Page:The American Cyclopædia (1879) Volume IV.djvu/747

ternate layers with which the pure coal is so closely mixed that these coal strata are not workable. It is to the increasing thickness of these interlying clay partings, as they are named by the miners, that the subdivision of the coal beds is mainly due. Frequently coal beds are seen in one bulk at one place, and in another, divided into two by a clay parting, which by and by thickens to 10 or 20 ft. or more, and thus separates the coal into two distinct beds.—As remarked above, the Permian, at least in America, has no coal. The characters of this formation are somewhat similar in the nature of the rock to those of the subcarboniferous, mainly consisting of red shale, limestone, and sandstone. With some species of ferns of the coal measures, its flora is represented by large calamites, a genus whose species have the texture and outside form of the horsetail plants of our time, but of an immense size, and especially with new types of conifers. In North America the Permian formations are mostly composed of thick strata of magnesian limestone, overlying the upper coal measures of the west, near and along the Missouri river, especially in Kansas, and also in the Rocky mountains, where it generally underlies the cretaceous formation. The triassic period, in ascending, follows the Permian. It is represented in North America, especially in Nova Scotia, Connecticut, New Jersey, and Pennsylvania, by deposits of red hard shale with few fossil remains, and in North Carolina and Virginia by the same kind of rocks, containing however beds of combustible mineral or of coal. The coal of Richmond, that of Deep river and Dan river, is worked out of beds 4 to 6 ft. thick, and is reported to be of good quality. In connection with the coal strata, and in their roof shales, remains of fossil plants are found in profusion, indeed in the same position and in the same abundance as the remains of plants are seen in the true coal measures; but they represent far different species. With some ferns of a peculiar type, the fossil plants of the trias are mainly species of conifers, and in the greatest proportion species of cycadeæ, a family which is predominant also in the flora of the Jurassic period. Of this period, which in Europe is a formation of immense thickness, we have little in America. If has been recognized in the Rocky mountains in some beds of limestone. In one of its divisions, the oölitic, it has some fine deposits of coal in England. These beds, like all the coal beds, are overlaid by black shale imbedding vegetable remains, which mostly represent species of cycadeæ and of ferns.—Over the Jurassic we find the cretataceous. Like those of the upper Devonian, the subcarboniferous, and the Permian, its strata are mostly marine and submarine (beach formations), denoting an epoch of subsidence to prepare, after the destruction of ancient races of plants and animals, a new organic world by great changes in the atmospheric and surface circumstances of the earth. Formations of this kind have no coal. Their vegetable remains are mostly marine, and in the cretaceous they are locally present in connection with strata impregnated with bitumen. This is the case in California. The cretaceous formations of North America are extensive, and are distributed from the upper Missouri to the base of the Rocky mountains, where their thickness reaches 2,000 to 3,000 ft. The lower part, that which has been named the Dakota group from its predominance in that territory, is composed of red shale, and is evidently a beach formation immediately overlying the Permian. Though this group has no coal, but merely thin streaks of black bituminous shale, it is remarkable for its remains of fossil plants, which most of all represent dicotyledonous species. As high as the base of the cretaceous no plants of this vegetable division have been recognized. From the first appearance of land plants at the base of the Devonian, up to the cretaceous, all the species represented by their remains are referable to cryptogams and gymnosperms, these comprising the cycads and the conifers. But all at once in this Dakota group, which in America is the lowest cretaceous, we find an abundance of leaves of dicotyledonous species, sassafras, sycamore, oak, tulip tree, &c., without any remains of the former precedent and prominent types. From the Dakota group upward there is a succession of cretaceous sandstone and black shale, whose fossils are all animals, mostly large shells of species indicating a deep marine formation; and upon this a succession of thick beds of coarse hard sandstone, somewhat similar by its compounds to the millstone grit of the carboniferous, containing, especially in the intermediate layers of shale, a great abundance of fucoids and broken fragments of wood, stems, &c., appearing as brought up and mixed with the sand of the shores by the waves.—Further west, and along the eastern base of the Rocky mountains, this sandstone formation is the lowest stage of the tertiary period. It points by its compounds to the slow upheaval of a new land, and opens a new epoch where the conditions for the production of coal appear nearly as favorable as they were during the period of the true or old carboniferous formation. Along the base of the Rocky mountains, and in the high valleys of the interior of the chain, from the Rio Grande in New Mexico along the Pacific slope to Alaska, coal beds of the lower tertiary cover wide areas, and are sometimes of great thickness. In Colorado the veins now actively worked vary in thickness from 4 to 15 ft. At Evanston, Utah, the main coal, interlaid with bands of slate, is 26 ft. thick. The succession of the strata and their distribution is also remarkably similar to that of the carboniferous measures. At Marshall's, for example, in Colorado, an exposed section of 450 ft. of measures indicates 60 ft. of coal distributed in nine or ten veins, the lowest 14 ft. thick, separated by strata of sandstone, iron ore, clay,