Page:A Treatise on Geology, volume 1.djvu/182

 166 on their edges. The striations on the slopes are very similar to those on the conical ironstone; and though the differences are in other respects great, they both probably depend on some general law of concretionary action, modified in operation by the nature of the substances acted on: but we are quite ignorant of the circumstances which determine this peculiar structure in coal.

Considered in its greatest generality, and with reference to countries where the masses appear in the greatest simplicity (as in the south of England), the carboniferous system consists of three formations: viz.—

This triple system becomes modified in the north of England, so as to constitute, in Derbyshire, a quadruple system, without any red sandstone, thus:—