ERUPTIVE ROCKS FROM MONTANA. 49 



b. Porphyritic augite and sanidine ; groiindmass augite 

 and sanidine. No. 40 of the original collection represents 

 this type well. The rock shows a greenish-gray ground- 

 mass in which are imbedded light flesh-colored feldspar 

 crystals of a tabular habit, 2-3 mm. long and about i m. 

 thick. Tlu)i section: Clear, typical sanidine crystals of 

 normal form and often appearing as Carlsbad twins ; the 

 rough separation parallel to ooPiX is often seen; contains 

 as inclusions round or hexagonal crystals, isotropic and 

 possibly related to the sodalite group ; this mineral dis- 

 solves readily in HCl and gives abundant crystals of NaCl. 

 Sparingly occur brown foils of apparentl}^ uniaxial biotite. 

 Large prisms of a deep green, pleochroitic pyroxene with 

 nearly colorless center. The axis of elasticity, in the 

 green shell, lying next to c is c, not a, as should be the 

 the case if the mineral were a?girine, and the angle c — i 

 is 30^ in the same direction as the corresponding angle of 

 the colorless center, that being nearly normal for augite 

 or 38^. The pleochroismus is c leek-green, b and a more 

 yellowish-green. From this it would appear that an au- 

 gite with admixture of the aegirine-molecule is present. 



The groundmass consists of needles of the same deep 

 green pyroxene, together with microlites of feldspar and 

 possibly some glass. 



This type may also be developed much more crystal- 

 line, as for example, in a heavy dike cutting the road to 

 Fort Benton a few miles north of the divide. It is a greenish 

 gray tine-grained rock, porphyritic by a great number of 

 thin tabular white sanidine crystals (maximum size 5 mm. 

 X 10 mm. X I mm.) arranged parallel to the walls of the 

 dike ; also containing augite prisms of the usual habit and 

 color, up to 3 mm. long. The groundmass contains augite, 

 feldspar, and small biotite foils. The sanidine crystals 

 were partly analyzed by Dr. F. A. Gooch, who obtained: 

 ['>J 



