462 M. D. Oldham — Gneissose Rocks of the Simalaya. 



gradually ceases to be porphyritic, and sometimes contains merely 

 a small proportion of felspatliic material. How complete is the 

 assumption of a gneissose and loss of the granitic structure may be 

 judged from the fact that specimens from one exposure, which can 

 be shown to be intrusive, have been examined by Col. McMahon, 

 and declared by him to be a true gneiss showing no signs of 

 intrusive origin.' 



Col. McMahon has shown that the microscopic structure of this 

 rock, as well as the inclusions of mica-schist, prove its intrusive 

 nature. I may add that the same is shown by its mode of occur- 

 rence, by the manner in which it cuts across the bedding of the 

 rocks among which it occurs, and by the invariable occurrence of 

 contact metamoi-phism in the neighbourhood of any large mass of it. 

 The intrusion, in many cases, does not seem either to have caused 

 or been accompanied by any considerable disturbance, but to have 

 taken place by a fusion (or solution) and absorption of the rocks 

 which it has replaced. These often continue with a perfectly steady, 

 low dip right up to a large mass of the gneissose granite, and end 

 abruptly there ; yet the intrusive nature of the granite is shown 

 by the presence of included masses of these very rocks. The same 

 thing is indicated by a study of the intrusive sheets; they con- 

 stantly thin out and thicken without any disturbance of the border- 

 ing beds, which simply run up to the edge of the gneissose granite, 

 and stop there abruptly ; the granite itself too becomes more mica- 

 ceous or more quartzose (according to the prevailing type of rock it 

 has passed through) and less felspathic the further it is traced from 

 its source, indicating a gradual increase of impurity. But the most 

 conclusive proof 1 know of is exhibited by the sections on the 

 eastern side of the Chor Mountain. The southern sections show a 

 considerable thickness of volcanic beds, altered to more or less 

 schistose hornblende rock ; while on the northern sections these 

 hornblende rocks are absent, but the granite has become so highly 

 hornblendic that the ground-mass is of a dark green colour, throwing 

 up the porphyritic crystals of white orthoclase in a most conspicuous 

 manner. 



Before passing on, it will be well to explain how these two 

 distinct rocks came to be confused with each other. In the Sutlej 

 Yalley, between Simla and the Wangtu bridge, there are extensive 

 exposures of gneissose rocks ; these are almost all the gneissose 

 granite, but owing to similarity of lithological appearance and abso- 

 lute continuity of outcrop, they were (erroneously) confounded by 

 Dr. Stoliczka with the granitoid gneiss which is exposed almost to 

 the exclusion of intrusive granite on the ascent from the Wangtu 

 bridge to the Babeh pass. In 1877 Col. McMahon published a 

 paper on the " Central gneiss " of the Simla Himalayas, in which he 

 (correctly) identified the rock of the Chor and the gneissose granite 

 . intrusions south of the Sutlej with the rock of the Sutlej Valley. 

 In 1883 he showed that while the rock of the Dhaoladhar, which he 

 had identified with that of the Chor, could not in a single case be 

 1 Rec. Geol. Surv. India, voL xvii. p. 60 ; vol. xix. p. 86. 



