Page:A Treatise on Geology, volume 2.djvu/136

 122 According to Mr. Sharpe, a cleavage system may be regarded as bounded by parallel lines along which the cleavage is vertical, and in all intermediate points less than vertical, in the middle of the space horizontal or nearly so; and he imagines these cleavage surfaces to be portions of great curves, everywhere perpendicular to pressures emanating from the axis of that space; that is to say, they would be so many parts of cylindrical sheets of uniform tension.

Upon this view we are not perhaps obliged to take into account any one of the axes of movement in a district, but the pressure on a whole district; and we are even released from referring the slaty cleavage to the date of their axes; it may be posterior to them all, and be only related to a general subterranean cause, of which they are some of the external manifestations.

A curious investigation of the component parts of non-fossiliferous slates has convinced Mr. Sharpe that the parts of such rocks have undergone that compression across the planes of cleavage, and extension in the direction of the dip, which had been inferred for other slates from evidence of altered fossils.

On the foundation of facts which have thus brought out the idea of internal pressure as an antecedent to the production of cleavage, Mr. Hopkins has endeavoured to point out the accurate mechanical conditions of the problem, and to indicate the points to which the attention of future observers should be specially directed for the purpose of ascertaining the data required for a complete theory. Though we cannot here give an analysis