PETROGRAPHY. 345 



and spherulitic structure was present in some places. In other 

 experiments where Daubree applied superheated steam, he 

 obtained orthoclase and a micaceous substance. These ex- 

 periments gave convincing evidence that the constituents of 

 granite could be of aquo-igneous origin. 



Almost simultaneously with Daubree's investigations, Sorby 

 was engaged in microscopic examination of thin sections of 

 granite. He demonstrated, in 1858, the presence of water 

 vesicles in quartz, and concluded that the granite magma had 

 been saturated with water and had solidified under great pres- 

 sure at a temperature not above a dull red glow. Delesse, 

 in 1857, drew attention to the great differences between the 

 phenomena of contact metamorphism produced by granite and 

 those produced by lavas, and argued from his observations 

 that the granites had not solidified from a state of dry fusion, 

 but from an eminently plastic magma, whose plasticity was due 

 to the presence of water under high pressure. The theory of 

 the aquo-igneous origin of granite, and of the granite-grained 

 massive rocks generally, began to win wider credence in geo- 

 logical circles. 



The rapid progress made by microscopic research after the 

 year 1860 entirely disproved all theories which had assumed 

 an aqueous origin for porphyritic rocks. Examination of 

 thin sections showed conclusively that basalt, phonolite, 

 trachyte, porphyry, etc., were identical in internal structure 

 and composition with true volcanic lavas. Corroborative 

 evidence was afforded by the experimental researches which 

 were conducted, more especially in France, with such eminent 

 success. The attempts to reproduce rock-forming minerals 

 artificially proved that the majority of the constituents in the 

 granitic rocks, such as quartz, orthoclase, microcline, potash 

 mica, tourmaline, hornblende, could be solidified from fused 

 materials by the admixture of water vapours, chlorine, and 

 other solvents, whereas the minerals occurring in volcanic 

 and porphyritic rocks, such as olivine, augite, enstatite, 

 hypersthene, wollastonite, the plagioclase varieties, melilite, 

 nepheline, leucite, magnesia mica, garnet, magnetite, spinel, 

 haematite, tridymite, etc., could solidify from a state of dry 

 fusion. 



In the year 1878, the efforts of Fouque and Michel-Le'vy to 

 reproduce eruptive rock without the aid of superheated water 

 were at last successful. The chemical elements were placed in 



