THE ORIGIN OF MINK HALS 247 



plex nature, and it is not possible for them to fonn at the temper- 

 ature of their free/ing point. Such minerals arc termed the low 

 temperature minerals. Their molecules are considered to be more 

 complex than those which form from direct fusion and without the 

 aid of minerali/ers, or the high temperature minerals. In the low 

 temperature minerals are included the amphibolcs, micas, sodalites, 

 nepheline, tourmaline, topaz, beryl, titanite, quartz, albite, and 

 orthoclase, and also many rare minerals of the pegmatites. In the 

 synthesis of this class a minerali/er must usually be present, either 

 to lower the fusing point or to reduce the viscosity of the melt, 

 while some, as topaz, tourmaline, and muscovite, have never been 

 produced artificially. 



That mineralizers have been present during the stage of crystal- 

 lization of such rocks as granite, syenite, and pegmatites is shown 

 by the simple fact that the quartz of such rocks always contains 

 numerous cavities holding liquid inclusions, and that the micas con- 

 tain hydroxyl and fluorine. The mineralizer is usually a volatile 

 substance or forms volatile compounds with the bases as the 

 fluorides, and therefore when the magma is extruded they escape, 

 and with their escape the tendency to quickly solidify and the for- 

 mation of glass is increased. Basalts are more often crystallized 

 than are the rhyolites and andesites, as in the latter the mineral- 

 i/ers have escaped, leaving the magma by nature too viscous to 

 crystallize. The coarse crystalline forms, granite and syenite, are 

 plutonic rocks and have crystallized under conditions which pre- 

 clude any escape of the volatile mineralizers, or they have escaped 

 very slowly. 



In the process of cooling, those compounds which are the more 

 insoluble have separated first, with the result that the remaining 

 liquid portion is a concentrated solution of the more soluble, and 

 the dykes, veins, and marginal masses connected with some granites 

 and known as pegmatites are the result of and represent the ulti- 

 mate concentration of some of the constituents of the original fused 

 magma. Crystals of pegmatites are large and well formed, and such 

 dykes contain minerals in quantity which are rare accessories in the 

 rock mass as a whole. There seems to be no good reason why the 

 condition should not change during crystallization from that at 

 the beginning, a fused magma, at a high temperature, far above the 

 critical temperature of water, 365 C., to that at the end, when the 

 final product of crystallization may be separated from a solution in 

 water, though hot and under pressure. 



