It has given me much satisfaction to see Mr. Harker's criticism of my theory of cleavage, because I think few stronger champions could be found for the older hypothesis. His remarks deserve a reply. I first give Mr. Harker's description of the theory which I have endeavoured to supersede. The changes which have induced cleavage “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, although he has not incorrectly described my theory, I prefer to restate it in the words I used at the end of my paper on “Faulting, Jointing, and Cleavage.” The internal movement, which has produced cleavage, “would have been accomplished by faulting, had not the friction been too great, owing to pressure, to allow of sliding along surfaces of separation. Viscous shearing therefore performed the office and produced cleavage surfaces.”