Page:Transactions of the Geological Society, 1st series, vol. 1.djvu/207

 angle of about 6°. It is composed of the usual members, namely, of quartzose sandstone, of indurated clay, of clay-porphyry, of slaty-clay, and of coal, alternating with each other without much regularity, except that each bed of coal is always immediately covered by indurated or slaty-clay and not by sandstone. The series is the most complete in the deep of Madeley colliery, where a pit has been sunk to the depth of seven hundred and twenty-nine feet through all the beds, eighty-six in number, that compose this formation.

The sandstones which make part of the first thirty strata, are fine grained, considerably micaceous, and often contain thin plates or minute fragments of coal. The thirty-first and thirty-third strata are coarse-grained sandstone entirely penetrated by petroleum; they are, both together, fifteen feet and a half thick, and have a bed of sandy slate-clay about four feet thick interposed between them. These strata are interesting, as furnishing the supply of petroleum that issues from the tar-spring at Coalport. By certain geologists this reservoir of petroleum has been supposed to be sublimed from the beds of coal that lie below; an hypothesis not easily reconcilable to present appearances, especially as it omits to explain how the petroleum in the upper of these beds could have passed through the interposed bed of clay so entirely as to leave no trace behind; it is also worthy of remark, that the nearest coal is only six inches thick, and is separated from the above beds by a mass ninety-six feet in thickness, consisting of sandstone and clay strata without any mixture of petroleum. At the depth of four hundred and thirty feet occurs the first bed of very coarse sandstone or grit; its thickness is about fifteen feet. The next bed of sandstone deserving notice occurs at the depth of five hundred and seventy-six feet, is about eighteen feet thick, is fine-grained and very hard, and often mixed with a little petroleum; the name given to it by the colliers is the big flint.