Col. McMahon — Granite of the Himalayas. 215 



scopic size, have a distinct individuality of tlieir own. Both varieties 

 will, in the following remarks, be referred to under the term crypto- 

 cry stall ine mica. 



This crypto-crystalline mica is present in all varieties of the 

 Dalhousie granite. It is to be seen in every thin slice under the 

 microscoj^e ; and in the foliated specimens it traverses them in 

 strings ; sometimes extremely attenuated ; at other times widening 

 out into comparatively broad lake-like expanses. It does not run in 

 straight lines, but flows in curves like those of rivers on a map ; now 

 approaching each other ; now diverging widely ; bending sharply at 

 one turn, and making a wide sweep at another. It frequently leads 

 up to, or embraces in its streams, large crystals of muscovite, quartz 

 grains, and other minerals ; and it occasionally traverses large 

 felspars. I shall have to allude to this crypto-crystalline mica 

 further on. 



Another striking characteristic of all varieties of the Dalhousie 

 granite is what I have termed in my papers fish-roe quartz ; that is 

 to say, quartz in grains of microscopic size clustered together in a way 

 to suggest the roe of a fish. This polysynthetic or fish-roe quartz 

 behaves very much as the crypto-crystalline mica behaves ; that is 

 to say, in the foliated specimens it flows about in streams round the 

 larger minerals ; fills the interstices between larger grains of quartz ; 

 and stops the fractures in felspars. I shall have to allude to this 

 further on. 



The most important questions which arise in connection with the 

 rock above described are : Is it an igneous eruptive rock? and if so, 

 was the foliation produced prior, or subsequent, to its final consolida- 

 tion ? I propose to consider each point separately. 



Evidence of the eruptive origin of the Gneissose- Granite. 



In the first place it seems necessary to meet the objection of those 

 who may be disposed to hold the view that it is an ancient, possibly 

 Pre-cambrian rock, which suffered erosion before Silurian times. 

 It would be impossible to discuss this position in detail in a brief 

 article that only proposes to give the results of the author's investi- 

 gations ; but I may say that I carefully considered this point in the 

 field at a time when I had formed no theories about the Dalhousie 

 rock in my own mind, and that I could not discover any evidence 

 to support it. Two considerations seem to offer serious obstacles to 

 the acceptance of this theory. In the first place, erosion which 

 reduced the gneissose-granite from a thickness of eleven miles to 

 that of two hundred and fifty feet would hardly have left a long 

 stretch of this slight thickness without having cut through it alto- 

 gether ; secondly, if this crystalline rock represents an ancient and 

 eroded land-surface, the granite to the north of the Eavi, where its 

 thickness is only two hundred and fifty feet, ought to be a fair sample 

 of the granite two or three miles to the south of the Ravi, where 

 its thickness is six miles and a half; but, on the contrary, the former 

 is a much more intensely foliated rock than any portion of the latter. 

 The thin band of granite north of the Eavi has evidently been sub- 



