346 HISTORY OF GEOLOGY AND PAL/EONTOLOGY. 



a platinum crucible and fused; the fused mass was then sub- 

 jected for forty-eight hours to a temperature nearly that of the 

 fusing-point, the material being afterwards allowed to cool 

 slowly. According to the ingredients that were introduced, 

 consolidated rock-material agreed completely with certain 

 augite-andesites, leucite and nepheline rocks, and contained 

 the majority of the minerals composing these rocks in the 

 form of well-developed crystals. 



Inasmuch as these important results showed that the por- 

 phyritic series of rocks could originate merely by the cooling 

 of a molten magma, they tended to widen the gulf between 

 the porphyritic and basaltic, and the granite-grained series. 

 Favour was given to Hutton's assumption that the latter owed 

 their distinctive characters to their subterraneous origin under 

 great pressure, and the Huttonian conception was made even 

 more emphatic by Rosenbusch in his classification of 1886. 

 Further confirmation was given by Gilbert's description of 

 intrusive masses of rock, so-called "laccolites" (ante, p. 274) 

 between sedimentary strata in the Henry mountains ; and also 

 by Reyer's investigations on massive flows and local differences 

 in the mineralogical composition and the texture of the con- 

 solidated rock. 



After the principle of the eruptive origin of the crystalline 

 massive rocks had been firmly established, the interest of 

 petrographers was directed to the investigation of the chemical 

 constitution of the rock-magmas and the processes effecting 

 their consolidation. The chemistry of rocks had been greatly 

 advanced by the researches of Abich, Delesse, Bischof, 

 and especially by Bunsen. As has been already mentioned 

 (ante, p. 328), Bunsen concluded from his examination of the 

 igneous rocks of Iceland that all the eruptive rocks of that 

 island in their composition presented either a normal trachyte 

 magma or a normal pyroxene magma, or a mixture of these 

 two varieties of rock-magma in varying proportions. According 

 to Bunsen, it is possible by means of a simple formula, being 

 given the amount of silica present in such a mixed rock, to 

 reckon the amount of the normal trachytic and normal pyro- 

 xenic material present in the rock. Streng, Kjerulf, and others 

 accepted Bunsen's conclusions and tried to apply them gene- 

 rally to all eruptive rocks. 



Sartorius von Waltershausen explained (1853) the chemical 

 difference of the Iceland eruptive rocks, not upon Bunsen's 



