4:76 Weed and Pirsson—ILgneous Rocks of Montana. 
lite, the appearance of these latter minerals in minute amount 
being clearly influenced by its association with the sodalite 
syenite. Its recognition as a distinct rock type is founded on 
its coarse granular structure and the predominance of the ferro- 
magnesian minerals over the feldspathic one, orthoclase. For 
purpose of comparison its analysis is quoted beside that of the 
type described above. 
From what has been said it will be seen that the West Knob 
type of Yogo agrees very closely with the shonkinite of Square 
Butte, and this agreement is even more marked in the variation 
with large micas than in the type analyzed. They agree in 
structure, mineral composition and chemically. In the hand 
specimen they have a somewhat different habit, owing to the 
fact that in the Square Butte type of shonkinite the augite crys- 
tals are much larger and more idiomorphic than in that from 
Yogo Peak, while the actual amount of biotite is somewhat less. 
Under the microscope they closely resemble each other and 
both have the broad, poikilitic orthoclase areas and the large 
skeleton biotites. It gives us pleasure to announce the occur- 
rence of this rock type from a second distinct locality. 
Differentiation at Yogo Peak.—¥ rom what has already been 
stated it is evident that at Yogo Peak we havea stock of intruded 
igneous rock of an oval shape which shows a progressive differ- 
entiation along its major axis. The rock mass contains essen- 
tially the same minerals throughout, but there is a progressive 
increase in the ferro-magnesian species toward the western end. 
This is brought out still more strongly by a comparison of the 
chief rock-making oxides. 
Syenite. Yogoite. Shonkinite, 
SiO, 2 alee Cee ee ty NG Es 54°42 48°98 
Al,O, ach ati lies Sele ao Oy, 14°28 12°29 
Pepsi pls ee ep, oe 2°03 3°O2 2°88 
ReOuy a3. saree Oe 4°13 Ted 
MgO PR ere pes NY 3°67 6°12 9°19 
CaO bak at age anaes 4°61 Lhe 9°65 
Na,O a ge sce ad eae 4°35 3°44 2°22 
1 Cae) hg ae, aaa i nates Merwe 4°50 4°22 4°96 
Here almost without exception the change is progressive. 
The silica, alumina and soda diminish ; the iron oxides, lime and 
magnesia increase; the potash remains the same or relatively 
increases. In this connection it is interesting to observe the 
conduct of the rarer, less essential oxides, which occur in 
amounts sufficiently large so that the differences between them 
are evidently not those which fall within the limits of ana- 
lytical error. 
