98 



GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



abundant. The lighter- colored varieties of the rock exhibit quartz. The 

 finest-grained porphyritic forms show only feldspar and pyroxene pheno- 

 crysts, but most of the rock is evenly granular, with no porphyritic structure. 

 When examined microscopically, the constituents are found to be hyper- 

 sthene, augite, hornblende, biotite, lime-soda feldspar, orthoclase, quartz. 

 These are not all in every modification of the rock. Their range of 

 variation is indicated in Table II, in which a, b, etc., represent different 

 mineralogical modifications of the rocks. 



Table II. — Mineral variation of the diorites at Electric Peak. 





Pyroxene. 



Hornblende. 



Biotite. 



Labradorite. 



Oligoclaae. 



Orthoclase 



Quartz, 



a .. 

 b... 



C. - 



d 







Little... 

 Some . . . 

 Much . . . 



Much . . . 

 Much . .. 



Much . .. 

 Much . . . 

 Some . .. 

 Some . .. 

 Little . . . 







Little. 

 Little. 

 Some. 

 Much. 

 Much. 

 Much. 

 Much. 



Much . . . 

 Little . . . 



Much . . . 

 Much . . . 

 Much . . . 

 Some . . . 

 Little... 







Some . . . 

 Some . . . 

 Much . . . 

 Much . . . 

 Much . . . 



Little . . . 

 Little . . . 

 Little . . . 

 Little... 

 Some ... 







f 























The diorite is traversed by veins or dikes of equally coarse-grained, 

 lighter-colored diorite, sometimes approaching granite in composition. In 

 one case the rock is fine-grained granite (fig. 1 of PL XVIII). In places 

 there are narrow seams of feldspar and quartz, which grade into rock 

 containing some biotite and hornblende, and finally into quartzose diorite. 

 Such seams of feldspar and quartz appear to be the extremities of fissures 

 or cracks in the early solidified magma, into which the fluid portion of sub- 

 sequently intruded magma has been forced. They are of truly igneous 

 origin, and consist of the most liquid portion of the magma, or that part 

 which is the last to crystallize. Segregations rich in ferromagnesian 

 minerals are abundant in some parts of the rock. 



The marginal portions of the diorite core are different in different 

 places. In some there are porphyry -like modifications, indicating the more 

 rapid cooling of the margin of the mass. In other places the coarse-grained 

 rock is directly in contact with the wall of the conduit, showing no indica- 

 tion of chilling. In the first case the surrounding rocks must have been 

 cooler than the igneous magma. In the latter case they must have been 



