620 /. H. L. VOGT 



atmospheric pressure of respectively o. 1 1 5 and o. 23°. A correspond- 

 ing rough calculation has been made by Fenner,^ who presumed 

 Vi — V2 = o.o57 (as at room temperature) and inversion heat = 15 cal., 

 which makes a rise of temperature per atmosphere of 0.105°. 

 Starting from these figures (15, 10, 5 cal.) we should have, on a 

 pressure of 270 atmospheres (or at the depth of i kilometer), a rise 

 of the inversion point of respectively 28, 38 and 76°, or from 875** 

 to respectively 908, 913 and 951° — and it may be that even the last 

 number is too low an estimate. On the pressure prevailing at the 

 depth of a few kilometers, the inversion point quite certainly will 

 most likely prove to be considerably above 1200°. 



The binary eutectics Qu:Or and Qu:Ab, at a pressure of one 

 atmosphere, I estimate at about 1075°-! 100°, repectively97S°-iooo°. 

 The ternary eu tactic Qu:Or:Ab must lie somewhat lower, prob- 

 ably at about 950° or 925°. 



If we choose for instance a common granite, quartz porphyry, 

 or rhyolite, the final crystallization must here take place as a 

 complex eutectic Qu:Or:Ab+An, with a little ferric oxide and 

 Mg, Fe-silicate, at a pressure of one atmosphere at a temperature 

 of presumably about 9oo°-95o°. And even if there occurs some 

 surplus quartz, so that the crystallization of this mineral begins 

 at a comparatively early stage, still the first crystallization of SiO, 

 will scarcely take place at a higher temperature than 950° or 1000°. 

 These values of temperature reckoned on the pressure of one atmos- 

 phere must certainly be increased somewhat, but only a little, for 

 crystallization at great depth and under high pressure, but, as we 

 shall mention later, must be decreased by the content of water, 

 etc., especially in the granitic deep-seated magma. 



As, on the other hand, the temperature of the inversion point 

 between a-quartz and trid3miite rises so considerably, SiOa will 

 never crystallize in deep-seated rocks as tridymite, but always as 

 quartz, and this quartz in granite, quartz-porphyry, and graphic 

 granite is, as pointed out by Wright and Larsen, always an a-quartz. 



Tridymite as a primary^ formation in igneous rocks is, as is 

 well known, limited to certain rhyolites, trachytes, andesites, etc., 



' Jour, of the Wash. Acad, of Sci., 2 (1912), p. 479 and Amtr. Jour, of Sci., XXXVI 

 (1913).?- 347- 



2 The hydrothermal formation of tridymite is here left out of consideration. 



