350 Lieut.- General C. A. McMahon — 



In the above remarks my chief object has been to bring before 

 the reader, as clearly as I could, what I believe to be the main 

 factor in the production of the foliation observable in the Himalayan 

 granite. Other causes contributed to produce the effects observed. 

 Traction when the rock was being moved into place, ^ and the 

 corrosive action of the acid residuum on the phenocrysts, might 

 be named ; but it is needless to dilate on them here, as I have 

 already written much in other papers on these themes. I am glad 

 to see that Mr. Middlemiss recognizes the validity of the last- 

 mentioned factor, for he remarks (p. 63) with reference to the 

 porphyritic crystals : " The larger ones have often only a very 

 vague outline in the rock, as if their boundaries had become in- 

 distinct from the reaction of the still molten magma upon them 

 whilst the rock was in a half-consolidated state." 



It is a great relief to be able to turn at this point in the discussion 

 to an interesting and important piece of work in the Scotch High- 

 lands by the officers of H.M.'s Geological Survey. I allude to the 

 explanation of Sheet 5, Memoirs of the Geological Survey, Scotland, 

 by Messrs. Home, Peach, and Teall, 1896. In describing the 

 granite of the south-east portion of the county of Kirkcudbright, 

 Mr. Home tells us at p. 23 that " the granite occurs mainly in the 

 massive form throughout this extensive area, but there are certain 

 limited tracts where it has a marked foliated character. The latter 

 variety is a marginal phenomenon. It does not, however, extend 

 continuously round the border of the igneous mass, being restricted 



to well-defined areas As the observer passes inwards 



towards the centre, he finds the foliated variety of the border tract 

 gradually to merge into the massive granite." 



The granite contains basic inclusions and acid veins, which are 

 also foliated, the foliation of the granite, basal inclusions, and acid 

 veins having a common direction. " The evidence obtained in the 

 field," Mr. Home writes (p. 24), "clearly points to the conclusion 

 that the foliation must have been superinduced on the massive 

 granite with the basic inclusions, after the formation of the acid 

 veins." 



I infer, from what follows, that the acid veins must have been 

 formed after the granite was moved into place, but before it cooled 

 down. The foliation of the basic inclusions was evidently due to the 

 cause that produced the foliation of the granite itself ; and as the latter 

 is attributable, as will be seen later, to pressure before cooling, 

 I infer that the basic inclusions were made plastic by the contact 

 action of the granite before the pressure was applied. 



" After the development of the foliation," Mr. Home continues 

 (p. 25), "the foliated granite was injected by a well-marked series 

 of dykes. .... These dykes run in a north-westerly direction, 

 and have fine-grained or chilled edges. No single instance has 

 been observed among them of any trace of foliation." 



^ Sir A. Geikie mentions interesting cases of "extreme mechanical deformation 

 by the movement of a still molten rock " in his paper on the Tertiary Basalt 

 Plateaux of N.W. Europe (Q.J.G.S., vol. lii, p. 333). 



