268 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



uniform distribution instead of the segregation of the heavier minerals 

 in feldspathic magmas, such as phenocrysts of augite and magnetite in 

 rhyolite. The specific gravity of the former, about 3.3 and 5, is so much 

 g'reater than that of even the solidified magma in the form of obsidian, 2.3, 

 that it is difficult to imagine how fairly large crystals of these minerals could 

 have remained suspended for any length of time in this matrix when it was 

 in a fluid state. 



Cross ' has called attention to certain large crystals of orthoclase in 

 dacite-porphyries ("quartz-porphy rites") and in granular diorites in Colo- 

 rado, which appear to have crystallized after the magmas of these rocks had 

 been erupted and had come to rest. And Pirsson has joresented, in a paper 

 read before the eleventh annual meeting of the Geological Society of 

 America, further evidence of the relatively late growth of phenocrysts in 

 many porphyritic rocks. 



1 Cross, W., The laccolitic mountain groups of Colorado, Utah, and Arizona : Fourteenth Ann. 

 Eept. U. S. Geo]. Survey, 1895, p. 229. 



