The Student's Elements of Geology
by
Sir Charles Lyell
Part 8 out of 14
COLUMN 2: FRENCH.
COLUMN 3: ENGLISH.
Keuper: Marnes irisees: Saliferous and gypseous shales and sandstone.
Muschelkalk: Muschelkalk, ou calcaire coquillier: Wanting in England.
Bunter-sandstein: Gres bigarre: Sandstone and quartzose conglomerate.
KEUPER.
(FIGURE 397. Equisetites columnaris. (Syn. Equisetum columnare.) Fragment of
stem, and a small portion of same magnified. Keuper.)
The first of these, or the Keuper, underlying the beds before described as
Rhaetic, attains in Wurtemberg a thickness of about 1000 feet. It is divided by
Alberti into sandstone, gypsum, and carbonaceous clay-slate. (Monog. des Bunter-
Sandsteins.) 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 (Figures 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.
ST. CASSIAN AND HALLSTADT BEDS (SEE MAP, FIGURE 398).
(FIGURE 398. Map of Tyrol and Styria showing St. Cassian and Hallstadt Beds.)
The sandstones and clay of the Keuper resemble the deposits of estuaries and a
shallow sea near the land, and afford, in the north-west 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.
(FIGURE 399. Scoliostoma, St. Cassian.)
(FIGURE 400. Platystoma Suessii, Hornes. From Hallstadt.)
(FIGURE 401. Koninckia Leonhardi, Wissmann.
a. Ventral view. Part of ventral valve removed to show the vascular impressions
of dorsal valve.
b. Interior of dorsal valve, showing spiral processes restored.
c. Vertical section of both valves. Part shaded black showing place occupied by
the animal, and the dorsal valve following the curve of the ventral.)
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, Figure 399, and Platystoma, Figure 400, among
the Gasteropoda; and Koninckia, Figure 401, among the Brachiopoda.
TABLE 21.1 GENERA OF FOSSIL MOLLUSCA IN THE ST. CASSIAN AND HALLSTADT BEDS.
COLUMN 1: COMMON TO OLDER ROCKS.
Orthoceras.
Bactrites.
Macrocheilus.
Loxonema.
Holopella.
Murchisonia.
Porcellia.
Athyris.
Retzia.
Cyrtina.
Euomphalus.
COLUMN 2: CHARACTERISTIC TRIASSIC GENERA.
Ceratites.
Cochloceras.
Choristoceras.
Rhabdoceras.
Aulacoceras.
Scoliostoma (reaches its maximum in the Trias, but passes down to older rocks).
Naticella.
Platystoma.
Ptychostoma.
Euchrysalis.
Halobia.
Hornesia.
Amphiclina.
Koninckia.
Cassianella. (Reach their maximum in the Trias, but pass up to newer rocks.)
Myophoria. (Reach their maximum in the Trias, but pass up to newer rocks.)
COLUMN 3: COMMON TO NEWER ROCKS.
Ammonites.
Chemnitzia.
Cerithium.
Monodonta.
Opis.
Sphoera.
Cardita.
Myoconcha.
Hinnites.
Monotis.
Plicatula.
Pachyrisma.
Thecidium.
Table 21.1 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 Palaeozoic and Mesozoic rocks are
supplied by the St. Cassian and Hallstadt beds.
The first column marks the last appearance of several genera which are
characteristic of Palaeozoic 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
Palaeozoic 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 Palaeozoic
affinities) is made out, and doubt whether they have yet been clearly proved to
be newer than the Muschelkalk.
MUSCHELKALK.
(FIGURE 402. Ceratites nodosus, Schloth. Muschelkalk, Germany. Side and front
view, showing the denticulated outline of the septa dividing the chambers.)
(FIGURE 403. Gervilia (Avicula) socialis, Schloth. Characteristic shell of the
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 (Figure 402) terminate in a
few small denticulations pointing inward. Among the bivalve crustacea, the
Estheria minuta, Bronn (see Figure 390), is abundant, ranging through the
Keuper, Muschelkalk, and Bunter-sandstein; and Gervillia socialis (Figure 403),
having a similar range, is found in great numbers in the Muschelkalk of Germany,
France, and Poland.
(FIGURE 404. Encrinus liliiformis, Schlott. Syn. E. moniliformis. Body, arms,
and part of stem.
a. Section of stem. Muschelkalk.)
(FIGURE 405. Aspidura loricata, Agassiz.
a. Upper side.
b. Lower side. Muschelkalk.)
(FIGURE 406. Palatal teeth of Placodus gigas. Muschelkalk.)
The abundance of the heads and stems of lily encrinites, Encrinus liliiformis
(Figure 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 (Figure 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 Figure 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.
BUNTER-SANDSTEIN.
(FIGURE 407. Voltzia heterophylla. (Syn. Voltzia brevifolia.)
b. Portion of same magnified to show fructification. Sulzbad. 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 Figure 407.) Out of thirty species of ferns, cycads,
conifers, and other plants, enumerated by M. Ad. Brongniart, in 1849, as coming
from the "Gres bigarre," 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.
(FIGURE 408. Footprints of a bird, Turner's Falls, Valley of the Connecticut.)
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.
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. (Hitchcock Mem. of the American
Academy New Series volume 3 page 129 1848.)
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 Figure 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 can not 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 palaeozoic 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.
(FIGURE 409. Triassic coal-shale, Richmond, Virginia.
a. Estheria ovata.
b. Young of same.
c. Natural size of a.
d. Natural size of b.)
The fossil fish are Ganoids, some of them of the genus Catopterus, others
belonging to the liassic genus Tetragonolepis (Aechmodus), see Figure 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 Figure 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 palaeozoic 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 Chapter 21; 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 degrees N., to that of North Carolina, 35
degrees 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.
PRIMARY OR PALAEOZOIC SERIES.
CHAPTER XXII.
PERMIAN OR MAGNESIAN LIMESTONE GROUP.
Line of Separation between Mesozoic and Palaeozoic 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 Palaeozoic 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
palaeontological 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 Palaeozoic 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 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. (Transactions of the Geological
Society London Second Series volume 3 page 37.) 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:
TABLE 22.1. THICKNESS OF PERMIAN STRATA IN NORTH OF ENGLAND.
COLUMN 1: NAME OF STRATA.
COLUMN 2: THICKNESS IN FEET IN N.W. OF ENGLAND.
COLUMN 3: THICKNESS IN FEET IN N.E. OF ENGLAND.
Upper Permian (Sedimentary): 600 : 50-100.
Middle Permian (Calcareous): 10-30 : 600.
Lower Permian (Sedimentary): 3000 : 100-250. (Edward Hull Ternary
Classification Quarterly Journal of Science No. 23 1869.)
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.
(FIGURE 410. Schizodus Schlotheimi, Geinitz. Permian crystalline limestone.)
(FIGURE 411. The hinge of Schizodus truncatus, King. Permian.)
(FIGURE 412. Mytilus septifer, King. Syn. Modiola acuminata, Sowerby. Permian
crystalline limestone.)
This formation is seen upon the coast of Durham and Yorkshire, between the Wear
and the Tees. Among its characteristic fossils are Schizodus Schlotheimi (Figure
410) and Mytilus septifer (Figure 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.
(FIGURE 413. Magnesian Limestone, Humbleton Hill, near Sunderland. (King's
Monograph Plate 2.)
a. Fenestella retiformis, Schlot, sp. Syn. Gorgonia infundibuliformis, Goldf.;
Retepora flustracea, Phillips.
b. Part of the same highly magnified.)
The magnesian limestone sometimes becomes very fossiliferous and includes in it
delicate bryozoa, one of which, Fenestella retiformis (Figure 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.
(FIGURE 414. Productus horridus, Sowerby. (P. calvus, Sowerby) Sunderland and
Durham, in Magnesian Limestone; Zechstein and Kupferschiefer, Germany.)
(FIGURE 415. Lingula Crednerii. (Geinitz.) Magnesian Limestone, and
Carboniferous Marl-slate, Durham; Zechstein, Thuringia.)
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 Palaeozoic, and the brachiopods are the only
group which have furnished species common to the more ancient or Carboniferous
rocks. Of these Lingula Crednerii (Figure 415) is an example. There are 25
Gasteropods and only one cephalopod, Nautilus Freieslebeni, which is also found
in the German Zechstein.
(FIGURE 416. Spirifera alata, Schloth. Syn. Trigonotreta undulata, Sowerby.,
King's Monograph. Magnesian Limestone.)
Shells of the genera Productus (Figure 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 (Figure 416), Lingula Crednerii (Figure 415), and
other brachiopoda of the true primary or palaeozoic 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 (Figure 410), and other
lamellibranchiate bivalves, are abundant, but spiral univalves are very rare.
(FIGURE 417. Restored outline of a fish of the genus Palaeoniscus, Agassiz.
Palaeothrissum, Blainville.)
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 Palaeoniscus 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.
(FIGURE 418. Shark. Heterocercal.)
(FIGURE 419. Shad. (Clupea. Herring tribe. Homocercal.)
The Palaeoniscus 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 Figure 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 Figure 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.
(FIGURES 420-425. SCALES OF FISH. MAGNESIAN LIMESTONE.
(FIGURE 420. Palaeoniscus comptus, Agassiz. Scale, magnified. Marl-slate.)
(FIGURE 421. Palaeoniscus elegans, Sedgwick. Under surface of scale, magnified.
Marl-slate.)
(FIGURE 422. Palaeoniscus glaphyrus, Agassiz. Under surface of scale, magnified.
Marl-slate.)
(FIGURE 423. Coelacanthus granulatus, Agassiz. Granulated surface of scale,
magnified. Marl-slate.)
(FIGURE 424. Pygopterus mandibularis. Agassiz. Marl-slate.
a. Outside of scale, magnified.
b. Under surface of same.)
(FIGURE 425. Acrolepis Sedgwickii, Agassiz. Outside of scale, magnified. Marl-
slate.))
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 Palaeozoic 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 22.1, 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. (Ramsay Quarterly Geological Journal 1855; and Lyell Principles of
Geology volume 1 page 223 10th edition.)
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.
PERMIAN FLORA.
(FIGURE 426. Walchia piniformis, Schloth. Permian, Saxony. (Gutbier, Die
Versteinerungen des Permischen Systemes in Sachsen volume 2 plate 10.)
a. Branch.
b. Twig of the same.
c. Leaf magnified.)
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.
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 Figure 426), a genus of Conifers,
called Lycopodites by some authors, are said by Geinitz to be common to the
coal-measures.
(FIGURE 427. Cardiocarpon Ottonis. Gutbier, Permian, Saxony. 1/2 diameter.)
(FIGURE 428. Neoggerathia cuneifolia. Brongniart. (Murchison's Russia volume 2
Plate A figure 3.)
Among the genera also enumerated by Colonel Gutbier are the fruit called
Cardiocarpon (see Figure 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
Figure 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. Coniferae, 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.
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; secondly, 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.
TABLE 23.1.
In the south-western part of our island, in Somersetshire and South Wales, the
three divisions usually spoken of are:
1. Coal-measures: strata of shale, sandstone, and grit, from 600 to 12,000 feet
thick, with occasional seams of coal.
2. 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.
3. 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 Figure 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 of the above Table 23.1.)
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.
(Edward Hull Quarterly Geological Journal volume 24 page 327.) 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 Stigmariae, 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
Figure 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 Sigillariae, Lepidodendra, and Stigmariae, 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. (Quarterly
Geological Journal volume 5 Mem. page 17.)
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 Figure 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 15 1/2 feet in circumference at the base, 7
1/2 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 Sigillariae 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.
(FIGURE 429. Ground-plan of a fossil forest, Parkfield Colliery, near
Wolverhampton, showing the position of 73 trees in a quarter of an acre.)
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
Figure 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.
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.
(FIGURE 430. Uniting of distinct coal-seams.)
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.
Let A-C (Figure 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 Stigmariae 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 Sigillariae 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. (Memoirs of the
Geological Survey pages 51, 255, etc.)
INTERCALATED MARINE BEDS IN COAL.
(FIGURE 431. Microconchus (Spirorbis) carbonarius, Murchison. Natural size and
magnified.
b. Variety of same.)
(FIGURE 432. Cythere (Leperditia) inflata. Natural size and magnified.
Murchison.)
(FIGURE 433. Goniatites Listeri, Martin. Coal-measures, Yorkshire and
Lancashire.)
(FIGURE 434. Aviculopecten papyraceus, Goldf. (Pecten papyraceus, Sowerby.))
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 (Figure 432), and
the microscopic shell of an annelid of an extinct genus called Microconchus
(Figure 431), allied to Spirorbis. In the coal-field of Yorkshire there are
fresh-water strata, some of which contain shells referred to the family
Unionidae; 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 (Figure 433), Orthoceras, and Aviculopecten papyraceus, Goldf. (Figure
434).
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
Curculionidae, a neuropterous insect of the genus Corydalis, and another related
to the Phasmidae, have been found.
(FIGURE 435. Wing of a Grasshopper. Gryllacris lithanthraca, Goldenberg. Coal,
Saarbruck, near Treves.)
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 Saarbruck, near
Treves. (Dunker and V. Meyer Palaeontology volume 4 page 17.) Among them are
several Blattinae, three species of Neuroptera, one beetle of the Scarabaeus
family, a grasshopper or locust, Gryllacris (see Figure 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 Saarbruck, in the
interior of a flattened Lepidodendron. It is much larger than that of any known
living species of the same genus.
BATRACHIAN REPTILES IN COAL.
(FIGURE 436. Archegosaurus minor, Goldfuss. Fossil reptile from the coal-
measures, Saarbruck.)
(FIGURE 437. Imbricated covering of skin of Archegosaurus medius, Goldf.
Magnified.)
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 (Chapter 21), 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 Figure 437).
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 vertebrae 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.
(FIGURE 438. Slab of sandstone from the coal-measures of Pennsylvania, with
footprints of air-breathing reptile and casts of cracks. Scale one-sixth the
original.)
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 Figure 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.
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 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. (Acadian Geology second edition 1868.) 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.
(FIGURE 439. Section of the cliffs of the South Joggins, near Minudie, Nova
Scotia (from north to south through coal with upright trees and sandstone and
shale).
c. Grindstone.
d, g. Alternations of sandstone, shale, and coal containing upright trees.
e, f. Portion of cliff, given on a larger scale in Figure 440.
f. Four-foot coal, main seam.
h, i. Shale with fresh-water mussels, see below.)
In the section in Figure 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
degrees 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 Sigillariae,
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.
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.
(FIGURE 440. Erect fossil trees. Coal-measures, Nova Scotia.)
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 in
Figure 439, the position of the four-foot coal will be perceived, and in Figure
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.
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, Figure 440, represented in
the bed e in the section, Figure 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, Figure 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
Sigillariae 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
Coniferae of the Coal period grew in the same swamps as Sigillariae, 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 (Figure 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
Sigillariae, 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 Figure 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 Figure
461), there is a remarkable absence from the coal-measures of any form of
vegetation properly aquatic, the true coal being a sub-aerial 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 Palaeozoic 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 Sigillariae 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 vertebrae of a reptile, probably about 2 1/2 feet in length
(Dendrerpeton Acadianum, Owen), were scattered through this stony matrix. The
shell, also, of a Pupa (see Figure 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. (Quarterly Geological Journal volume 9
page 58.) 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, Figure 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. (Dawson Air-Breathers of the Coal in Nova Scotia Montreal
1863.)
(FIGURE 441. Xylobius Sigillariae, Dawson. Coal, Nova Scotia.
a. Natural size.
b. Anterior part, magnified.
c. Caudal extremity, magnified.)
(FIGURE 442. Pupa vetusta, Dawson.
a. Natural size.
b. Magnified.)
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 (Figure 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 Figure 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.
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.
(FIGURE 443. Zonites (Conulus) priscus, Carpenter.
a. Natural size.
b. Magnified.)
The surface striae 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
Dr. Dawson discovered in this lower bed, so full of the Pupa, another land-shell
of the genus Helix (sub-genus Zonites), see Figure 443. (Dawson Acadian Geology
1868 page 385.)
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 vertebrae 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 can not 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.
(FIGURES 444 and 445. On green shale, from Cape Breton, Nova Scotia.
(FIGURE 444. Carboniferous rain-prints with worm-tracks (a, b) on green shale,
from Cape Breton, Nova Scotia. Natural size.)
(FIGURE 445. Casts of rain-prints on a portion of the same slab (Figure 444),
seen to project on the under side of an incumbent layer of arenaceous shale.
Natural size. The arrow represents the supposed direction of the shower.))
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, Figure 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.
FOLDING AND DENUDATION OF THE BEDS INDICATED BY THE NOVA SCOTIA COAL-STRATA.
(FIGURE 446. Cone and branch of Lepidodendron corrugatum. Lower Carboniferous,
New Brunswick.)
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 above). 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 Figure 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.
(FIGURE 447. Diagram section from north, through Minudie, S. Joggins, Shoulie R.
and Cobequid Mountains, south, showing the curvature and supposed denudation of
the Carboniferous strata in Nova Scotia.
A. Anticlinal axis of Minudie.
B. Synclinal of Shoulie River.
1. Coal-measures.
2. Lower Carboniferous.)
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, Figure 447.
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, Figure 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 Sigillariae and Coniferae 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
degrees 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 Figure
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 degrees to one of more than 40 degrees, 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.
CHAPTER XXIV.
FLORA AND FAUNA OF THE CARBONIFEROUS PERIOD.
Vegetation of the Coal Period.
Ferns, Lycopodiaceae, Equisetaceae, Sigillariae, Stigmariae, Coniferae.
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, so great
appears to have been the numerical preponderance of flowerless or cryptogamic
plants of the families of ferns, club-mosses, and horse-tails. (For botanical
nomenclature see Chapter 17.) 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 phaenogamous plants were
constitute any feature in the coal are the coniferae; 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.
(FIGURE 448. Pecopteris elliptica, Bunbury. (Sir C. Bunbury Quarterly Geological
Journal volume 2 1845.) Frostburg.)
We are struck at the first glance with the similarity of the ferns to those now
living. In the fossil genus Pecopteris, for example (Figure 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.
(FIGURE 449. Caulopteris primaeva, 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 Figure 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 Figure 449) by Lindley, and the Psaronius of the upper or
newest coal-measures, before alluded to (Chapter 22). All the recent tree-ferns
belong to one tribe (Polypodiaceae), 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.
(FIGURES 450, 451 and 452. Living tree-ferns of different genera. (Ad. Brong.)
(FIGURE 450. Tree-fern from Isle of Bourbon.)
(FIGURE 451. Cyathea glauca, Mauritius.)
(FIGURE 452. Tree-fern from Brazil.))
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.
LYCOPODIACEAE-- LEPIDODENDRON.
(FIGURES 453, 454 and 455. Lepidodendron Sternbergii. Coal-measures, near
Newcastle.
(FIGURE 453. Branching trunk, 49 feet long, supposed to have belonged to L.
Sternbergii. (Foss. Flo. 203.))
(FIGURE 454. Branching stem with bark and leaves of L. Sternbergii. (Foss. Flo.
4.)
(FIGURE 455. Portion of same nearer the root. Natural size. (Ibid.)))
(FIGURE 456. Lycopodium densum.
a. Living species. New Zealand.
b. Branch; natural size.
c. Part of same, magnified.)
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 Figure 454). They belong to the
Lycopodiaceae, bearing sporangia and spores similar to those of the living
representatives of this family (Figure 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. The Figures 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. The living club-mosses, of which there
are about 200 species, are most abundant in tropical climates. They usually
creep on the ground, but some stand erect, as the Lycopodium densum from New
Zealand (see Figure 456), which attains a height of three feet.
(FIGURE 457. Lepidostrobus ornatus, Brong. Shropshire.
a. (Body) half natural size.
b. Portion of a section, showing the large sporangia in their natural position,
and each supported by its bract or scale.
c. Spores in these sporangia, highly magnified. (Hooker Mem. Geological Survey
volume 2 part 2 page 440.)
In the Carboniferous strata of Coalbrook Dale, and in many other coal-fields,
elongated cylindrical bodies, called fossil cones, named Lepidostrobus by M.
Adolphe Brongniart, are met with. (See Figure 457.) They often form the nucleus
of concretionary balls of clay-ironstone, and are well preserved, exhibiting a
conical axis, around which a great quantity of scales were compactly imbricated.
The opinion of M. Brongniart that the Lepidostrobus is the fruit of
Lepidodendron has been confirmed, for these strobili or fruits have been found
terminating the tip of a branch of a well-characterised Lepidodendron in
Coalbrook Dale and elsewhere.
EQUISETACEAE.
(FIGURE 458. Calamites Sucowii, Brong.; natural size. Common in coal throughout
Europe.)
(FIGURE 459. Stem of Figure 458, as restored by Dr. Dawson.)
(FIGURE 460. Radical termination of a Calamite. Nova Scotia.)
To this family belong two fossil genera of the coal, Equisetites and Calamites.
The Calamites were evidently closely related to the modern horse-tails
(Equiseta) differing principally in their great size, the want of sheaths at the
joints, and some details of fructification. They grew in dense brakes on sandy
and muddy flats in the manner of modern Equisetaceae, and their remains are
frequent in the coal. Seven species of this plant occur in the great Nova Scotia
section before described, where the stems of some of them five inches in
diameter, and sometimes eight feet high, may be seen terminating downward in a
tapering root (see Figure 460).
(FIGURE 461. Asterophillites foliosus. (Foss. Flo. 25.) Coal-measures,
Newcastle.)
(FIGURE 462. Annularia sphenophylloides, Dawson.)
(FIGURE 463. Sphenophyllum erosum, Dawson.)
Botanists are not yet agreed whether the Asterophyllites, a species of which is
represented in Figure 461, can form a separate genus from the Calamite, from
which, however, according to Dr. Dawson, its foliage is distinguished by a true
mid-rib, which is wanting in the leaves known to belong to some Calamites.
Figures 462 and 463 represent leaves of Annularia and Sphenophyllum, common in
the coal, and believed by Mr. Carruthers to be leaves of Calamites. Dr.
Williamson, who has carefully studied the Calamites, thinks that they had a
fistular pith, exogenous woody stem, and thick smooth bark, which last having
always disappeared, leaves a fluted stem, as represented in Figure 459.
SIGILLARIA.
(FIGURE 464. Sigillaria laevigata, Brong.)
A large portion of the trees of the Carboniferous period belonged to this genus,
of which as many as 28 species are enumerated as British. The structure, both
internal and external, was very peculiar, and, with reference to existing types,
very anomalous. They were formerly referred, by M. Ad. Brongniart, to ferns,
which they resemble in the scalariform texture of their vessels and, in some
degree, in the form of the cicatrices left by the base of the leaf-stalks which
have fallen off (see Figure 464). But some of them are ascertained to have had
long linear leaves, quite unlike those of ferns. They grew to a great height,
from 30 to 60, or even 70 feet, with regular cylindrical stems, and without
branches, although some species were dichotomous towards the top. Their fluted
trunks, from one to five feet in diameter, appear to have decayed more rapidly
in the interior than externally, so that they became hollow when standing; and
when thrown prostrate, they were squeezed down and flattened. Hence, we find the
bark of the two opposite sides (now converted into bright shining coal)
constitute two horizontal layers, one upon the other, half an inch, or an inch,
in their united thickness. These same trunks, when they are placed obliquely or
vertically to the planes of stratification, retain their original rounded form,
and are uncompressed, the cylinder of bark having been filled with sand, which
now affords a cast of the interior.
Dr. Hooker inclined to the belief that the Sigillariae may have been
cryptogamous, though more highly developed than any flowerless plants now
living. Dr. Dawson having found in some species what he regards as medullary
rays, thinks with Brongniart that they have some relation to gymnogens, while
Mr. Carruthers leans to the opinion that they belong to the Lycopodiaceae.
STIGMARIA.
(FIGURE 465. Stigmaria attached to a trunk of Sigillaria.)
This fossil, the importance of which has already been pointed out in Chapter 23,
was originally conjectured to be an aquatic plant. It is now ascertained to be
the root of Sigillaria. The connection of the roots with the stem, previously
suspected, on botanical grounds, by Brongniart, was first proved, by actual
contact, in the Lancashire coal-field, by Mr. Binney. The fact has lately been
shown, even more distinctly, by Mr. Richard Brown, in his description of the
Stigmariae occurring in the under-clays of the coal-seams of the Island of Cape
Breton, in Nova Scotia. In a specimen of one of these, represented in Figure
465, the spread of the roots was sixteen feet, and some of them sent out
rootlets, in all directions, into the surrounding clay.
(FIGURE 466. Stigmaria ficoides, Brong. 1/4 natural size. (Foss. Flo. 32.))
(FIGURE 467. Stigmaria ficoides, Brong. Surface of another individual of same
species, showing form of tubercles. (Foss. Flo. 34.))
In the sea-cliffs of the South Joggins in Nova Scotia, I examined several erect
Sigillariae, in company with Dr. Dawson, and we found that from the lower
extremities of the trunk they sent out Stigmariae as roots. All the stools of
the fossil trees dug out by us divided into four parts, and these again
bifurcated, forming eight roots, which were also dichotomous when traceable far
enough. The cylindrical rootlets formerly regarded as leaves are now shown by
more perfect specimens to have been attached to the root by fitting into deep
cylindrical pits. In the fossil there is rarely any trace of the form of these
cavities, in consequence of the shrinkage of the surrounding tissues. Where the
rootlets are removed, nothing remains on the surface of the Stigmaria but rows
of mammillated tubercles (see Figures 466, 467), which have formed the base of
each rootlet. These protuberances may possibly indicate the place of a joint at
the lower extremity of the rootlet. Rows of these tubercles are arranged
spirally round each root, which have always a medullary axis and woody system
much resembling that of Sigillaria, the structure of the vessels being, like it,
scalariform.
CONIFERAE.
(FIGURE 468. Fragment of coniferous wood, Dadoxylon, of Endlicher, fractured
longitudinally; from Coalbrook Dale. W.C. Williamson. (Manchester Philosophical
Mem. volume 9 1851.)
a. Bark.
b. Woody zone or fibre (pleurenchyma).
c. Medulla or pith.
d. Cast of hollow pith or "Sternbergia.")
(FIGURE 469. Fragment of coniferous wood, Dadoxylon, of Endlicher. Magnified
portion of Figure 468; transverse section.
b-b. Woody fibre.
c. Pith.
e, e, e. Medullary rays.)
The coniferous trees of this period are referred to five genera; the woody
structure of some of them showing that they were allied to the Araucarian
division of pines, more than to any of our common European firs. Some of their
trunks exceeded forty-four feet in height. Many, if not all of them, seem to
have differed from living Coniferae in having large piths; for Professor
Williamson has demonstrated the fossil of the coal-measures called Sternbergia
to be the pith of these trees, or rather the cast of cavities formed by the
shrinking or partial absorption of the original medullary axis (see Figures 468,
469). This peculiar type of pith is observed in living plants of very different
families, such as the common Walnut and the White Jasmine, in which the pith
becomes so reduced as simply to form a thin lining of the medullary cavity,
across which transverse plates of pith extend horizontally, so as to divide the
cylindrical hollow into discoid interspaces. When these interspaces have been
filled up with inorganic matter, they constitute an axis to which, before their
true nature was known, the provisional name of Sternbergia (d, d, Figure 468)
was given. In the above specimen the structure of the wood (b, Figures 468 and
469) is coniferous, and the fossil is referable to Endlicher's fossil genus
Dadoxylon.
(FIGURE 470. Trigonocarpum ovatum, Lindley and Hutton. Peel Quarry, Lancashire.)
(FIGURE 471. Trigonocarpum olivaeforme, Lindley, with its fleshy envelope.
Felling Colliery, Newcastle.)
The fossil named Trigonocarpon (Figures 470 and 471), formerly supposed to be
the fruit of a palm, may now, according to Dr. Hooker, be referred, like the
Sternbergia, to the Coniferae. Its geological importance is great, for so
abundant is it in the coal-measures, that in certain localities the fruit of
some species may be procured by the bushel; nor is there any part of the
formation where they do not occur, except the under-clays and limestone. The
sandstone, ironstone, shales, and coal itself, all contain them. Mr. Binney has
at length found in the clay-ironstone of Lancashire several specimens displaying
structure, and from these, says Dr. Hooker, we learn that the Trigonocarpon
belonged to that large section of existing coniferous plants which bear fleshy
solitary fruits, and not cones. It resembled very closely the fruit of the
Chinese genus Salisburia, one of the Yew tribe, or Taxoid conifers.
ANGIOSPERMS.
(FIGURE 472. Antholithes. Felling Colliery, Newcastle.)
The curious fossils called Antholithes by Lindley have usually been considered
to be flower spikes, having what seems a calyx and linear petals (see Figure
472). Dr. Hooker, after seeing very perfect specimens, also thought that they
resembled the spike of a highly-organised plant in full flower, such as one of
the Bromeliaceae, to which Professor Lindley had at first compared them. Mr.
Carruthers, who has lately examined a large series in different museums,
considers it to be a dicotyledonous angiosperm allied to Orobanche (broom-rape),
which grew, not on the soil, but parasitically on the trees of the coal forests.
(FIGURE 473. Pothocites Grantonii, Pat. Coal-measures, Edinburgh.
c. Stem and spike; 1/2 natural size.
b. Remains of the spathe magnified.
c. Portion of spike magnified.
d. One of the calyces magnified.)
In the coal-measures of Granton, near Edinburgh, a remarkable fossil (Figure
473) was found and described in 1840, by Dr. Robert Paterson. (Transactions of
the Botanical Society of Edinburgh volume 1 1844.) It was compressed between
layers of bituminous shale, and consists of a stem bearing a cylindrical spike,
a, which in the portion preserved in the slate exhibits two subdivisions and
part of a third. The spike is covered on the exposed surface with the four-cleft
calyces of the flowers arranged in parallel rows. The stem shows, at b, a little
below the spike, remains of a lateral appendage, which is supposed to indicate
the beginning of the spathe. The fossil has been referred to the Aroidiae, and
there is every probability that it is a true member of this order. There can at
least be no doubt as to the high grade of its organisation, and that it belongs
to the monocotyledonous angiosperms. Mr. Carruthers has carefully examined the
original specimen in the Botanical Museum, Edinburgh, and thinks it may have
been an epiphyte.
CLIMATE OF THE COAL PERIOD.
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