216 Col. McMahon — Granite of the Simalayas. 



jected to much more intense squeezing than the massive portion, 

 and field observations favour the conclusion that the intensity of 

 the foliation and the narrowness of the outcrop are directly connected 

 with each other. 



Without pausing to discuss this question further, I pass on to 

 state briefly the evidence which the rock presents of eruptive 

 origin. 



Stratigraphical evidence. — First, the granite has produced a certain 

 amount of contact metamorphism on the rocks touching it. The 

 slates exhibit a marked increase in induration and they pass more 

 or less into a crystalloid condition as they approach the granite. 

 Schorl, garnets, dark mica, muscovite, and magnetite have been 

 formed in the slates by contact action ; foliation has sometimes 

 resulted from that action — at other times " spotted schists " have 

 been formed. 



Secondly, tongues and intrusive veins have been sent from the 

 granite into the adjoining rocks : in other places the granite appears 

 in sheets between the beds of the sedimentary rocks at some distance 

 from the junction of the latter with the main mass of the granite ; 

 and in some cases these sheets, or dykes, have cut through the beds 

 and passed from one horizon to another. 



Thirdly, the main mass of the granite appears at different geolo- 

 gical horizons. 



Fourthly, the granite contains veins similar to those caused by 

 shrinkage on cooling in granites of admittedly eruptive origin. 



Fifthly, it contains fragments of slates and schists imbedded in it. 

 A striking photograph, or rather a print produced by the helio- 

 gravure process from a photograph, of a schist enclosed in the main 

 mass of the granite, half a mile from its contact with the slates, will 

 be found at p. 175, Eecords Geological Survey of India, vol. xvii. 

 (with a paper thereon ) ; which, in conjunction with the evidence 

 afforded by the microscope, completely sets at rest any doubts that 

 might have existed, previous to the discovery of this specimen, as to 

 whether or not these inclusions are true fragments of foreign rocks 

 caught up in the granite. 



Microscopic evidence. — The evidence afforded by the study of 

 numerous thin slices under the microscope confirms the conclusion 

 arrived at by the stratigraphical evidence, and indicates the eruptive 

 origin of the gneissose-granite. I select the following instances for 

 enumeration : — 



First, the presence of microliths containing contraction cavities 

 pi'ecisely similar to those in microliths to be seen in undoubted erup- 

 tive rocks. As an illustration I would point to fig. 11 of the plate 

 at page 158, Records G.S.I, vol. xvi., which gives the sketch of a 

 microlith seen in one of the Aden lavas, but which serves equally 

 well as an illustration of some of those seen in the gneissose-granites ; 

 vide Records G.S.I, vol. xvii. p. 60. 



Secondly, cracks in microliths formed by contraction on cooling. 

 In some cases the fractured portions have been floated to a little dis- 

 tance from each other. Fig. 5 of the plate at p. 72, Records G.S.I. 



