168 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Jan. 31, 



Where cleavage is previously existing in rocks, it appears very 

 reasonable to suppose that the folia would arrange themselves along 

 the lines of cleavage, which may be regarded as so many cracks or 

 vacuities extremely convenient for the development of crystalline 

 matter. These spaces probably may be much more diffused than 

 would appear at first sight. For we may suppose matter compressed so 

 as to give it a cleavage-structure, and this pressure then removed or 

 only relaxed : the elasticity of the mass itself (however small) 

 would naturally cause a certain degree of relaxation throughout its 

 entire body, giving it, in consequence, a general porous structure ; 

 the pores being of course elongated and flattened in the direction of 

 the planes of cleavage, or at right angles to the compressing force. 

 Afterwards, when the foliating forces, which cannot be other than 

 chemical, come into action in a mass in this state, we may fairly 

 expect that the lines of fohation will be identical with these lines ; — 

 which seems to be the reason that we so often find the lines of 

 cleavage and foliation parallel. In case the chemical action here 

 alluded to was so intense as to obliterate the cleavage-planes, then 

 we have no guarantee that the lines of foliation will follow these 

 planes. 



In some cases possibly the cleavage may have taken place after 

 foliation, and in some measure accommodated itself to these lines as 

 offering the least resistance. 



The superincumbent pressure, likewise, must be taken into account 

 in considering the arrangement of foliation ; as it appears likely that 

 this would act against any vertical position of the crystals ; and 

 the elongated and flattened appearance generally seen in the cry- 

 stalline minerals inducing foliation appears doubtless the result of this 

 action. 



Foliation and cleavage have both, I believe, been referred by some 

 to electric or magnetic action ; but I am afraid that it is too general 

 to refer to these causes effects that we do not at first sight compre- 

 hend. If it be found, as I believe to be the case, that electricity 

 traverses more easily in the direction of the cleavage-planes than 

 across them, I think tliis argument has no further weight than as 

 confirming the porous structure here supposed as consequent on the 

 relaxation of a previously applied pressure ; as it seems very probable 

 that the resistance offered to the current in this direction would, in 

 consequence, be less than across the grain, — and we know that most 

 non-metallic mineral substances are very imperfect conductors, 

 whereas space, or air rarefied by heat or exhaustion, are conductors, 

 though not equal to the metals themselves. 



Darwin and Sharpe (the latter in his paper on the Highlands) 

 put forward the view that cleavage and foliation are identical, — that 

 is, are parts of one and the same process, the former being but the 

 first stage of the latter. To this I must object, and I believe that 

 they are not only distinct phsenomena, but that the causes producing 

 them are also distinct ; as in foliation we must have chemical action 

 brought into operation, whilst in cleavage no such action is requisite, 

 and the phsenomena admit of a mechanical explanation. 



