IN his critique of Dr. Becker's theory of slaty cleavage in NATURE of May 4, “A. H.” says that it is substantially the same as mine, and rightly objects that, “if the cleavage plane were a plane of shearing it would correspond with a circular section of the ellipsoid” of distortion. It is true that I made that suggestion in the body of my first paper on cleavage in the Geological Magazine, 1884, but in a postscript to that paper I stated that a conversation with Mr. Harker had led me to the conclusion that the cleavage surfaces are determined by the position of the principal axes of the ellipsoids of distortion produced by a shearing movement, and to this view I have ever since adhered.