CAUSES OF MINERALIZATION AND METHYLOSIS. 35 



The anhydrous character of the minerals referred to seems to 

 favour the idea that heat and pressure, more than water, have 

 been the agents in effecting the change we have assumed. On 

 this account the change may be termed xerothermal, parti- 

 cularly as the aquosity of the rocks in question seems to be but 

 slightly more than would be an accompaniment of dry heat. 



The methylosed section, there can be little or no doubt, has 

 been largely under the influence of water ; for a great portion 

 of the minerals composing its members are more or less hydrous, 

 This fact, and a number of relevant considerations, warrant us 

 in assuming it to be extremely probable that water derived from 

 extraneous or foreign sources, as seas and lakes, has copiously 

 permeated mineralized rocks, thus giving rise to various 

 chemical reactions within them, ending in their methylosis. 

 The change produced in methylosed rocks, compared with that 

 which has taken place in the mineralized section, justifies the 

 use of the generally adopted term hydrothermal*. 



Since the important discoveries made by Daubree, showing 

 that various minerals (zeolites, hyalite, calcite, diopside, &c.) 

 have been developed among the bricks and mortar of the old 

 concrete work of the Roman baths at Plombieres, in the Vosges, 

 through the action of subterranean alkaline water, with a tem- 

 perature of from 59 to 172^ F.f, it can no longer be held as 

 an unwarranted assumption that similar chemical changes have 

 been effected in rocks through the latter becoming saturated 

 with superadded heated watery solutions J. 



ing to Sterry Hunt, show that water was not excluded from the original 

 granite paste (Cheni. & Geol. Essays, p. 5). We regard these minerals as of 

 secondary origin, resulting from the admission of water into the granite 

 after its formation. 



* We find that Bunsen has designated this change hydatothennic, and its 

 opposite pyrocaustic. 



t Ann. des Mines, 1868, &c. 



t Daubree has already suggested that the water mechanically contained in 

 rocks, commonly termed quarry-water ("eau de carriere"), appears to be all 

 that is required to develop, when assisted by heat, very energetic action 

 (Quart. Journ. Geol. Soc. vol. xvii. p. xlix). 



