14 StrO.MKVKR, The Fonnation of Minerals in Granite. 



heat ; the teinpcrature gradient would in this case be a 

 steep one, say i^^C. per 40 feet, the depth at which granite 

 would commence to solidify would not be great, the 

 pressure would not be great, and most probably the quartz 

 would crystallise first, and then the felspars, etc., forming, 

 say " quartz porphyry," i)rovided, of course, that there is 

 an excess of silica. 



II. The rock resting on the molten granite is heavy, 

 containing, say, much iron oxide, and would probaby be a 

 good conductor of heat ; the temperature gradient would 

 not be stee{), say i '^C. per 200 feet. The depth at which the 

 granite would commence to solidify would be much greater 

 than in the above case and the pressure much greater, 

 most probably the quartz would remain fluid long after the 

 felspars had crystallised, forming, say, " felspar porphyry." 



Every intermediate condition is now conceivable, 

 and rocks in which quartz is missing, although the 

 chemical analysis shows plenty of silica, have probably 

 been formed under conditions where the melting points of 

 quartz and the other minerals are almost equal. 



Although we have as yet no experiments which throw 

 light on this subject, and although it may perhaps be 

 impossible to carry out really reliable ones, it is hoped 

 that the possible influence which pressure exerts on the 

 melting temperature of rocks, and therefore on the forma- 

 tion of their minerals, will permit of a rough classification 

 on a somewhat more systematic plan than is now the 

 case. In this respect, probably more would be learnt by 

 a careful study of variations in a single large mass of 

 granite than by comparing samples from different parts 

 of the world ; and although our range, as regards depth of 

 formation, is limited to at most 10 per cent, of the actual 

 depth at which solidification may have taken place, 

 there is at least a reasonable probability that marked 



