400 Rev. 0. Fisher — On Cleavage and Distortion. 



with the dotted restoration in his section, which is evidentlj' 

 imaginary, and shows the mesial fold too much to the left. It comes 

 out clearly, on comparing the two diagrams, that the diametral plane 

 of the ellipsoid corresponds with yy, the plane of cleavage in 

 Sharpe's figure. The plane of cleavage, therefore, does not lie in 

 the direction of movement among the ultimate pai'ticles of the rock, 

 but is inclined to it. It will be shown later on that the inclination 

 may be as great as 45"^, and diminishing from that as a maximum, 

 the two directions may become nearly, though never quite, identical 

 as the shear is increased. 



An inspection of the ellipse shows that it will have two diameters 

 which are equal to the diameter of the circle. One of these will be 

 in the direction of the shear, and the other similarly situated on the 

 other side of the minor axis. Objects found along these diameters 

 will not be distorted, while those lying nearer to the major axis will 

 be lengthened, and those nearer to the minor shortened. 



Our ellipse evidently corresponds with that given by Phillips ' in 

 illustration of Dr. Sorby's views regarding unequiaxed particles. 

 And if such particles were originally lying at all angles in the circle, 

 they would become packed together most closely about the diametral 

 plane of the ellipsoid, and thus, according to his theory, that ought 

 to be a cleavage plane, as we have seen that it is. 



It is important to notice that in our diagram there is not any com- 

 pression whatever represented, so that it appears that lateral pressure 

 is not required for the geometrical phenomena of distortion by 

 cleavage. Nevertheless, if the reasoning of Part IV. is sound, 

 which I believe to be the case ^ (excepting the assumption that the 

 cleavage and shear coincide), a pressure across the tract is mechani- 

 cally requisite to avoid faulting. The effect of this will be to con- 

 dense the rock while it is being converted into slate. We must 

 therefore use the smaller ellipse in Fig. 2, if we wish accurately to 

 delineate the distortion. This, however, will in no way alter the 

 general character of the phenomena. 



The bedding of the strata will of course be in general distorted, 

 as well as the fossils. And the very remarkable phenomena of 

 close folds on a small scale accompanied by cleavage planes, such as 

 is represented in Dr. Sorby's diagram,^ may according to the present 

 theory be explained. We observe that the axes of the folds are 

 nearly parallel to the cleavage. We may therefore regard each 

 semi-fold as approximately a semi-ellipse, which has arisen from the 

 distortion of a semi-fold much less compressed. If, then, this rock 

 was more rigid than the matrix in which it was inclosed, it would 

 not, like it, yield entirely by viscous shearing, but partly by fracture, 



^ Phil. Mag. Jan. 1856. 



2 The argument (p. 273) regarding the circumstances which would lead to a viscous 

 shearing as distinguished from faulting sup^DOses that a horizontal stress causes a 

 shearing of the parallelepiped. We now regard the shearing as causing a stress. The 

 relations of the forces will remain unaltered, although the directions of the motions 

 will he reversed. 



3 Edin. New Phil. Journ. 1853, vol. iv. p. 138, copied by Professors Tyndall and 

 Phillips. 



