464 R, D. Oldham — Gneissose Hocks of the Simalaya. 



The exact causes productive of foliation are not thoroughly under- 

 stood, but it appears to be in some way related to the parallel 

 structure due to stratification or cleavage, as the case may be. In 

 the case of an intrusive sheet of granite, there would be neither 

 stratification nor cleavage ; but friction against the sides of the 

 channel it flowed in would be sufficient to produce a slight parallel 

 structure in a viscid mass ; while it is not inconceivable that the 

 very fact of the minerals, in the rock on either side, arranging them- 

 selves in a laminated structure, would induce similar rearrangement 

 of the minerals in the gradually cooling granite. In short, I believe 

 that the very slight foliation of the larger masses is principally a 

 fluxion structure, while the more developed structure of the thinner 

 bands, and near the margins of the larger masses, was produced in 

 the solid but still heated granite by the same causes — whatever they 

 be — that led to the foliation of the adjacent sedimentary beds.^ 



Before leaving this subject, there is one point that may be referred 

 to with advantage. In Mr. Lydekker's memoir on the geology of 

 Cashmir it is stated, both on the map and in the text, that part of 

 the gneiss of that region consists of metamorphosed Silurian strata ; 

 this statement will not, I fear, be borne out by a more detailed 

 examination of the ground. I have had a tolerably extensive, if 

 fragmentary experience of the Himalayas, during which I have 

 never seen a case of beds, which occur elsewhere as slates, having 

 been converted into gneiss ; but I have seen sections, similar to those 

 described by Mr. Lydekker, where there appears to be a gradual 

 passage from slate to gneiss. It appears not to be uncommon that 

 near the boundary of a crystalline area there should be sections 

 showing a considerable thickness of gneissic rocks, with small 

 intercalations of non-gneissic beds, which can occasionally be 

 recognized as belonging to some definite horizon in the slaty series. 

 The sections, at first sight, seem to indicate an extreme metamorphism 

 of the beds, a few of which have so far escaped metamorphism as 

 still to be recognizable. Apart, however, from the fact that the 

 gneiss exhibits those features especially characteristic of the gneissose 

 granite, where I have been able to trace the horizontal extension of 

 the rocks into unaltered slates, the change has not been by a gradual 

 diminution in the metamorphism of the rocks as a whole, but by a 

 gradual diminution of the gneissose beds, those which remain being 

 as distinctly crystalline as before, till, where they have diminished 

 in thickness and the schistose beds prevail, they can be distinctly 

 recognized as intrusive gneissose granite. The sections indicate, 

 not an extreme degree of metamorphism of the slaty rocks, but their 

 more or less complete obliteration by gneissose granite. 



In Western Garhwal there is a considerable development of 

 arkose beds which have become foliated, but are still recognizable as 

 foliated arkose. It is quite conceivable that similar beds might be so 

 metamorphosed as to be undistinguishable from gneiss, but with this 



^ In 1884 Colonel McMahon seems to have held an opinion somewhat similar to 

 this (see Eecords Geol. Surv. India, vol. xvii. p. 721, but so far as I can understand 

 Ids paper in the May Number of this Magazine, he has now abandoned it. 



