438 



GEOLOGY OF THE YELLOWSTONE NATIONAL PAKK. 



Analysis of basalt from the north base of Prospect Peak. 



Constituent. 



SiO„ 



TiO* 



A1 2 3 



Fe 2 Oi 



FeO 



MnO 



MgO 



CaO 



Na 2 



K 2 



P 2 5 



S0 3 



Li 2 



H 2 



Total 



Per cent. 



48.49 



2.19 



18.35 



7.63 



1.21 



None. 



6.72 



10.40 



3.02 



.57 



.20 



.52 



.02 



.67 



99.99 



The basalt from another exposure on Mount Everts (618) and one from 

 Falls River Basin (1757) have a little ophitic augite and much that is in 

 grains. It is in excess of feldspar. A basalt from the south side of Lava 

 Creek below Undine Falls (613) is richer in feldspar and the olivine is 

 altered; otherwise it is similar to the basalts just described. The basalt 

 from 4 miles above the mouth of Boone Creek (1739) and one from the 

 Snake River Valley (1760, 1761) have no ophitic augite, but the prismatic 

 labradorite is idiomorphic and the general character of the rock is the same 

 as that of those just described; there is, however, considerable microlitic 

 base, and ilmenite rods are numerous. Olivines are small, and the dark- 

 colored minerals are in excess of the feldspar. The basalt from Iris Falls, 

 Bechler River (1759), is very fine grained and partly ophitic, with minute 

 feldspar phenocrysts. 



The basalt overlying dacite-porphyry east of Bunsen Peak (606) is 

 very much like the ophitic basalts. The lath-shaped labradorite is similar 

 to that in the foregoing, but in places shows a slightly parallel arrange- 

 ment. Basalts from numerous localities in the Park (582, 584, 589, 591, 

 598, 603, 614, 615, 624, 627, 632, 655, 663, 671) are quite the same as the 

 one last mentioned, but vary slightly in the relative proportions of augite 

 and feldspar and in the size of the component crystals. All are holocrystal- 

 line, or nearly so. In some modifications of these rocks (605, 614) the 



