852 THE JOURNAL OF GEOLOGY. 
The main mass of mountains consists of basaltic breccias resting 
upon Cretaceous sediments, and also upon acidic tuffs and breccias, 
which are earlier than the basaltic rocks. The radial disposition of the 
dikes is one of the most marked characteristics of the geologic struc- 
ture, and is well shown on the map. The soft Cretaceous strata consist 
of sandstones and clayey shales belonging to at least two groups ; the 
lower referred to the Kootenia, the upper possibly representing a 
southward development of the Belly River formation. ‘The strata are 
nearly horizontal or but slightly inclined away from the mountains. 
In the vicinity of the volcanic cores the sedimentary rocks are meta- 
_morphosed into dense hornstones and quartzites, quite like the baked 
Algonkian slates or Cambrian shales of Castle Mountain, or the meta- 
morphosed Livingston beds about the volcanic core in the Crazy 
Mountains. 
The chief interest of the region is in the character and occurrence 
of the igneous rocks, and in the differentiations of magmatic material 
that has taken place at each of the volcanic cores. The South and 
Highwood cores consist of syenite; from them radiate dikes of basaltic 
rocks, in part leucite or analcite-basalt ; in part dark rocks with large 
plates of black biotite. Complementary rocks in the form of por- 
phyries also occur. About the Highwood core the first rocks erupted 
were acidic tuffs and breccias, intermingled with flows of felsite and 
possibly phonolite. These were succeeded by breccias and lavas of 
basaltic material similar to the dikes just mentioned. The east core 
is of syenite, partly surrounded by basaltic lavas and breccias. 
The Shonkin core is the largest in the district and consists of gran- 
ular rock, in one place breaking up through basaltic breccia. The 
Arnoux core is of similar granular rock, which breaks through acidic 
tuffs and basaltic lavas. 
Palisade Butte is a volcanic core of coarsely crystallized basic rock 
like that in Square Butte, and called shonkinite. It is a columnar 
mass topped by syenite which appears to have been extruded through 
the shonkinite. The rocks mentioned in connection with these cores 
have not yet been described in detail but are said to be of novel types 
and of great interest petrologically. At Square Butte, an eastern 
outlying mountain of this group, the character of the igneous rocks is 
such that they have been specially described. The butte is a flat- 
topped mass rising 1700 feet above its base. The igneous rocks form 
a laccolite once covered by Cretaceous strata, now almost completely 
bared of its covering. The topographic character of the mountain and 
