G. JB . Becker — Current Theories of Slaty Cleavage. 7 



planes and must crystallize there in obedience to some law. 

 Sharpe believed the micas secondary and that they crystallized 

 most rapidly in the direction of least resistance.""" I entirely 

 agree with him. 



It would appear from Mr. Leith's discussion that as a matter 

 of fact the relation sought to be proved is not that mica scales 

 form normally to local stress, but that they are arranged per- 

 pendicularly to the shortest axis of the strain ellipsoid. He 

 offers a variety of evidence that this relation exists in those 

 rocks which have undergone what he designates as flow cleav- 

 age, and this evidence is discussed in the fifth chapter of part 

 one. He takes up first the distortion of pebbles in conglom- 

 erates, which he alleges are elongated in a direction parallel to 

 the schistosity of the matrix. A schistose conglomerate is not 

 a particularly favorable rock for a discussion of this description, 

 because the schistose lamellse wind in and out between the 

 pebbles, and it is impossible to assign to them an average direc- 

 tion with any degree of accuracy .f My experience, however, 

 does not coincide with his, so far as observation is concerned. 

 Where conglomerates have been rendered schistose, and the 

 pebbles are not so abundant as to touch one another, it is in some 

 instances possible to break them out with adherent portions of 

 the matrix. I have thus extracted many scores of pebbles from 

 schistose conglomerates where the conditions appeared favor- 

 able, and I have found that each pebble came out with an 

 appendage of schist, a sort of beard, which in almost all cases 

 stood at a sensible angle to the major axis of the pebble. Never- 

 theless, I do not for a moment undertake to deny that there are 

 conglomerates where there is sensible coincidence between these 

 directions. The question is what such a concidence would in- 

 dicate. I do not think that Mr. Leith has put the correct 

 interpretation upon it. In conglomerates, as everyone knows, 

 there is a strong tendency for the pebbles to arrange themselves 

 with their shortest axes perpendicular to the plane of bedding, 

 though there is usually some imbrication or shingling. If the 

 plane of bedding were parallel to the plane of fixed resistance, 

 and if also a force were to act on the conglomerate at 90° to 

 the plane of bedding, then the elongation of such pebbles as 

 lay quite flat would coincide with the normal to the least axis 

 of the strain ellipsoid. But each of these conditions, must be 

 of very rare occurrence, and that both should be fulfilled at 

 once is in the highest degree improbable. 



*Geol. Soc. Journ., vol. v, p. 129, 1849. 



fOn p. 116, Mr. Leith asserts that in a rock undergoing flow " the general 

 effect of rigid particles is to transmit stresses locally in directions normal to 

 themselves." I know of no such theorem in mechanics, and believe the 

 statement incorrect. 



