250 
of heat, and in fact, I must admit that in the case of coarse 
grained highly quartzose granites, there is so very little evidence 
of igneous fusion, and such overwhelming proof of the action 
of water, that it is impossible to draw a line between them and 
those veins where, in all probability, mica, felspar, and quartz 
have been deposited from solution in water, without there being 
any definite genuine igneous fusion, like that in the case of 
furnace slags or erupted lavas.” While from the fact that 
schorl melts readily at a bright red heat and multitudes of hair 
like crystals of schor] are enclosed in the quartz of Cornwall, 
it is inferred that the granite did not become finally solid at a 
temperature much higher than a dull red heat. From the fluid 
cavities, the temperature inferred for an elvan dyke is 608 F., 
which indicates a pressure of 18,100 feet. But most of the 
observations on Cornish elvans gave a pressure of 40,300 feet, 
while the quartz porphyry dykes of the Highlands of Scotland 
indicate on similar evidence, a pressure of 69,000 feet. The 
granite of St. Austell in the same way indicates a temperature 
of 490° feet, and a pressure of 32,400 feet, while near Penzance 
the pressure corresponds to 63,600 feet; the mean pressure 
indicated by Cornish granites is 50,000 feet. The mean pressure 
of the Aberdeen granite is about 76,000 feet, while the centre 
of the main mass of the granite of Aberdeen requires a pressure 
of 78,000 feet. Whence we learn that the inferred temperatures 
under which these rocks were produced, are scarcely higher 
than would be reached at corresponding depths beneath the 
surface by the mere natural augmentation of the earth’s heat, 
so that if anything like 50,000 or 70,000 feet of rock has been 
denuded to expose the granite, all difficulty as to the temperature 
vanishes; and the water though greatly heated, was in most 
cases caught up by the crystals in a fluid state, more or less 
saturated with the alkalies which enter into the composition of 
the minerals forming the rock. For a discussion on the nature 
of the evidence, see Sorby, Q.J.G.S., vol. xiv. p. 453. 
Mention has been made of compression at high temper- . 
atures, and so a short note on the faculty of conducting heat 
possessed by certain bodies, may well follow, at least to some 
