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.]