Vol. 57.] ANNIVERSARY ADDRESS OP THE 1»RESIDENT. Ixvii 



middle of the century these facts were generally recognized for all 

 those masses which occur outside areas of crystalline schist. But 

 their recognition, although conclusive as against the view that 

 granite was everywhere a primordial sediment, by no means 

 involved the necessity of accepting the Huttonian theory that it 

 resulted from the consolidation of a mass of matter in a state 

 of pure igneous fusion. 



The earlier phases of the discussion as to the origin of granite 

 centred mainly round the tectonic relations of the rock-masses ; but 

 the later phases had reference rather to the composition and struc- 

 ture of the rock itself. The papers on this subject, and especially 

 the discussion between Scheerer and Durocher, are well worthy of 

 the attention of modern petrologists. 



Scheerer maintained that the purely igneous origin of granite was 

 disproved by 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 slowly 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 endeavoured 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 ad- 

 mitted that the objections to Pournet's theory were rendered less 

 forcible by a consideration 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 affect 

 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 siHca as the quantity 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 petrosilex. 



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

 granites of the peculiar pyrognomic minerals, such as gadolinite, 

 was also brought forward by Scheerer. If a chip of isotropic gado- 



