The Student's Elements of Geology Part 50

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 ma.s.s of strata more than 1200 feet thick. As there were 21 seams of coal in this intervening ma.s.s, 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-sh.e.l.ls 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 sh.e.l.ls. They were all originally entire when imbedded, but are most of them now crushed, flattened, and distorted by pressure; they must have been acc.u.mulated, 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-sh.e.l.l 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 cla.s.s 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-sh.e.l.l, 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 inc.u.mbent 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 sh.o.r.es 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 a.s.sociated 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 sh.o.r.es 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 pa.s.s 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 pa.s.sing 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 pa.s.sing 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 const.i.tuted nearly the whole flora, but also because they were, on the whole, more highly developed than any belonging to the same cla.s.s now existing, and united some forms of structure now only found separately and in distinct orders. The only phaenogamous plants were const.i.tute 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 cla.s.s 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.