142 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



CORRELATION OF THE ROCKS ON A CHEMICAL BASIS. 



Correlating the two groups of rocks according to their chemical 

 composition and arranging them as in Table XVI, we see that the 

 hornblende-miea-andesites, Nos. 487 and 494, are the equivalents of the 

 quartz-mica-diorites, Nos. 313, 311, 303, 323, and 321, and of the quartz- 

 pyroxene-mica-diorite, No. 309. The dacites, Nos. 521 and 523, are the 

 equivalents of the quartz-mica-diorite-porphyries, Nos. 329 and 326. The 

 hornblende-pyroxene-andesites and the pyroxene-andesites, Nos. 421, 471, 

 407, 386, and 409, are the equivalents of the coarse-grained pyroxene-mica- 

 diorite, No. 295, with variable percentage of quartz, and of the fine-grained 

 diorites, Nos. 272 and 273, and of a fine-grained variety, No. 267. 



The dacites and hornblende-mica-andesites included within this cor- 

 relation are intruded bodies within the breccia of Sepulchre Mountain, and 

 have the same mineral composition as the corresponding porphyries and 

 diorites of Electric Peak. Thev differ from them in structure and degree 

 of crystallization, as already described. 



The glassy andesites, with pyroxene and hornblende phenocrysts, 

 however, present the utmost contrast to the chemically equivalent coarsely 

 crystalline diorites. In the former the hypersthene, augite, hornblende, and 

 plagioclase are sharply defined idiomorphic crystals in a groundmass of 

 glass, which is crowded with microlites of plagioclase and pyroxene, besides 

 grains of magnetite. The hornblende is brown, occasionally red, and the 

 other phenocrysts have all the microscopical characters which distinguish 

 their occurrence in glassy rocks. In the diorite the hornblende is green; in 

 some cases brown ; and the hypersthene, augite, and hornblende are accom- 

 panied by biotite, and are all intergrown in the most intricate manner, with 

 evidence that they commenced to crystallize in the order just given. The 

 labradorite is often clouded with minute opaque particles, which are charac- 

 teristic of its occurrence in many diorites. It is surrounded by a shell of 

 more alkaline plagioclase, which, with occasional individuals of orthoclase 

 and considerable quartz, closed the crystallization of the magma. Magne- 

 tite, apatite, and zircon are the accessory minerals. The quartz contains 

 fluid inclusions, which complete the correspondence of this diorite with 

 typical diorites of other regions. 



From the structure of this region, which has been so finely exposed 



