292 THE EVOLUTION OF PETKOLOGICAL IDEAS. 



Scheerer maintained that the purel}' igneous origin of granite was 

 disproved ))y three lines of argument. Thus he contended that the 

 very presence of quartz was opposed to the theory, for this mineral 

 could not be formed by igneous fusion, and was absent from lavas 

 containing an excess of silica, such as obsidian, even when these lavas 

 must have cooled more slowlj'^ than some granite veins. 



Again, the order of consolidation of the minerals, as determined by 

 mutual interference, was not the order of their fusibilities. Fournet 

 had endeavored to remove this objection by supposing that quartz, like 

 water and sulphur, could be cooled below its proper melting point. 

 But this theory of the surfusion of quartz was untenable, because the 

 amount of overcooling was too great and the complete rest which was 

 necessary could not be postulated. Scheerer admitted that the o])jec- 

 tions to Fournet's theory were rendered less forcible by a considera- 

 tion of the fact pointed out by Durocher, that the magma of granite 

 did not contain the material of the separate minerals in a fused state, 

 but consisted of a homogeneous liquid — a solution as we should say — 

 so that the overcooling did not ati'ect quartz as such. This, however, 

 in his opinion, did not justify the theory, for, to use a free translation 

 of his own words: ""It is evident that the point of solidification of the 

 silicate forming the magma out of which the different compounds are 

 separated ought to approach the fusion point of silica as the quantit}' 

 of bases in the liquid portion decreases." 



According to Durocher's view, the ultimate base of granite should 

 consist, not of quartz, but of a substance like petrosiiex. 



A third line of argument, founded on the occurrence in some gran- 

 ites of the peculiar p^a-ognomic minerals, such as gadolinite, was also 

 brought forward by Scheerer. If a chip of isotropic gadolinite be 

 heated to redness, a sudden and remarkable change takes place. It 

 glows brightly for a few moments, and after cooling is found to have 

 become denser and strongly birefringent. Thus gadolinite occurs in 

 two phases; a lighter isotropic phase and a denser birefringent phase. 

 The change from the former to the latter takes place at a red heat, 

 and the reaction is accompanied by a considerable loss of energy, but 

 little or no loss of material. The occurrence of this mineral in some 

 granites proves, therefore, according to Scheerer. that they nuist have 

 consolidated below a red heat. 



These three lines of argument, based on the presence of ((uartz. on 

 the mutual relations of the constituents, and on the presence in some 

 granites of pyrognomic minerals, concur, he considers, in disproving 

 the theory of pure igneous fusion. 



Scheerer then propounds his own theory of a(|uco-igneous fusion, 

 basing it on the fact that some of the granitic minerals, such as mica, 

 contain water. The presence of even small quantities of water would, 

 he maintains, lower the consolidation point considerably, and during 



