B. D. Oldham — Gneissose Hocks of the Himalaya. 463 



sTaown to be a true gneiss, it presented all the characters of an 

 igneous rock. In this paper he dropped the term granitoid gneiss 

 in favour of g-neissose OTanite, and added that " it is for consideration 

 whether the terra ' central gneiss,' introduced by the late lamented 

 Dr. Stoliczka, and since used to denote the ' granitoid gneiss ' of the 

 North-West Himalayas, should not be discontinued in future." 

 Consideration is entirely in favour of this proposition, but against 

 the total abandonment of the term " central gneiss," which may still 

 be conveniently used for the fundamental gneiss, the oldest rock, 

 in the Himalayas. 



The separation of these two rocks in the field will often be a 

 matter of difficulty, and not always possible, except where the 

 gneissose or the intrusive characters are well developed ; the micro- 

 scope will not aid in this, for, as I have remarked above, in one case 

 it has declared what can be shown to be an intrusive rock to be a 

 metamorjohic gneiss, while some of the more granitoid forms of the 

 central gneiss show so little foliation and are so granitoid as seen in 

 a hand specimen, that I doubt whether even microscopic examination 

 would give a decided answer as to whether they are granite or gneiss. 

 The general result then is the satisfactory one (for it is always more 

 satisfactory to confirm than to refute a previous observer) that Dr. 

 Stoliczka was correct in describing the oldest rock he observed as 

 a gneiss, while Col. McMahon is equally correct in maintaining that 

 the rocks of the Sutlej Valley and the Dhaoladhar are granite. I 

 may now pass on to consider the cause of the foliation of this granite. 



In considering this question, it is necessary to distinguish between 

 the large, slightly foliated masses and the distinctly foliated sheets. 

 In the former case the obscure foliation is probably in the main a 

 form of fluxion structure, but the well-developed foliation of the 

 thin sheets, which are occasionally sufficiently fissile to be used as 

 flags or roofing slates, cannot be solely due to this cause. 



When we find intrusive sheets of a few feet, or even a few tens of 

 feet, in thickness, extending for miles without more than mere local 

 variations of thickness, it must be evident that their fluidity cannot 

 have been in any great degree due to excess of temperature ; in the 

 case of the thinner sheets I do not think that it can have been in any 

 degree due to this cause, but rather to a difference of composition 

 which enabled the granite to maintain some degree of fluidity, while 

 the rocks into which it was intruded remained solid. Whatever this 

 temperatiire may have been, it was sufficient to metamorphose the 

 sedimentary beds which have alwaj'S been converted into more or 

 less perfect schists, that do not exhibit any marked increase of nieta- 

 morphism near the sheets of gneissose granite intruded into them.^ 



^ Contact metamorphistn is only conspicuous in the case of large intrusive masses. 

 The statement in the text may seem inconsistent with that of Col. McMahon regard- 

 ing the slightly metamorphosed condition of the slates in contact with the outer band 

 of gneissose granite in the Dalhousie region ; but the particular slated referred to are 

 everywhere characterized by a much greater power of resisting metamorphism than 

 those above and below them. I have more than once observed the total absence of 

 metamorphism, or the mere development of a micaceous glaze on the bedding-planes, 

 ■where associated beds were converted into distinct schists. 



