The received theory of Slaty Cleavage has generally been held to afford a complete explanation of the observed phenomena. The proximate cause of the structure was shown by Dr. Sorby to be a superinduced arrangement of the flat and long-shaped fragments constituting the rock, in virtue of which they tend to lie in, or nearly in, the planes of cleavage; this arrangement being assisted, as Mr. D. Sharpe advocated, by a flattening of those particles themselves. These changes were ascribed to great lateral compression of the rock in the direction perpendicular to the cleavage-planes, together with some expansion along those planes in the line of their dip; and a great mass of evidence was brought forward to support this theory.