Page:The American Cyclopædia (1879) Volume IV.djvu/745

Rh marine animals, attesting the dependence of animal upon vegetable life, or at least their relation to it. Rich deposits of mineral oil are found also in the rocks of this period, especially in the Niagara limestone. At Chicago, for example, this rock is completely saturated with oil. The Salina and lower Helderberg periods are generally composed of deep marine formations, where of course remains of plants are very rare, as seaweeds do not live at a great depth. Even the Salina formations have few animal remains. But the last period of the Silurian formation, the Oriskany, manifests its vegetable life not only by remains of marine plants, but near its upper part by the first traces of land vegetation, recognized in fragments of a species referable to the lycopods or club-moss family, and similar in size at least to the ground pine of our woods, lycopodium selago and L. lucidulum. They are the earliest representatives as yet known of a family of plants to which belong the genera lepidodendron, sigillaria, &c., which come later, and, by the great number of their species and the enormous size of the trees generally representing them, are called to play at a subsequent period a remarkable part in the formation of coal. From the beginning of the Devonian, the corniferous, vegetable remains appear in the strata in a far greater abundance, especially those of marine plants, together with a proportionately increased supply of mineral oil. It is the epoch of the cauda-galli grit, so called from the distribution in the rocks of a large fucoid which covers with its debris wide surfaces of shales and fills strata of great thickness. This plant passes up through the whole Devonian to the base of the carboniferous. The bitumen has been during this period treasured in large crevices of the rocks, in cavities wherefrom it is now pumped out and utilized. The oil wells of Canada come from deposits in the cauda-galli formation; in New York the cavities of the corniferous limestone, even those which have been formed by the decomposition or destruction of fossil remains, are filled with mineral oil. Above the corniferous, the black shales, the Marcellus and Genesee shale of the Hamilton period, are everywhere impregnated with bitumen, from which they receive their black color. The combustible matter is abundant enough to percolate through the fissure with water; it is also obtained from the shale by distillation. Even these black shales have been used for fuel, giving a bright flame, though they do not consume. They have been constantly searched for coal; but no coal has been found in this formation, which from its fossils is evidently of marine origin. With the bones and teeth of large fishes, it has also the remains of seaweeds to which its bituminization is due. Some scattered trunks, of conifers especially, and mostly silicified, have been dug out of the black shales of the Hamilton period. They seem to have been floated and deposited along low shores by the waves

or the current of the sea. In a few localities in Canada ferns and other plants have been observed seemingly deposited in place. But nowhere has the land vegetation been luxuriant enough to produce coal, though the genera, if not the species, represented by these plants are closely allied to those of the carboniferous period. The upper Devonian, especially the Chemung, presents by its vegetable remains the same character as those of the former period; but the land vegetation becomes more and more predominant, yet not enough to produce coal. Moreover, most of the upper Devonian strata represent beach formations by their red shales. These formations have a distinct flora, far different in its typical characters from that which is recognized in the composition of coal. These extensive mud beds washed by the tides, alternately above and under water, were preparing a solid basis for the land, or a ground for the remarkable period which was to follow.—The carboniferous period, as the word implies, is essentially that of the coal formation. Its lowest part, the subcarboniferous, is its foundation, laid upon the mud beds of the Devonian. In the east of the North American basins, it is composed of thick strata of hard red shale and sandy rocks; in the west, of beds of hard compact limestone and sandstone, overlaid by the successive stages of the carboniferous in a thickness of 2,000 to 3,000 ft. From the base of the subcarboniferous measures, the land plants already in abundance appear, in ascending, in a constantly increasing proportion, while the marine plants disappear in the same degree. The composition of the first or of the lowest bed of coal indicates only the remains of land plants, which henceforth, in the whole thickness of the coal formations, constitute the vegetation; at least no remains of marine plants have been found in connection with the coal, or appear to have contributed to its composition. From the millstone grit to the Permian, the shales are mostly covered and filled with fragments of plants, which without exception belong to the species of the coal or land flora; and in some beds of sandstone also we find local deposits of trunks now silicified or petrified, all representing species of ferns, lepidodendron, sigillaria, catamites, &c., to which the coal plants are referable. The first coal beds appear in the upper part of the subcarboniferous, or somewhat lower, a short distance below the millstone grit, a formation composed of sand and pebbles agglomerated, and for this reason named conglomerate, which generally underlies the productive measures as well in Europe as in America. Under the millstone grit a few beds of coal have been formed locally, two, rarely three, not of great thickness nor of wide extent, sometimes merely in pockets, as they are called, the matter transformed into coal having filled deep hollows of an irregular bottom and of small area. In Arkansas, however, the whole coal-bearing measures are under the