In our last lecture stress was laid on the fact that coal-beds, unlike mineral veins, are stratified—not injected, or filling cracks in the earth as metals do. And when we use the term stratified, we mean that the materials we are considering—coal, ironstone, sandstone, clay, shale—were all deposited sheet over sheet, layer over layer, principally by the agency of water.In scarcely any other way, except by water, can we conceive of materials being spread abroad over vast surfaces, in that even and regular manner which we call “stratified.” As a rule, the matters ejected from the mouths of fiery volcanos are only rudely heaped up, and unless they fall into the sea, do not undergo this smoothing, spreading-out process. The sand of the soa-shore however, and the pebbles on its margin, and the mud of its great depths, are truly “stratified;” and if a fertile plain, or a marshy district were submerged in the waters, the materials on that surface would be soon covered over by the ooze and sand and shingle, and would then be said to be “interstratified” with them. In this way coal-beds occur among beds of sandstone and other rocks.It is seldom that any coal-field contains more than twenty-five or thirty workable seams: and perhaps these altogether do not amount to above eighty or one hundred feet at the utmost, while in South Wales the coal strata are twelve thousand feet thick. The mass, you see, is rock.
Quality of economically extractable coal beds in the Gillette coal field as compared with other Tertiary coal beds in the Powder River basin, Wyoming and Montana
