264 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



very little, if any, quartz. The eleventh is a pyroxene-diorite-porphyry 

 approaching- monzonite-porphyry, which is very fine grained and is com- 

 posed of plagioclase, augite, hypersthene, magnetite, and some biotite and 

 a little quartz, with no olivine, but paramorphs after olivine, now consisting 

 of augite, biotite, and magnetite. It also contains microscopic feldspars, 

 which are in part orthoclase. 



The twelfth analysis is still higher in silica, with somewhat lower 

 alumina, lower lime, and about the same alkalies. The rock is a dike of 

 hornblende-mica-andesite-porphyry with phenocrysts of andesine-labra- 

 dorite, hornblende, and biotite, besides a small amount of augite and 

 hypersthene. The groundmass is microcrystalline, and consists of feldspar 

 and the ferromagnesian minerals just named, with considerable magnetite. 

 There is a little chlorite or serpentine. It is interesting to compare this 

 rock with that of the Indian Creek laccolith. 



The next three analyses (13, 14, 15) are of diorites that form part 

 of the core of the volcano. They have nearly the same percentage of 

 silica, but that with lowest silica has highest alumina and lime and low- 

 est magnesia and potash. This rock also differs from the others miner- 

 alogically. It is a very fine-grained rock without phenocrysts. It consists 

 of labradorite-andesine and a small amount of orthoclase and quartz, 

 besides a moderate amount of augite, hypersthene, and magnetite, and 

 very little biotite. The second of the three, with 63.97 per cent of silica, 

 is of a coarser-grained quartz-mica-diorite, composed of andesine with a 

 nearly equal amount of orthoclase and quartz, besides considerable biotite 

 and hornblende, some magnetite, and a little pyroxene. The third of these 

 three analyses is of a quartz-mica-diorite-porphyry, with abundant quartz. 

 The phenocrysts are biotite, andesine, and quartz, and occasionally ortho- 

 clase. 



It corresponds chemically to the diorite last described, and is almost 

 exactly the same mineralogically and structurally as one of the quartz- 

 mica-diorite-porphyries (331) of Electric Peak, and chemically it resembles 

 another of those rocks (329). 



The sixteenth analysis is of a fine-grained granite which forms a 

 10-inch vein in the diorite. It consists of quartz, orthoclase, and oligo- 

 clase, with biotite and magnetite, besides a very little hornblende and some 

 chlorite. 



