472 Weed and Pursson—Igneous Rocks of Montana. 
comparatively large amount of soda present cannot be entirely 
in the oligoclase, and the unstriated feldspar undoubtedly con- 
tains the albite molecule to some extent. The barium and 
strontium are possibly present in the feldspar. The mineral 
and chemical nature of the rock show it to have a somewhat 
dioritic tendency. | 
Yogoite. Middle knob rock type.-—West of the eastern 
knob of Yogo Peak the rock forming the summit of the 
mountain gradually changes in character until it assumes the 
type which characterizes the middle knob and which is so well 
displayed in the crags which form this point. The rock has a 
parting similar to that of the syenite described above, and is 
like it in its general characters. Ona freshly fractured surface 
the rock is, however, seen to be of a very much darker gray 
in color, with a greenish tone, and to be more coarsely erystal- 
lized, so that it possesses a mottled appearance, recalling a 
diorite in its habit. Even at a casual glance the rock is seen 
to be more basic than the syenite, and the ferro-magnesian 
minerals appear to make up half the bulk of the rock. The 
reflection of light from numerous cleavage plates of small 
biotite crystals is also noticeable. 
Microscopically, the minerals are seen to be the same as 
those in the syenite, but with the following differences: The 
augite, which was a nearly colorless diopside, is here a clear, 
light green mineral, quite idiomorphic, and having the usual 
cleavage and appearance, and containing frequent shreds of 
biotite and granules of iron ore as inclusions. It is very 
abundant in rather small, stout crystals, and constitutes, in 
fact, the main ferro-magnesian component. Paramorphs of 
augite and hornblende are still seen, but they are rare, and the 
hornblende has dwindled to a very small amount. [ron ore is 
more abundant than in the syenite and the grains are larger. 
Biotite is quite abundant in strongly pleochroic tablets of the 
-usual type. The ratio of the plagioclase to the orthoclase is 
nearly the same as in the syenite. It is, however, somewhat 
more basic, approaching andesine. The orthoclase is like that 
of the syenite, but shows a more marked preference to sur- 
round the plagioclase and to appear in broader plates. Quartz 
is wholly wanting. The two predominating minerals are augite 
and orthoclase, and the great increase of the former over the 
latter, as compared with the ratio prevailing in the syenite, is 
disclosed in the chemical composition, as shown in the follow- 
ing analysis by Dr. Hillebrand : 
