TRANSACTIO^•S OF SECTION C. 580 



Skction C. -GEOLOGY. 

 Peesident of the Sectiox — Lieut.-Gen. C. A. McMahox, F.R.S., V.P.G.S. 



TFIURSDAY, SEPTEMBER 11. 



Tlie rresidcnt delivered the following Address: — 



Rock Metamoijjhism. 



I WISH to offer some observations to-day on some aspects of rock metamor- 

 pbisni ; and as tliis is a complex subject, and the time at my disposal is brief, 1 

 purpose to deal with it in simple language, and to avoid as far as possible all petro- 

 logical technicalities. 



A short description of a granite in the Satlej Valley of the Himalayas will, I 

 think, introduce us by a short cut to the consideration of ' contact metamorphism,' 

 an important branch of the subject under consideration. 



The granite I allude to is an intruder in the normal gneissose-granite of the 

 Himalayas, and cuts through it at right angles to its foliation. 



The intruder, which is some yards wide, did not rise through a simple crack or 

 fissure, for its passage upwards was interrupted by a sheet of dark intrusive 

 diorite, older than itself, which ran, roughly speaking, parallel to the foliation of 

 the gneissose-granite. 



This sheet of diorite offered considerable resistance to the rising granite. 



The granite zigzagged backwards and forwards across the diorite and ran along 

 its edges for fifty yards or more, con veJ ting it into a mica trap. 



It then tore itself away and continued its upward course. The granite T am 

 describing was in a molten or fluid condition at the time of its eruption, as I hope 

 to show in my subsequent remarks. 



I may pause here, however, to consider in passing what was the probable 

 temperature reached by a granite such as that above described. 



The question is one of very great difficulty, as we know so little about the 

 plutonic conditions of igneous rocks, and can only arrive at an answer to our ques- 

 tion by indirect evidence. 



The melting-point of quartz ranges from 1425° to 14.50° C, but the fusion 

 point of granite need not necessarily be as high as this, inasmuch as the presence 

 of water at high temperature materially lowers the melting or solution point. 



The fusion point of the other constituents of granite may here be mentioned : 

 that of orthoclase ranges from 1164° to 1168°; microcline, 1169°; albite, 

 1172°; augite and hornblende. 1188° to 1200°; apatite, 1221°. Zircon, which 

 is commonly found in granites, and is one of the first minerals to separate out of 

 the magma, is shown by Ralph (Jusack to have probably a melting-point of 1760° ; 

 whilst topaz, a not uncommon mineral in granite, is infusible up to the melting- 

 point of platinum, namel)', 1770° L\ 



If we consider, therefore, the melting-points of the mineral constituents of 

 granite, we can hardly avoid the conclusion that for the magma to have attained 

 perfect fluidity it must have reached a temperature of at least 1200° C, 



