Page:The American Cyclopædia (1879) Volume IV.djvu/746

730 millstone grit, and generally contain two beds of coal, even sometimes three of wide extent, and varying in thickness from 1 to 4 ft. In eastern Kentucky and Tennessee, also in Indiana, two lower coal beds 2 to 3 ft. thick are found under the conglomerate, in near proximity to its base; and in Illinois a lowest bed of coal has been observed under the Chester limestone, an upper division of the great limestone formation which there underlies the coal measures. Coal is found in Nova Scotia, in Scotland, and in Russia also below the millstone grit. But it is only from the millstone grit upward, and in a thickness of 300 to 1,000 ft. of measures, according to the localities, that coal beds are formed of great thickness and of wide extent. The second bed of coal above the conglomerate (B) is generally composed of two or three different beds, either united in one or separated by beds of shale or even of sandstone. This bed is found over nearly the whole extent of the coal measures from Missouri to eastern Pennsylvania, and even to Massachusetts and Rhode Island. Its thickness is rarely under 3 ft., more generally 6 to 7 ft.; at Straitsville in Ohio it is 12 ft.; it is 30 ft. or more in some localities of the anthracite basin of Pennsylvania. From the millstone grit upward, in a thickness of from 500 to nearly 1,000 ft., coal beds are formed of various thickness and extent, alternating with strata of shale, limestone, sandstone, &c., up to the great Pittsburgh coal, which is, near the end of the productive carboniferous formation, what the big vein is near its beginning. The Pittsburgh bed covers, in Pennsylvania, Virginia, and Ohio, an area estimated at 20,000 sq. m., preserving generally a thickness of 4 to 6 ft., and often greater. Between the millstone grit and the Pittsburgh coal, five to twelve veins are generally found in the measures. Of course the productiveness of the measures is locally very variable. It is at its utmost in the anthracite region of Pennsylvania. At Plymouth, near Wilkesbarre, for example, where the big vein is 30 ft. thick, a section of 300 ft. of measures has 50 ft. of coal in five veins. At Scranton a section of 350 ft. indicates 60 ft. of coal in eight or nine beds. Here the largest vein is 14 ft. In the same region a section of 800 ft. has nearly 70 ft. of coal distributed in 15 beds. In the bituminous coal region of Pennsylvania and Ohio, as well as in the coal basin of Indiana and Illinois, the thickness and number of the beds of coal are somewhat reduced, as is also the thickness of the measures; but there is a remarkable relation between the formation of the coal beds and that of the intermediate strata. In the north at least, or with the exception of the Virginia coal fields, the relation indicated above is, with little difference, as one foot of coal in 18 to 22 ft. of measures; or in 500 ft. of coal measures there is generally an average thickness of 25 ft. of coal, distributed of course in several beds of various thickness. From the Pittsburgh coal

above and for nearly 1,000 ft. of measures the productiveness of the carboniferous formation diminishes in such a way that, though several coal beds have been observed, they have scarcely been found anywhere of a workable thickness. The reverse of the fact remarked in the subcarboniferous, where as the land plants become more numerous the deposits of coal also increase, is seen in passing higher up above the Pittsburgh coal to the end of the carboniferous. The plants which by their components and their great size contributed most to the formation of the coal, the lepidodendron, sigillaria, &c., decrease in numbers and even disappear. Several species of the carboniferous period are seen still persisting later, even in the Permian epoch; but these species are merely ferns and calamites, which, though large and abundant enough, do not seem to have formed any dense agglomerations of woody material, like those which evidently covered the surface of the earth when the vegetation was in its full vigor.—The strata intermediate to the beds of coal are composed of sandstone, sometimes hard enough to be cut in blocks and used for building, sometimes soft and passing to shale; of limestone, iron ore, clay, and bituminous shale. Most of the coal beds rest upon a bottom of grayish or whitish soft clay, generally mixed with remains of the stigmaria, a floating plant, the most common in the coal measures, which seems to have entered largely into the composition of the clay and of some kinds of coal. The bottom clay varies in thickness from a few inches to more than 30 ft. When penetrated by ferruginous solutions, it becomes reddish, and is sometimes hardened to such a degree that it resembles limestone and breaks under the hammer like hard rock. It is the rotten limestone of the miners. Very rarely the coal beds are in immediate superposition to sandstone; none have been seen as yet upon limestone without an intermediate bed of clay. In most cases, as a coal bed becomes at its base mixed with clay which at the point of contact is black, hard, and bituminous, it passes at its upper surface into black bituminous shale or slate differently composed from that of the bottom, laminated like the coal, or in thin layers and mixed with remains of the plants which have entered into the composition of the coal. These shales, however, do not always occur, and therefore the roof of the coal is often, sandstone, or more rarely limestone. With the remains of plants, the roof shales have also fossil shells, bones, or teeth of fishes. The limestone beds of the coal have rarely if ever remains of plants, but often a great quantity of animal remains, coral, crinoids, fishes, and especially shells. As will be seen hereafter, the distribution of all these kinds of strata is normal and explainable, as is also the formation of the shales, which are seen not only as the roof of the coal beds, but interlying them, and unfortunately sometimes in numerous