394 GEOLOGY OP THE YELLOWSTONE NATIONAL PARK. 



uniformly and generally through large masses of rock, without reference to 

 the boundaries of the rock body — that is, they are uniformly scattered 

 through the pumiceous, compact, glassy, and lithoidal portions of the lava 

 flow in a given locality, or are entirely absent from all of them. It is quite 

 as evident that the second group of crystallization products came into exist- 

 ence after the lava reached the surface, and to a great extent after it had 

 ceased to flow. The reasons for this will appear upon studying the micro- 

 structure of the various forms of rhyolite, for they will be found to exhibit 

 a definite relation to the form of the body of the rock. 



It does not follow from this that the porphyritical crystals or pheno- 

 crysts were developed prior to the act of eruption of the lava; on the con- 

 trary, it has been shown elsewhere 1 that the phenocrysts of rocks were 

 probably crystallized during the act of eruption, and that in some cases 

 their growth continued uninterruptedly into the period of final crystalliza- 

 tion of the whole magma. But in the case of the rhyolite of this region 

 there is nothing to indicate what were the conditions of crystallization in 

 different parts of the rhyolite lava previous to its arrival at the surface of 

 the earth. There is simply the fact that in- places, or in particular flows, 

 phenocrysts are entirely wanting, and that in others they are few or are 

 very abundant, and that they may be small in some cases and large in 

 others. The rhyolitic lava of Obsidian Cliff is an example of a compara- 

 tively small flow in which no phenocrysts have been developed. The main 

 body of rhyolite, however, is variable in this respect; but the variations 

 obtain for larger areas of lava. Let us first consider the microscopical 

 characteristics of the phenocrysts. 



PHENOCRYSTS. 



The minerals that have crystallized porphyritically are quartz, sanidine, 

 plagioclase, augite, and magnetite. In this category may be included zir- 

 con, though its crystals are always microscopic in size. Its period of crys- 

 tallization is the same as that of the phenocrysts. The same may be said 

 of pseudobrookite, which is closely associated with the iron Ores and zircon, 

 and also of apatite and allanite, which occur sporadically. 



1 Chapter III, p. 105, and Chapter VII, p. 267. 



