mentioned in the last chapter (p. 356) that the base of the Lower Lias
is characterised, both in England and Germany, by beds containing
distinct species of Ammonites, the lowest subdivision having been
called the zone of _Ammonites planorbis._ Below this zone, on the
boundary line between the Lias and the strata of which we are about to
treat, called “Trias,” certain cream-coloured limestones devoid of
fossils are usually found. These white beds were called by William
Smith the White Lias, and they have been shown by Mr. Charles Moore to
belong to a formation similar to one in the Rhætian Alps of Bavaria, to
which Mr. Gumbel has applied the name of Rhætic. They have also long
been known as the Koessen beds in Germany, and may be regarded as beds
of passage between the Lias and Trias. They are named the Penarth beds
by the Government surveyors of Great Britain, from Penarth, near
Cardiff, in Glamorganshire, where they sometimes attain a thickness of
fifty feet.
The principal member of this group has been called by Dr. Wright the
_Avicula contorta_ bed,[1] as this shell is very abundant, and has a
wide range in Europe. General Portlock first described the formation as
it occurs at Portrush, in Antrim, where the _ Avicula contorta_ is
accompanied by _Pecten Valoniensis,_ as in Germany.
The best known member of the group, a thin band or bone-breccia, is
conspicuous among the black shales in the neighbourhood of Axmouth in
Devonshire, and in the cliffs of Westbury-on-Severn, as well as at Aust
and other places on the borders of the Bristol Channel. It abounds in
the remains of saurians and fish, and was formerly classed as the
lowest bed of the Lias; but Sir P. Egerton first pointed out, in 1841,
that it should be referred to the Upper New Red Sandstone, because it
contained an assemblage of fossil fish which are either peculiar to
this stratum, or belong to species well-known in the Muschelkalk of
Germany. These fish belong to the genera _Acrodus, Hybodus, Gyrolepis,_
and _Saurichthys._
Fig. 383: Cardium rhæticum. Fig. 384: Pecten Valoniensis. Fig. 385:
Avicula contorta. Fig. 386: Hybodus plica ilis. Fig. 387: Saurichthys
apicalis. Fig. 388: Gyrolepsis tenuistriatus.
Among those common to the English bone-bed and the Muschelkalk of
Germany are _Hybodus plicatilis_ (Fig. 386), _Saurychthys apicalis_
(Fig. 387), _Gyrolepis tenuistriatus_ (Fig. 388), and _G. Albertii._
Remains of saurians, _Plesiosaurus_ among others, have also been found
in the bone-bed, and plates of an _Encrinus._ It may be questioned
whether some of those fossils which have the most Triassic character
may not have been derived from the destruction of older strata, since
in bone-beds, in general, many of the organic remains are undoubtedly
derivative.
Fig. 389: Microlestes antiquus, molar tooth.
_Triassic Mammifer._—In North-western Germany, as in England, there
occurs beneath the Lias a remarkable bone breccia. It is filled with
shells and with the remains of fishes and reptiles, almost all the
genera of which, and some even of the species, agree with those of the
subjacent Trias. This breccia has accordingly been considered by
Professor Quenstedt, and other German geologists of high authority, as
the newest or uppermost part of the Trias. Professor Plieninger found
in it, in 1847, the molar tooth of a small Triassic mammifer, called by
him _ Microlestes antiquus._ He inferred its true nature from its
double fangs, and from the form and number of the protuberances or
cusps on the flat crown; and considering it as predaceous, probably
insectivorous, he called it _Microlestes_ from micros, little, and
lestes, a beast of prey. Soon afterwards he found a second tooth, also
at the same locality, Diegerloch, about two miles to the south-east of
Stuttgart.
No anatomist had been able to give any feasible conjecture as to the
affinities of this minute quadruped until Dr. Falconer, in 1857,
recognised an unmistakable resemblance between its teeth and the two
back molars of his new genus _Plagiaulax_ (Fig. 306), from the Purbeck
strata. This would lead us to the conclusion that Microlestes was
marsupial and plant-eating.
In Würtemberg there are two bone-beds, namely, that containing the
Microlestes, which has just been described, which constitutes, as we
have seen, the uppermost member of the Trias, and another of still
greater extent, and still more rich in the remains of fish and
reptiles, which is of older date, intervening between the Keuper and
Muschelkalk.
The genera _Saurichthys, Hybodus,_ and _Gyrolepis_ are found in both
these breccias, and one of the species, _ Saurichthys Mongeoti,_ is
common to both bone-beds, as is also a remarkable reptile called
_Nothosaurus mirabilis._ The saurian called _Belodon_ by H. von Meyer,
of the Thecodont family, is another Triassic form, associated at
Diegerloch with Microlestes.
TRIAS OF ENGLAND.
Between the Lias and the Coal (or Carboniferous group) there is
interposed, in the midland and western counties of England, a great
series of red loams, shales, and sandstones, to which the name of the
“New Red Sandstone formation” was first given, to distinguish it from
other shales and sandstones called the “Old Red,” often identical in
mineral character, which lie immediately beneath the coal. The name of
“Red Marl” has been incorrectly applied to the red clays of this
formation, as before explained (p. 38), for they are remarkably free
from calcareous matter. The absence, indeed, of carbonate of lime, as
well as the scarcity of organic remains, together with the bright red
colour of most of the rocks of this group, causes a strong contrast
between it and the Jurassic formations before described.
The group in question is more fully developed in Germany than in
England or France. It has been called the Trias by German writers, or
the Triple Group, because it is separable into three distinct
formations, called the “Keuper,” the “Muschelkalk,” and the
“Bunter-sandstein.” Of these the middle division, or the Muschelkalk,
is wholly wanting in England, and the uppermost (Keuper) and lowest
(Bunter) members of the series are not rich in fossils.
Upper Trias or Keuper.—In certain grey indurated marls below the
bone-bed Mr. Boyd Dawkins has found at Watchet, on the coast of
Somersetshire, a molar tooth of Microlestes, enabling him to refer to
the Trias strata formerly supposed to be Liassic. Mr. Charles Moore had
previously discovered many teeth of mammalia of the same family near
Frome, in Somersetshire, in the contents of a vertical fissure
traversing a mass of carboniferous limestone. The top of this fissure
must have communicated with the bed of the Triassic sea, and probably
at a point not far from the ancient shore on which the small marsupials
of that era abounded.
This upper division of the Trias called the Keuper is of great
thickness in the central counties of England, attaining, according to
Mr. Hull’s estimate, no less than 3450 feet in Cheshire, and it covers
a large extent of country between Lancashire and Devonshire.
In Worcestershire and Warwickshire in sandstone belonging to the
uppermost part of the Keuper the bivalve crustacean _Estheria minuta_
occurs. The member of the English “New Red” containing this shell, in
those parts of England, is, according to Sir Roderick Murchison and Mr.
Strickland, 600 feet thick, and consists chiefly of red marl or slate,
with a band of sandstone. Ichthyodorulites, or spines of _ Hybodus,_
teeth of fishes, and footprints of reptiles were observed by the same
geologists in these strata.
Fig. 390: Estheria minuta.
Fig. 391: Hyperodapedon Gordoni. Left Plate, Maxillary.
In the Upper Trias or Keuper the remains of two saurians of the order
Lacertilia have been found. The one called _ Rhynchosaurus_ occurred at
Grinsell near Shrewsbury, and is characterised by having a small
bird-like skull and jaws without teeth. The other _Hyperodapedon_ (Fig.
391) was first noticed in 1858, near Elgin, in strata now recognised as
Upper Triassic, and afterwards in beds of about the same age in the
neighbourhood of Warwick. Remains of the same genus have been found
both in Central India and Southern Africa in rocks believed to be of
Triassic age. The Hyperodapedon has been shown by Professor Huxley to
be a terrestrial reptile having numerous palatal teeth, and closely
allied to the living Sphenodon of New Zealand.
The recent discoveries of a living saurian in New Zealand so closely
allied to this supposed extinct division of the Lacertilia seems to
afford an illustration of a principle pointed out by Mr. Darwin of the
survival in insulated tracts, after many changes in physical geography,
of orders of which the congeners have become extinct on continents
where they have been exposed to the severer competition of a larger
progressive fauna.
Fig. 392: Tooth of Labyrinthodon.
Teeth of Labyrinthodon (Fig. 392) found in the Keuper in Warwickshire
were examined microscopically by Professor Owen, and compared with
other teeth from the German Keuper. He found after careful
investigation that neither of them could be referred to true saurians,
although they had been named _Mastodonsaurus_ and _Phytosaurus_ by
Jäger. It appeared that they were of the _Batrachian_ order, and of
gigantic dimensions in comparison with any representatives of that
order now living. Both the Continental and English fossil teeth
exhibited a most complicated texture, differing from that previously
observed in any reptile, whether recent or extinct, but most nearly
analogous to the _Ichthyosaurus._ A section of one of these teeth
exhibits a series of irregular folds, resembling the labyrinthic
windings of the surface of the brain; and from this character Professor
Owen has proposed the name Labyrinthodon for the new genus. Fig. 393 of
part of one is given from his “Odontography,” plate 64, A. The entire
length of this tooth is supposed to have been about three inches and a
half, and the breadth at the base one inch and a half.
Fig. 393: Transverse section of upper part of tooth of Labyrinthodon
Jaegeri.
_Rock-salt._—In Cheshire and Lancashire there are red clays containing
gypsum and salt of the age of the Trias which are between 1000 and 1500
feet thick. In some places lenticular masses of pure rock-salt nearly
100 feet thick are interpolated between the argillaceous beds. At the
base of the formation beneath the rock-salt occur the Lower Sandstones
and Marl, called provincially in Cheshire “water-stones,” which are
largely quarried for building. They are often ripple-marked, and are
impressed with numerous footprints of reptiles.
The basement beds of the Keuper rest with a slight unconformability
upon an eroded surface of the “Bunter” next to be described.
Fig. 394: Single footstep of Cheirotherium.
Lower Trias or Bunter.—The lower division or English representative of
the “Bunter” attains a thickness of 1500 feet in the counties last
mentioned, according to Professor Ramsay. Besides red and green shales
and red sandstones, it comprises much soft white quartzose sandstone,
in which the trunks of silicified trees have been met with at Allesley
Hill, near Coventry. Several of them were a foot and a half in
diameter, and some yards in length, decidedly of coniferous wood, and
showing rings of annual growth.[2] Impressions, also, of the footsteps
of animals have been detected in Lancashire and Cheshire in this
formation. Some of the most remarkable occur a few miles from
Liverpool, in the whitish quartzose sandstone of Storton Hill, on the
west side of the Mersey. They bear a close resemblance to tracks first
observed in this member of the Upper New Red Sandstone, at the village
of Hesseberg, near Hildburghausen, in Saxony. For many years these
footprints have been referred to a large unknown quadruped,
provisionally named _Cheirotherium_ by Professor Kaup, because the
marks both of the fore and hind feet resembled impressions made by a
human hand. (See Fig. 394.) The foot-marks at Hesseberg are partly
concave, and partly in relief, the former, or the depressions, are seen
upon the upper surface of the sandstone slabs, but those in relief are
only upon the lower surfaces, being, in fact, natural casts, formed in
the subjacent footprints as in moulds. The larger impressions, which
seem to be those of the hind foot, are generally eight inches in
length, and five in width, and one was twelve inches long. Near each
large footstep, and at a regular distance (about an inch and a half)
before it, a smaller print of a fore foot, four inches long and three
inches wide, occurs. The footsteps follow each other in pairs, each
pair in the same line, at intervals of fourteen inches from pair to
pair. The large as well as the small steps show the great toes
alternately on the right and left side; each step makes the print of
five toes, the first, or great toe, being bent inward like a thumb.
Though the fore and hind foot differ so much in size, they are nearly
similar in form.
Fig. 395: Line of footsteps on slab of sandstone.
As neither in Germany nor in England had any bones or teeth been met
with in the same identical strata as the footsteps, anatomists
indulged, for several years, in various conjectures respecting the
mysterious animals from which they might have been derived. Professor
Kaup suggested that the unknown quadruped might have been allied to the
_Marsupialia_; for in the kangaroo the first toe of the fore foot is in
a similar manner set obliquely to the others, like a thumb, and the
disproportion between the fore and hind feet is also very great. But M.
Link conceived that some of the four species of animals of which the
tracks had been found in Saxony might have been gigantic _Batrachians,_
and when it was afterwards inferred that the Labyrinthodon was an
air-breathing reptile, it was conjectured by Professor Owen that it
might be one and the same as the Cheirotherium.
Dolomitic Conglomerate of Bristol.—Near Bristol, in Somersetshire, and
in other counties bordering the Severn, the lowest strata belonging to
the Triassic series consist of a conglomerate or breccia resting
unconformably upon the Old Red Sandstone, and on different members of
the Carboniferous rocks, such as the Coal Measures, Millstone Grit, and
Mountain Limestone. This mode of superposition will be understood by
reference to the section below Dundry Hill (Fig. 85), where No. 4 is
the dolomitic conglomerate. Such breccias may have been partly the
result of the subÆrial waste of an old land-surface which gradually
sank down and suffered littoral denudation in proportion as it became
submerged. The pebbles and fragments of older rocks which constitute
the conglomerate are cemented together by a red or yellow base of
dolomite, and in some places the encrinites and other fossils derived
from the Mountain Limestone are so detached from the parent rocks that
they have the deceptive appearance of belonging to a fauna
contemporaneous with the dolomitic beds in which they occur. The
imbedded fragments are both rounded and angular, some consisting of
sandstone from the coal-measures, being of vast size, and weighing
nearly a ton. Fractured bones and teeth of saurians which are truly of
contemporaneous origin are dispersed through some parts of the breccia,
and two of these reptiles called Thecodont saurians, named from the
manner in which the teeth were implanted in the jawbone, obtained great
celebrity because the patches of red conglomerate in which they were
found, near Bristol, were originally supposed to be of Permian or
Palæozoic age, and therefore the only representatives in England of
vertebrate animals of so high a grade in rocks of such antiquity. The
teeth of these saurians are conical, compressed, and with finely
serrated edges (see Fig. 396); they are referred by Professor Huxley to
the Dinosaurian order.
Fig. 396: Tooth of Thecodontosaurus.
Origin of Red Sandstone and Rock-salt.—In various parts of the world,
red and mottled clays and sandstones, of several distinct geological
epochs, are found associated with salt, gypsum, and magnesian
limestone, or with one or all of these substances. There is, therefore,
in all likelihood, a general cause for such a coincidence.
Nevertheless, we must not forget that there are dense masses of red and
variegated sandstones and clays, thousands of feet in thickness, and of
vast horizontal extent, wholly devoid of saliferous or gypseous matter.
There are also deposits of gypsum and of common salt, as in the
blue-clay formation of Sicily, without any accompanying red sandstone
or red clay.
These red deposits may be accounted for by the decomposition of gneiss
and mica schist, which in the eastern Grampians of Scotland has
produced a mass of detritus of precisely the same colour as the Old Red
Sandstone.
It is a general fact, and one not yet accounted for, that scarcely any
fossil remains are ever preserved in stratified rocks in which this
oxide of iron abounds; and when we find fossils in the New or Old Red
Sandstone in England, it is in the grey, and usually calcareous beds,
that they occur. The saline or gypseous interstratified beds may have
been produced by submarine gaseous emanations, or hot mineral springs,
which often continue to flow in the same spots for ages. Beds of
rock-salt are, however, more generally attributed to the evaporation of
lakes or lagoons communicating at intervals with the ocean. In Cheshire
two beds of salt occur of the extraordinary thickness of 90 or even 100
feet, and extending over an area supposed to be 150 miles in diameter.
The adjacent beds present ripple-marked sandstones and footprints of
animals at so many levels as to imply that the whole area underwent a
slow and gradual depression during the formation of the red sandstone.
Major Harris, in his “Highlands of Ethiopia,” describes a salt lake,
called the Bahr Assal, near the Abyssinian frontier, which once formed
the prolongation of the Gulf of Tadjara, but was afterwards cut off
from the gulf by a broad bar of lava or of land upraised by an
earthquake. “Fed by no rivers, and exposed in a burning climate to the
unmitigated rays of the sun, it has shrunk into an elliptical basin,
seven miles in its transverse axis, half filled with smooth water of
the deepest cærulean hue, and half with a solid sheet of glittering
snow-white salt, the offspring of evaporation.” “If,” says Mr. Hugh
Miller, “we suppose, instead of a barrier of lava, that sand-bars were
raised by the surf on a flat arenaceous coast during a slow and equable
sinking of the surface, the waters of the outer gulf might occasionally
topple over the bar, and supply fresh brine when the first stock had
been exhausted by evaporation.”
The Runn of Cutch, as I have shown elsewhere,[3] is a low region near
the delta of the Indus, equal in extent to about a quarter of Ireland,
which is neither land nor sea, being dry during part of every year, and
covered by salt water during the monsoons. Here and there its surface
is incrusted over with a layer of salt caused by the evaporation of
sea-water. A subsiding movement has been witnessed in this country
during earthquakes, so that a great thickness of pure salt might result
from a continuation of such sinking.
TRIAS OF GERMANY.
In Germany, as before hinted, chapter 21, the Trias first received its
name as a Triple Group, consisting of two sandstones with an
intermediate marine calcareous formation, which last is wanting in
England.
NOMENCLATURE OF TRIAS.
German French English Keuper Marnes irisées Saliferous and
gypseous
shales and sandstone. Muschelkalk Muschelkalk, on calcaire
coquillier Wanting in England. Bunter-sandstein Grès
bigarré Sandtone and quartzose conglomerate.
Keuper.—The first of these, or the Keuper, underlying the beds before
described as Rhætic, attains in Würtemberg a thickness of about 1000
feet. It is divided by Alberti into sandstone, gypsum, and carbonaceous
clay-slate.[4] Remains of reptiles called _Nothosaurus_ and
_Phytosaurus,_ have been found in it with Labyrinthodon; the detached
teeth, also, of placoid fish and of Rays, and of the genera
_Saurichthys_ and _Gyrolepis_ (Figs. 387, 388). The plants of the
Keuper are generically very analogous to those of the oolite and lias,
consisting of ferns, equisetaceous plants, cycads, and conifers, with a
few doubtful monocotyledons. A few species such as _Equisetites
columnaris,_ are common to this group and the oolite.
Fig. 397: Equisetites columnaris.
_St. Cassian and Hallstadt Beds_ (see Map, Fig. 398).— The sandstones
and clay of the Keuper resemble the deposits of estuaries and a shallow
sea near the land, and afford, in the N.W. of Germany, as in France and
England, but a scanty representation of the marine life of that period.
We might, however, have anticipated, from its rich reptilian fauna,
that the contemporaneous inhabitants of the sea of the Keuper period
would be very numerous, should we ever have an opportunity of bringing
their remains to light. This, it is believed, has at length been
accomplished, by the position now assigned to certain Alpine rocks
called the “St. Cassian beds,” the true place of which in the series
was until lately a subject of much doubt and discussion. It has been
proved that the Hallstadt beds on the northern flanks of the Austrian
Alps correspond in age with the St. Cassian beds on their southern
declivity, and the Austrian geologists, M. Suess of Vienna and others,
have satisfied themselves that the Hallstadt formation is referable to
the period of the Upper Trias. Assuming this conclusion to be correct,
we become acquainted suddenly and unexpectedly with a rich marine fauna
belonging to a period previously believed to be very barren of organic
remains, because in England, France, and Northern Germany the upper
Trias is chiefly represented by beds of fresh or brackish water origin.
Fig. 398: Map of Tyrol and Styria showing St. Cassian and Hallstadt
Beds.
Fig. 399: Scoliotoma. Fig. 400: Koninckia Leonhardi.
About 600 species of invertebrate fossils occur in the Hallstadt and
St. Cassian beds, many of which are still undescribed; some of the
Mollusca are of new and peculiar genera, as _Scoliostoma,_ Fig. 399,
and _Platystoma,_ Fig. 400, among the Gasteropoda; and _Koninckia,_
Fig. 401, among the Brachiopoda.
Fig. 401: Koninckia Leonhardi.
The following table of genera of marine shells from the Hallstadt and
St. Cassian beds, drawn up first on the joint authority of M. Suess and
the late Dr. Woodward, and since corrected by Messrs. Etheridge and
Tate, shows how many connecting links between the fauna of primary and
secondary Palæozoic and Mesozoic rocks are supplied by the St. Cassian
and Hallstadt beds.
GENERA OF FOSSIL MOLLUSCA IN THE ST. CASSIAN AND HALLSTADT BEDS.
Common to Older Rocks Characteristic Triassic Genera Common to
Newer Rocks Orthoceras
Bactrites
Macrocheilus
Loxonema
Holopella
Murchisonia
Porcellia
Athyris
Retzia
Cyrtina
Euomphalus Ceratites
Cochloceras
Choristoceras
Rhabdoceras
Aulacoceras
Scoliostoma [5]
Naticella
Platystoma
Ptychostoma
Euchrysalis
Halobia
Hornesia
Amphiclina
Koninckia
Cassianella [6]
Myophoria [6] Ammonites
Chemnitzia
Cerithium
Monodonta
Opis
Sphoera
Cardita
Myoconcha
Hinnites
Monotis
Plicatula
Pachyrisma
Thecidium
The first column marks the last appearance of several genera which are
characteristic of Palæozoic strata. The second shows those genera which
are characteristic of the Upper Trias, either as peculiar to it, or, as
in the three cases marked by asterisks, reaching their maximum of
development at this era. The third column marks the first appearance in
Triassic rocks of genera destined to become more abundant in later
ages.
It is only, however, when we contemplate the number of species by which
each of the above-mentioned genera are represented that we comprehend
the peculiarities of what is commonly called the St. Cassian fauna.
Thus, for example, the Ammonite, which is not common to older rocks, is
represented by no less than seventy-three species; whereas Loxonema,
which is only known as common to older rocks, furnishes fifteen
Triassic species. Cerithium, so abundant in tertiary strata, and which
still lives, is represented by no less than fourteen species. As the
Orthoceras had never been met with in the marine Muschelkalk, much
surprise was naturally felt that seven or eight species of the genus
should appear in the Hallstadt beds, assuming these last to belong to
the Upper Trias. Among these species are some of large dimensions,
associated with large Ammonites with foliated lobes, a form never seen
before so low in the series, while the Orthoceras had never been seen
so high.
On the whole, the rich marine fauna of Hallstadt and St. Cassian, now
generally assigned to the lowest members of the Upper Trias or Keuper,
leads us to suspect that when the strata of the Triassic age are better
known, especially those belonging to the period of the Bunter
sandstone, the break between the Palæozoic and Mesozoic Periods may be
almost effaced. Indeed some geologists are not yet satisfied that the
true position of the St. Cassian beds (containing so great an admixture
of types, having at once both Mesozoic and Palæozoic affinities) is
made out, and doubt whether they have yet been clearly proved to be
newer than the Muschelkalk.
Muschelkalk.—The next member of the Trias in Germany, the
_Muschelkalk,_ which underlies the _Keuper_ before described, consists
chiefly of a compact greyish limestone, but includes beds of dolomite
in many places, together with gypsum and rock-salt. This limestone, a
formation wholly unrepresented in England, abounds in fossil shells, as
the name implies. Among the Cephalopoda there are no belemnites, and no
ammonites with foliated sutures, as in the Lias, and Oolite, and the
Hallstadt beds; but we find instead a genus allied to the Ammonite,
called _Ceratites_ by de Haan, in which the descending lobes (Fig. 402)
terminate in a few small denticulations pointing inward. Among the
bivalve crustacea, the _Estheria minuta,_ Bronn (see Fig. 390), is
abundant, ranging through the Keuper, Muschelkalk, and
Bunter-sandstein; and _Gervillia socialis_ (Fig. 403), having a similar
range, is found in great numbers in the Muschelkalk of Germany, France,
and Poland.
Fig. 402: Ceratites nodosus. Fig. 403: Gervillia (Avicula) socialis.
Fig. 404: Enerinus liliiformis. Fig. 405: Aspidura loricata.
The abundance of the heads and stems of lily encrinites, _ Encrinus
liliiformis_ (Fig. 404), (or _Encrinites moniliformis_), shows the slow
manner in which some beds of this limestone have been formed in clear
sea-water. The star-fish called _Aspidura loricata_ (Fig. 405) is as
yet peculiar to the Muschelkalk. In the same formation are found the
skull and teeth of a reptile of the genus _Placodus_ (see Fig. 406),
which was referred originally by Munster, and afterwards by Agassiz, to
the class of fishes. But more perfect specimens enabled Professor Owen,
in 1858, to show that this fossil animal was a Saurian reptile, which
probably fed on shell-bearing mollusks, and used its short and flat
teeth, so thickly coated with enamel, for pounding and crushing the
shells.
Fig. 406: Palatal teeth of Placodus gigas.
Fig. 407: Voltzia heterophylla.
Bunter-sandstein.—The _Bunter-sandstein_ consists of various-coloured
sandstones, dolomites, and red clays, with some beds, especially in the
Hartz, of calcareous pisolite or roe-stone, the whole sometimes
attaining a thickness of more than 1000 feet. The sandstone of the
Vosges is proved, by its fossils, to belong to this lowest member of
the Triassic group. At Sulzbad (or Soultz-les-bains), near Strasburg,
on the flanks of the Vosges, many plants have been obtained from the
“bunter,” especially conifers of the extinct genus _Voltzia,_ of which
the fructification has been preserved. (See Fig. 407.) Out of thirty
species of ferns, cycads, conifers, and other plants, enumerated by M.
Ad. Brongniart, in 1849, as coming from the “Grès bigarré,” or Bunter,
not one is common to the Keuper.
The footprints of Labyrinthodon observed in the clays of this formation
at Hildburghausen, in Saxony, have already been mentioned. Some idea of
the variety and importance of the terrestrial vertebrate fauna of the
three members of the Trias in Northern Germany may be derived from the
fact that in the great monograph by the late Hermann von Meyer on the
reptiles of the Trias, the remains of no less than eighty distinct
species are described and figured.
TRIAS OF THE UNITED STATES.
New Red Sandstone of the Valley of the Connecticut River.—In a
depression of the granitic or hypogene rocks in the States of
Massachusetts and Connecticut strata of red sandstone, shale, and
conglomerate are found, occupying an area more than 150 miles in length
from north to south, and about five to ten miles in breadth, the beds
dipping to the eastward at angles varying from 5 to 50 degrees. The
extreme inclination of 50 degrees is rare, and only observed in the
neighbourhood of masses of trap which have been intruded into the red
sandstone while it was forming, or before the newer parts of the
deposit had been completed. Having examined this series of rocks in
many places, I feel satisfied that they were formed in shallow water,
and for the most part near the shore, and that some of the beds were
from time to time raised above the level of the water, and laid dry,
while a newer series, composed of similar sediment, was forming.
Fig. 408: Foot-prints of a bird, Turner’s Falls, Valley of the
Connecticut.
According to Professor Hitchcock, the footprints of no less than
thirty-two species of bipeds, and twelve of quadrupeds, have been
already detected in these rocks. Thirty of these are believed to be
those of birds, four of lizards, two of chelonians, and six of
batrachians. The tracks have been found in more than twenty places,
scattered through an extent of nearly 80 miles from north to south, and
they are repeated through a succession of beds attaining at some points
a thickness of more than 1000 feet.[7]
The bipedal impressions are, for the most part, trifid, and show the
same number of joints as exist in the feet of living tridactylous
birds. Now, such birds have three phalangeal bones for the inner toe,
four for the middle, and five for the outer one (see Fig. 408); but the
impression of the terminal joint is that of the nail only. The fossil
footprints exhibit regularly, where the joints are seen, the same
number; and we see in each continuous line of tracks the three-jointed
and five-jointed toes placed alternately outward, first on the one
side, and then on the other. In some specimens, besides impressions of
the three toes in front, the rudiment is seen of the fourth toe behind.
It is not often that the matrix has been fine enough to retain
impressions of the integument or skin of the foot; but in one fine
specimen found at Turner’s Falls, on the Connecticut, by Dr. Deane,
these markings are well preserved, and have been recognised by
Professor Owen as resembling the skin of the ostrich, and not that of
reptiles.
The casts of the footprints show that some of the fossil bipeds of the
red sandstone of Connecticut had feet four times as large as the living
ostrich, but scarcely, perhaps, larger than the Dinornis of New
Zealand, a lost genus of feathered giants related to the Apteryx, of
which there were many species which have left their bones and almost
entire skeletons in the superficial alluvium of that island. By
referring to what was said of the Iguanodon of the Wealden, the reader
will perceive that the Dinosaur was somewhat intermediate between
reptiles and birds, and left a series of tridactylous impressions on
the sand.
To determine the exact age of the red sandstone and shale containing
these ancient footprints, in the United States, is not possible at
present. No fossil shells have yet been found in the deposit, nor
plants in a determinable state. The fossil fish are numerous and very
perfect; but they are of a peculiar type, called _Ischypterus,_ by Sir
Philip Egerton, from the great size and strength of the fulcral rays of
the dorsal fin, from ischus, strength, and pteron, a fin.
The age of the Connecticut beds cannot be proved by direct
superposition, but may be presumed from the general structure of the
country. That structure proves them to be newer than the movements to
which the Appalachian or Allegheny chain owes its flexures, and this
chain includes the ancient or palæozoic coal-formation among its
contorted rocks.
Coal-field of Richmond, Virginia.—In the State of Virginia, at the
distance of about 13 miles eastward of Richmond, the capital of that
State, there is a coal-field occurring in a depression of the granite
rocks, and occupying a geological position analogous to that of the New
Red Sandstone, above-mentioned, of the Connecticut valley. It extends
26 miles from north to south, and from four to twelve from east to
west.
The plants consist chiefly of zamites, calamites, equiseta, and ferns,
and, upon the whole, are considered by Professor Heer to have the
nearest affinity to those of the European Keuper.
The equiseta are very commonly met with in a vertical position more or
less compressed perpendicularly. It is clear that they grew in the
places where they are now buried in strata of hardened sand and mud. I
found them maintaining their erect attitude, at points many miles
apart, in beds both above and between the seams of coal. In order to
explain this fact, we must suppose such shales and sandstones to have
been gradually accumulated during the slow and repeated subsidence of
the whole region.
Fig. 409: Triassic coal-shale, Richmond, Virginia.
The fossil fish are Ganoids, some of them of the genus _ Catopterus,_
others belonging to the liassic genus _ Tetragonolepis (Æchmodus),_ see
Fig. 376. Two species of _ Entomostraca_ called _Estheria_ are in such
profusion in some shaly beds as to divide them like the plates of mica
in micaceous shales (see Fig. 409).
These Virginian coal-measures are composed of grits, sandstones, and
shales, exactly resembling those of older or primary date in America
and Europe, and they rival, or even surpass, the latter in the richness
and thickness of the coal-seams. One of these, the main seam, is in
some places from 30 to 40 feet thick, composed of pure bituminous coal.
The coal is like the finest kinds shipped at Newcastle, and when
analysed yields the same proportions of carbon and hydrogen—a fact
worthy of notice, when we consider that this fuel has been derived from
an assemblage of plants very distinct specifically, and in part
generically, from those which have contributed to the formation of the
ancient or palæozoic coal.
Triassic Mammifer.—In North Carolina, the late Professor Emmons has
described the strata of the Chatham coal-field, which correspond in age
to those near Richmond, in Virginia. In beds underlying them he has met
with three jaws of a small insectivorous mammal which he has called
_Dromatherium sylvestre,_ closely allied to _Spalacotherium._ Its
nearest living analogue, says Professor Owen, “is found in Myrmecobius;
for each ramus of the lower jaw contained ten small molars in a
continuous series, one canine, and three conical incisors—the latter
being divided by short intervals.”
Low Grade of Early Mammals favourable to the Theory of Progressive
Development.—There is every reason to believe that this fossil
quadruped is at least as ancient as the Microlestes of the European
Trias described in p. 368; and the fact is highly important, as proving
that a certain low grade of marsupials had not only a wide range in
time, from the Trias to the Purbeck, or uppermost oolitic strata of
Europe, but had also a wide range in space, namely, from Europe to
North America, in an east and west direction, and, in regard to
latitude, from Stonesfield, in 52° N., to that of North Carolina, 35°
N.
If the three localities in Europe where the most ancient mammalia have
been found—Purbeck, Stonesfield, and Stuttgart—had belonged all of them
to formations of the same age, we might well have imagined so limited
an area to have been peopled exclusively with pouched quadrupeds, just
as Australia now is, while other parts of the globe were inhabited by
placentals; for Australia now supports one hundred and sixty species of
marsupials, while the rest of the continents and islands are tenanted
by about seventeen hundred species of mammalia, of which only forty-six
are marsupial, namely, the opossums of North and South America. But the
great difference of age of the strata in each of these three localities
seems to indicate the predominance throughout a vast lapse of time
(from the era of the Upper Trias to that of the Purbeck beds) of a low
grade of quadrupeds; and this persistency of similar generic and
ordinal types in Europe while the species were changing, and while the
fish, reptiles, and mollusca were undergoing great modifications, would
naturally lead us to suspect that there must also have been a vast
extension in space of the same marsupial forms during that portion of
the Secondary or Mesozoic epoch which has been termed “the age of
reptiles.” Such an inference as to the wide geographical range of the
ancient marsupials has been confirmed by the discovery in the Trias of
North America of the above-mentioned Dromatherium. The predominance in
earlier ages of these mammalia of a low grade, and the absence, so far
as our investigations have yet gone, of species of higher organisation,
whether aquatic or terrestrial, is certainly in favour of the theory of
progressive development.
[1] Dr. Wright, on Lias and Bone Bed, Quart. Geol. Journ., 1860, vol.
xvi.
[2] Buckland, Proc. Geol. Soc., vol. ii, p. 439; and Murchison and
Strickland, Geol. Trans., Second Series., vol. v, p. 347.
[3] Principles of Geology, chap. xxvii.
[4] Monog. des Bunter-Sandsteins.
[5] Reaches its maximum in the Trias, but passes down to older rocks.
[6]
Reach their maximum in the Trias, but pass up to newer rocks.
[7] Hitchcock, Mem. of Amer. Acad., New Series, vol. iii, p. 129,
1848.
PRIMARY OR PALÆOZOIC SERIES
CHAPTER XXII.
PERMIAN OR MAGNESIAN LIMESTONE GROUP.
Line of Separation between Mesozoic and Palæozoic Rocks. — Distinctness
of Triassic and Permian Fossils. — Term Permian. — Thickness of
calcareous and sedimentary Rocks in North of England. — Upper, Middle,
and Lower Permian. — Marine Shells and Corals of the English Magnesian
Limestone. — Reptiles and Fish of Permian Marl-slate. — Foot-prints of
Reptiles. — Angular Breccias in Lower Permian. — Permian Rocks of the
Continent. — Zechstein and Rothliegendes of Thuringia. — Permian Flora.
— Its generic Affinity to the Carboniferous.
In pursuing our examination of the strata in descending order, we have
next to pass from the base of the Secondary or Mesozoic to the
uppermost or newest of the Primary or Palæozoic formations. As this
point has been selected as a line of demarkation for one of the three
great divisions of the fossiliferous series, the student might
naturally expect that by aid of lithological and palæontological
characters he would be able to recognise without difficulty a distinct
break between the newer and older group. But so far is this from being
the case in Great Britain, that nowhere have geologists found more
difficulty in drawing the line of separation than between the Secondary
and Primary series. The obscurity has arisen from the great resemblance
in colour and mineral character of the Triassic and Permian red marls
and sandstones, and the scarcity and often total absence in them of
organic remains. The thickness of the strata belonging to each group
amounts in some places to several thousand feet; and by dint of a
careful examination of their geological position, and of those fossil,
animal, and vegetable forms which are occasionally met with in some
members of each series, it has at length been made clear that the older
or Permian rocks are more connected with the Primary or Palæozoic than
with the Secondary or Mesozoic strata already described.
The term Permian has been proposed for this group by Sir R. Murchison,
from Perm, a Russian province, where it occupies an area twice the size
of France, and contains a great abundance and variety of fossils, both
vertebrate and invertebrate. Professor Sedgwick in 1832[1] described
what is now recognised as the central member of this group, the
Magnesian limestone, showing that it attained a thickness of 600 feet
along the north-east of England, in the counties of Durham, Yorkshire,
and Nottinghamshire, its lower part often passing into a fossiliferous
marl-slate and resting on an inferior Red Sandstone, the equivalent of
the Rothliegendes of Germany. It has since been shown that some of the
Red Sandstones of newer date also belong to the Permian group; and it
appears from the observations of Mr. Binney, Sir R. Murchison, Mr.
Harkness, and others, that it is in the region where the limestone is
most largely developed, as, for example, in the county of Durham, that
the associated red sandstones or sedimentary rocks are thinnest,
whereas in the country where the latter are thickest the calcareous
member is reduced to thirty, or even sometimes to ten feet. It is
clear, therefore, says Mr. Hull, that the sedimentary region in the
north of England area has been to the westward, and the calcareous area
to the eastward; and that in this group there has been a development
from opposite directions of the two types of strata.
In illustration of this he has given us the following table:
THICKNESS OF PERMIAN STRATA IN NORTH OF ENGLAND.
N.W. of England N.E. of England Feet Feet Upper Permian
(Sedimentary) 600 50–100 Middle Permian
(Calcareous) 10–30 600 Lower Permian
(Sedimentary) 3000 100–250[2]
Upper Permian.—What is called in this table the Upper Permian will be
seen to attain its chief thickness in the north-west, or on the coast
of Cumberland, as at St. Bee’s Head, where it is described by Sir
Roderick Murchison as consisting of massive red sandstones with gypsum
resting on a thin course of Magnesian Limestone with fossils, which
again is connected with the Lower Red Sandstone, resembling the upper
one in such a manner that the whole forms a continuous series. No
fossil footprints have been found in this Upper as in the Lower Red
Sandstone.
Middle Permian—Magnesian Limestone and Marl-slate.—This formation is
seen upon the coast of Durham and Yorkshire, between the Wear and the
Tees. Among its characteristic fossils are _Schizodus Schlotheimi_
(Fig. 410) and _Mytilus septifer_ (Fig. 412). These shells occur at
Hartlepool and Sunderland, where the rock assumes an oolitic and
botryoidal character. Some of the beds in this division are
ripple-marked. In some parts of the coast of Durham, where the rock is
not crystalline, it contains as much as 44 per cent of carbonate of
magnesia, mixed with carbonate of lime. In other places—for it is
extremely variable in structure—it consists chiefly of carbonate of
lime, and has concreted into globular and hemispherical masses, varying
from the size of a marble to that of a cannon-ball, and radiating from
the centre. Occasionally earthy and pulverulent beds pass into compact
limestone or hard granular dolomite. Sometimes the limestone appears in
a brecciated form, the fragments which are united together not
consisting of foreign rocks but seemingly composed of the breaking-up
of the Permian limestone itself, about the time of its consolidation.
Some of the angular masses in Tynemouth cliff are two feet in diameter.
Fig. 410: Schidozus Schlotheimi, Permian crystalline limestone. Fig.
411: The hinge of Schizodus truncatus, Permian. Fig. 412: Mytilus
septifer, Permian crystalline limestone.
The magnesian limestone sometimes becomes very fossiliferous and
includes in it delicate bryozoa, one of which, _Fenestella retiformis_
(Fig. 413), is a very variable species, and has received many different
names. It sometimes attains a large size, single specimens measuring
eight inches in width. The same bryozoan, with several other British
species, is also found abundantly in the Permian of Germany.
The total known fauna of the Permian series of Great Britain at present
numbers 147 species, of which 77, or more than half, are mollusca. Not
one of these is common to rocks newer than the Palæozoic, and the
brachiopods are the only group which have furnished species common to
the more ancient or Carboniferous rocks. Of these _Lingula Crednerii_
(Fig. 415) is an example. There are 25 Gasteropods and only one
cephalopod, _Nautilus Freieslebeni,_ which is also found in the German
Zechstein.
Fig. 413: Magnesian Limestone. Fig. 413: Magnesian Limestone, Humbleton
Hill, near Sunderland.[3]
Shells of the genera _Productus_ (Fig. 414) and _ Strophalosia_ (the
latter of allied form with hinge teeth), which do not occur in strata
newer than the Permian, are abundant in the ordinary yellow magnesian
limestone, as will be seen in the valuable memoirs of Messrs. King and
Howse. They are accompanied by certain species of _Spirifera_ (Fig.
416), _Lingula Crednerii_ (Fig. 415), and other brachiopoda of the true
primary or palæozoic type. Some of this same tribe of shells, such as
Camarophoria, allied to Rhynchonella, Spiriferina, and two species of
_Lingula,_ are specifically the same as fossils of the carboniferous
rocks. _Avicula, Arca,_ and _Schizodus_ (Fig. 410), and other
lamellibranchiate bivalves, are abundant, but spiral univalves are very
rare.
Fig. 414: Productus horridus. Fig. 415: Lingula Crednerii. Fig. 416:
Spirifera alata.
Beneath the limestone lies a formation termed the marl-stone, which
consists of hard calcareous shales, marl-slate, and thin-bedded
limestones. At East Thickley, in Durham, where it is thirty feet thick,
this slate has yielded many fine specimens of fossil fish—of the genera
_Palæoniscus_ ten species, _Pygopterus_ two species, _Coelacanthus_ two
species, and _Platysomus_ two species, which as genera are common to
the older Carboniferous formation, but the Permian species are
peculiar, and, for the most part, identical with those found in the
marl-slate or copper-slate of Thuringia.
Fig. 417: Restored outline of a fish of the genus Palæoniscus. Fig.
418: Shark, Heterocercal. Fig. 419: Shad. (Clupea. Herring tribe.)
Homocereal.
The _Palæoniscus_ above mentioned belongs to that division of fishes
which M. Agassiz has called “Heterocercal,” which have their tails
unequally bilobate, like the recent shark and sturgeon, and the
vertebral column running along the upper caudal lobe. (See Fig. 418.)
The “Homocercal” fish, which comprise almost all the 9000 species at
present known in the living creation, have the tail-fin either single
or equally divided; and the vertebral column stops short, and is not
prolonged into either lobe. (See Fig. 419.) Now it is a singular fact,
first pointed out by Agassiz, that the heterocercal form, which is
confined to a small number of genera in the existing creation, is
universal in the magnesian limestone, and all the more ancient
formations. It characterises the earlier periods of the earth’s
history, whereas in the secondary strata, or those newer than the
Permian, the homocercal tail predominates.
A full description has been given by Sir Philip Egerton of the species
of fish characteristic of the marl-slate, in Professor King’s monograph
before referred to, where figures of the ichthyolites, which are very
entire and well preserved, will be found. Even a single scale is
usually so characteristically marked as to indicate the genus, and
sometimes even the particular species. They are often scattered through
the beds singly, and may be useful to a geologist in determining the
age of the rock.
Fig. 420: Palæoniscus comptus. Fig. 421: Palæoniscus elegans. Fig. 422:
Palæoniscus glaphyrus. Fig. 423: Cœlacanthus granulatus. Fig. 424:
Pygopterus mandibularis. Fig. 425: Acrolepis Sedgwickii.
We are indebted to Messrs. Hancock and Howse for the discovery in this
marl-slate at Midderidge, Durham, of two species of _ Protosaurus,_ a
genus of reptiles, one representative of which, _P. Speneri,_ has been
celebrated ever since the year 1810 as characteristic of the
Kupfer-schiefer or Permian of Thuringia. Professor Huxley informs us
that the agreement of the Durham fossil with Hermann von Meyer’s figure
of the German specimen is most striking. Although the head is wanting
in all the examples yet found, they clearly belong to the Lacertian
order, and are therefore of a higher grade than any other vertebrate
animal hitherto found fossil in a Palæozoic rock. Remains of
Labyrinthodont reptiles have also been met with in the same slate near
Durham.
Lower Permian.—The inferior sandstones which lie beneath the marl-slate
consist of sandstone and sand, separating the Magnesian Limestone from
the coal, in Yorkshire and Durham. In some instances, red marl and
gypsum have been found associated with these beds. They have been
classed with the Magnesian Limestone by Professor Sedgwick, as being
nearly co-extensive with it in geographical range, though their
relations are very obscure. But the principal development of Lower
Permian is, as we have seen by Mr. Hull’s table p. 386, in the
northwest, where the Penrith sandstone, as it has been called, and the
associated breccias and purple shales are estimated by Professor
Harkness to attain a thickness of 3000 feet. Organic remains are
generally wanting, but the leaves and wood of coniferous plants, and in
one case a cone, have been found. Also in the purple marls of
Corncockle Muir near Dumfries, very distinct footprints of reptiles
occur, originally referred to the Trias, but shown by Mr. Binney in
1856 to be Permian. No bones of the animals which they represent have
yet been discovered.
_Angular Breccias in Lower Permian._—A striking feature in these beds
is the occasional occurrence, especially at the base of the formation,
of angular and sometimes rounded fragments of Carboniferous and older
rocks of the adjoining districts being included in a paste of red marl.
Some of the angular masses are of huge size.
In the central and southern counties, where the Middle Permian or
Magnesian Limestone is wanting, it is difficult to separate the upper
and lower sandstones, and Mr. Hull is of opinion that the patches of
this formation found here and there in Worcestershire, Shropshire, and
other counties may have been deposited in a sea separated from the
northern basin by a barrier of Carboniferous rocks running east and
west, and now concealed under the Triassic strata of Cheshire. Similar
breccias to those before described are found in the more southern
counties last mentioned, where their appearance is rendered more
striking by the marked contrast they present to the beds of well-rolled
and rounded pebbles of the Trias occupying a large area in the same
region.
Professor Ramsay refers the angular form and large size of the
fragments composing these breccias to the action of floating ice in the
sea. These masses of angular rock, some of them weighing more than half
a ton, and lying confusedly in a red, unstratified marl, like stones in
boulder-drift, are in some cases polished, striated, and furrowed like
erratic blocks in the moraine of a glacier. They can be shown in some
cases to have travelled from the parent rocks, thirty or more miles
distant, and yet not to have lost their angular shape.[4]
Permian Rocks of the Continent.—Germany is the classic ground of the
Magnesian Limestone now called Permian. The formation was well studied
by the miners of that country a century ago as containing a thin band
of dark-coloured cupriferous shale, characterised at Mansfield in
Thuringia by numerous fossil fish. Beneath some variegated sandstones
(not belonging to the Trias, though often confounded with it) they came
down first upon a dolomitic limestone corresponding to the upper part
of our Middle Permian, and then upon a marl-slate richly impregnated
with copper pyrites, and containing fish and reptiles (Protosaurus)
identical in species with those of the corresponding marl-slate of
Durham. To the limestone they gave the name of Zechstein, and to the
marl-slate that of Mergel-schiefer or Kupfer-schiefer. Beneath the
fossiliferous group lies the Rothliegendes or Rothtodt-liegendes,
meaning the red-lyer or red-dead-lyer, so-called by the German miners
from its colour, and because the copper had _died out_ when they
reached this underlying non-metalliferous member of the series. This
red under-lyer is, in fact, a great deposit of red sandstone, breccia,
and conglomerate with associated porphyry, basalt, and amygdaloid.
According to Sir R. Murchison, the Permian rocks are composed, in
Russia, of white limestone, with gypsum and white salt; and of red and
green grits, occasionally with copper ore; also magnesian limestones,
marl-stones, and conglomerates.
Fig. 426: Walchia piniformis.
Permian Flora.—About 18 or 20 species of plants are known in the
Permian rocks of England. None of them pass down into the Carboniferous
series, but several genera, such as _ Alethopteris, Neuropteris,
Walchia,_ and _Ullmania,_ are common to the two groups. The Permian
flora on the Continent appears, from the researches of MM. Murchison
and de Verneuil in Russia, and of MM. Geinitz and von Gutbier in
Saxony, to be, with a few exceptions, distinct from that of the coal.
Fig. 27: Cardiocarpon Ottonis.
In the Permian rocks of Saxony no less than 60 species of fossil plants
have been met with. Two or three of these, as _Calamites gigas,
Sphenopteris erosa,_ and _S. lobata,_ are also met with in the
government of Perm in Russia. Seven others, and among them _Neuropteris
Loshii, Pecopteris arborescens,_ and _P. similis,_ and several species
of _Walchia_ (see Fig. 426), a genus of Conifers, called _Lycopodites_
by some authors, are said by Geinitz to be common to the coal-measures.
Fig. 428: Noeggerathia cuneifolia. Fig. 428: Noeggerathia cuneifolia.
Brongniart.[5]
Among the genera also enumerated by Colonel Gutbier are the fruit
called _Cardiocarpon_ (see Fig. 427), _ Asterophyllites,_ and
Annularia, so characteristic of the Carboniferous period; also
_Lepidodendron,_ which is common to the Permian of Saxony, Thuringia,
and Russia, although not abundant. _Neoggerathia_ (see Fig. 428), the
leaves of which have parallel veins without a midrib, and to which
various generic synonyms, such as _Cordaites, Flabellaria,_ and _
Poacites,_ have been given, is another link between the Permian and
Carboniferous vegetation. Coniferæ, of the Araucarian division, also
occur; but these are likewise met with both in older and newer rocks.
The plants called _Sigillaria_ and _ Stigmaria,_ so marked a feature in
the Carboniferous period, are as yet wanting in the true Permian.
Among the remarkable fossils of the Rothliegendes, or lowest part of
the Permian in Saxony and Bohemia, are the silicified trunks of
tree-ferns called generically _Psaronius._ Their bark was surrounded by
a dense mass of air-roots, which often constituted a great addition to
the original stem, so as to double or quadruple its diameter. The same
remark holds good in regard to certain living extra-tropical
arborescent ferns, particularly those of New Zealand.
Upon the whole, it is evident that the Permian plants approach much
nearer to the Carboniferous flora than to the Triassic; and the same
may be said of the Permian fauna.
[1] Trans. Geol. Soc. Lond., Second Series, vol. iii, p. 37.
[2] Edward Hull, Ternary Classification, Quart. Journ. Science, No.
xxiii, 1869.
[3] King’s Monograph, pl. 2.
[4] Ramsay, Quart. Geol. Journ., 1855; and Lyell, Principles of
Geology, vol. i, p. 223, 10th edit.
[5] Murchison’s Russia, vol. ii, pl. A, fig. 3.
CHAPTER XXIII.
THE COAL OR CARBONIFEROUS GROUP.
Principal Subdivisions of the Carboniferous Group. — Different
Thickness of the sedimentary and calcareous Members in Scotland and the
South of England. — Coal-measures. — Terrestrial Nature of the Growth
of Coal. — Erect fossil Trees. — Uniting of many Coal-seams into one
thick Bed. — Purity of the Coal explained. — Conversion of Coal into
Anthracite. — Origin of Clay-ironstone. — Marine and brackish-water
Strata in Coal. — Fossil Insects. — Batrachian Reptiles. —
Labyrinthodont Foot-prints in Coal-measures. — Nova Scotia
Coal-measures with successive Growths of erect fossil Trees. —
Similarity of American and European Coal. — Air-breathers of the
American Coal. — Changes of Condition of Land and Sea indicated by the
Carboniferous Strata of Nova Scotia.
Principal Subdivisions of the Carboniferous Group.—The next group which
we meet with in the descending order is the Carboniferous, commonly
called “The Coal,” because it contains many beds of that mineral, in a
more or less pure state, interstratified with sandstones, shales, and
limestones. The coal itself, even in Great Britain and Belgium, where
it is most abundant, constitutes but an insignificant portion of the
whole mass. In South Wales, for example, the thickness of the
coal-bearing strata has been estimated at between 11,000 and 12,000
feet, while the various coal seams, about 80 in number, do not,
according to Professor Phillips, exceed in the aggregate 120 feet.
The Carboniferous formation assumes various characters in different
parts even of the British Islands. It usually comprises two very
distinct members: first, the sedimentary beds, usually called the
Coal-measures, of mixed fresh-water, terrestrial, and marine origin,
often including seams of coal; second, that named in England the
Mountain or Carboniferous Limestone, of purely marine origin, and made
up chiefly of corals, shells, and encrinites, and resting on shales
called the shales of the Mountain Limestone.
In the south-western part of our island, in Somersetshire and South
Wales, the three divisions usually spoken of are:
Coal-measures: Strata of shale, sandstone, and grit, from 600 to 12,000
feet thick, with occasional seams of coal.
Millstone grit: A coarse quartzose sandstone passing into a
conglomerate, sometimes used for millstones, with beds of shale;
usually devoid of coal; occasionally above 600 feet thick.
Mountain or Carboniferous Limestone: A calcareous rock containing
marine shells, corals, and encrinites; devoid of coal; thickness
variable, sometimes more than 1500 feet.
If the reader will refer to the section in Fig. 85, he will see that
the Upper and Lower Coal-measures of the coal-field near Bristol are
divided by a micaceous flaggy sandstone called the Pennant Rock. The
Lower Coal-measures of the same section rest sometimes, especially in
the north part of the basin, on a base of coarse grit called the
Millstone Grit (No. 2 on the previous page).
In the South Welsh coal-field Millstone Grit occurs in like manner at
the base of the productive coal. It is called by the miners the
“Farewell Rock,” as when they reach it they have no longer any hopes of
obtaining coal at a greater depth in the same district. In the central
and northern coal-fields of England this same grit, including quartz
pebbles, with some accompanying sandstones and shales containing coal
plants, acquires a thickness of several thousand feet, lying beneath
the productive coal-measures, which are nearly 10,000 feet thick.
Below the Millstone Grit is a continuation of similar sandstones and
shales called by Professor Phillips the Yoredale series, from Yoredale,
in Yorkshire, where they attain a thickness of from 800 to 1000 feet.
At several intervals bands of limestone divide this part of the series,
one of which, called the Main Limestone or Upper Scar Limestone,
composed in great part of encrinites, is 70 feet thick. Thin seams of
coal also occur in these lower Yoredale beds in Yorkshire, showing that
in the same region there were great alternations in the state of the
surface. For at successive periods in the same area there prevailed
first terrestrial conditions favourable to the growth of pure coal,
secondly, a sea of some depth suited to the formation of Carboniferous
Limestone, and, thirdly, a supply of muddy sediment and sand,
furnishing the materials for sandstone and shale. There is no clear
line of demarkation between the Coal-measures and the Millstone Grit,
nor between the Millstone Grit and underlying Yoredale rocks.
On comparing a series of vertical sections in a north-westerly
direction from Leicestershire and Warwickshire into North Lancashire,
we find, says Mr. Hull, within a distance of 120 miles an augmentation
of the sedimentary materials to the extent of 16,000 feet.
Leicestershire and Warwickshire 2,600 feet North
Staffordshire 9,000 feet South Lancashire 12,130 feet North
Lancashire 18,700 feet
In central England, where the sedimentary beds are reduced to about
3000 feet in all, the Carboniferous Limestone attains an enormous
thickness, as much as 4000 feet at Ashbourne, near Derby, according to
Mr. Hull’s estimate. To a certain extent, therefore, we may consider
the calcareous member of the formation as having originated
simultaneously with the accumulation of the materials of grit,
sandstone, and shale, with seams of coal; just as strata of mud, sand,
and pebbles, several thousand feet thick, with layers of vegetable
matter, are now in the process of formation in the cypress swamps and
delta of the Mississippi, while coral reefs are forming on the coast of
Florida and in the sea of the Bermuda islands. For we may safely
conclude that in the ancient Carboniferous ocean those marine animals
which were limestone builders were never freely developed in areas
where the rivers poured in fresh water charged with sand or clay; and
the limestone could only become several thousand feet thick in parts of
the ocean which remained perfectly clear for ages.
The calcareous strata of the Scotch coal-fields, those of Lanarkshire,
the Lothians, and Fife, for example, are very insignificant in
thickness when compared to those of England. They consist of a few beds
intercalated between the sandstones and shales containing coal and
ironstone, the combined thickness of all the limestones amounting to no
more than 150 feet. The vegetation of some of these northern
sedimentary beds containing coal may be older than any of the
coal-measures of central and southern England, as being coeval with the
Mountain Limestone of the south. In Ireland the limestone predominates
over the coal-bearing sands and shales. We may infer the former
continuity of several of the coal-fields in northern and central
England, not only from the abrupt manner in which they are cut off at
their outcrop, but from their remarkable correspondence in the
succession and character of particular beds. But the limited extent to
which these strata are exposed at the surface is not merely owing to
their former denudation, but even in a still greater degree to their
having been largely covered by the New Red Sandstone, as in Cheshire,
and here and there by the Permian strata, as in Durham.
It has long been the opinion of the most eminent geologists that the
coal-fields of Yorkshire and Lancashire were once united, the upper
Coal-measures and the overlying Millstone Grit and Yoredale rocks
having been subsequently removed; but what is remarkable, is the
ancient date now assigned to this denudation, for it seems that a
thickness of no less than 10,000 feet of the coal-measures had been
carried away before the deposition even of the lower Permian rocks
which were thrown down upon the already disturbed truncated edges of
the coal-strata.[1] The carboniferous strata most productive of
workable coal have so often a basin-shaped arrangement that these
troughs have sometimes been supposed to be connected with the original
conformation of the surface upon which the beds were deposited. But it
is now admitted that this structure has been owing to movements of the
earth’s crust of upheaval and subsidence, and that the flexure and
inclination of the beds has no connection with the original
geographical configuration of the district.
COAL-MEASURES.
I shall now treat more particularly of the productive coal-measures,
and their mode of origin and organic remains.
Coal formed on Land.—In South Wales, already alluded to, where the
coal-measures attain a thickness of 12,000 feet, the beds throughout
appear to have been formed in water of moderate depth, during a slow,
but perhaps intermittent, depression of the ground, in a region to
which rivers were bringing a never-failing supply of muddy sediment and
sand. The same area was sometimes covered with vast forests, such as we
see in the deltas of great rivers in warm climates, which are liable to
be submerged beneath fresh or salt water should the ground sink
vertically a few feet.
In one section near Swansea, in South Wales, where the total thickness
of strata is 3246 feet, we learn from Sir H. De la Beche that there are
ten principal masses of sandstone. One of these is 500 feet thick, and
the whole of them make together a thickness of 2125 feet. They are
separated by masses of shale, varying in thickness from 10 to 50 feet.
The intercalated coal-beds, sixteen in number, are generally from one
to five feet thick, one of them, which has two or three layers of clay
interposed, attaining nine feet. At other points in the same coal-field
the shales predominate over the sandstones. Great as is the diversity
in the horizontal extent of individual coal-seams, they all present one
characteristic feature, in having, each of them, what is called its
_underclay._ These underclays, co-extensive with every layer of coal,
consist of arenaceous shale, sometimes called fire-stone, because it
can be made into bricks which stand the fire of a furnace. They vary in
thickness from six inches to more than ten feet; and Sir William Logan
first announced to the scientific world in 1841 that they were regarded
by the colliers in South Wales as an essential accompaniment of each of
the eighty or more seams of coal met with in their coal-field. They are
said to form the _floor_ on which the coal rests; and some of them have
a slight admixture of carbonaceous matter, while others are quite
blackened by it.
All of them, as Sir William Logan pointed out, are characterised by
inclosing a peculiar species of fossil vegetable called _ Stigmaria,_
to the exclusion of other plants. It was also observed that, while in
the overlying shales, or “roof” of the coal, ferns and trunks of trees
abound without any _ Stigmariæ,_ and are flattened and compressed,
those singular plants of the underclay most commonly retain their
natural forms, unflattened and branching freely, and sending out their
slender rootlets, formerly thought to be leaves, through the mud in all
directions. Several species of _Stigmaria_ had long been known to
botanists, and described by them, before their position under each seam
of coal was pointed out, and before their true nature as the roots of
trees (some having been actually found attached to the base of
_Sigillaria_ stumps) was recognised. It was conjectured that they might
be aquatic, perhaps floating plants, which sometimes extended their
branches and leaves freely in fluid mud, in which they were finally
enveloped.
Now that all agree that these underclays are ancient soils, it follows
that in every instance where we find them they attest the terrestrial
nature of the plants which formed the overlying coal, which consists of
the trunks, branches, and leaves of the same plants. The trunks have
generally fallen prostrate in the coal, but some of them still remain
at right angles to the ancient soils (see Fig. 440). Professor Goppert,
after examining the fossil vegetables of the coal-fields of Germany,
has detected, in beds of pure coal, remains of plants of every family
hitherto known to occur fossil in the carboniferous rocks. Many seams,
he remarks, are rich in _Sigillariæ, Lepidodendra,_ and _Stigmariæ,_
the latter in such abundance as to appear to form the bulk of the coal.
In some places, almost all the plants were calamites, in others
ferns.[2]
Between the years 1837 and 1840, six fossil trees were discovered in
the coal-fields of Lancashire, where it is intersected by the Bolton
railway. They were all at right angles to the plane of the bed, which
dips about 15 degrees to the south. The distance between the first and
the last was more than 100 feet, and the roots of all were imbedded in
a soft argillaceous shale. In the same plane with the roots is a bed of
coal, eight or ten inches thick, which has been found to extend across
the railway, or to the distance of at least ten yards. Just above the
covering of the roots, yet beneath the coal-seam, so large a quantity
of the _Lepidostrobus variabilis_ was discovered inclosed in nodules of
hard clay, that more than a bushel was collected from the small
openings around the base of some of the trees (see Fig. 457 of this
genus). The exterior trunk of each was marked by a coating of friable
coal, varying from one-quarter to three-quarters of an inch in
thickness; but it crumbled away on removing the matrix. The dimensions
of one of the trees is 15½ feet in circumference at the base, 7½ feet
at the top, its height being eleven feet. All the trees have large
spreading roots, solid and strong, sometimes branching, and traced to a
distance of several feet, and presumed to extend much farther.
In a colliery near Newcastle a great number of _ Sigillariæ_ occur in
the rock as if they had retained the position in which they grew. No
less than thirty, some of them four or five feet in diameter, were
visible within an area of 50 yards square, the interior being
sandstone, and the bark having been converted into coal. Such vertical
stems are familiar to our coal-miners, under the name of coal-pipes.
They are much dreaded, for almost every year in the Bristol, Newcastle,
and other coal-fields, they are the cause of fatal accidents. Each
cylindrical cast of a tree, formed of solid sandstone, and increasing
gradually in size towards the base, and being without branches, has its
whole weight thrown downward, and receives no support from the coating
of friable coal which has replaced the bark. As soon, therefore, as the
cohesion of this external layer is overcome, the heavy column falls
suddenly in a perpendicular or oblique direction from the roof of the
gallery whence coal has been extracted, wounding or killing the workman
who stands below. It is strange to reflect how many thousands of these
trees fell originally in their native forests in obedience to the law
of gravity; and how the few which continued to stand erect, obeying,
after myriads of ages, the same force, are cast down to immolate their
human victims.
It has been remarked that if, instead of working in the dark, the miner
was accustomed to remove the upper covering of rock from each seam of
coal, and to expose to the day the soils on which ancient forests grew,
the evidence of their former growth would be obvious. Thus in South
Staffordshire a seam of coal was laid bare in the year 1844, in what is
called an open work at Parkfield colliery, near Wolverhampton. In the
space of about a quarter of an acre the stumps of no less than 73 trees
with their roots attached appeared, as shown in Fig. 429, some of them
more than eight feet in circumference. The trunks, broken off close to
the root, were lying prostrate in every direction, often crossing each
other. One of them measured 15, another 30 feet in length, and others
less. They were invariably flattened to the thickness of one or two
inches, and converted into coal. Their roots formed part of a stratum
of coal ten inches thick, which rested on a layer of clay two inches
thick, below which was a second forest resting on a two-foot seam of
coal. Five feet below this, again, was a third forest with large stumps
of _Lepidodendra, Calamites,_ and other trees.
Fig. 429: Ground plan of fossil forest, Parkfield Colliery, near
Wolverhampton, showing the position of 73 trees in a quarter of an ace.
Blending of Coal-seams.—Both in England and North America seams of coal
are occasionally observed to be parted from each other by layers of
clay and sand, and, after they have been persistent for miles, to come
together and blend in one single bed, which is then found to be equal
in the aggregate to the thickness of the several seams. I was shown by
Mr. H. D. Rogers a remarkable example of this in Pennsylvania. In the
Shark Mountain, near Pottsville, in that State, there are thirteen
seams of anthracite coal, some of them more than six feet thick,
separated by beds of white quartzose grit and a conglomerate of quartz
pebbles, often of the size of a hen’s egg. Between Pottsville and the
Lehigh Summit Mine, seven of these seams of coal, at first widely
separated, are, in the course of several miles, brought nearer and
nearer together by the gradual thinning out of the intervening
coarse-grained strata and their accompanying shales, until at length
they successively unite and form one mass of coal between forty and
fifty feet thick, very pure on the whole, though with a few thin
partings of clay. This mass of coal I saw quarried in the open air at
Mauch Chunk, on the Bear Mountain. The origin of such a vast thickness
of vegetable remains, so unmixed, on the whole, with earthy
ingredients, can be accounted for in no other way than by the growth,
during thousands of years, of trees and ferns in the manner of peat—a
theory which the presence of the Stigmaria _in situ_ under each of the
seven layers of anthracite fully bears out. The rival hypothesis, of
the drifting of plants into a sea or estuary, leaves the
non-intermixture of sediment, or of clay, sand, and pebbles, with the
pure coal wholly unexplained.
The late Mr. Bowman was the first who gave a satisfactory explanation
of the manner in which distinct coal-seams, after maintaining their
independence for miles, may at length unite, and then persist
throughout another wide area with a thickness equal to that which the
separate seams had previously maintained.
Fig. 430: Uniting of distinct coal-seams.
Let A C (Fig. 430) be a three-foot seam of coal originally laid down as
a mass of vegetable matter on the level area of an extensive swamp,
having an under-clay, _f g,_ through which the Stigmariæ or roots of
the trees penetrate as usual. One portion, B C, of this seam of coal is
now inclined; the area of the swamp having subsided as much as 25 feet
at E C, and become for a time submerged under salt, fresh, or brackish
water. Some of the trees of the original forest A B C fell down, others
continued to stand erect in the new lagoon, their stumps and part of
their trunks becoming gradually enveloped in layers of sand and mud,
which at length filled up the new piece of water C E.
When this lagoon has been entirely silted up and converted into land,
the forest-covered surface A B will extend once more over the whole
area A B E, and a second mass of vegetable matter, D E, forming three
feet more of coal, will accumulate. We then find in the region E C two
seams of coals, each three feet thick, with their respective
under-clays, with erect buried trees based upon the surface of the
lower coal, the two seams being separated by 25 feet of intervening
shale and sandstone. Whereas in the region A B, where the growth of the
forest has never been interrupted by submergence, there will simply be
one seam, two yards thick, corresponding to the united thickness of the
beds B E and B C. It may be objected that the uninterrupted growth of
plants during the interval of time required for the filling up of the
lagoon will have caused the vegetable matter in the region D A B to be
thicker than the two distinct seams E and C, and no doubt there would
actually be a slight excess representing one or more generation of
trees and plants forming the undergrowth; but this excess of vegetable
matter, when compressed into coal, would be so insignificant in
thickness that the miner might still affirm that the seam D A
throughout the area D A B was equal to the two seams C and E.
Cause of the Purity of Coal.—The purity of the coal itself, or the
absence in it of earthy particles and sand, throughout areas of vast
extent, is a fact which appears very difficult to explain when we
attribute each coal-seam to a vegetation growing in swamps. It has been
asked how, during river inundations capable of sweeping away the leaves
of ferns and the stems and roots of _Sigillariæ_ and other trees, could
the waters fail to transport some fine mud into the swamps? One
generation after another of tall trees grew with their roots in mud,
and their leaves and prostrate trunks formed layers of vegetable
matter, which was afterwards covered with mud since turned to shale.
Yet the coal itself, or altered vegetable matter, remained all the
while unsoiled by earthy particles. This enigma, however perplexing at
first sight, may, I think, be solved by attending to what is now taking
place in deltas. The dense growth of reeds and herbage which
encompasses the margins of forest-covered swamps in the valley and
delta of the Mississippi is such that the fluviatile waters, in passing
through them, are filtered and made to clear themselves entirely before
they reach the areas in which vegetable matter may accumulate for
centuries, forming coal if the climate be favourable. There is no
possibility of the least intermixture of earthy matter in such cases.
Thus in the large submerged tract called the “Sunk Country,” near New
Madrid, forming part of the western side of the valley of the
Mississippi, erect trees have been standing ever since the year
1811-12, killed by the great earthquake of that date; lacustrine and
swamp plants have been growing there in the shallows, and several
rivers have annually inundated the whole space, and yet have been
unable to carry in any sediment within the outer boundaries of the
morass, so dense is the marginal belt of reeds and brush-wood. It may
be affirmed that generally, in the “cypress swamps” of the Mississippi,
no sediment mingles with the vegetable matter accumulated there from
the decay of trees and semi-aquatic plants. As a singular proof of this
fact, I may mention that whenever any part of a swamp in Louisiana is
dried up, during an unusually hot season, and the wood set on fire,
pits are burnt into the ground many feet deep, or as far down as the
fire can descend without meeting with water, and it is then found that
scarcely any residuum or earthy matter is left. At the bottom of all
these “cypress swamps” a bed of clay is found, with roots of the tall
cypress (_Taxodium distichum_), just as the under-clays of the coal are
filled with _Stigmaria._
Conversion of Coal into Anthracite.—It appears from the researches of
Liebig and other eminent chemists, that when wood and vegetable matter
are buried in the earth exposed to moisture, and partially or entirely
excluded from the air, they decompose slowly and evolve carbonic acid
gas, thus parting with a portion of their original oxygen. By this
means they become gradually converted into lignite or wood-coal, which
contains a larger proportion of hydrogen than wood does. A continuance
of decomposition changes this lignite into common or bituminous coal,
chiefly by the discharge of carbureted hydrogen, or the gas by which we
illuminate our streets and houses. According to Bischoff, the
inflammable gases which are always escaping from mineral coal, and are
so often the cause of fatal accidents in mines, always contain carbonic
acid, carbureted hydrogen, nitrogen, and olefiant gas. The
disengagement of all these gradually transforms ordinary or bituminous
coal into anthracite, to which the various names of glance-coal, coke,
hard-coal, culm, and many others, have been given.
There is an intimate connection between the extent to which the coal
has in different regions parted with its gaseous contents, and the
amount of disturbance which the strata have undergone. The coincidence
of these phenomena may be attributed partly to the greater facility
afforded for the escape of volatile matter, when the fracturing of the
rocks has produced an infinite number of cracks and crevices. The gases
and water which are made to penetrate these cracks are probably
rendered the more effective as metamorphic agents by increased
temperature derived from the interior. It is well known that, at the
present period, thermal waters and hot vapours burst out from the earth
during earthquakes, and these would not fail to promote the
disengagement of volatile matter from the Carboniferous rocks.
In Pennsylvania the strata of coal are horizontal to the westward of
the Alleghany Mountains, where the late Professor H. D. Rogers pointed
out that they were most bituminous; but as we travel south-eastward,
where they no longer remain level and unbroken, the same seams become
progressively debitumenized in proportion as the rocks become more bent
and distorted. At first, on the Ohio River, the proportion of hydrogen,
oxygen, and other volatile matters ranges from forty to fifty per cent.
Eastward of this line, on the Monongahela, it still approaches forty
per cent, where the strata begin to experience some gentle flexures. On
entering the Alleghany Mountains, where the distinct anticlinal axes
begin to show themselves, but before the dislocations are considerable,
the volatile matter is generally in the proportion of eighteen or
twenty per cent. At length, when we arrive at some insulated
coal-fields associated with the boldest flexures of the Appalachian
chain, where the strata have been actually turned over, as near
Pottsville, we find the coal to contain only from six per cent of
volatile matter, thus becoming a genuine anthracite.
Clay-ironstone.—Bands and nodules of clay-ironstone are common in
coal-measures, and are formed, says Sir H. De la Beche, of carbonate of
iron mingled mechanically with earthy matter, like that constituting
the shales. Mr. Hunt, of the Museum of Practical Geology, instituted a
series of experiments to illustrate the production of this substance,
and found that decomposing vegetable matter, such as would be
distributed through all coal strata, prevented the further oxidation of
the proto-salts of iron, and converted the peroxide into protoxide by
taking a portion of its oxygen to form carbonic acid. Such carbonic
acid, meeting with the protoxide of iron in solution, would unite with
it and form a carbonate of iron; and this mingling with fine mud, when
the excess of carbonic acid was removed, might form beds or nodules of
argillaceous ironstone.[3]
Intercalated Marine Beds in Coal.—Both in the coal-fields of Europe and
America the association of fresh, brackish-water, and marine strata
with coal-seams of terrestrial origin is frequently recognised. Thus,
for example, a deposit near Shrewsbury, probably formed in brackish
water, has been described by Sir R. Murchison as the youngest member of
the coal-measures of that district, at the point where they are in
contact with the overlying Permian group. It consists of shales and
sandstones about 150 feet thick, with coal and traces of plants;
including a bed of limestone varying from two to nine feet in
thickness, which is cellular, and resembles some lacustrine limestones
of France and Germany. It has been traced for 30 miles in a straight
line, and can be recognised at still more distant points. The
characteristic fossils are a small bivalve, having the form of a
_Cyclas_ or _Cyrena,_ also a small entomostracan, _Cythere inflata_
(Fig. 432), and the microscopic shell of an annelid of an extinct genus
called _ Microconchus_ (Fig. 431), allied to _Spirorbis._ In the
coal-field of Yorkshire there are fresh-water strata, some of which
contain shells referred to the family Unionidæ; but in the midst of the
series there is one thin but very widely-spread stratum, abounding in
fishes and marine shells, such as _ Goniatites Listeri_ (Fig. 433),
_Orthoceras,_ and _ Aviculopecten papyraceus,_ Goldf. (Fig. 434).
Fig. 431: Microconchus (Spirorbis) carbonarius. Fig. 432: Cythere
(Leperditia) inflata. Fig. 433: Goniatites Listeri. Fig. 434:
Aviculopecten papyraceus.
Insects in European Coal.—Articulate animals of the genus Scorpion were
found by Count Sternberg in 1835 in the coal-measures of Bohemia, and
about the same time in those of Coalbrook Dale by Mr. Prestwich, were
also true insects, such as beetles of the family _Curculionidæ,_ a
neuropterous insect of the genus _Corydalis,_ and another related to
the _Phasmidæ,_ have been found.
From the coal of Wetting, in Westphalia, several specimens of the
cockroach or _Blatta_ family, and the wing of a cricket (_Acridites_)
have been described by Germar. Professor Goldenberg published, in 1854,
descriptions of no less than twelve species of insects from the nodular
clay-ironstone of Saarbrück, near Trèves.[4] Among them are several _
Blattinæ,_ three species of _Neuroptera,_ one beetle of the _Scarabæus_
family, a grasshopper or locust, _ Gryllacris_ (see Fig. 435), and
several white ants or Termites. Professor Goldenberg showed me, in
1864, the wing of a white ant, found low down in the productive
coal-measures of Saarbrück, in the interior of a flattened
Lepidodendron. It is much larger than that of any known living species
of the same genus.
Fig. 435: Wing of a Grasshopper. Gryllacris lithanthraca.
Fig. 436: Archegosaurus minor. Fossil reptile from the coal-measures,
Saarbrück.
Batrachian Reptiles in Coal.—No vertebrated animals more highly
organised than fish were known in rocks of higher antiquity than the
Permian until the year 1844, when the _Apateon pedestris,_ Meyer, was
discovered in the coal-measures of Munster-Appel in Rhenish Bavaria,
and three years later, in 1847, Professor von Dechen found three other
distinct species of the same family of Amphibia in the Saarbruck
coal-field above alluded to. These were described by the late Professor
Goldfuss under the generic name of _Archegosaurus._ The skulls, teeth,
and the greater portions of the skeleton, nay, even a large part of the
skin, of two of these reptiles have been faithfully preserved in the
centre of spheroidal concretions of clay-ironstone. The largest of
these, _Archegosaurus Decheni,_ must have been three feet six inches
long. Figure 436 represents the skull and neck bones of the smallest of
the three, of the natural size. They were considered by Goldfuss as
saurians, but by Herman von Meyer as most nearly allied to the
_Labyrinthodon_ before mentioned (p. 371), and the remains of the
extremities leave no doubt they were quadrupeds, “provided,” says Von
Meyer, “with hands and feet terminating in distinct toes; but these
limbs were weak, serving only for swimming or creeping.” The same
anatomist has pointed out certain points of analogy between their bones
and those of the _Proteus anguinus_; and Professor Owen has observed
that they make an approach to the _Proteus_ in the shortness of their
ribs. Two specimens of these ancient reptiles retain a large part of
the outer skin, which consisted of long, narrow, wedge-shaped,
tile-like, and horny scales, arranged in rows (see Fig. 437).
Fig. 437: Imbricated covering of skin of Archegosaurus medius.
In 1865, several species belonging to three different genera of the
same family of perennibranchiate Batrachians were found in the
coal-field of Kilkenny in bituminous shale at the junction of the coal
with the underlying Stigmaria-bearing clay. They were, probably,
inhabitants of a marsh, and the large processes projecting from the
vertebræ of their tail imply, according to Professor Huxley, great
powers of swimming. They were of the Labyrinthodont family, and their
association with the fish of the coal, of which so large a proportion
are ganoids, reminds us that the living perennibranchiate amphibia of
America frequent the same rivers as the ganoid Lepidostei or bony
pikes.
_Labyrinthodont footprints in coal-measures._—In 1844, the very year
when the Apateon, before mentioned, of the coal was first met with in
the country between the Moselle and the Rhine, Dr. King published an
account of the footprints of a large reptile discovered by him in North
America. These occur in the coal-strata of Greensburg, in Westmoreland
County, Pennsylvania; and I had an opportunity of examining them when
in that country in 1846. The footmarks were first observed standing out
in relief from the lower surface of slabs of sandstone, resting on thin
layers of fine unctuous clay. I brought away one of these masses, which
is represented in Fig. 438. It displays, together with footprints, the
casts of cracks (_a, a′_) of various sizes. The origin of such cracks
in clay, and casts of the same, has before been explained, and referred
to the drying and shrinking of mud, and the subsequent pouring of sand
into open crevices. It will be seen that some of the cracks, as at _b,
c,_ traverse the footprints, and produce distortion in them, as might
have been expected, for the mud must have been soft when the animal
walked over it and left the impressions; whereas, when it afterwards
dried up and shrank, it would be too hard to receive such indentations.
Fig. 438: Slab of sandstone from the coal-measures of Pennsylvania,
with foot-prints of air-breathing reptile and casts of cracks.
We may assume that the reptile which left these prints on the ancient
sands of the coal-measures was an air-breather, because its weight
would not have been sufficient under water to have made impressions so
deep and distinct. The same conclusion is also borne out by the casts
of the cracks above described, for they show that the clay had been
exposed to the air and sun, so as to have dried and shrunk.
Nova Scotia Coal-measures.—The sedimentary strata in which thin seams
of coal occur attain a thickness, as we have seen, of 18,000 feet in
the north of England exclusive of the Mountain Limestone, and are
estimated by Von Dechen at over 20,000 feet in Rhenish Prussia. But the
finest example in the world of a natural exposure in a continuous
section ten miles long, occurs in the sea-cliffs bordering a branch of
the Bay of Fundy, in Nova Scotia. These cliffs, called the “South
Joggins,” which I first examined in 1842, and afterwards with Dr.
Dawson in 1845, have lately been admirably described by the
last-mentioned geologist[5] in detail, and his evidence is most
valuable as showing how large a portion of this dense mass was formed
on land, or in swamps where terrestrial vegetation flourished, or in
fresh-water lagoons. His computation of the thickness of the whole
series of carboniferous strata as exceeding three miles, agrees with
the measurement made independently by Sir William Logan in his survey
of this coast.
There is no reason to believe that in this vast succession of strata,
comprising some marine as well as many fresh-water and terrestrial
formations, there is any repetition of the same beds. There are no
faults to mislead the geologist, and cause him to count the same beds
over more than once, while some of the same plants have been traced
from the top to the bottom of the whole series, and are distinct from
the flora of the antecedent Devonian formation of Canada. Eighty-one
seams of coal, varying in thickness from an inch to about five feet,
have been discovered, and no less than seventy-one of these have been
actually exposed in the sea-cliffs.
In the section (Fig. 439), which I examined in 1842, the beds from _c_
to _i_ are seen all dipping the same way, their average inclination
being at an angle of 24° S.S.W. The vertical height of the cliffs is
from 150 to 200 feet; and between _d_ and _g_—in which space I observed
seventeen trees in an upright position, or, to speak more correctly, at
right angles to the planes of stratification—I counted nineteen seams
of coal, varying in thickness from two inches to four feet. At low tide
a fine horizontal section of the same beds is exposed to view on the
beach, which at low tide extends sometimes 200 yards from the base of
the cliff. The thickness of the beds alluded to, between _d_ and _g,_
is about 2500 feet, the erect trees consisting chiefly of large
_Sigillariæ,_ occurring at ten distinct levels, one above the other.
The usual height of the buried trees seen by me was from six to eight
feet; but one trunk was about 25 feet high and four feet in diameter,
with a considerable bulge at the base. In no instance could I detect
any trunk intersecting a layer of coal, however thin; and most of the
trees terminated downward in seams of coal. Some few only were based on
clay and shale; none of them, except _ Calamites,_ on sandstone. The
erect trees, therefore, appeared in general to have grown on beds of
vegetable matter. In the underclays _Stigmaria_ abounds.
Fig. 439: Section of the cliffs of the South Joggins, near Minudie,
Nova Scotia.
These root-bearing beds have been found under all the coal-seams, and
such old soils are at present the most destructible masses in the whole
cliff, the sandstones and laminated shales being harder and more
capable of resisting the action of the waves and the weather.
Originally the reverse was doubtless true, for in the existing delta of
the Mississippi those clays in which the innumerable roots of the
deciduous cypress and other swamp trees ramify in all directions are
seen to withstand far more effectually the undermining power of the
river, or of the sea at the base of the delta, than do beds of loose
sand or layers of mud not supporting trees. It is obvious that if this
sand or mud be afterwards consolidated and turned to sandstone and hard
shale, it would be the least destructible.
In regard to the plants, they belonged to the same genera, and most of
them to the same species, as those met with in the distant coal-fields
of Europe. Dr. Dawson has enumerated more than 150 species, two-thirds
of which are European, a greater agreement than can be said to exist
between the same Nova Scotia flora and that of the coal-fields of the
United States. By referring to the section, Fig. 439, the position of
the four-foot coal will be perceived, and in Fig. 440 (a section made
by me in 1842 of a small portion) that from _e_ to _f_ of the same
cliff is exhibited, in order to show the manner of occurrence of erect
fossil trees at right angles to the planes of the inclined strata.
Fig. 440: Erect fossil trees, Coal-measures, Nova Scotia.
In the sandstone which filled their interiors, I frequently observed
fern-leaves, and sometimes fragments of _Stigmaria,_ which had
evidently entered together with sediment after the trunk had decayed
and become hollow, and while it was still standing under water. Thus
the tree, _a,_ Fig. 440, represented in the bed _e_ in the section,
Fig. 439, is a hollow trunk five feet eight inches in length,
traversing various strata, and cut off at the top by a layer of clay
two feet thick, on which rests a seam of coal (_b,_ Fig. 440) one foot
thick. On this coal again stood two large trees (_c_ and _d_), while at
a greater height the trees _f_ and _g_ rest upon a thin seam of coal
(_e_), and above them is an underclay, supporting the four-foot coal.
Occasionally the layers of matter in the inside of the tree are more
numerous than those without; but it is more common in the coal-measures
of all countries to find a cylinder of pure sandstone—the cast of the
interior of a tree—intersecting a great many alternating beds of shale
and sandstone, which originally enveloped the trunk as it stood erect
in the water. Such a want of correspondence in the materials outside
and inside, is just what we might expect if we reflect on the
difference of time at which the deposition of sediment will take place
in the two cases; the imbedding of the tree having gone on for many
years before its decay had made much progress. In many places distinct
proof is seen that the enveloping strata took years to accumulate, for
some of the sandstones surrounding erect sigillarian trunks support at
different levels roots and stems of _Calamites_; the _Calamites_ having
begun to grow after the older _Sigillariæ_ had been partially buried.
The general absence of structure in the interior of the large fossil
trees of the Coal implies the very durable nature of their bark, as
compared with their woody portion. The same difference of durability of
bark and wood exists in modern trees, and was first pointed out to me
by Dr. Dawson, in the forests of Nova Scotia, where the Canoe Birch
(_Betula papyracea_) has such tough bark that it may sometimes be seen
in the swamps looking externally sound and fresh, although consisting
simply of a hollow cylinder with all the wood decayed and gone. When
portions of such trunks have become submerged in the swamps they are
sometimes found filled with mud. One of the erect fossil trees of the
South Joggins fifteen feet in height, occurring at a higher level than
the main coal, has been shown by Dr. Dawson to have a coniferous
structure, so that some _Coniferæ_ of the Coal period grew in the same
swamps as _Sigillariæ,_ just as now the deciduous Cypress (_Taxodium
distichum_) abounds in the marshes of Louisiana even to the edge of the
sea.
When the carboniferous forests sank below high-water mark, a species of
_Spirorbis_ or _Serpula_ (Fig. 431), attached itself to the outside of
the stumps and stems of the erect trees, adhering occasionally even to
the interior of the bark—another proof that the process of envelopment
was very gradual. These hollow upright trees, covered with innumerable
marine annelids, reminded me of a “cane-brake,” as it is commonly
called, consisting of tall reeds, _Arundinaria macrosperma,_ which I
saw in 1846, at the Balize, or extremity of the delta of the
Mississippi. Although these reeds are fresh-water plants, they were
covered with barnacles, having been killed by an incursion of
salt-water over an extent of many acres, where the sea had for a season
usurped a space previously gained from it by the river. Yet the dead
reeds, in spite of this change, remained standing in the soft mud,
enabling us to conceive how easily the larger _ Sigillariæ,_ hollow as
they were but supported by strong roots, may have resisted an incursion
of the sea.
The high tides of the Bay of Fundy, rising more than 60 feet, are so
destructive as to undermine and sweep away continually the whole face
of the cliffs, and thus a new crop of erect fossil trees is brought
into view every three or four years. They are known to extend over a
space between two and three miles from north to south, and more than
twice that distance from east to west, being seen in the banks of
streams intersecting the coal-field.
_Structure of Coal._—The bituminous coal of Nova Scotia is similar in
composition and structure to that of Great Britain, being chiefly
derived from sigillarioid trees mixed with leaves of ferns and of a
Lycopodiaceous tree called _ Cordaites_ (_Noeggerathia,_ etc., for
genus, see Fig. 428), supposed by Dawson to have been deciduous, and
which had broad parallel veined leaves without a mid-rib. On the
surface of the seams of coal are large quantities of mineral charcoal,
which doubtless consist, as Dr. Dawson suggests, of fragments of wood
which decayed in the open air, as would naturally be expected in swamps
where so many erect trees were preserved. Beds of cannel-coal display,
says Dr. Dawson, such a microscopical structure and chemical
composition as shows them to have been of the nature of fine vegetable
mud such as accumulates in the shallow ponds of modern swamps. The
underclays are loamy soils, which must have been sufficiently above
water to admit of drainage, and the absence of sulphurets, and the
occurrence of carbonate of iron in them, prove that when they existed
as soils, rain-water, and not sea-water, percolated them. With the
exception, perhaps, of _Asterophyllites_ (see Fig. 461), there is a
remarkable absence from the coal-measures of any form of vegetation
properly aquatic, the true coal being a sub-aërial accumulation in soil
that was wet and swampy but not permanently submerged.
Air-breathers of the Coal.—If we have rightly interpreted the evidence
of the former existence at more than eighty different levels of forests
of trees, some of them of vast extent, and which lasted for ages,
giving rise to a great accumulation of vegetable matter, it is natural
to ask whether there were not many air-breathing inhabitants of these
same regions. As yet no remains of mammalia or birds have been found, a
negative character common at present to all the Palæozoic formations;
but in 1852 the osseous remains of a reptile, the first ever met with
in the carboniferous strata of the American continent, were found by
Dr. Dawson and myself. We detected them in the interior of one of the
erect Sigillariæ before alluded to as of such frequent occurrence in
Nova Scotia. The tree was about two feet in diameter, and consisted of
an external cylinder of bark, converted into coal, and an internal
stony axis of black sandstone, or rather mud and sand stained black by
carbonaceous matter, and cemented together with fragments of wood into
a rock. These fragments were in the state of charcoal, and seem to have
fallen to the bottom of the hollow tree while it was rotting away. The
skull, jaws, and vertebræ of a reptile, probably about 2½ feet in
length (_Dendrerpeton Acadianum,_ Owen), were scattered through this
stony matrix. The shell, also, of a _ Pupa_ (see Fig. 442), the first
land-shell ever met with in the coal or in beds older than the
tertiary, was observed in the same stony mass. Dr. Wyman of Boston
pronounced the reptile to be allied in structure to _Menobranchus_ and
_Menopoma,_ species of batrachians, now inhabiting the North American
rivers. The same view was afterwards confirmed by Professor Owen, who
also pointed out the resemblance of the cranial plates to those seen in
the skull of _Archegosaurus_ and _Labyrinthodon._[6] Whether the
creature had crept into the hollow tree while its top was still open to
the air, or whether it was washed in with mud during a flood, or in
whatever other manner it entered, must be matter of conjecture.
Footprints of two reptiles of different sizes had previously been
observed by Dr. Harding and Dr. Gesner on ripple-marked flags of the
lower coal-measures in Nova Scotia (No. 2, Fig. 447), evidently made by
quadrupeds walking on the ancient beach, or out of the water, just as
the recent Menopoma is sometimes observed to do. The remains of a
second and smaller species of Dendrerpeton, _D. Oweni,_ were also found
accompanying the larger one, and still retaining some of its dermal
appendages; and in the same tree were the bones of a third small
lizard-like reptile, _Hylonomus Lyelli,_ seven inches long, with stout
hind limbs, and fore limbs comparatively slender, supposed by Dr.
Dawson to be capable of walking and running on land.[7]
Fig. 441: Xylobius Sigillariæ. Coal, Nova Scotia.
In a second specimen of an erect stump of a hollow tree 15 inches in
diameter, the ribbed bark of which showed that it was a Sigillaria, and
which belonged to the same forest as the specimen examined by us in
1852, Dr. Dawson obtained not only fifty specimens of Pupa vetusta
(Fig. 442), and nine skeletons of reptiles belonging to four species,
but also several examples of an articulated animal resembling the
recent centipede or gally-worm, a creature which feeds on decayed
vegetable matter (see Fig. 441). Under the microscope, the head, with
the eyes, mandible, and labrum, are well seen. It is interesting, as
being the earliest known representative of the myriapods, none of which
had previously been met with in rocks older than the oolite or
lithographic slate of Germany.
Fig. 442: Pupa vetusta.
Some years after the discovery of the first Pupa, Dr. Dawson, carefully
examining the same great section containing so many buried forests in
the cliffs of Nova Scotia, discovered another bed, separated from the
tree containing Dendrerpeton by a mass of strata more than 1200 feet
thick. As there were 21 seams of coal in this intervening mass, the
length of time comprised in the interval is not to be measured by the
mere thickness of the sandstones and shales. This lower bed is an
underclay seven feet thick, with stigmarian rootlets, and the small
land-shells occurring in it are in all stages of growth. They are
chiefly confined to a layer about two inches thick, and are unmixed
with any aquatic shells. They were all originally entire when imbedded,
but are most of them now crushed, flattened, and distorted by pressure;
they must have been accumulated, says Dr. Dawson, in mud deposited in a
pond or creek.
Fig. 443: Zonites (Conulus) priseus.
The surface striæ of _Pupa vetusta,_ when magnified 50 diameters,
present exactly the same appearance as a portion corresponding in size
of the common English _Pupa juniperi,_ and the internal hexagonal
cells, magnified 500 diameters, show the internal structure of the
fossil and recent Pupa to be identical. In 1866[8] Dr. Dawson
discovered in this lower bed, so full of the Pupa, another land-shell
of the genus Helix (sub-genus Zonites), see Fig. 443.
None of the reptiles obtained from the coal-measures of the South
Joggins are of a higher grade than the Labyrinthodonts, but some of
these were of very great size, two caudal vertebræ found by Mr. Marsh
in 1862 measuring two and a half inches in diameter, and implying a
gigantic aquatic reptile with a powerful swimming tail.
Except some obscure traces of an insect found by Dr. Dawson in a
coprolite of a terrestrial reptile occurring in a fossil tree, no
specimen of this class has been brought to light in the Joggins. But
Mr. James Barnes found in a bed of shale at Little Grace Bay, Cape
Breton, the wing of an Ephemera, which must have measured seven inches
from tip to tip of the expanded wings—larger than any known living
insect of the Neuropterous family.
That we should have made so little progress in obtaining a knowledge of
the terrestrial fauna of the Coal is certainly a mystery, but we have
no reason to wonder at the extreme rarity of insects, seeing how few
are known in the carboniferous rocks of Europe, worked for centuries
before America was discovered, and now quarried on so enormous a scale.
These European rocks have not yet produced a single land-shell, in
spite of the millions of tons of coal annually extracted, and the many
hundreds of soils replete with the fossil roots of trees, and the erect
trunks and stumps preserved in the position in which they grew. In many
large coal-fields we continue as much in the dark respecting the
invertebrate air-breathers then living, as if the coal had been thrown
down in mid-ocean. The early date of the carboniferous strata cannot
explain the enigma, because we know that while the land supported a
luxuriant vegetation, the contemporaneous seas swarmed with life—with
Articulata, Mollusca, Radiata, and Fishes. The perplexity in which we
are involved when we attempt to solve this problem may be owing partly
to our want of diligence as collectors, but still more perhaps to
ignorance of the laws which govern the fossilisation of land-animals,
whether of high or low degree.
Carboniferous Rain-prints.—At various levels in the coal measures of
Nova Scotia, ripple-marked sandstones, and shales with rain-prints,
were seen by Dr. Dawson and myself, but still more perfect impressions
of rain were discovered by Mr. Brown, near Sydney, in the adjoining
island of cape Breton. They consist of very delicate markings on
greenish slates, accompanied by worm-tracks (_a, b,_ Fig. 444), such as
are often seen between high and low water mark on the recent mud of the
Bay of Fundy.
The great humidity of the climate of the Coal period had been
previously inferred from the number of its ferns and the continuity of
its forests for hundreds of miles; but it is satisfactory to have at
length obtained such positive proofs of showers of rain, the drops of
which resembled in their average size those which now fall from the
clouds. From such data we may presume that the atmosphere of the
Carboniferous period corresponded in density with that now investing
the globe, and that different currents of air varied then as now in
temperature, so as to give rise, by their mixture, to the condensation
of aqueous vapour.
Fig. 444: Carboniferous rain-prints with worm tracks on green shale,
from Cape Breton, Nova Scotia. Fig. 445: Casts of rain-prints on a
portion of the same slab (Fig. 444), seen to project on the underside
of an incumbent layer of arenaceous shale.
Folding and Denudation of the Beds indicated by the Nova Scotia
Coal-strata.—The series of events which are indicated by the great
section of the coal-strata in Nova Scotia consist of a gradual and
long-continued subsidence of a tract which throughout most of the
period was in the state of a delta, though occasionally submerged
beneath a sea of moderate depth. Deposits of mud and sand were first
carried down into a shallow sea on the low shores of which the
footprints of reptiles were sometimes impressed (see p. 407).
Fig. 446: Cone and branch of Lepidodendron corrugatum.
Though no regular seams of coal were formed, the characteristic
imbedded coal-plants are of the genera _Cyclopteris_ and _
Alethopteris,_ agreeing with species occurring at much higher levels,
and distinct from those of the antecedent Devonian group. The
_Lepidodendron corrugatum_ (see Fig. 446), a plant predominating in the
Lower Carboniferous group of Europe, is also conspicuous in these
shallow-water beds, together with many fishes and entomostracans. A
more rapid rate of subsidence sometimes converted part of the sea into
deep clear water, in which there was a growth of coral which was
afterwards turned into crystalline limestone, and parts of it,
apparently by the action of sulphuric acid, into gypsum. In spite of
continued sinking, amounting to several thousand feet, the sea might in
time have been rendered shallow by the growth of coral, had not its
conversion into land or swampy ground been accelerated by the pouring
in of sand and the advance of the delta accompanied with such
fluviatile and brackish-water formations as are common in lagoons.
The amount to which the bed of the sea sank down in order to allow of
the formation of so vast a thickness of rock of sedimentary and organic
origin is expressed by the total thickness of the Carboniferous strata,
including the coal-measures, No. 1, and the rocks which underlie them,
No. 2, Fig. 447.
Fig. 447: Diagram showing the curvature and supposed denudation of the
Carboniferous strata in Nova Scotia.
After the strata No. 2 had been elaborated, the conditions proper to a
great delta exclusively prevailed, the subsidence still continuing so
that one forest after another grew and was submerged until their
under-clays with roots, and usually seams of coal, were left at more
than eighty distinct levels. Here and there, also, deposits bearing
testimony to the existence of fresh or brackish-water lagoons, filled
with calcareo-bituminous mud, were formed. In these beds (_h_ and _i,_
Fig. 439) are found fresh-water bivalves or mussels allied to Anodon,
though not identical with that or any living genus, and called
_Naiadites carbonarius_ by Dawson. They are associated with small
entomostracous crustaceans of the genus Cythere, and scales of small
fishes. Occasionally some of the calamite brakes and forests of
Sigillariæ and Coniferæ were exposed in the flood season, or sometimes,
perhaps, by slight elevatory movements to the denuding action of the
river or the sea.
In order to interpret the great coast section exposed to view on the
shores of the Bay of Fundy, the student must, in the first place,
understand that the newest or last-mentioned coal formations would have
been the only ones known to us (for they would have covered all the
others), had there not been two great movements in opposite directions,
the first consisting of a general sinking of three miles, which took
place during the Carboniferous Period, and the second an upheaval of
more limited horizontal extent, by which the anticlinal axis A was
formed. That the first great change of level was one of subsidence is
proved by the fact that there are shallow-water deposits at the base of
the Carboniferous series, or in the lowest beds of No. 2.
Subsequent movements produced in the Nova Scotia and the adjoining New
Brunswick coal-fields the usual anticlinal and synclinal flexures. In
order to follow these, we must survey the country for about thirty
miles round the South Joggins, or the region where the erect trees
described in the foregoing pages are seen. As we pass along the cliffs
for miles in a southerly direction, the beds containing these fossil
trees, which were mentioned as dipping about 18° south, are less and
less inclined, until they become nearly horizontal in the valley of a
small river called the Shoulie, as ascertained by Dr. Dawson. After
passing this synclinal line the beds begin to dip in an opposite or
north-easterly direction, acquiring a steep dip where they rest
unconformably on the edges of the Upper Silurian strata of the Cobequid
Hills, as shown in Fig. 447. But if we travel northward towards Minudie
from the region of the coal-seams and buried forests, we find the dip
of the coal-strata increasing from an angle of 18° to one of more than
40°, lower beds being continually exposed to view until we reach the
anticlinal axis A and see the lower Carboniferous formation, No. 2, at
the surface. The missing rocks removed by denudation are expressed by
the faint lines at A, and thus the student will see that, according to
the principles laid down in the seventh chapter, we are enabled, by the
joint operations of upheaval and denudation, to look, as it were, about
three miles into the interior of the earth without passing beyond the
limits of a single formation.
[1] Edward Hull, Quart. Geol. Journ., vol. xxiv, p. 327.
[2] Quart. Geol. Journ., vol. v, Mem., p. 17.
[3] Memoirs of the Geol. Survey, pp. 51, 255, etc.
[4] Dunker and V. Meyer, Palæont., vol. iv, p. 17.
[5] Acadian Geology, 2nd edit., 1868.
[6] Quart. Geol. Journ., vol. ix, p. 58.
[7] Dawson, Air-Breathers of the Coal in Nova Scotia, Montreal, 1863.
[8] Dawson, Acadian Geology, 1868, p. 385.
CHAPTER XXIV.
FLORA AND FAUNA OF THE CARBONIFEROUS PERIOD.
Vegetation of the Coal Period. — Ferns, Lycopodiaceæ, Equisetaceæ,
Sigillariæ, Stigmariæ, Coniferæ. — Angiosperms. — Climate of the Coal
Period. — Mountain Limestone. — Marine Fauna of the Carboniferous
Period. — Corals. — Bryozoa, Crinoidea. — Mollusca. — Great Number of
fossil Fish. — Foraminifera.
Vegetation of the Coal Period.—In the last chapter we have seen that
the seams of coal, whether bituminous or anthracitic, are derived from
the same species of plants, and Goppert has ascertained that the
remains of every family of plants scattered through the shales and
sandstones of the coal-measures are sometimes met with in the pure coal
itself—a fact which adds greatly to the geological interest of this
flora.
The coal-period was called by Adolphe Brongniart the age of
Acrogens,[1] so great appears to have been the numerical preponderance
of flowerless or cryptogamic plants of the families of ferns,
club-mosses, and horse-tails. He reckoned the known species in 1849 at
500, and the number has been largely increased by recent research in
spite of reductions owing to the discovery that different parts of even
the same plants had been taken for distinct species. Notwithstanding
these changes, Brongniart’s generalisation concerning this flora still
holds true, namely, that the state of the vegetable world was then
extremely different from that now prevailing, not only because the
cryptogamous plants constituted nearly the whole flora, but also
because they were, on the whole, more highly developed than any
belonging to the same class now existing, and united some forms of
structure now only found separately and in distinct orders. The only
phænogamous plants were constitute any feature in the coal are the
coniferæ; monocotyledonous angiosperms appear to have been very rare,
and the dicotyledonous, with one or two doubtful exceptions, were
wanting. For this we are in some measure prepared by what we have seen
of the Secondary or Mesozoic floras if, consistently with the belief in
the theory of evolution, we expect to find the prevalence of simpler
and less specialised organisms in older rocks.
Ferns.—We are struck at the first glance with the similarity of the
ferns to those now living. In the fossil genus _Pecopteris,_ for
example (Fig. 448), it is not easy to decide whether the fossils might
not be referred to the same genera as those established for living
ferns; whereas, in regard to some of the other contemporary families of
plants, with the exception of the fir tribe, it is not easy to guess
even the class to which they belong. The ferns of the Carboniferous
period are generally without organs of fructification, but in the few
instances in which these do occur in a fit state for microscopical
investigations they agree with those of the living ferns.
Fig. 448: Pecopteris elliptica. Fig. 449: Caulopteris primæva. Fig.
448: _Pecopteris elliptica_, Bunbury.[2] Frostburg.
Fig. 449: _Caulopteris primæva_, Lindley.
When collecting fossil specimens from the coal-measures of Frostburg,
in Maryland, I found in the iron-shales several species with
well-preserved rounded spots or marks of the sori (see Fig. 448). In
the general absence of such characters they have been divided into
genera distinguished chiefly by the branching of the fronds and the way
in which the veins of the leaves are disposed. The larger portion are
supposed to have been of the size of ordinary European ferns, but some
were decidedly arborescent, especially the group called _Caulopteris_
(see Fig. 449) by Lindley, and the _Psaronius_ of the upper or newest
coal-measures, before alluded to (p. 393). All the recent tree-ferns
belong to one tribe (_Polypodiaceæ_), and to a small number only of
genera in that tribe, in which the surface of the trunk is marked with
scars, or cicatrices, left after the fall of the fronds. These scars
resemble those of _Caulopteris._
No less than 130 species of ferns are enumerated as having been
obtained from the British coal-strata, and this number is more than
doubled if we include the Continental and American species. Even if we
make some reduction on the ground of varieties which have been
mistaken, in the absence of their fructification, for species, still
the result is singular, because the whole of Europe affords at present
no more than sixty-seven indigenous species.
Living tree-ferns of different genera. Fig. 450: Tree-fern from Isle of
Bourbon. Fig. 451: Cyathea glauca, Mauritius. Fig. 452: Tree-fern from
Brazil.
Lycopodiaceæ—_Lepidodendron._—About forty species of fossil plants of
the Coal have been referred to this genus, more than half of which are
found in the British coal-measures. They consist of cylindrical stems
or trunks, covered with leaf-scars. In their mode of branching, they
are always dichotomous (see Fig. 454). They belong to the
_Lycopodiaceæ,_ bearing sporangia and spores similar to those of the
living representatives of this family (Fig. 457); and although most of
the Carboniferous species grew to the size of large trees, Mr.
Carruthers has found by careful measurement that the volume of the
fossil spores did not exceed that of the recent club-moss, a fact of
some geological importance, as it may help to explain the facility with
which these seeds may have been transported by the wind, causing the
same wide distribution of the species of the fossil forests in Europe
and America which we now observe in the geographical distribution of so
many living families of cryptogamous plants.
Lepidodendrum Sternbergii. Coal-measures, near Newcastle. Fig. 453:
Branching trunk, 49 feet long, supposed to have belonged to L.
Sternbergii. Fig. 454: Branching stem with bark and leaves of L.
Sternbergii. Fig. 455: Portion of same, nearer the root.
Fig. 456: a. Lycopodium densum. Living species, New Zealand; b. Branch;
c. Part of same, magnified.
The Figs. 453–455 represent a fossil _Lepidodendron,_ 49 feet long,
found in Jarrow Colliery, near Newcastle, lying in shale parallel to
the planes of stratification. Fragments of others, found in the same
shale, indicate, by the size of the rhomboidal scars which cover them,
a still greater magnitude.