Ixxvi PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



We have also to regard the varied resistances which heds of dissimilar 

 cohesion and composition would offer to the bending or contorting 

 force, the slipping of beds on each other (a very common circum- 

 stance in some contorted countries), and, in certain regions, the com- 

 plication arising from beds moved more than once and in different 

 directions. 



Whatever, therefore, our views may be as to the cause of cleavage 

 planes, the effects which can be produced by mechanical action alone 

 demand our attention ; and hence the Society stands much indebted 

 to Mr. Sharpe for bringing this subject so pointedly before it. The 

 hypothesis he supports Mr. Sharpe considers sufficient to account for 

 the facts he has noticed, and those who were present here w^hen 

 the discussion upon this communication took place, must have fully 

 appreciated the desire of our colleague to arrive at the truth, regard- 

 less of his own hypothesis or that of any one else, so that the facts 

 be explained. 



AVe had occasion in the address of last year, while alluding to the 

 memoir of Mr. Hopkins, before noticed, to express our participation 

 in the view that cleavage was due to the action of a force by which 

 the component particles of the rock were arranged in a manner 

 analogous to crystallization, pointing to the discoveries of Faraday 

 and others respecting the properties of matter as affording grounds 

 for such a view. At the same time we fully admit, with Mr. Sharpe, 

 the importance of ascertaining the laminating effects which may be 

 simply due to mechanical pressure, and the necessity of not attributing 

 to one cause the effects which may really be due to another. 



It may be fairly asked, if the casts of fossils are found elongated 

 in the planes of cleavage, why not included fragments of rocks also, 

 supposing a motion of their component particles, from some efficient 

 cause ? In the first place it would be desirable to ascertain, though 

 the fragments may be in the planes of the cleavage, if they may not 

 have been deposited in the planes consequent on the original drift of 

 detritus, these fragments included, along the bottom of water, so that 

 they were accumulated in planes sloping from the upper surface of 

 the bed to its base, in the manner commonly known as * false bedding.' 

 If the angle of the plane of cleavage (to which such fragments are 

 parallel) be too great for this view, then we have to regard the 

 lengthening of the apparent fragments, and how far they may have 

 really been hard when acted upon ; and in doing so we have to con- 

 sider, if the fragments be those of some hardened rock or rocks, the 

 effects that would follow from the pressure of bodies of unequal 

 resistance in the bed of which they constituted the parts, as for ex- 

 ample clay beds holding fragments of previously consolidated slates, 

 or pebbles of sandstone or hard trappean rocks. Mr. Sharpe has 

 called attention to the shape of the component particles of fine roof- 

 ing-slates, and the probability of their having been lengthened in the 

 lines of cleavage ; a very important fact, whatever our views of the 

 kind of force employed. 



It would be out of place further to dwell on this subject, one of 

 great geological interest, more particularly when combined with the 



