Page:A Treatise on Geology, volume 1.djvu/206

 190 question, which appears in my 'Illustrations of the Geology of Yorkshire,' I have found it necessary to analyse the phenomena, so as to be able to inquire separately into the local origin of the three substances of principal importance—limestone, sandstone, shale: the former is of oceanic origin, for it contains only marine exuviæ, and when in greatest thickness and purity, was evidently deposited by water in a state of great tranquillity, or slow decomposition. In the same south-eastern direction that the limestone grows thicker from a certain point in the district, the sandstone and shales grows thinner: in the opposite direction they thicken, but not equally; the sandstones thicken toward the north, the shales toward the west, and in this direction certain limestones and sandstones totally vanish. With these sandstones the coal beds also vanish; where the sandstones thicken and grow numerous toward the north, the coal beds also augment in number and thickness; and the limestones change gradually from an undivided mass to many distinct members, separated by sandstones, shales, coal, and ironstone.

Thus to any point A, in diagram No. 52., where a series of limestone, sandstone, shales, coal, ironstone, occurs, the limestone may be supposed to have been brought by