468 Cleavage and Foliation. pakt n. 



tary origin, without it be further shown in each case, 

 that the folia not only strike, but dip throughout in 

 parallel planes with those of the true stratification. 



As in some cases it appears that where a fissile rock 

 has been exposed to partial metamorphic action — for 

 instance, from the irruption of granite — the foliation 

 has supervened on the already existing cleavage-planes ; 

 so perhaps, in some instances, the foliation of a rock may 

 have been determined by the original planes of deposi- 

 tion or of oblique current-lam inae : I have, however, 

 myself, never seen such a case, and I must maintain that 

 in most extensive metamorphic areas, the foliation is 

 the extreme result of that process, of which cleavage 

 is the first effect. That foliation may arise without any 

 previous structural arrangement in the mass, we may 

 infer from injected, and therefore once liquefied, rocks, 

 both of volcanic and plutonic origin, sometimes having 

 a 'grain' (as expressed by Professor Sedgwick), and 

 sometimes being composed of distinct folia or laminae of 

 different compositions. In the earlier chapters of the 

 present work, I have given several instances of this struc- 

 ture in volcanic rocks, and it is not uncommonly seen in 

 plutonic masses — thus, in the Cordillera of Chile, there 

 are gigantic mountain-like masses of red granite, which 

 have been injected whilst liquefied, and which neverthe- 

 less, display in parts a decidedly laminar structure. 1 



Finally, we have seen that the planes of cleavage 

 and of foliation — that is, of the incipient process and of 

 the final result — generally strike parallel to the principal 



1 As remarked in a former part of this chapter, I suspect that 

 the boldly conical mountains of gneiss-granite, near Bio de Janeiro, 

 in which the constituent minerals are arraDged in parallel planes, 

 are of intrusive origin. We must not, however, forget the lesson of 

 caution taught by tbe curious clay-stone porphyries of Port Desire, 

 in which we have seen that the breaking up and aggregation of a 

 thinly stratified tufaceous mass, has yielded a rock semi-porphyritic 

 ■with crystals of feldspar, arranged in the planes of original deposition. 



