ORIGIN OP CRYSTALLINE ROOKS. 17 



undergo permaueut jphysical and chemical changes, Scheerer, moreover, argued that the 

 temperature of formation of the granitic veins holding these minerals could not have been 

 very high. ■^- 



This notion of hydroplutonic eruptions, thus set forth by Sorope, Scheerer and Élie de 

 Beaumont, has received a still ftirther extension of late. The hydrated rock, serpentine, is 

 supposed by some of those who maintain its exoplutonic derivation to have come u.p from 

 below as an anhydrous silicate, and to have been sïibsequently hydrated. Daubrée, how- 

 ever, has suggested that it had already passed into the hydrated condition before its ejec- 

 tion.^ Akin to this is the view of some modern Italian geologists, who explain the strati- 

 form character of this rock by supposing that it was ejected from below as an aqueous 

 magma, chiefly of hydrated silicates of magnesia and iron mingled in some cases with felds- 

 pathic matter ; from which, by crystallization and rearrangement, the masses of serpentine 

 and their associated euphotides have been formed, as well as the accompanying anhydrous 

 silicates, olivine and enstatite. By this hypothesis " the serpentines are considered as 

 eruptive without being truly igneous, inasmuch as they do not contain in their composi- 

 tion any mineral which has been submitted to igneous fusion," though " the magma may 

 have had a temiierature of several hundred degrees." ^' 



The coucei^tion of hydroplutonic eruptions, whether applied by Scrope to lavas, by 

 Scheerer to granites, by Belt to metalliferous quartz lodes, or by Daubrée and some Italian 

 geologists to serpentines and euphotides, is instructive as a phase in the development of 

 that geological hypothesis, according to which a volcano is a deus ex machina, which is 

 invoked to solve every knotty problem that presents itself in studying the origin of 

 rock-masses. 



§ 35. Writing in 1883 of the extravagances of the exoplutonic or volcanic doctrine, I 

 spoke of it as " the belief in a subterranean providence which could send forth at will 

 from its reservoirs " alike granite and basalt, olivine-rock and limestone, quartz-rock and 

 magnetite. *' An otherwise friendly critic '"^ speaks of this language as " a kind of device 

 for producing à false impression, by associating rocks for the most part of erui)tive origin 

 with others which are not so." This, however, is precisely what the plutonic school in 

 question has done, and is still doing. Eminent teachers in geology of our time, some of 

 them still living, have included with granites and basalts, not only serpentines, but lime- 

 stones, magnetite, auriferous quartz, buhrstone, rock-salt, anhydrite, hydrous iron-ores, and 

 even certain clays and sands, among the substances which have been thrown iip from 

 the depths of the earth. 



The obvious question, as to the origin of these sïipposed accumulations of various 

 and unlike substances in the under-world, has been one to perplex the thoughtful geolo- 

 gists of this school, and for those who did not admit that such might come from buried 

 deposits, once superficial, presented difficulties which it was sought to overcome by a 



^' For an analysis of these views of Scheerer and Élie de Beaumont, and references to tlie controversies to 

 which they gave rise, see Hunt, Chemical and Geological Essays, pp. 5, 6, and 188, 189. 



'■'' Géologie Expérimentale, p. 542. 



^*See, for an account of this hypothesis as maintained by Issel and Capacci, Hunt, on The Geological History 

 of Serpentines, Trans- Roy. Soc. Can., vol. i., part iv., p. 198. 



'' Ibid, vol. i., part iv., p. 206. 



^* Geological Magazine for June, 1884, page 278. 



Sec. in., 1884. 3. 



