ABSAROKITE-SHOSHONITE-BANAKITE SERIES. 941 
the groundmass of the central part in mineral composition. Bio- 
tite is not present, and the only feldspars are microlites of plagio- 
clase, which may correspond to the prismatic cores in the small 
orthoclases of the central part of the dike. 
When compared with the leucite-absarokite from Ishawooa 
Canyon it is found that the phenocrysts of olivine and augite are 
not so abundant as in that rock, while there is much more augite 
in the groundmass, besides abundant biotite. The microscopic 
feldspars are not so large and distinct, and leucite is wanting. 
Chemically the rock just described is richer in potash, with about 
the same soda. Alumina is slightly higher; and magnesia and 
lime slightly lower. Silica is a little higher. The high percent- 
age of water indicates a hydrous silicate in the groundmass. 
The rock, whose chemical composition is shown by analysis 
3, forms a 3-foot dike on the high ridge south of Clark’s Fork 
River, southeast of Index Peak. The rock corresponding to anal- 
ysis 4 forms a surficial lava flow east of the head of Raven Creek 
on Mirror Plateau. Chemically they are almost identical, and 
nearly the same as the dike rock from the divide between Lamar 
River and Crandall Creek. They have slightly less magnesia, 
and the lava flow has 3.4 per cent. more silica than the dike rock. 
The dike rock is dark greenish gray, aphanitic, with small mega- 
scopic phenocrysts of augite, and occasional large crystals of 
quartz with augite shells. In thin section the few megascopic 
augites resemble those in the rock last described. All the other 
constituents may be considered as parts of the holocrystalline 
.groundmass. They are abundant pale green augite, brown biotite 
and magnetite with a subordinate amount of feldspathic matrix. 
There are larger augites, colorless at the center and green at the 
margin, besides many small serpentinized olivines. The feldspar 
is partly lath-shaped orthoclase with prismatic cores of lime- 
soda-feldspars. They are often in radiating groups. There is 
also much cloudy microcryptocrystalline material with no definite 
form, except a frequent approach to the outline of an isometric 
mineral. These are often darker colored at the center, and sug- 
gest altered leucites. The large loss upon ignition and the pres- 
