436 GEOLOGY OF THE YELLOWSTONE NATIONAL PARK. 



(c) Very fine grained, with small feldspar phenocrysts. 

 It must not be forgotten, however, that these are often merely phases 

 of crystallization that may sometimes occur in one and the same rock body. 



OPHITIC BASALT. 



The basalts with ophitic structure are very fine-grained, holocrystalline 

 rocks, with very few small phenocrysts of lime-soda feldspar, in some cases 

 none. They occur in all parts of the Park, having been found at a number 

 of places on Yellowstone River, in the sheet opposite Tower Falls (661), 

 near Garnet Hill (652), in the sheets at the mouth of Blacktail Deer Creek 

 (625), and in those near Bear Gulch and elsewhere. It is a structural facies 

 of the basalt underlying rhyolite on Mount Everts (622) and occurs in the 

 basalt on the rhyolite plateau (588, 596). It also forms a facies of the basalt 

 of Falls River Basin (1758). The proportions of the mineral constituents — 

 augite, labradorite, olivine, and magnetite — vary in different rocks. In 

 some, augite is in excess of feldspar; in others, they are nearly equal to each 

 other. The ophitic augites range from 0.5 to 1 mm. in diameter, while the 

 inclosed feldspars average 0.1 mm. in length. The basalt overlying gneiss 

 on the summit of Hellroaring Peak, just north of the Park boundary (658), 

 has the greatest amount of augite and magnetite and rod-shaped iron oxide 

 (ilmenite ?), with subordinate labradorite and olivine. The olivine is in 

 small, nearly idiomorphic crystals, and also in a few small phenocrysts, 

 besides a few of labradorite. A basalt from the plateau southwest of the 

 Gallatin Mountains (588) is similar, but the labradorite is more abundant, 

 and olivine in minute crystals, slightly reddened on the surface, is very 

 abundant. Augite and magnetite are plentiful. There are no megascopic 

 phenocrysts. Others are similar, but are finer grained. In one case (1041) 

 augite and labradorite (about An 3 Ab 2 ) are in nearly equal proportions, and 

 magnetite and olivine are less than in the two previous cases. There are 

 no megascopic phenocrysts. This structure is shown in PL LIX, fig. 1. 

 The basalt from north of the mouth of Mountain Ash Creek, Falls River 

 Basin (1758), is like the last in composition, but has small phenocrysts of 

 long prismatic labradorite and olivine. These four rocks are almost per- 

 fectly fresh, and are holocrystalline, with numerous small pores. The basalts 

 occurring near Junction Butte and opposite Tower Fall (652, 654, 661) are 

 also holocrystalline, but have relatively more labradorite in tabular crys- 



