Since a “shear” is mathematically the same as a compression in one direction with a compensating expansion in a direction atright angles to it, the consideration whether slaty cleavage can be ascribed to movements of this character resolves itself into the question whether the cleaved rocks have or have not suffered a total diminution of bulk. The most convenient way of treating thequestion is by discussing, as I did in my former paper (p. 15), the form of the “ellipsoid of distortion.” The ellipsoid produced by a pure shear would be one of three unequal axes, of which the second or mean axis would be a geometric mean between the other two: whereas, if the expansion did not compensate the compression, the second axis would be greater than this geometric mean, and if the expansion were slight, the second and greatest axes would be nearly equal. The facts seem to accord with the latter supposition, and the diminution of bulk thus indicated agrees with what might be a priori expected.