348 Lieut. -General 0. A. McMahon — 



rook in whicli crystallization had commenced prior to its being 

 moved into place, would, if the pressure were kept up during the 

 subsequent stages of its cooling, whilst it was " gradually pro- 

 gressing towards rigidity," inevitably result in the production of 

 foliation, and would account for the " undulose extinction, fractured 

 minerals, and crumpled micas, and such like evidence of strain and 

 mechanical action," to be found in the Himalayan gneissose-granite 

 (Pres. Address, loc. cit., pp. 288-9). Given tangential pressure 

 and strain exerted on an intruded granite during its gradual 

 cooling, these results must follow. 



The above explanation, at all events, oflfers a definite theory to 

 explain the foliation and other phenomena to be observed in 

 the Himalayan granite, and it is no answer to this theory 

 to point to evidence of strain and mechanical action in the 

 rock ; for the existence of strain and mechanical action during the 

 critical period in the history of the granite, is an essential part of 

 the theory itself. 



I have looked in vain through Mr. Middlemiss's Memoir for 

 evidence, other than the above, to rebut my theory. My second 

 Presidential Address, dealing with the structural characters of the 

 granite, probably did not reach the author until after his Memoir 

 was set up in type, but my address was, in the main, a condensed 

 statement of a theory long held by me, and to be found in my 

 previous papers. The one published in this Magazine in May, 1887, 

 alluded to above, for instance, contains an abstract statement of the 

 theory in question. 



It may be as well to remark here that so far as the foliation of the 

 Himalayan granite is concerned, the foliation set up was a change 

 of structure only, and did not involve any metasomatic change in 

 the chemical composition of the minerals. The foliation was, in its 

 main features, a parallelism in the orientation of the minerals which 

 alreadjr existed in the still plastic magma, and was superinduced by 

 pressure applied before the final consolidation of the rock. The 

 mineral principally concerned in producing the foliation was mica, 

 an original mineral abundantly present in the granite before 

 foliation set in. 



Given an abundance of mica in a still plastic granite, T do not see 

 how great pressure could be applied without causing the platy 

 leaves of mica to arrange themselves with their flat basal faces 

 at right angles to the direction of the pressure. We have here, 

 then, a vera causa to account for the foliation of an igneous rock. 

 Given a sufficient amount of mica, or other mineral of similar habit, 

 and sufficient pressure applied between the formation of the mica 

 and the consolidation of the rock, and we have a sufficient explana- 

 tion of the foliation it may exhibit. 



It may be as well to point out that the tendency of platy minerals 

 to arrange themselves under pressure with their flat faces at right 

 angles to the direction of the pressure, was probably aided by other 

 contributory causes. Even after the granite was moved into place, 

 and the crystallization of some of its minerals, begun in plutonio 



