CARBONIFEROUS GROUP--_continued_.
Coal-fields of the United States--Section of the country between the
Atlantic and Mississippi--Position of land in the carboniferous period
eastward of the Alleghanies--Mechanically formed rocks thinning out
westward, and limestones thickening--Uniting of many coal-seams into
one thick one--Horizontal coal at Brownsville, Pennsylvania--Vast
extent and continuity of single seams of coal--Ancient river-channel
in Forest of Dean coal-field--Absence of earthy matter in
coal--Climate of carboniferous period--Insects in coal--Rarity of
air-breathing animals--Great number of fossil fish--First discovery of
the skeletons of fossil reptiles--Footprints of reptilians--Mountain
limestone--Its corals and marine shells.
It was stated in the last chapter that a great uniformity prevails in the
fossil plants of the coal-measures of Europe and North America; and I may
add that four-fifths of those collected in Nova Scotia have been identified
with European species. Hence the former existence at the remote period
under consideration (the carboniferous) of a continent or chain of islands
where the Atlantic now rolls its waves seems a fair inference. Nor are
there wanting other and independent proofs of such an ancient land situated
to the eastward of the present Atlantic coast of North America; for the
geologist deduces the same conclusion from the mineral composition of the
carboniferous and some older groups of rocks as they are developed on the
eastern flanks of the Alleghanies, contrasted with their character in the
low country to the westward of those mountains.
The annexed diagram (fig. 379.) will assist the reader in understanding
the phenomena now alluded to, although I must guard him against
supposing that it is a true section. A great number of details have of
necessity been omitted, and the scale of heights and horizontal
distances are unavoidably falsified.
[Illustration: Fig. 379. Diagram explanatory of the geological structure of
a part of the United States between the Atlantic and the Mississippi.
Length from E. to W. 850 miles.
Appalachian Coal Field. Alleghanies, or Appalachians.
Same section--_continued_.
Mississippi. Illinois Coal Field. Cincinnati. Appalachian Coal Field.
A B. Atlantic plain.
B C. Atlantic slope.
C D. Alleghanies or Appalachian chain.
D E. Appalachian coal-field west of the mountains.
E F. Dome-shaped outcrop of strata on the Ohio, older than the coal.
F G. Illinois coal-field.
_h._ Falls and rapids of the rivers at the junction of the hypogene and
newer formations.
_i_, _k_, _l_, _m_. Parallel folds of Appalachians becoming successively
more open, and flatter in going from E. to W.
_References to the different Formations._
1. Miocene tertiary.
2. Eocene tertiary.
3. Cretaceous strata.
4. Red sandstone with ornithichnites (new red or trias?) usually much
invaded by trap.
5. Coal-measures (bituminous coal).
5' Anthracitic coal-measures.
5'' Carboniferous limestone of the Illinois coal-field, wanting in the
Appalachian.
6. Old red or Devonian, Olive slate, &c.
7. Primary fossiliferous or Silurian strata.
8. Hypogene strata, or gneiss, mica schist, &c., with granite veins.
_Note._ The dotted lines at _i_ and _k_ express portions of rock removed
by denudation, the amount of which may be estimated by supposing similar
lines prolonged from other points where different strata end abruptly
at the surface.
_N.B._ The lower section at ** joins on to the upper one at *.]
Starting from the shores of the Atlantic, on the eastern side of the
Continent, we first come to a low region (A B), which was called the
alluvial plain by the first geographers. It is occupied by tertiary and
cretaceous strata, before described (pp. 171. 206. and 224.), which are
nearly horizontal. The next belt, from B to C, consists of granitic rocks
(hypogene), chiefly gneiss and mica-schist, covered occasionally with
unconformable red sandstone, No. 4. (New Red or Trias?), remarkable for its
ornithichnites (see p. 327.). Sometimes, also, this sandstone rests on the
edges of the disturbed paleozoic rocks (as seen in the section). The region
(B C), sometimes called the "Atlantic Slope," corresponds nearly in average
width with the low and flat plain (A, B), and is characterized by hills of
moderate height, contrasting strongly, in their rounded shape and altitude,
with the long, steep, and lofty parallel ridges of the Alleghany mountains.
The outcrop of the strata in these ridges, like the two belts of hypogene
and newer rocks (A B, and B C), above alluded to, when laid down on a
geological map, exhibit long stripes of different colours, running in a
N.E. and S.W. direction, in the same way as the lias, chalk, and other
secondary formations in the middle and eastern half of England.
The narrow and parallel zones of the Appalachians here mentioned, consist
of strata, folded into a succession of convex and concave flexures,
subsequently laid open by denudation. The component rocks are of great
thickness, all referable to the Silurian, Devonian, and Carboniferous
formations. There is no principal or central axis, as in the Pyrenees and
many other chains--no nucleus to which all the minor ridges conform; but
the chain consists of many nearly equal and parallel foldings, having what
is termed an anticlinal and synclinal arrangement (see above, p. 48.). This
system of hills extends, geologically considered, from Vermont to Alabama,
being more than 1000 miles long, from 50 to 150 miles broad, and varying in
height from 2000 to 6000 feet. Sometimes the whole assemblage of ridges
runs perfectly straight for a distance of more than 50 miles, after which
all of them wheel round together, and take a new direction, at an angle of
20 or 30 degrees to the first.
We are indebted to the state surveyors of Virginia and Pennsylvania, Prof.
W. B. Rogers and his brother Prof. H. D. Rogers, for the important
discovery of a clue to the general law of structure prevailing throughout
this range of mountains, which, however simple it may appear when once made
out and clearly explained, might long have been overlooked; amidst so great
a mass of complicated details. It appears that the bending and fracture of
the beds is greatest on the south-eastern or Atlantic side of the chain,
and the strata become less and less disturbed as we go westward, until at
length they regain their original or horizontal position. By reference to
the section (fig. 379.), it will be seen that on the eastern side, or in
the ridges and troughs nearest the Atlantic, south-eastern dips
predominate, in consequence of the beds having been folded back upon
themselves, as in _i_, those on the north-western side of each arch having
been inverted. The next set of arches (such as _k_) are more open, each
having its western side steepest; the next (_l_) opens out still more
widely, the next (_m_) still more, and this continues until we arrive at
the low and level part of the Appalachian coal-field (D E).
In nature or in a true section, the number of bendings or parallel folds is
so much greater that they could not be expressed in a diagram without
confusion. It is also clear that large quantities of rock have been removed
by aqueous action or denudation, as will appear if we attempt to complete
all the curves in the manner indicated by the dotted lines at _i_ and _k_.
The movements which imparted so uniform an order of arrangement to this
vast system of rocks must have been, if not contemporaneous, at least
parts of one and the same series, depending on some common cause. Their
geological date is well defined, at least within certain limits, for
they must have taken place after the deposition of the carboniferous
strata (No. 5.), and before the formation of the red sandstone (No. 4.).
The greatest disturbing and denuding forces have evidently been exerted
on the south-eastern side of the chain; and it is here that igneous or
plutonic rocks are observed to have invaded the strata, forming dykes,
some of which run for miles in lines parallel to the main direction of
the Appalachians, or N.N.E. and S.S.W.
The thickness of the carboniferous rocks in the region C is very great, and
diminishes rapidly as we proceed to the westward. The surveys of
Pennsylvania and Virginia show that the south-east was the quarter whence
the coarser materials of these strata were derived, so that the ancient
land lay in that direction. The conglomerate which forms the general base
of the coal-measures is 1500 feet thick in the Sharp Mountain, where I saw
it (at C) near Pottsville; whereas it has only a thickness of 500 feet,
about thirty miles to the north-west, and dwindles gradually away when
followed still farther in the same direction, till its thickness is reduced
to 30 feet.[329-A] The limestones, on the other hand, of the coal-measures,
augment as we trace them westward. Similar observations have been made in
regard to the Silurian and Devonian formations in New York; the sandstones
and all the mechanically-formed rocks thinning out as they go westward, and
the limestones thickening, as it were, at their expense. It is, therefore,
clear that the ancient land was to the east, where the Atlantic now is; the
deep sea, with its banks of coral and shells to the west, or where the
hydrographical basin of the Mississippi is now situated.
In that region, near Pottsville, where the thickness of the coal-measures
is greatest, there are thirteen seams of anthracitic coal, several of them
more than 2 yards thick. Some of the lowest of these alternate with beds of
white grit and conglomerate of coarser grain than I ever saw elsewhere,
associated with pure coal. The pebbles of quartz are often of the size of a
hen's egg. On following these pudding-stones and grits for several miles
from Pottsville, by Tamaqua, to the Lehigh Summit Mine, in company with Mr.
H. D. Rogers, in 1841, he pointed out to me that the coarse-grained strata
and their accompanying shales gradually thin out, until seven seams of
coal, at first widely separated, are brought nearer and nearer together,
until they successively unite; so that at last they form one mass, between
40 and 50 feet thick. I saw this enormous bed of anthracitic coal quarried
in the open air at Mauch Chunk (or the Bear Mountain), the overlying
sandstone, 40 feet thick, having been removed bodily from the top of the
hill, which, to use the miner's expression, had been "scalped." The
accumulation of vegetable matter now constituting this vast bed of
anthracite, may perhaps, before it was condensed by pressure and the
discharge of its hydrogen, oxygen, and other volatile ingredients, have
been between 200 and 300 feet thick. The origin of such a vast thickness of
vegetable remains, so unmixed with earthy ingredients, can, I think, 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 absence of sediment, or, in this
case, of sand and pebbles, wholly unexplained.
[Illustration: Fig. 380. Cross section.]
[Illustration: Fig. 381. Cross section.]
But the student will naturally ask, what can have caused so many seams of
coal, after they had been persistent for miles, to come together and blend
into one single seam, and that one equal in the aggregate to the thickness
of the several separate seams? Often had the same question been put by
English miners before a satisfactory answer was given to it by the late Mr.
Bowman. The following is his solution of the problem. Let _a a'_, fig.
380., be a mass of vegetable matter, capable, when condensed, of forming a
3-foot seam of coal. It rests on the underclay _b b'_, filled with roots of
trees _in situ_, and it supports a growing forest (C D). Suppose that part
of the same forest D E had become submerged by the ground sinking down 25
feet, so that the trees have been partly thrown down and partly remain
erect in water, slowly decaying, their stumps and the lower parts of their
trunks being enveloped in layers of sand and mud, which are gradually
filling up the lake D F. When this lake or lagoon has at length been
entirely silted up and converted into land, say, in the course of a
century, the forest C D will extend once more continuously over the whole
area C F, as in fig. 381., and another mass of vegetable matter (_g g'_),
forming 3 feet more of coal, may accumulate from C to F. We then find in
the region F, two seams of coal (_a'_ and _g'_) each 3 feet thick, and
separated by 25 feet of sandstone and shale, with erect trees based upon
the lower coal, while, between D and C, we find these two seams united into
a 2-yard coal. It may be objected that the uninterrupted growth of plants
during the interval of a century will have caused the vegetable matter in
the region C D to be thicker than the two distinct seams _a'_ and _g'_ at
F; and no doubt there would actually be a slight excess representing one
generation of trees with the remains of other plants, forming half an inch
or an inch of coal; but this would not prevent the miner from affirming
that the seam _a g_, throughout the area C D, was equal to the two seams
_a'_ and _g'_ at F.
The reader has seen, by reference to the section (fig. 379. p. 327.), that
the strata of the Appalachian coal-field assume an horizontal position west
of the mountains. In that less elevated country, the coal-measures are
intersected by three great navigable rivers, and are capable of supplying
for ages, to the inhabitants of a densely peopled region, an inexhaustible
supply of fuel. These rivers are the Monongahela, the Alleghany, and the
Ohio, all of which lay open on their banks the level seams of coal. Looking
down the first of these at Brownsville, we have a fine view of the main
seam of bituminous coal 10 feet thick, commonly called the Pittsburg seam,
breaking out in the steep cliff at the water's edge; and I made the
accompanying sketch of its appearance from the bridge over the river (see
fig. 382.). Here the coal, 10 feet thick, is covered by carbonaceous shale
(_b_), and this again by micaceous sandstone (_c_). Horizontal galleries
may be driven everywhere at very slight expense, and so worked as to drain
themselves, while the cars, laden with coal and attached to each other,
glide down on a railway, so as to deliver their burden into barges moored
to the river's bank. The same seam is seen at a distance, on the right bank
(at _a_), and may be followed the whole way to Pittsburg, fifty miles
distant. As it is nearly horizontal, while the river descends it crops out
at a continually increasing, but never at an inconvenient, height above the
Monongahela. Below the great bed of coal at Brownsville is a fire-clay 18
inches thick, and below this, several beds of limestone, below which again
are other coal seams. I have also shown in my sketch another layer of
workable coal (at _d d_), which breaks out on the slope of the hills at a
greater height. Here almost every proprietor can open a coal-pit on his own
land, and the stratification being very regular, he may calculate with
precision the depth at which coal may be won.
The Appalachian coal-field, of which these strata form a part (from C
to E, section, fig. 379., p. 327.), is remarkable for its vast area;
for, according to Professor H. D. Rogers, it stretches continuously from
N.E. to S.W., for a distance of 720 miles, its greatest width being
about 180 miles. On a moderate estimate, its superficial area amounts to
63,000 square miles.
[Illustration: Fig. 382. View of the great Coal Seam on the Monongahela at
Brownsville, Pennsylvania, U. S.
_a._ Ten-foot seam of coal.
_b._ Black bituminous or carbonaceous shale, 10 feet thick.
_c._ Micaceous sandstone.
_d d._ Upper seam of coal, 6 feet thick.]
This coal formation, before its original limits were reduced by
denudation, must have measured 900 miles in length, and in some places more
than 200 miles in breadth. By again referring to the section (fig. 379., p.
327.), it will be seen that the strata of coal are horizontal to the
westward of the mountains in the region D E, and become more and more
inclined and folded as we proceed eastward. Now it is invariably found, as
Professor H. D. Rogers has shown by chemical analysis, that the coal is
most bituminous towards its western limit, where it remains level and
unbroken, and that it becomes progressively debituminized as we travel
south-eastward towards the more bent and distorted rocks. Thus, on the
Ohio, 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 (5', fig. 379.) 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
to twelve per cent. of bitumen, thus becoming a genuine anthracite.[333-A]
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 carburetted 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, carburetted hydrogen, nitrogen, and olefiant gas.
The disengagement of all these gradually transforms ordinary or bituminous
coal into anthracite, to which the various names of splint coal, glance
coal, culm, and many others, have been given.
We have seen that, in the Appalachian coal-field, there is an intimate
connection between the extent to which the coal has 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,
where the fracturing of the rocks had produced an infinite number of
cracks and crevices, and also to the heat of the gases and water
penetrating these cracks, when the great movements took place, which
have rent and folded the Appalachian strata. 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.
_Continuity of seams of coal._--As single seams of coal are continuous over
very wide areas, it has been asked, how forests could have prevailed
uninterruptedly over such wide spaces, without being oftener flooded by
turbid rivers, or, when submerged, denuded by marine currents. It appears,
from the description of the Cape Breton coal-field, by Mr. Richard Brown,
that false stratification is common in the beds of sand, and some partial
denudation of these, at least, must often have taken place during the
accumulation of the carboniferous series.
In the Forest of Dean, ancient river-channels are found, which pass through
beds of coal, and in which rounded pebbles of coal occur. They are of older
date than the overlying and undisturbed coal-measures. The late Mr. Buddle,
who described them to me, told me he had seen similar phenomena in the
Newcastle coal-field. Nevertheless, instances of these channels are much
more rare than we might have anticipated, especially when we remember how
often the roots of trees (_Stigmariae_) have been torn up, and drifted in
broken fragments into the grits and sandstones. The prevalence of a
downward movement is, no doubt, the principal cause which has saved so many
extensive seams of coal from destruction by fluviatile action.
The purity of the coal, or its non-intermixture with earthy matter,
presents another theoretical difficulty to many geologists, who are
inclined to believe that the trees and smaller plants of the
carboniferous period grew in extensive swamps, rather than on land not
liable to be inundated. It appears, however, that in the alluvial plain
and delta of the Mississippi, extensive "cypress swamps," as they are
called, densely covered with various trees, occur, into which no matter
held in mechanical suspension is ever introduced during the greatest
inundations, inasmuch as they are all surrounded by a dense marginal
belt of reeds, canes, and brushwood. Through this thick barrier the
river-water must pass, so that it is invariably well filtered before it
can reach the interior of the forest-covered area, within which,
vegetable matter is continually accumulating from the decay of trees and
semi-aquatic plants. In proof of this, I may observe, that whenever any
part of a swamp 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.[334-A]
At the bottom of these "cypress swamps" of the Mississippi, a bed of
clay is found, with roots of the tall cypress (_Taxodium distichum_),
just as the underclays of the coal are filled with _Stigmaria_.
_Climate of Coal Period._--So long as the botanist taught that a tropical
climate was implied by the carboniferous flora, geologists might well be at
a loss to reconcile the preservation of so much vegetable matter with a
high temperature; for heat hastens the decomposition of fallen leaves and
trunks of trees, whether in the atmosphere or in water.[335-A] It is well
known that peat, so abundant in the bogs of high latitudes, ceases to grow
in the swamps of warmer regions. It seems, however, to have become a more
and more received opinion, that the coal-plants do not, on the whole,
indicate a climate resembling that now enjoyed in the equatorial zone.
Tree-ferns range as far south as the southern part of New Zealand, and
Araucarian pines occur in Norfolk Island. A great predominance of ferns and
lycopodiums indicates warmth, moisture, equability of temperature, and
freedom from frost, rather than intense heat; and we know too little of the
sigillariae, calamites, asterophyllites, and other peculiar forms of the
carboniferous period, to be able to speculate with confidence on the kind
of climate they may have required.
No doubt, we are entitled to presume, from the corals and cephalopoda of
the mountain limestone, that a warm temperature characterized the northern
seas in the carboniferous era; but the absence of cold may have given rise
(as at present in the seas of the Bermudas, under the influence of the gulf
stream) to a very wide geographical range of stone-building corals and
shell-bearing cuttle-fish, without its being necessary to call in the aid
of tropical heat.[335-B]
CARBONIFEROUS REPTILES.
Where we have evidence in a single coal-field, as in that of Nova Scotia,
or South Wales, of fifty or even a hundred ancient forests buried one above
the other, with the roots of trees still in their original position, and
with some of the trunks still remaining erect, we are apt to wonder that
until the year 1844 no remains of contemporaneous air-breathing creatures,
except a few insects, had been discovered. No vertebrated animals more
highly organized than fish, no mammalia or birds, no saurians, frogs,
tortoises, or snakes, were yet known in rocks of such high antiquity. In
the coal-field of Coalbrook Dale mention had been made of two species of
beetles of the family _Curculionidae_, and of a neuropterous insect
resembling the genus _Corydalis_, with another related to the
_Phasmidae_.[335-C] In other coal-measures in Europe we find notice of a
scorpion and of a moth allied to _Tinea_, also of one air-breathing
crustacean, or land-crab. Yet Agassiz had already described in his great
work on fossil fishes more than one hundred and fifty species of
ichthyolites from the coal strata, ninety-four belonging to the families of
shark and ray, and fifty-eight to the class of ganoids. Some of these fish
are very remote in their organization from any now living, especially
those of the family called _Sauroid_ by Agassiz; as _Megalichthys_,
_Holoptychius_, and others, which are often of great size, and all
predaceous. Their osteology, says M. Agassiz, reminds us in many respects
of the skeletons of saurian reptiles, both by the close sutures of the
bones of the skull, their large conical teeth striated longitudinally (see
fig. 383.), the articulations of the spinous processes with the vertebrae,
and other characters. Yet they do not form a family intermediate between
fish and reptiles, but are true _fish_, though doubtless more highly
organized than any living fish.[336-A]
[Illustration: Fig. 383. _Holoptychius Hibberti_, Ag. Fifeshire
coal-field; natural size.]
The annexed figure represents a large tooth of the _Megalichthys_,
found by Mr. Horner in the Cannel coal of Fifeshire. It probably
inhabited an estuary, like many of its contemporaries, and frequented
both rivers and the sea.
[Illustration: Fig. 384. _Archegosaurus minor_, Goldfuss. Fossil reptile
from the coal-measures, Saarbrueck.]
At length, in 1844, the first skeleton of a true reptile was announced from
the coal of Muenster-Appel in Rhenish Bavaria, by H. von Meyer, under the
name of _Apateon pedestris_, the animal being supposed to be nearly related
to the salamanders. Three years later, in 1847, Prof. von Dechen found in
the coal-field of Saarbrueck, at the village of Lebach, between Strasburg
and Treves, the skeletons of no less than three distinct species of
air-breathing reptiles, which were described by the late Prof. Goldfuss
under the generic name of _Archegosaurus_. The ichthyolites and plants
found in the same strata, left no doubt that these remains belonged to the
true coal period. 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-iron-stone. The largest of these lizards, _Archegosaurus Decheni_,
must have been 3 feet 6 inches long. The annexed drawing represents 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_, and therefore connected with the batrachians, as well as
the lizards. The remains of the extremities leave no doubt that 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 Mr. Owen has
observed to me that they make an approach to the _Proteus_ in the shortness
of their ribs. Two of these ancient reptiles retain a large part of the
outer skin, which consisted of long, narrow, wedge-shaped, tile-like, and
horny scales, arranged in rows (see fig. 385.).
[Illustration: Fig. 385. Imbricated covering of skin of _Archegosaurus
medius_, Goldf.; magnified.[337-A]]
_Cheirotherian footprints in coal measures, United States._--In 1844,
the very year when the Apateon or Salamander 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 in 1846. I was at once convinced of their genuineness, and declared
my conviction on that point, on which doubts had been entertained both
in Europe and the United States. 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 the accompanying drawing (fig.
386.). 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.
No less than twenty-three footsteps were observed by Dr. King in the
same quarry before it was abandoned, the greater part of them so
arranged (see fig. 387.) on the surface of one stratum as to imply that
they were made successively by the same animal. Everywhere there was a
double row of tracks, and in each row they occur in pairs, each pair
consisting of a hind and fore foot, and each being at nearly equal
distances from the next pair. In each parallel row the toes turn the one
set to the right, the other to the left. In the European
_Cheirotherium_, before mentioned (p. 290.), both the hind and fore feet
have each five toes, and the size of the hind foot is about five times
as large as the fore foot. In the American fossil the posterior
footprint is not even twice as large as the anterior, and the number of
toes is unequal, being five in the hinder and four in the anterior foot.
In this, as in the European _Cheirotherium_, one toe stands out like a
thumb, and these thumb-like toes turn the one set to the right, and the
other to the left. The American _Cheirotherium_ was evidently a broader
animal, and belonged to a distinct genus from that of the triassic
age in Europe.[338-A]
[Illustration: Fig. 386. _Scale one-sixth the original._ Slab of sandstone
from the coal-measures of Pennsylvania, with footprints of air-breathing
reptile and casts of cracks.]
We may assume that the reptile which left these prints on the ancient
sands of the coal-measures was an air-breather, because its weight would
not have been sufficient under water to have made impressions so deep and
distinct. The same conclusion is also borne out by the casts of the cracks
above described, for they show that the clay had been exposed to the air
and sun, so as to have dried and shrunk.
[Illustration: Fig. 387. Series of reptilian footprints in the coal-strata
of Westmoreland County, Pennsylvania.
_a._ Mark of nail?]
The geological position of the sandstone of Greensburg is perfectly clear,
being situated in the midst of the Appalachian coal-field, having the main
bed of coal, called the Pittsburg seam, above mentioned (p. 331.), 3 yards
thick, 100 feet above it, and worked in the neighbourhood, with several
other seams of coal at lower levels. The impressions of _Lepidodendron_,
_Sigillaria_, _Stigmaria_, and other characteristic carboniferous plants,
are found both above and below the level of the reptilian footsteps.
Analogous footprints of a large reptile of still older date have since
been found (1849), by Mr. Isaac Lea, in the lowest beds of the coal
formation at Pottsville, near Philadelphia, so that we may now be said
to have the footmarks of two reptilians of the coal period, and the
skeletons of four.[340-A]
CARBONIFEROUS OR MOUNTAIN LIMESTONE.
We have already seen that this rock lies sometimes entirely beneath the
coal-measures, while, in other districts, it alternates with the shales and
sandstone of the coal. In both cases it is destitute of land plants, and
usually charged with corals, which are often of large size; and several
species belong to the lamelliferous class of Lamarck, which enter largely
into the structure of coral reefs now growing. There are also a great
number of _Crinoidea_ (see fig. 388.), and a few _Echinoderms_, associated
with the zoophytes above mentioned. The _Brachiopoda_ constitute a large
proportion of the Mollusca, many species being referable to two extinct
genera, _Spirifer_ (or _Spirifera_) (fig. 389.), and _Productus_
(_Leptaena_) (fig. 390.).
[Illustration: Fig. 388. _Cyathocrinites planus_, Miller.
Mountain limestone.]
[Illustration: Fig. 389. _Spirifer glaber_, Sow. Mountain limestone.]
[Illustration: Fig. 390. _Productus Martini_, Sow. (_P. semireticulatus_,
Flem.) Mountain limestone.]
Among the spiral univalve shells the extinct genus _Euomphalus_ (see fig.
391.) is one of the commonest fossils of the Mountain limestone. In the
interior it is often divided into chambers (see fig. 391. _d_); the septa
or partitions not being perforated, as in foraminiferous shells, or in
those having siphuncles, like the Nautilus. The animal appears, like the
recent _Bulimus decollatus_, to have retreated at different periods of its
growth, from the internal cavity previously formed, and to have closed all
communication with it by a septum. The number of chambers is irregular, and
they are generally wanting in the innermost whorl.
[Illustration: Fig. 391. _Euomphalus pentagulatus_, Min. Con.
Mountain limestone.
_a._ Upper side; _b._ lower, or umbilical side; _c._ view showing mouth
which is less pentagonal in older individuals; _d._ view of polished
section, showing internal chambers.]
[Illustration: Fig. 392. Portion of _Orthoceras laterale_,
Phillips. Mountain limestone.]
There are also many univalve and bivalve shells of existing genera in the
Mountain limestone, such as _Turritella_, _Buccinum_, _Patella_,
_Isocardia_, _Nucula_, and _Pecten_.[341-A] But the _Cephalopoda_ depart,
in general, more widely from living forms, some being generically distinct
from all those found in strata newer than the coal. In this number may be
mentioned _Orthoceras_, a siphuncled and chambered shell, like a _Nautilus_
uncoiled and straightened. Some species of this genus are several feet long
(fig. 392.). The _Goniatite_ is another genus, nearly allied to the
_Ammonite_, from which it differs in having the lobes of the septa free
from lateral denticulations, or crenatures; so that the outline of these is
continuous and uninterrupted (see _a_, fig. 393.). Their siphon is small,
and in the form of the striae of growth they resemble _Nautili_. Another
extinct generic form of Cephalopod, abounding in the Mountain limestone,
and not found in strata of later date, is the _Bellerophon_ (fig. 394.), of
which the shell, like the living Argonaut, was without chambers.
[Illustration: Fig. 393. _Goniatites evolutus_, Phillips.[342-A]
Mountain limestone.]
[Illustration: Fig. 394. _Bellerophon costatus_, Sow.[342-B]
Mountain limestone.]
FOOTNOTES:
[329-A] H. D. Rogers, Trans. Assoc. Amer. Geol., 1840-42, p. 440.
[333-A] Trans. of Ass. of Amer. Geol., p. 470.
[334-A] Lyell's Second Visit to the U. S., vol. ii. p. 245. American
Journ. of Sci., 2d series, vol. v. p. 17.
[335-A] Principles of Geol., p. 696.
[335-B] For changes in climate, see Principles of Geol., chaps.
vii. and viii.
[335-C] Geol. Trans., 2d series, vol. vi. p. 330.
[336-A] Agassiz, Poiss. Foss., lib. 4. p. 62. and liv. 5. p. 88.
[337-A] Goldfuss, Neue Jenaische Lit. Zeit., 1848; and Von Meyer, Quart.
Geol. Journ., vol. iv. p. 51., memoirs.
[338-A] See Lyell's Second Visit, &c., vol. ii. p. 305.
[340-A] These impressions, found by Mr. Lea, were imagined to be in a rock
as ancient as the old red sandstone; but, according to Mr. H. D. Rogers,
they are in the lowest part of the coal formation.
[341-A] Phillips, Geol. of Yorksh., vol. ii. p. 208.
[342-A] Phillips, Geol. of Yorksh., pl. 20. fig. 65.
[342-B] Ibid., pl. 17. fig. 15.