270 Rev. 0. Fisher — On Faulting, Jointing, and Cleavage. 



sirable in the case of the green spots seen on many purple slates ; for 

 their elliptic form cannot be relied upon to prove elongation, unless 

 we knew tliat they were originally spherical. The author has seen 

 a family of these spots covering a slate laid down as a paving stone, 

 in which the spots are of various shapes ; and most of them contain 

 traces of some foreign body as a nucleus, which has probably deter- 

 mined, not only their shapes, but also their positions : for the dark 

 streak representing the fragment of former foreign matter lies as 

 usual with its length parallel to the cleavage. This would lead one 

 to suppose that the discolouration took place after the shearing.^ 



Fig. I 



The diagram (Fig. 1) illustrates the effect of a shear in altering 

 the forms of a square, and of a circle, both without and with com- 

 pression. We may regard the square as a section of a cube, and the 

 circle as that of a sphere, and we see how they will be distorted into 

 rhomboids and ellipsoids respectively. It will be remarked that 

 every section of either rhomboid, parallel to the shear, and per- 

 pendicular to the plane of the paper, will be a square, equal to the 

 section by the same plane of the original cube ; and every section 

 so made of the uncompressed ellipsoid will be a circle equal to the 

 section of the sphere, and, in the case of the compressed ellipsoid, 

 it will be a circle equal to the section, made by the same plane, of 

 the oblate spheroid, into which a like compression would deform the 

 sphere. The corresponding distortion of fossils may then be readily 

 comprehended. 



Professor Phillips wrote, " Cleavage is a peculiar structure im- 

 pressed on certain rocks and in certain regions by the operation of 

 some very extensive cause, operating after the stratified rocks had 

 undergone great dis2:)lacement. For this fundamental generalization 

 we are, I believe, entirely indebted to Sedgwick." ^ The principal 

 generalizations stated in his report are, that the direction of the 

 cleavage usually coincides with the mean strike of the beds ; and 

 that the dip, or hade, of the cleavage is greatest where the disturb- 

 ance has been greatest. He, however, told the author in conversa- 

 tion, during the meeting of the Brit. Assoc, in 1862, that the two 

 great laws of cleavage are, that its strike is always very nearly 

 coincident with the strike of the beds, and at anticlinals and 

 synclinals it is normal to their surface and varies slightly from 



^ The slate is in the yard of the Eagle Tavern at Cambridge. 



' Phillips on Slaty Cleavage, Brit. Assoc. Reports of the meeting, 1856. Reports 

 1857, p. 373. — See Sedgwick, "On the Structure of large mineral masses," Trans. 

 Geol. Soc. 2nd series, vol. iii. 1835, pp. 473, 474. 



