Gymnosperms, 6.
Haliserites, 39.
Hartt, Prof., 53.
Heer, Dr., 108, 181.
Helderberg period, Sea of, 250.
Heterangium, 77.
Hicks, Dr., 21.
Hunt, Dr. Sterry, 13, 143.
Huxley, Prof., 53.
Hymenæa, 204.
Insects, Erian, 83.
Juglans, 196.
Jurassic flora, 177.
Kainozoic flora, 191, 208, 214, 219.
Kidston, Mr. R., 128, 273.
King, Mr. Clarence, 211.
Komé, Plants of, 242, 281.
Laramie flora, 209, 215.
Laurentian plants, 8.
Laurentian, Climate of, 17.
Laurophyllum, 193.
Laws of introduction of plants, 237, 266.
Leda clay, Flora of, 232.
Lepidodendron, 120, 156, 162.
Lepidophloios, 121, 157, 165.
Leptophleum, 157.
Lesquereux, Mr. L., 169, 214.
Licrophycus, 30.
Lignitic series of America, 208.
Liquidambar, 197.
Liriodendron, 199.
Lower Carboniferous flora, 277.
Logan, Sir W., 48.
Lyell on climate, 249.
Magnolia, 200.
McConnell, Mr., 209.
McNab, Prof., 169.
Megalopteris, 76.
Megaphyton, 129.
Mesozoic flora, 175.
Climate of, 178.
Migrations of plants, 240, 245.
Miller, Hugh, 98.
Miocene flora, 220.
Miocene, Supposed, 242.
Modern flora, 219.
Modern plants, how modified, 269.
Modifications of plants, 266.
Nathorst, Dr., 26, 196.
Nematodendreæ, 25.
Nematophycus, 23.
Nematophyton, 21, 22, 42.
Newberry, Dr., 200, 203, 214.
Newfoundland, Fossil plants of, 242.
Newton, Mr., 52.
Nicholson, Dr. A., 20.
Niobrara series, 243, 246.
Noeggerathia, 130.
Northern origin of plants, 238.
Origin of plants, 237.
Orton, Prof., 51.
Pachytheca, 21.
Palæanthus, 205.
Palæochorda, 30.
Palæophycus, 30, 38.
Palæozoic floras compared, 273.
Palms, 188, 194.
Pandanus, 188.
Patoot beds, 282.
Peach, Mr., 98.
Petroleum, Origin of, 56.
Phymatoderma, 29.
Plants, Classification of, 6.
Platanus, 198.
Platyphyllum, 74.
Pleistocene climate, 227, 230.
Pleistocene flora, 223, 227.
Pliocene climate, 223.
Podozamites, 178.
Poles, Supposed change of, 248.
Populus, 191, 228.
Potamogeton, 229.
Potentilla, 228.
Protannularia, 21.
Protichnites, 27.
Protophyllum, 199.
Protosalvinia, 52.
Protostigma, 20.
Prototaxites, 21.
Psaronius, 93.
Psilophyton, 64.
Ptilophyton, 62, 86.
Quercus, 197.
Rhizocarps, 48.
Rill-marks, 33.
Rusichnites, 28.
Saccamina, 57.
Salisburia, 180.
Salter, Mr., 98.
Salvinia, 54.
Saporta, Count de, 26, 193.
Saportea, 57.
Sassafras, 199.
Scalariform tissue, 70.
Schimper, Dr., 116, 169, 208.
Scolithus, 30.
Scottish Devonian, 98.
Sequoia, 181.
Shrinkage cracks, 33.
Sigillaria, 71, 112, 154.
Southern Hemisphere, 217, 273.
Carboniferous in, 147.
Tertiary in, 217.
Sphenophyllum, 61, 122, 171.
Spirophyton, 38.
Spitzbergen, 241.
Sterculites, 193.
Sternbergia, 137, 152.
Stigmaria, 115.
Stur, Dr., on Sigillaria, 116.
Symphorocarpus, 214.
Syringodendron, 156.
Syringoxylon, 82.
Table of formations, 4.
Tasmania, Fossil plants of, 217, 246.
Tasmanite, 57.
Tertiary period, Flora of, 191, 208, 214, 219.
Tertiary of Australia, 217.
Thallogens, 6.
Thomas, Mr., 51.
Thuja, 213, 229.
Time, Geological, 5.
Trapa, 196.
Tree-ferns, 90, 129.
Triassic flora, 176.
Trigonocarpum, 136, 153.
Tyndall, Prof., 138.
Ulrich, Prof., 57.
Unartok beds, 281.
Ursa stage of Heer, 108, 241.
Walchia, 134, 138.
Ward, Mr. L. T., 192, 212, 215.
Wethered, Mr. E., 52.
White, Dr., 215.
Williams, Prof., 51.
Williamson, Dr., 26, 31, 71, 167.
Williamsonia, 188.
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Forel, McCook, and others, is that he has carefully watched and
marked particular insects, and has had their nests under observation
for long periods--one of his ants' nests having been under constant
inspection ever since 1874 His observations are made principally
upon ants, because they show more power and flexibility of mind; and
the value of his studies is that they belong to the department of
original research.""
"We have no hesitation in saying that the author has presented us
with the most valuable series of observations on a special subject
that has ever been produced, charmingly written, full of logical
deductions, and, when we consider his multitudinous engagements,
a remarkable illustration of economy of time. As a contribution
to insect psychology, it will be long before this book finds a
parallel."--_London Athenæum._
-----------------------
New York: D. APPLETON & CO., 1, 3, & 5 Bond Street.
_D. APPLETON & CO.'S PUBLICATIONS._
------------------------------------------------------------------
Dr. H. ALLEYNE NICHOLSON'S WORKS.
=TEXT-BOOK OF ZOÖLOGY,= for Schools and Colleges. 12mo. Half roan,
$1.60.
=MANUAL OF ZOÖLOGY,= for the Use of Students, with a General
Introduction to the Principles of Zoölogy. Second edition.
Revised and enlarged, with 243 Woodcuts. 12mo. Cloth, $2 50.
=TEXT-BOOK OF GEOLOGY,= for Schools and Colleges. 12mo. Half roan,
$1.25.
=INTRODUCTION TO THE STUDY OF BIOLOGY,= Illustrated. 12mo. Cloth, 60
cents.
=THE ANCIENT LIFE-HISTORY OF THE EARTH.= A Comprehensive Outline of
the Principles and Leading Facts of Palæontological Science.
12mo. Cloth, $2.00.
"A work by a master in the science who understands the significance
of every phenomenon which he records, and knows how to make it reveal
its lessons. As regards its value there can scarcely exist two
opinions. As a text-book of the historical phase of palæontology is
will be indispensable to students, whether specially pursuing geology
or biology; and without it no man who aspires even to an outline
knowledge of natural science can deem his library complete."--_The
Quarterly Journal of Science_.
"The Professor of Natural History in the University of St. Andrews
has, by his previous works on zoology and paleontology, so fully
established his claim to be an exact thinker and a close reasoner,
that scarcely any recommendation of ours can add to the interest
with which all students in natural history will receive the present
volume. It is, as its second title expresses it; a comprehensive
outline of the principles and leading facts of palæontoiogical
science. Numerous woodcut illustrations very delicately executed, a
copious glossary, and an admirable index, add much to the value of
this volume."--_Athenæum_.
-----------------------
New York: D. APPLETON & CO., 1, 3, & 5 Bond Street.
* * * * *
Transcriber Notes
All images were moved so as to not split paragraphs.
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