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Weed and Pirsson—Igneous Rocks of Montana. 471 
common with a brownish-green hornblende. The two min- 
erals are very frequently found together in stout, ill-shaped 
erystals from 1 to 2™™ long, the pyroxene forming a core, sur- 
rounded by the hornblende. In such cases the amount of 
pyroxene is inversely proportional to that of the hornblende. 
The appearance and association of these two minerals clearly 
show that the hornblende is paramorphic after the pyroxene. 
The latter rarely occurs alone, while the hornblende frequently 
does so. Biotite is rare and occurs only as occasional brown 
pleochroic shreds. 
Orthoclase is the predominant feldspar, occurring in irregu- 
lar masses. A much smaller quantity of plagioclase is also 
present, the optical characters proving it to be oligoclase. It 
is more idiomorphie than orthoclase and is, indeed, often sur- 
rounded by a mantle of the latter mineral. A small amount 
of interstitial quartz completes the list of minerals. 
In structure the rock is hypidiomorphic, but only partly so, 
as the pyroxene and hornblende are themselves rather ill- 
formed and irregular, and the tendency is toward an allotrio- 
morphic structure. ‘The average size of grain is about 1™™. 
The following analysis shows the chemical composition of 
this rock. It has been made by Dr. W. F. Hillebrand of the 
U. S. Geological Survey, a chemist whose skill and painstaking 
accuracy have been of such vast service to American petrog- 
raphy. 
Syenite from Yogo Peak. 
SC RE pete mn nee ae 61°65 
RO) ae pe Ae epee tere conte atars 56 
ENING) Te Pie pe AES eS em, Shy OOM 
Ose ee ee ah yo bt trace 
Lio 10 ems Sak ae, eae eSB 
JK ieee alta ee ree aan een 205 
ICUS Gh pial als Ele ii gall Mata tea? ‘09 
MgO sg Sel Ee RN TS eS ee eee 3°67 
SEO ee we ee Pg te Od 
Pete ee See es ee ees on 7 
eee: en eee. UR Pee ee pe ys 
yp OMG i 20 2. SRL Eee) 4°35 
Bees se aes SAE S 4°50 
ROS ihe fe BOM Re 24 trace 
GOLA? arise ae A 26 
He@rabove: IM Orica ane sesiue "41 
ES Oa adil at akin it eee Gskd. yt! 788 
100°15 
The above analysis is that of a syenite with moderately re 
lime, iron and magnesia for a rock of the syenite group. The 
