350 ORIGIN OF ERUPTIVE AND PRIMARY ROCKS. 



which it contains huge masses and smaller fragments of other 

 rocks. This is one of the most conclusive proofs of the power of 

 the forces at work during the protrusion of the granite, and taken 

 in connection with its lbnns of deposition furnishes incontrover- 

 tible evidence of its eruptive origin* Among the objections which 

 have been made to this view, the most important is that founded 

 on the circumstance that the quartz of granite has been the last 

 of its constituents to solidify. Many theories have been proposed 

 to account for this circumstance but it would seem necessary 

 before attempting its explanation to enquire whether this alleged 

 behavior of the quartz is really the fact. It is doubted by a few 

 geologists, and altogether denied by Sartorius von Waltershausen, 

 who remarks that according to his experience in the primary 

 rocks especially in granite as well as in the volcanic rocks, quartz 

 corundum and periclase have always first been separated. " For 

 instance," he says, " I have minutely examined the granites from 

 " Baveno, from various districts of the Grimsel, from Mont Blanc, 

 a from the Oker valley (Harz), from the Island of Mull and many 

 " other places, and those rocks show that the quartz solidified 

 " first, then the mica and finally the feldspar."* In the face of 

 such a distinct statement it might not be safe to regard as 

 thoroughly established the fact whereon the above objection to the 

 eruptive origin of granite is founded. 



With regard to the chemical composition of granite, its content 

 in silica, according to 18 analyses mentioned by Bischof, ranges 

 from 63.3 to 76.02 per cent., and averages 69.33 per cent. Only 

 two analyses of syenite are on record, the silica being estimated as 

 61.72 and 66.39 per cent. ;• average 64.05. If we compare these 

 figures with the average content in silica of other eruptive rocks 

 we find generally a diminution in the quantity of silica as the rocks 

 become more and more recent, provided always that their classi- 

 fication into two great series of siliceous and more basic rocks is 

 kept in sihgt. Thus the acid series comprehend: 



Granites 69.33 per cent, of siliea. 

 Porphyries 67. 11 " " 



Trathytes 62.91 " " 



The basic series on the other hand consist of: 



Syenites • 64.04 per cent, silica. 



Greenstone and Melaphyrs 50.65 " " 

 Dolerites and Melaphyrs 46.16 " " 



* Die Yulcanische Gesteine 2 &c 3 p. 225. 



