Weed and Pirsson — Bearjpaw Mountains^ Montana. 137 



The cBgirite-augite is in stout to slender prisms not of good 

 idiomorphic shapes; it is often diopside-like in the center and 

 increases in gegirite to the edge ; at other times small anhe- 

 drons are seen of quite pure aegirite, as shown bj the extinc- 

 tion angle. 



The, feldspar has a tendency to a broad lath-shaped form, 

 with the interspaces filled by smaller rounded grains of the 

 same mineral. It is sometimes single, sometimes twinned 

 according to the carlsbad law ; more rarely baveno twins are 

 seen. Examined with very low powers, it appears nearly 

 homogeneous ; with very high powers, it is seen to be a mixture 

 of albite and orthoclase in very fine little specks that mottle 

 each other and fade away indefinitely, giving the surface a 

 flamed, mottled appearance. Ko albite or pericline twinning 

 is seen in it. It is filled with minute microlites of pyroxene. 

 It presents the common characters of the soda-orthoclase seen in 

 rocks of this class and needs no further mention. 



The sodalite is rather rare and occurs only as minute masses 

 filling small angular interspaces. Its presence was proved by 

 a chemical test for chlorine. 



The presence of an occasional apatite grain, shred of brown 

 pleochroic biotite, and of a little greenish brown hornblende was 

 also noted, as well as small characteristic crystals of titanite 

 and, very rarely indeed, a little iron ore. 



The structure is the characteristic one of abyssal rocks, com- 

 posed of formless interlocking grains ; it is hypidiomorphic. 

 The rock contains numerous angular inclusions of basic rock, 

 some finer grained than the syenite and others as crystalline. 

 They probably represent fragments of a partly consolidated 

 basic rock that is one of the products of the difierentiation of 

 the intrusive mass. 



The presence of this alkali syenite in connection with the 

 other types described in tlie Bearpaw Mountains is very inter- 

 esting and indicates that a study of this core or stock may be 

 expected to yield very interesting petrologic results. 



Intrusive Trachyte, near Lloyd. 



The postofiice of Lloyd is situated in the northeastern part 

 of the Bearpaw Mountains, on the headwaters of Snake Creek. 

 The mail route from Chinook passes Bean Butte, an isolated 

 hill of basaltic breccias, and crosses a low gap in the rough 

 chain of basaltic hills lying between Bean and Snake creeks, 

 and through the meadows of a hill-environed basin cut in soft 

 Cretaceous shales. A steep hillside immediately east of the 

 postoffice is seen to be capped by a massive cliff, whose talus 

 slopes extend down to the road. The cliff is formed of a mass 

 of igneous rock intruded in tilted argillaceous shales. 



