Alfred Harher — On the Cause of Slaty Cleavage. 15 



the structure of the Dinosaurian skeleton. In a subsequent paper 

 we propose to give a notice of some of the more important points 

 which M. Dollo's researches have helped to elucidate. In the mean- 

 time we cannot but recall with feelings of the warmest gratitude 

 and admiration those earlier palEeontological labourers, who, with 

 exceedingly imperfect materials at their command, succeeded in 

 demonstrating the existence of this great Sub-Class of Animals, 

 which Prof. Huxley has clearly shown occupy a position between 

 Birds and Eeptiles, and help us most materially to complete the 

 grand chain by which the Animal Kingdom is linked together. 



EXPLANATION OF PLATE I. 

 Fig. 1. Iguanodon Mantelli, Meyer; reproduction on a reduced scale oP M. Dollo's 

 beautiful plate, already referred to (Bull. Mus Eoy. d'Hist. Nat. de 

 Belg. 1884, t. iii. pi. vii.) i, the ischium: il. the iliiini : p, the pubis. 

 ,, 2. Skull of I. Mantelli (seen in profile) 4th nat. size, p, the prsesymphisial 

 bone (see observations on this remarkable bone by Mr. J. W. Hulke, 

 F.E.S. (Pres. G. Soc. 1884), in his Anniversary Address, Quart. Journ. 

 Geol. Soc. 1884, vol. xl. pp. 47-51.) 



III. — The Cause op Slaty Cleavage : Compression v. Shearing. 



By Alfred Harker, B.A., F.G.S., 



of St. John's College, and Demonstrator in Petrology in the Woodwardian 



Museum, Cambridge. 



THE received theory of Slaty Cleavage has generally been held 

 to aiford 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 them- 

 selves. These changes were ascribed to great lateral compression of 

 the rock in the direction perpendicular to the cleavage-planes, to- 

 gether 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. 



In papers recently published in the Geological Magazine (1884, 

 pp. 268, 396, etc.) the Eev. 0. Fisher rejects this compression and 

 would substitute shearing, i.e. a creeping motion such as would dis- 

 tort a cube into an oblique parallelepiped without change of volume 

 (see Fig. 3, p. 399). He indeed admits a certain compression in 

 addition, but it is so slight that for simplicity we may disregard it. 



Mr. Fisher contends that the kind of distortion presented by the 

 fossils of cleaved rocks is such as would be produced by a shearing 

 motion, and he seems to imply that these appearances could not be 

 brought about by compression and expansion. But it is easy to see 

 that a shear of given amount is precisely equivalent, so far as dis- 

 tortion is concerned, to a compression of proper amount in a certain 

 direction with a corresponding expansion in a direction perpen- 

 dicular to it. For the distorting effect of any sti-ain or system 

 of strains can be fully represented by the 'ellipsoid of distortion,' 

 i.e. the ellipsoid into which a sphere would be distorted by the 



