A. It. Sunt — The CrystaUimtion of Granite. 395 



a temperature of 200° C. Being, however, an undoubted crock, 

 I am extremely disinclined to come between the geological pots and 

 the chemical kettles ; and I need only say that my sympathies are 

 with the chemical kettles, because they seem to me to account for 

 all the phenomena which I have observed with my antiquated 

 microscopes. 



It has always been a very great perplexity to me that, whereas 

 I have ever relied on the evidence of the critical temperatures, 

 the fluid inclusions, and the deposited chlorides, the great geological 

 authorities, to whom, as captain of a golf club, I bow with 

 genuine humility, scarcely ever refer to or notice these things f 

 and, in consequence, I am in eternal uncertainty. The question 

 alwaj'S arises, have the geologists disproved and rejected the 

 physical evidence, or do they only ignore it ? 



Not very long ago two eminent men disputed over the relative 

 fusion temperatures of granite and basalt ; but surely the question 

 really is as to the relative crystallising temperatures of these rocks ; 

 and, as we know for a fact that basalt can be artificially produced 

 by drj'- fusion without the intervention of water, and as we also 

 know that granite almost invariably contains water in one or more 

 of its minerals, the dry-fusion temperatures of the two rocks become 

 a mere academic question. Is this not so ? 



The main wrangle is as to what becomes of the water-vapour 

 above the critical temperature. Well, being a gas it may well be 

 dissolved in the liquid magma, and there it may stop until the 

 lowering of the temperature allows it to resume its liquid state, 

 when it will begin to dissolve the minerals instead of being itself 

 dissolved ; but, seeing the rarity of chlorides as rock - forming 

 minerals, it seems improbable that there would be much chlorine 

 or hydrogen in the magma at the cooling stages to enter into 

 combination with the metals, especially as we know for a fact, 

 as has been already noticed, that both those gases are got rid of 

 by volcanoes. The suggestion of Dana that the volcanic hydrogen 

 arises from the dissociation of water is rather negatived by the fact 

 that, while volumes of steam are emitted by volcanoes, hydrogen 

 only occurs in small quantity, and is therefore more likely to be due 

 to the dissociation first of the chlorides and then of the hydrochloric 

 acid, H CI. Were the water itself dissociated, we might expect 

 volcanic flames on a magnificent scale if the hydrogen recombined 

 in the atmosphere, or constant explosions if the hydrogen recombined 

 in the lava column. As a matter of fact, volcanoes often emit steam 

 very quietly. 



Petrologists are much more interested in the proportions of 

 potash, soda, and lime in a felspar, and the consequent angle of 

 extinction of the crystal, than in its crystallising temperature. 

 Yet the crystallising temperatures of the same felspar often vary 

 exceedingly. For instance, General McMahon in his address spoke 

 of the fusing temperature of albite as 1,172° C. Subsequently, 

 when I read my paper, I exhibited a lantern slide showing secondary 

 albite in one of the Devonshire green-schists, which albite contains 



