190 Weed and Pirsson—Bearpaw Mountains of Montana. 



it shows that thej were formed in place, as noted later, and 

 are not of intratelluriG origin. 



In the hand Specimen the rock has a strong green color of a 

 grayish tint on fresh fracture, and has a pronounced spotted 

 appearance, owing to the great number of large feldspar pheno- 

 crysts with which the dark groundmass is thickly crowded. 

 These feldspar phenocrysts are of a very pale gray color or 

 white, are usually quite idiomorphic, and generally show a short, 

 thick tabular habit ; in size they vary from 5 to 15°"™ in length 

 by half that in breadth. They are often single, sometimes 

 grouped ; some are simple individuals, others are carlsbad 

 twins. While usually more or less opaque, they are often 

 clear, glassy, and of sanidine habit. They have excellent 

 cleavages, that parallel to o'(OOl) being very perfect. 



The green groundmass examined with the lens is seen to 

 be full of small glittering black augite prisms rarely more 

 than 1™"" long ; these can be frequently seen lying in the feld- 

 spar phenocrysts. The base in which these lie cannot be 

 resolved by the lens, but is evidently crystalline and of a dark 

 green color with a dull, greasy luster. An occasional speck of 

 yellow chalcopyrite and of a resinous brown mineral, which is 

 thought to be titanite, completes the list of minerals which are 

 megascopically visible. 



Microscopically the minerals seen are apatite^ cegirite^ 

 augite, alkali feldspar, nephelite, cancrinite, a little sodalite, 

 and a doubtful fibrous hornblende. 



The pyroxenes are very peculiar. They consist largely of 

 segirite-augite and segirite ; instead of gegirite forming an 

 exterior mantle, as is usually the case where these two occur 

 together, there are alternate zonal bands and patches of deep 

 green material and paler yellowish green, as if the formation 

 of the segirite molecule had varied in amount from time to 

 time. Another striking peculiarity of these augites consists 

 in the fact that they generally contain interior cores of a per- 

 fectly colorless pyroxene-like mineral. In no case is there a 

 gradual transition from the colorless core to the green gegirite ; 

 the line of juncture is perfectly sharp and well-defined. The 

 cores also have jagged irregular outlines, and are not of one 

 piece but consist of bundles of irregular strips, staves and 

 kernels, which vary in their optical orientation, and around 

 these colorless masses the deep green segirite has grown, filling 

 their interspaces and completing the crystal form. The min- 

 eral has a prismatic habit, two cleavages at nearly 90° ; like 

 pyroxene, both the refraction and birefraction are high. In 

 the length section, which is also that of the compound pyrox- 

 ene, the strips extinguish nearly parallel ; in cross sections the 

 extinction cuts the cleavages at an angle. The vertical axis is 



