MAGMATIC DIFFERENTIATION OF IGNEOUS ROCKS 621 



consequently to effusive rocks and to some dike rocks crystallized 

 at a small depth. The crystallizing temperature will here have 

 been above, probably indeed only a little above, the inversion point 

 between a-quartz and tridymite in force at the pressure in question, 

 and accordingly at depths of some hundred m., viz., at about 900° 

 or perhaps a little higher. 



By stating the depths at which SiOz, in the effusives and dike 

 rocks here mentioned, occurs as quartz or as tridymite, our knowl- 

 edge of the influence of pressure on the rise of the inversion point 

 between a-quartz and tridymite will be increased. Fenner {Amer. 

 Jour, of Sci., XXXVI, p. 348) describes a volcanic rock in which 

 quartz phenocrysts were first formed and were afterward trans- 

 formed into tridymite. Fenner suggests the explanation that the 

 quartz phenocrysts were formed at higher pressure a little below 

 the inversion point in force at the pressure in question, and that the 

 magma with its phenocrysts was afterwards brought nearer the 

 surface, viz., to a diminished pressure, lying a little above the 

 inversion point at the given temperature. The phenocrysts of 

 a-quartz first formed at the greater depth must consequently now 

 be transformed into trid3rm.ite. Fenner, however, expresses some 

 doubt as to this explanation, which, after all, in my opinion, is the 

 natural one. 



According to what has been stated by Wright and Larsen, the 

 quartz in the graphic-granite from granite-pegmatites was originally 

 formed as a-quartz, and accordingly at a relatively high tempera- 

 ture, viz., between the inversion point between /?- to a-quartz and 

 a-quartz to tridymite. But, besides, there sometimes occurs, in the 

 granite-pegmatites, quartz that "in all probability has never been 

 heated above the inversion temperature between a- and /3-quartz." 

 "These large masses of quartz were in certain cases definitely stated 

 by the field relations to be the last portions of the pegmatite to 

 crystallize out. ' ' This /3-quartz which is formed below the inversion 

 point between a- and /3-quartz, and the formation of which, at the 

 enormous pressure, must have taken place at a temperature below 

 600 or 625°, must be supposed to have been separated from a par- 

 ticular solution of Si02-FH20 (see following chapter on the influ- 

 ence of the light volatile compounds). 



