Weed and Pirsson — Bearpaw Mountains, Montana. 289 



long, black prisms of augite of about the same size or larger, 

 and round white or very pale greenish grains of leucite, which 

 thickly pepper the dark ground-mass and which vary in size 

 from mere white dots up to 0*5 mm across. The rock strongly 

 recalls certain of the south Italian leucite rocks in which the 

 leucite phenocrysts are very small. In thin section the rock 

 is composed of the above minerals thickly crowded, so that 

 there is little base. 



The augite is of the usual character in rocks of this class ; 

 in section it becomes very pale in color and of a brownish 

 rather than greenish tone and has a strong zonal structure, is 

 very idiomorphic, frequently twinned on a (100), has a wide 

 angle of extinction up to 40° or more, shows excellent cleav- 

 age and is very fresh. 



The olivine is generally idiomorphic though occurring at 

 times in poly somatic groups ; the large crystals are fresh and 

 clear ; the smaller ones frequently changed into a deep-brown 

 or reddish-brown colored substance, which appears to be simi- 

 lar to alterations of olivine seen in other occurrences,* in this 

 change the smaller crystals are completely altered, larger ones 

 only partially, and in the largest the alteration appears only as 

 red-brown bands following cleavage planes. We believe this 

 to be due to a change in the iron oxide in the mineral, and that 

 it is especially iron-rich olivines which are liable to it. 



The leucites under the microscope are seen to be very 

 thickly crowded, composing the bulk of the rock. They are 

 of all sizes, from very minute individuals up to those previ- 

 ously mentioned. They are of rounded forms though not 

 generally bounded by an absolutely sharp, definite line, but 

 fade out into the ground-mass in a rather ragged indefinite 

 way. They do not contain any of the inclusions zonally 

 arranged which are so frequently seen in this mineral, but are, 

 like the larger leucite phenocrysts in the leucitic rocks, entirely 

 free from them ; like these larger phenocrysts they are fre- 

 quently cracked or appear in grouped polysomatic forms. 

 They are not colorless and limpid like the leucites of the fresh 

 Italian rocks, but are of a light-brown color and appear with a 

 low power very like a kaolinized orthoclase in character ; they 

 are perfectly isotropic between crossed nicols. Studied with 

 high powers they appear filled with excessively fine granules, 

 shreds and leaves of some substance so fine that they can 

 barely be discriminated ; this material is unevenly distributed 

 through them and does not act on polarized light. It is possi- 



* Rosenbusch, N. Jahrb., 1872, p. 59, Phys. d. Min., 1892, p. 472; Michel 

 Levy, Bull. Geol. Soc. France, 3d Ser. xviii, 1890, p. 831 ; IddiDgs, Geol. Eureka 

 Dist., Mon. xx., U. S., Geol. Surv., AppeDdix B, pp. 388-390; Pirsson, this 

 Journal, vol. xlv, 1893, p. 381 ; Lawson, Geol. Carmelo Bay, Bull. Univ. Gal., 

 voli, 1893, p. 31. 



