Weed and Pirsson—Igneous Rocks of Montana. 475 
the potash predominating. The analysis is essentially that of 
a minette, but recalls also the composition of certain tephrites 
and leucitites. The rock is, however, a rather coarse, massive, 
evenly granular, stock form, consisting chiefly of augite and 
orthoclase with considerable accessory biotite, iron ore, and 
plagioclase, with a lesser amount of olivine and apatite. 
At the west end of Yogo Peak a variation of the above type 
is found that forms large, irregular masses near the contact, the 
rock being noticeable for the very large, spongy, biotite erys- 
tals which it carries. These biotites are at times 1™ across 
a cleavage face. They are made up of a number of smaller, 
nearly similarly oriented individuals mixed in with other con- 
stituents. Although the mica is subordinate in amount, it has 
the appearance of being predominantly present, and the rock 
appears at first glance to be almost wholly made up of these 
coarse biotite crystals. Examined in thin section under the 
microscope, it is seen to be composed of the same minerals as 
the type last described, but the olivine is fresh, showing little 
or none of the change into resorption pseudomorphs, and the 
total amount of iron ore, augite, and biotite is greater. The 
augite is still the predominating ferro-magnesian mineral. The 
orthoclase shows a still greater tendency toward the poikilitic 
structure, occurring in broad plates, enclosing other minerals— 
that is, filling the interspaces between them in similarly ori- 
ented areas. It is noticed also that the biotite occurs at times 
in this manner with respect to olivine, iron ore, and augite, 
and hence it is later in formation. Sometimes the olivine is 
surrounded by orthoclase, an uncommon association of these two 
minerals. In its period of formation, however, the olivine ante- 
dates all the other minerals. 
As yet, no analysis has been made of this type, but from a 
study of the section and its comparison with the foregoing types, 
and of their analyses with one another, it is clearly evident that 
the process of differentiation which is under description has 
advanced somewhat further taan in the last analysis, and that 
this rock would show still lower silica, alumina, and soda, with 
higher lime, iron and magnesia and considerable potash domi- 
nating the soda. 
Recently under the name of Shonkinite* we described a rock 
type from Square Butte in the Highwood Mountains of Mon- 
tana, some forty miles to the north of Yogo Peak, where it 
occurs, as the outer differentiated zone of a laccolite composed 
chiefly of sodalite syenite. 
It is a rather coarse, granular rock composed chiefly of ortho- 
clase and augite with accessory iron ore, apatite and biotite with 
a very small accessory amount of sodalite and traces of nephe- 
* Bull. Geol. Soc. Am., vol. vi, pp. 400-422, 1895. 
