R. D. Oldham — Gneissose Boclis of the Himalaya. 461 



V. — The Gneissose Eocks of the Himalaya. 

 By E. D. Oldham, A.E.S.M., F.G.S., 



of the Geological Survey of India. 



IT had not been my intention to publish anything on this subject 

 at present, as I have in preparation a review of the present state 

 of our knowledge of Himalayan geology ; but as Col. MoMahon has 

 started the subject, and his paper is not exhaustive, I trust the follow- 

 ing outline of that part which relates to the gneissose rocks may 

 prove of interest. 



The gneissose or granitoid rocks of the Himalayas may be divided 

 into three groups : — 



(1) The fundamental or "Central" gneiss of Dr. Stoliczka. 



(2) The orthoclase, usually porphyritic and gneissose, granites. 



(3) The oligoclase granite. 



The last of these appears to be distinct from and of later date than 

 the first two, and will not be further referred to here. 



The fundamental, or, to use the term under which it was first 

 described, the central, gneiss consists of a great thickness of crystal- 

 line rocks. In the little disturbed sections of the Upper Pabar 

 Valley in Bissahir it is seen to unconformably underlie rocks which 

 I have little doubt are the equivalents of Dr. Stoliczka's Babeh series 

 of Silurian age. Both by lithological structure and mode of occur- 

 rence, these beds are shown to be of metamorphic, as opposed to 

 intrusive origin ; they contain beds varying from almost pure 

 felspar to almost pure quartzite or mica-schist, but felspar is seldom 

 altogether absent ; some of the beds are augen gneiss, the eyes being 

 lenticular in form, lying in accord with the planes of foliation, and, 

 as regards their internal structure, consisting of a single twinned 

 crystal of orthoclase. 



On the section over the Babeh Pass, that first examined by Dr. 

 Stoliczka, the beds are more disturbed and more metamorphosed. 

 Highly foliated beds are rare, and the rock is for the most part a 

 more or less fine-grained granitoid gneiss : some beds are augen gneiss, 

 in which the eyes still maintain their lenticular form, though, as 

 a rule, they have more or less acquired an outline in conformity with 

 their crystalline structure ; these crystals, however, still lie along 

 the planes of foliation or very slightly oblique to them. 



There can be little doubt that it is from the fusion of these beds 

 that the gneissose granite was derived. Typically this rock consists 

 of a somewhat fine-grained, slightly-foliated matrix, through which 

 porphyritic crystals of orthoclase are scattered ; the crystals exhibit- 

 ing no definite orientation, but being scattered about with their axes 

 pointing in every direction, as described by Col. McMahon. It 

 occurs in large intrusive masses, or in veins of various thickness, 

 usually intruded parallel to the bedding planes ; the former usually 

 show very slight signs of foliation, except near their boundaries, 

 while the latter are almost, if not quite, invariably distinctly foliated. 

 As the veins are traced away from their parent masses, the 

 larger crystals of felspar appear to be left behind, and the rock 



