490 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



to decide whether or no it exercised an important influence over the 

 crystalline processes that took place during the consolidation of sucli 

 rocks as granite. The comparatively large quantity of alkaline chlo- 

 rides and sulphates, dissolved in those portions caught up in the 

 growing crystals, indicates that the amount cannot have heen un- 

 limited ; but, bearing in mind the facts I alluded to when describing 

 the fluid-cavities in the blocks ejected from modern volcanos, and 

 knowing, as we do, that the action of highly heated water is so very 

 energetic, I cannot think that its influence was unimportant. On 

 the contrary, seeing that the fluid-cavities in the quartz of quartz- 

 veins contain the selfsame salts and acids as those in the granite, as 

 though it had been deposited from portions of the liquid which had 

 passed from the granite up fissures, I think the amount, though 

 limited, must nevertheless have been considerable, and that its presence 

 will serve to account for the connexion between granite and quartz- 

 veins, and the very intimate relation of both to the metamorphic rocks, 

 and explain many peculiarities in the arrangement of the minerals in 

 the cavities in granite or in the solid rock, even if it was not the effec- 

 tive cause of their elimination and crystallization. These analytical de- 

 ductions have been confirmed in a most striking manner by the ad- 

 mirable experiments of M. Daubree (Observations sur le metamor- 

 phisme, &c, Annales des Mines, 5 e ser. t. xii. p. 289), who, in having 

 produced felspar and quartz artificially, by the action of water at a 

 similar temperature to that I have deduced from the fluid-cavities, 

 has removed some of the principal objections that might have been 

 urged against my conclusions. I therefore must confess myself to 

 be a very strong adherent to the views of Scrope (Treatise on Vol- 

 canos ; and on the nature of the liquidity of lava, Quarterly Journal 

 of the Geological Society, vol. xii. p. 338), Elie de Beaumont (Note 

 sur les emanations volcaniques, ut supra), and Scheerer (Discussion 

 sur la nature plutonique du granite, &c, Bulletin de la Societe Geolo- 

 gique de France, 2 ser. vol. iv. p. 468, and vol. vi. p. 644), though, 

 as will be perceived, I by no means agree with them in every par- 

 ticular. 



c. Temperature and pressure under which granitic ?'Ocks have been 



formed. 



In studying the fluid-cavities in elvans and granite, it is particu- 

 larly necessary to bear in mind the influence of pressure. As already 

 shown, the temperature requisite to expand the fluid so as to fill the 

 cavities is that at which the crystal was formed only when the pres- 

 sure was not greater than the elastic force of the vapour, and when 

 in equation (9) p=0 ; but if the pressure was very great, that tem- 

 perature would necessarily be far short of the actual heat. There- 

 fore, as already described, the true heat can only be determined when 

 the approximate value of the pressure is known ; and the pressure 

 cannot be deduced unless we can in some way or other approximate 

 to the temperature. The trachyte of Ponza was probably formed 

 under so small a pressure that it scarcely need be taken into account ; 

 but, in the case of granites, such a supposition would lead to the con- 



