PETROGRAPHY. 343 



Geological Society in 1847. His chief arguments were: (i) the 

 occurrence of separated quartz ; this, according to Scheerer, is 

 impossible in the case of consolidation from a fluid mixture of 

 silicates; (2) the order of succession in the separation of 

 felspar and quartz ; Scheerer ascribes no weight to Fournet's 

 "surfusion" theory, which supposes that quartz can remain 

 longer in solution than the more easily fusible felspar, as this 

 is a hypothesis which has not been tested experimentally for 

 silicate mixtures ; (3) the presence of so-called pyrognomic 

 minerals (orthite, gadolinite), whose physical properties are 

 altered at comparatively low temperatures. 



Scheerer also drew attention to the fact that water is held 

 in chemical combination with several of the constituents of 

 granite. This water he regarded as originally present in the 

 magma from which the granite solidified. But if the magma, 

 as might be safely assumed, was subjected to high pressure, 

 which prevented the escape of the superheated water, then 

 very probably the influence of the water might enable the 

 granite magma to remain fluid at temperatures much lower 

 than would be the case under the influence of dry heat. 

 When solidification set in, the minerals with the strongest 

 tendency to crystallise were the first to separate from the pasty 

 granite mass, and the water concentrated itself in the remaining 

 ground-mass, which always became more acid, and owing to 

 the superfluity of water the separation of quartz and the 

 pyrognomic minerals might under some circumstances be 

 suspended until the temperature of the mass was below that 

 of a red heat. 



Although Durocher still upheld the pyrogenetic origin of 

 granite against the objections raised by Scheerer, the hydato- 

 pyrogenetic or aquo-igneous doctrine ef Scheerer rapidly gained 

 ground in literature. Probably its strongest antagonist was 

 Bischof, whose explanation of the origin of granite, syenite, 

 porphyry, and even basalt, showed a reversion to Neptunistic 

 teaching. In the second volume of his Physical and Chemical 

 Geology (1851), Bischof, after a full discussion of the rock- 

 forming minerals, came to the conclusion that all except 

 augite and leucite could take origin from aqueous solutions 

 without increased temperature and under normal pressure, 

 and that their origin from fused rock-masses was quite excep- 

 tional. Moreover, the resemblance between the composition 

 of many eruptive rocks and that of certain sedimentary rocks 



