Weed and Pirsson — Beai'jpavj ^fountains, Mont<;ma, 139 



clearly shown by a section parallel to ^(010), which shows the 

 outline of a{100) and c(OOl). The angle c(OOl) on ^(100) was 

 measured as 65°, while C3r. ang. ^ = 63° 56^ The section 

 shows a positive obtuse bisectrix centered in the field, and the 

 extinction is 5° from the trace of the cleavage <?(001) in a pos- 

 itive sense. Cleavage fragments show that the basal cleavage 

 (?(001) is much better than that of 5(010); such fragments 

 placed on this face have an extinction so nearly as can be told 

 exactly parallel to the edge of c(OOl) on 5(010). These are the 

 exact optical characters of orthoclase ; we do not know, how- 

 ever, how much of soda in the shape of the albite molecule 

 the phenocrysts may contain ; toward the periphery they are 

 zonally built as best seen in the clinopinacoid section, and as 

 this occurs the angle of extinction rises gradually from 5° to 

 10° in a positive sense, the latter angle obtaining at the outer 

 border, the birefraction rising at the same time. This appears 

 to point clearly to an increase in the albite molecule as the 

 periphery is reached. In other sections flame-shaped tongues 

 and patches of this feldspar substance of higher birefraction 

 penetrate and spot the orthoclase. The only twinning observed 

 was that according to the carlsbad law, and the phenocrysts 

 are often bunched and grown together in groups. 



The feldspar composing the groundmass is in allotriomorphic 

 granules, which average 0*25™°^ in diameter. It appears quite 

 similar in character to the large phenocrysts, but is much more 

 turbid from kaolinization so that it does not lend itself well to 

 optical investigation ; it is untwinned and may safely be 

 assumed as soda orthoclase or anorthoclase. A few sections 

 were observed which showed in very minute laminae the 

 albite twinning; they were too few and the twinning too fine 

 to afford material for investigation. They may be of anortho- 

 clase. 



Between the feldspars are occasional minute anhedra of 

 quartz, evidently primary and the last product of crystalliza- 

 tion. 



Of the dark minerals the hornblende^ in small, rare patches, 

 is deep green and pleochroic into yellow. It has a small angle 

 of extinction and its birefraction is very low ; it appears arf- 

 vedsonite-like in its character. It nearly always appears as if 

 partly resorbed and is surrounded by borders of a very pale 

 reddish-brown variety with a similar orientation, and which is 

 more or less fibrous in character. 



The augite is present in idiomorphic prismatic crystals, 

 which vary from long slender columns to short thick ones. 

 The largest seen are about l^""" long. The color is quite vari- 

 able ; some are almost colorless, while others are a clear light- 

 green and faintly pleochroic ; the two colors are often found 



Am. Joub. Sci. — Fourth Series, Yol. II, No 8. — August, 1896. 

 10 



