General C. A. McMahon's Address. 459 



resistance to the rising granite. The granite zigzagged backwards 

 and forwards across the diorite and ran along its edges for fifty- 

 yards or more, converting it into a mica trap. It then tore itself 

 away and continued its upward course. The granite I 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 question by indirect evidence. The 

 melting-point of quartz ranges from 1425° to 1450° 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 Ealph Cusack 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, 

 namely, 1770° C. 



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° 0. Vernadsky has shown that 

 kyanite is transformed into sillimanite, a well-known product of 

 contact-metamorphism, at a temperature of 1320° to 1380°. If 

 rocks in contact with granitic masses have been raised to this 

 temperature, it follows that the granite itself must have been still 

 more heated. Vernadsky's observations have been relied on by 

 Mr. George Barrow in his well-known paper " On an Intrusion of 

 Muscovite-biotite Gneiss " in the S.E. Highlands of Scotland to 

 account for the presence of sillimanite in the inner zone of meta- 

 morphism between the kyanite schists and the granite, and he con- 

 sidered that the temperature attained by the " central masses of the 

 Highland rocks " was probably higher than the figures indicated by 

 Yernadsky. 



Bearing all these considerations in mind, including the influence of 

 water and alkali in reducing, and of pressure in raising, the melting- 

 point, I think we may safely infer that granites, such as the 

 Himalayan granite alluded to above, must have been raised at 

 plutonic depth to a temperature midway between red and white 

 heat, that is to say, to at least 1200° C. 



To return to the granite of the Satlej Valley under consideration, 

 I wish to draw attention to its condition just before crystallisation 

 commenced. A study of the mineral beryl will, it seems to me, 

 throw light on this point. Beryl is an important accessory mineral 

 of the granite under description. It is clearly an original mineral. 



