Page:A Treatise on Geology, volume 2.djvu/127

 CHAP. VII. strata, under the influence of heat or other causes competent to induce particular arrangements,—prismatic, cubical, rhomboidal, &c., according to the nature, thickness, and position of the rocks, the degree in which the polarities of their particles are controlled by the different qualities of neighbouring mineral masses, and other important circumstances. Another case which also fell under the author's notice at Coley Hill, near Newcastle, appears strongly to confirm the view here presented, and at the same time to remove part of the obscurity which has always been supposed to overhang the origin of the "cleavage" of slate (see Vol. I. p. 67, &c.). In the annexed cut, d, is a basaltic dyke, nearly vertical, and between twenty and thirty feet across, ranging east and west, and appearing at the surface.

s, is the ordinary coal shale, which is, as usual, very much laminated at a moderate distance (a few yards) from the dyke, and contain fern leaves and other plants between the laminæ.