Vol. 4] Smith. — Upper Region of Main Walker Hirer. 



21 



Specimen 100 occurs at the south margin of the map PL 1, in 

 Sing-ats'-e Ridge. It is an even-grained rock, made up of about 

 equal amounts of light and dark colored minerals. The lighl 

 colored minerals are feldspar and quartz. Biotite and hornblende 

 are easily recognizable. The slide contains plagioclase feldspar', 

 whose extinction angle in the zone normal to the braehypinacoid 

 ooP 56 (010) is symmetrically 14°, but the sign was not deter- 

 minable, and this one measured was not a Carlsbad twin, hence 

 the tables of Michel Levy could not be used. Some of the plagio- 

 clase attains a length of 2 mm. Microeline and quartz of about 

 equal grain are present, but the plagioclase predominates. Green 

 hornblende of a length of 2 mm. is present and in size about 

 equal to the plagioclase. There is less of the biotite than horn- 

 blende, and there is less iron oxide in this slide than in the pre- 

 vious slides. Sphene in part idiomorphic toward microeline and 

 allotrimorphie toward plagioclase is a constituent. This rock 

 may be termed a biotite granite. No. 120 is a similar rock. It 

 occurs farther south, off the map PI. 1, in the same ridge and 

 under the basalt cap west of Nordyke. In the hand-specimen 

 the feldspars are of two colors, brownish and white. These are 

 about of the same dimensions and about equally abundant. The 

 remainder of the rock, about one-third of it, is hornblende. The 

 average size of the first named minerals is a little less than 0.5 

 cm., and that of the hornblende about 3 mm. The contrast of 

 colors readily characterizes the rock in the field. In the slide the 

 brownish colored feldspar is orthoclase. Its color is due to dust- 

 like impurities. The plagioclase is clear for the most part. Bio- 

 tite is absent and the hornblende is of a greenish color. No min- 

 eral is idiomorphic except some accessories. The iron ore has 

 irregular shape and shows a preference for the hornblende The 

 slide as a whole shows considerable fracturing of the rock, and 

 thus indicates the disturbance to which it has been submitted, as 

 seen in the field. The disturbance evidently was not long con- 

 tinued, for no mineral indicating dynamic metamorphism is rec- 

 ognized, and this is equally true of all the slides thus far exam- 

 ined, unless epidote and microeline be so regarded. The neces- 

 sary water for hydration of the iron and lime to produce epidote 

 may be derived from the water found in the bubble inclusions 



