474. Weed and Pirsson—Igneous Rocks of Montana. 
abundance of small augites are also seen in the feldspathic con- 
stitnent. 
Under the microscope the same minerals are present as in 
those of the two types just noted, but the augite, biotite, and 
iron ore have greatly increased in amount. The increasing 
tendency of the orthoclase to surround the plagioclase has here 
resulted in producing broad plates in which small, stout plagio- 
clase laths lie scattered about unoriented. The plagioclase has 
become more basic and is now andesine. Its amount, compared 
with that of the orthoclase, has gradually diminished from 
the syenite down. The broad areas of orthoclase not only con- 
tain the plagioclase, but also the other constituents, in a poiki- 
litic manner. Pseudomorphs after olivine are sometimes seen, 
with an occasional unaltered remnant of olivine substance still 
remaining. The augite is darker than in the preceding types, 
and occasional paramorphoses to hornblende still occur. The 
biotite is fresh, clear, and strongly pleochroie, at times chang- 
ing toa clear, dark green variety. Apatite is more abundant 
and isin larger crystals. The structure is a hypidiomorphie, 
granitoid one. The analysis by Dr. Hillebrand is as follows: 
Shonkinite of Shonkinite of 
Yogo Peak. Square Butte. 
SiO. fa ea ee Pr ee el 46°73 
DE 8 apes baa NN ore: 1°44 78 
ACO eer a. eae 12°29 10°05 
Big 8 eka eee elie Speghpecuneet trace Bi bs 
ee eee 2°88 3°53 
1 ANE Osa thatetpa teed aie. ALdach 2 Cari 82.0 
Mn) ts Pee 08 28 
NiO at. BolT ms 9°19 9°68 
Cas ee Fa 9°65 13°22 
Ba@) you oie "43 undet 
REG) tious bat eee 08 undet 
NawO. peg dived aaa eres 1:80 
KilOle. cat vied gta eeeegG 3°76 
tye dis ape ae trace trace 
Oat lacs = 5 eae 26 ae 
H,O above 110° --- 56 
ih 0 A ROB spice ‘98 151 
AU 2p PRN Bil ub ho ea Aig.) tire Ae 18 
99°99 100°97 
C= 708 O=Cl 04 
99°91 L009 
The consideration of this analysis shows the relatively low 
silica and alumina and the very high amounts of iron, lime and 
magnesia. The alkalies are still present in considerable amount, 
