CRYSTALLIZATION. 265 



From the foregoing it is evident that magmas which may crystallize 

 into extrusive rocks whose essential minerals are plagioclase, augite, and 

 olivine may crystallize into coarsely granular rocks with plagioclase, 

 augite, hypersthene, and biotite, with a small amount of orthoclase and 

 quartz, with or without hornblende; and that olivine may be present in 

 some cases, when the other magnesian minerals will be less abundant. 



Hornblende, which is so important a constituent of the diorite of 

 Electric Peak, plays a very subordinate role in the granular rocks of Hurri- 

 cane Mesa. Biotite becomes more pronounced as the rocks become more 

 granular. Hypersthene also develops under the same conditions. And 

 orthoclase and quartz make their appearance in the granular equivalents of 

 many basalts. Thus we find minerals that are characteristic of more and 

 more siliceous members of the series of extrusive rocks developed in basic 

 magmas under conditions which render the magmas more highly crystalline 

 and more or less granular. 



These minerals, then, are in part functions of the chemical composition 

 of igneous magmas, while in part they are functions of the phase of crystal- 

 lization of chemically identical or similar magmas. This is another demon- 

 stration of the law that "the molecules in a chemically homogeneous fluid 

 magma combine in various ways and form quite different associations of 

 silicate minerals, producing mineralogically different rocks." 



CRYSTALLIZATION. 



It may be well to call attention to some of the conditions under which 

 the molten magmas within the dikes and the core of the Crandall volcano 

 must have solidified. Referring to the profile sections of the district and 

 the probable outline of the ancient volcano (PI. XXXII), it is evident that 

 the magmas which cooled within that portion of the core which is now 

 exposed, and those in the dikes within a radius of 2 miles, must have occu- 

 pied positions at nearly the same distance beneath the surface of the volcano. 

 And if the former may be considered to have solidified 10,000 feet below 

 this surface, then the latter must have solidified 10,000 feet below the surface. 

 The one is as deep seated as the other, and yet then degrees of crystallization 

 range from glassy to coarsely granular. The influence of pressure alone on 

 the crystallization of these rocks is not recognizable in the size of the grain 

 or in the phase of crystallization. 



