470 Weed and Pirsson—Igneous Rocks of Montana. 
has ruptured the sedimentary series, and extends for 10 miles 
to the northeast in a somewhat irregular line. This fracture is 
marked by a series of upbreaks of igneous magmas of various 
types which form intrusions of varying width, accompanied 
by laceolitic masses, intruded sheets and numerous dikes. The 
type of rock immediately adjacent to the Yogo Peak mass is a 
quartz syenite-porphyry, forming the low saddle northeast of 
the peak and which is succeeded eastward by a repetition of 
Yogo Peak rock types. Although this rock comes in actual con- 
tact with that of Yogo Peak, the line is hidden by debris and 
its character cannot be determined. 
The rock types whose varying phases are discussed in the 
present article are from the Yogo Peak mass itself, and as the 
stock shows a constant variation and gradation in its chemical 
and mineralogical composition along its east and west axis, 
three types have been selected to show the variation, the 
specimens coming from the east, middle and western points of 
the summit of the peak. For purposes of convenience, thesé 
will be designated as types from the east, middle, and west 
knobs respectively. 
Petrography of the Yogo Peak Rocks. 
Syenite. Hast knob rock type-—The rock mass composing 
the high eastern shoulder of Yogo Peak possesses a platy part- 
ing which causes it to split readily and to form piles of debris 
and talus slopes, above which project the low and much-jointed 
exposures of the rock in place. The joint blocks are short, 
stout rhomboids, or heavy plates a foot or so long. They are 
very hard and tough, ring sonorously under the hammer, and 
are broken with difficulty, the rock being unaltered and fresh. 
These characters prevail for the entire Yogo Peak mass. 
On a freshly fractured surface the rock appears evenly gran- 
_ ular, of moderately fine grain, and is compact in character and 
with few miarolitie cavities. The color is a medium gray with 
a.strong pinkish tone. The rock is clearly a feldspathic one, 
and of syenitic aspect. Examined with the lens, it is seen to 
be chiefly composed of light-colored feldspar, dotted with small, 
dark, formless spots of green pyroxene or hornblende. 
The microscope shows the following minerals to be present: 
apatite, titanite and iron ore, pyroxene, hornblende and biotite, 
orthoclase, oligoclase and quartz. The apatite and titanite are 
of the usual characters common to such rocks. The iron ore 
is not abundant.and occurs in small grains of about 1™” in 
diameter. The pyroxene is a very pale green diopside and is 
much cracked and broken up. It frequently appears like a 
bundle of rods. It is rarely alone and generally occurs in 
