290 



Lindgren and Melville — Sodalite- Syenite 



mica, and lastly white isometric crystals up to two millimeters 

 in diameter. The original character of these white crystals 

 cannot be made out as they are completely converted into 

 secondary aggregates. The groundmass, which is quite fine- 

 grained, contains much augite, besides other indeterminable 

 minerals. 



The second type is similar to the " analcite-basalts," but the 

 specimens are not fresh and the white mineral is again com- 

 pletely decomposed. This type is more coarsely crystalline 

 and not so pronouncedly porphyritic. 



The third type is coarsely granular and in composition ap- 

 proaches the theralite described by Mr. J. E. Wolff from Mar- 

 tindale near the Crazy Mountains. 



The main mass of Square Butte is represented by a light 

 yellowish grey, rather coarse grained granular rock of miarolitic 

 appearance (No. 28705, U. S. National Museum, Summit Square 

 Butte). Macroscopically it consists of grains and broad lath- 

 shaped crystals of feldspar, often five millimeters long, black 

 glistening hornblende prisms reaching three millimeters in 

 length, and sodalite which appears as small grains of a pale 

 brownish color. Under the microscope the rock is shown 

 to be hypidiomorphic granular and the following constituents 

 are noted in their order of formation : apatite, hornblende, 

 orthoclase (with some plagioclase), sodalite, and analcite. 



Fig. l. 



Sodalite-syenite magnified 7 diameters. 

 D analcite. 



A hornblende, B orthoclase, C sodalite, 



The predominant constituent is orthoclase, occurring both as 

 irregular grains and lath-shaped crystals ; individuals partly or 

 wholly imbedded in sodalite show terminal faces. Many of 



