144 Weed and Pirsson — Bearjpaw Mountains^ Montana. 



appears to be part of a lava flow intercalated in the breccias, 

 and proves to be a leucitite or olivine free leucite- basalt. As 

 but few American occurrences of leucite rocks are known, it 

 is of unusual interest. 



The rock is very dark gray on a fresh surface, quite dense, 

 with conchoidal fracture, and has a pronounced basaltic habit. 

 It does not contain either pores or the open structure due to 

 vapor. In the dark, compact groundmass occasional pheno- 

 crysts are seen. Yery rarely hiotite occurs in rather thick, 

 well formed bright, bronzy-brown tablets which vary from 

 5™°" to 10™"^ in diameter. Much more common are prisms of a 

 green divpside^ mostly very small, and round white sections of 

 leucite which are from 1 to 2""°^ across. 



In the thin sections the minerals are found to be phenocrysts 

 of hiotite, augite, and leucite in a groundmass of thickly 

 crowded leucites with a very little glass base. One or two 

 large ore grains were seen. No olivine is present and the 

 rock is therefore a leucitite. 



The hiotite is so rare that only one or two flakes have been 

 seen, and one large phenocryst that should have appeared has, 

 unfortunately, been almost destroyed in grinding. Sufficient 

 remains, however, to show that it is strongly pleochroic 

 between a peculiar dark reddish yellow and nearly colorless. 

 Cleavage plates furnishing basal sections are found not to be 

 entirely dark during revolution, but to indicate a considerable 

 obliquity of the axes of elasticity. It shows in convergent 

 light a pronounced biaxial character and is a meroxene : with 

 the movable thread micrometer ocular there was determined 



2E = 38°, 



which was found to be the same on two plates. This is a large 

 angle for a biotite, but Tschermak * gives 2E = 56° for one 

 which is also from a leucitic rock, the tuff of the Monti 

 Albani in Italy. 



These biotites are partly resorbed and bordered by masses 

 of opacite grains ; everything, indeed, points to an earlier 

 intratelluric period of formation for the mineral. 



The augite appears in section of a very pale olive brown ; 

 it is idiomorphic, has a good cleavage, is fresh, unaltered, 

 rarely contains glass inclusions, and there is not very much of 

 it. It has a wide extinction angle and appears present in only 

 one generation. 



The leucite phenocrysts are quite well crystallized, very 

 clear, limpid and fresh ; they are free from inclusions, as is 



*Ber. Ak. Wien., Ixxviii (1), p. 21, 1887. 



