94 



GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



II. The main body of the stock, most of its contact forms, and most 

 of the rocks that have broken through it, with some apophyses, probably 

 contemporaneous with the main mass. 



III. The quartz-mica-diorite-porphyry that broke through the stock 

 and also produced some dikes. 



I. THE DIKE ROCKS AND CERTAIN CONTACT FORMS OF THE STOCK. 



porphyries. — The porphyries forming most of the dikes, which are from 

 1 to 25 feet in width, have a generally uniform habit. They are dense 

 and aphanitic, filled with small phenocrysts of feldspar and ferromagnesian 

 silicates, mostly hornblende and biotite. They have a uniformly speckled 

 appearance, with occasional spots of white feldspar or black ferromagnesian 

 silicates. Variations in habit are due to differences in color, caused b}' the 

 relative proportions of light and dark colored phenocrysts, and to the nature 

 and amount of the groundmass. The color varies from dark greenish and 

 purplish gray to light gray of different tints. The dike rocks are in some 

 cases fresh and compact, in others decomposed and disintegrated. 



In thin sections under the microscope the groundmass of all these dike 

 rocks is found to be holocrystalline, and the phenocrysts are lime-soda feld- 

 spar and hornblende, generally with biotite, occasionally with pyroxene. 

 The relative proportions of these minerals vary gradually among the rock 

 bodies, so that the specimens collected may be arranged in the follow- 

 ing subdivisions, according to the relative amounts of the ferromagnesian 

 silicates : 



Table I. — Mineral variation of the andesite-porphyries of Electric Peak. 



Subdivisions. 



Biotite. 



Hornblende. 



Pyroxene. 





Much 



Much 



Some 



Little 







]) 



Much 







Much 





a 



/ 











Much 



Much 



Some 



Some 



Much . . 



Much 



Much 



h 













Besides the phenocrysts of biotite there are shreds of this mineral 

 that belong to the period of crystallization of the groundmass. There is a 



