174 Rev. 0. Fisher— The Cause of Slaty Cleavage. 



found examples of them, not only in several parts of the British 

 Islands, but among specimens from Western Africa, India, and the 

 Pacific Islands. 



I am now preparing descriptions of the rocks containing this 

 interesting mineral, in which I shall fully discuss its physical 

 characters, and the analyses which have been made of it. As the 

 mineral appears to be by no means uncommon, however, I have 

 thought it well to call attention to it in this preliminary note. 



"VIII. — The Cause of Slaty Clicatage. Sheaking v. Compression. 



A Eeply to Mr. Harker. 



BytheEev. 0. Fisher, M.A., F.G.S. 



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." ^ 



Mr. Harker truly says that I " contend that the kind of distortion 

 presented by the fossils of cleaved rocks is such as would be pro- 

 duced by a shearing motion." But I did not mean to deny the alter- 

 native, viz. that these appearances could equally have been brought 

 about by compression combined with expansion, as indeed appears 

 from my remark that the ellipsoid caused by shearing " corresponds 

 with that given by Phillips " in his diagram ; ^ and we know that he 

 was attributing distortion to compression combined with expansion. 



Mr. Harker evidently refers to Dr. Haughton's calculations,' where 

 he says that the ellipsoid of distortion comes out something very like 

 a flat oblate spheroid, which my theory would not explain. Dr. 

 Haughton's method of calculating the form of the ellipsoid of dis- 

 tortion is ingenious, and I now believe correct in theory ; but there 

 are points connected with the author's application of it to actual 

 instances, which I think present great difficulties. His law I. appears 

 to assert, that the greatest " distortion or elongation " of a fossil is 

 in the direction of the intersection of the cleavage and of the 

 bedding, which must be the strike of the rocks. Does this agree 

 with the experience of other observers'? Also he brings out the 



1 Geol. Mag. Dec. III. Vol. II. p. 15. 



2 Gkol. Mag. Dec. III. Vol. I. p. 276. 



3 Phil. Mag. Jan. 1856. * I'liil. Mag. 1856, vol. xii. p. 409. 



