Rev. 0. Fisher — T/te Cause of Slaty Cleavage. 175 



ellipsoid of distortion to be commonly an oblate spheroid. In cases 

 where the bedding coincided with the cleavage plane, this would 

 cause fossils to be scarcely at all distorted ; whereas in those I have 

 seen they are usually much elongated in the direction of dip of 

 cleavage. The great condensation of the material required by his 

 result is likewise scarcely conceivable, except under the exceptional 

 circumstance of its having been a shale before cleavage was induced. 

 In such a case, the chief condensation would have occurred previously 

 to the cleavage being set up, and ought not to be included in the 

 results of that action. But Dr. Haughton's method of calculation 

 does not afford any means of separating the two. The Tintagel 

 fossils have much the appearance of shells which have been origi- 

 nally preserved in a shale, after the manner of the fossils from the 

 Oxford Clay at Christian Malford. 



If cleavage be, as I argue, due to a shearing action, the question 

 whether the cleavage planes will be in the direction of the shear, or 

 in that of the longer diametral plane of the ellipsoid of distortion 

 produced by it, is perhaps an open one. In my first paper I sug- 

 gested that the two sets of planes would coincide,^ and I note that 

 Mr. Hai'ker would incline to this view, could he admit the theory at 

 all. In ray second paper, influenced by Sharpe's figures, I concluded 

 the other way. It is possible this later conclusion may be too sweep- 

 ing. But I feel pretty certain ; that if the rock be of such a nature, 

 that its constituent particles are all distorted, then cleavage must on 

 the whole follow the distortion. If, however, it consists of bard 

 particles imbedded in a less rigid matrix, so that those particles resist 

 deformation and the matrix shears past them, then the cleavage may 

 be less perfect, and may follow the shear. Those particles, eye- 

 shaped in section, which occur in coarse slates and in schists, point 

 to this kind of action ; and their outline indicates stream-lines, in 

 which the matrix has flowed past them in opposite directions on 

 their opposite surfaces, and abraded or dissolved away their substance 

 until they have been reduced to the lenticular forms which now 

 guide the cleavage. There may be degrees of action intermediate 

 between these extremes. 



There is no difficulty in accounting for contorted layers of harder 

 rock, interbedded with more yielding rock which shows cleavage, as 

 at Ilfracombe. The mode in which this would be efifected is excel- 

 lently illustrated in some diagrams given by Stapff.^ The effect pro- 

 duced would depend upon the inclination of the layers to the direction 

 of the shear. If that were such that they made at first an acute 

 angle with the direction of the sheai', they would be more and more 

 crunqded as the shear went on, until such time as the axis of the folds 

 became perpendicular to the direction of the shear. When that position 

 had been passed, they would begin to be straightened out again, were 

 the material capable of sustaining tension. But as it is not, they would 

 be pulled to pieces, and afterwards worn down into flattened frag- 



1 Geol. Mag., Dec. III. Vol. I. p. 269. 



? Wie am Mte. Piottiuo die I'arallelstruktur des Gneisses in Scliiclitung iibergcht. 

 Neues Jaliibuch fiir Miueralogie, etc., 1882, I. Band, pp. 92, 93. 



