590 REPORT— 1902. 



Vernadaky 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 Scot- 

 land to account for the presence of sillimanite in the inner zone of metamorphism 

 between the kyanite schists and the granite, and he considered that the tempera- 

 ture attained by the ' central masses of the Highland rocks ' was probably higher 

 than tlie figures indicated by Vernadsky. 



Bearing all 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, and it is material to note that it was the first mineral 

 to crystallise out of the magma of the Satlej granite. This is shown by several 

 circumstances. 



In the first place the beryl preserved its perfect crystallographic shape, showing 

 that its molecules during the entire period of crystallisation possessed comparative 

 freedom of motion, and were not interfered with or molested by other solid 

 minerals. In the second place all the essential minerals of the granite when they 

 suljsequently crystallised out of the magma were deposited on the crystals of beryl. 

 I have specimens of the granite showing crystals of beryl enclosed in felspar, in 

 muscovite, and in quartz. 



The beryl, therefore, having been the first mineral to crystallise, the examina- 

 tion of thin slices of it under the microscope ouglit to give us a clue to the con- 

 dition of the magma at the time the beryl was formed. 



I have made such an examination, and I find that the beryl is crowded with 

 liquid and gas cavities, the former containing movable bubbles and deposited 

 crystals as well as water. 



The bubbles are of substantial size relative to the area of the cavities, showing 

 that the water suffered considerable contraction after it was sealed up in the 

 beryl. 



Scrope long ago suggested that the fluidity of lavas below the melting-point 

 was due chiefly to the water they contained, and attributed the liquidity of granite 

 to the same cause. 



Scrope, however, in ascribing the mobility of an igneous rock to the presence 

 of water seems to have had regard principally or wholly to its mechanical action 

 in furnishing an elastic medium in the interstices between the crystals or grains of 

 the rock. He observes that a lava consists ' of more or less granular or crystalline 

 matter, containing minute quantities of either red-hot water, or steam in a state of 

 extreme condensation, and consequent tension, disseminated interstitially among 

 the crystals or granules, so as to communicate a certain mobility to them, and an 

 imperfect liquidity to the compound itself,' and he quotes Scheerer and Delesse, both 

 of whom assert that water exists in mechanical combination with all crystalline 

 rocks, ' its minute molecules being intercalated between the crystals.' 



Nowadays one would attribute the liquidity of an igneous rock not so much 

 to the mechanical action of the water present in it as to the combination of the 

 water with the mineral contents of the lava, producing a state of solution. 



Sorby's investigations supported Scrope's observations, for he proved that the ■ 

 liquid contained in the inclusions in granite is water, and showed that it was 

 caught up during the formation of the crystals, ' and was not introduced subse- 

 quent to the consolidation of the rock,' 



The water now contained in cavities in the beryl was probably held in solu- 

 tion by the constituents of that mineral at the time of its formation, and as it 



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