calkins.] Petrography of tlif Joint Day Basin. 



12] 



Serpentine of Desolation Creel'. — The rock from this locality 

 differs markedly from that just described, and possesses some 

 unusual characteristics. Macroscopically examined, it is seen to 

 consist mainly of a greenish gray substance of oily lustre, and 

 apparently crystalline. There arc a few veins of a substance of 

 similar texture and lustre, but with a yellowish green color similar 

 to that of epidote. The surface of the specimen is dotted with 

 red grains of about 1 mm. in average diameter, sometimes with 

 a hollow centre, their charactei being revealed under the micro- 

 scope . 



On microscopical examination, serpentine of the scaly variety, 

 antigorite, is found to make up the bulk of the rock. The 

 structure, which is best seen in polarized light, gives hardly a 

 clue to the origin of this mineral. It occurs in elongated scales 

 that generally overlap and cross one another in all directions so 

 as to form a kind of feltwork. In certain small areas, however, 

 the scales are rudely parallel, and the thought is suggested that 

 these areas may represent altered pyroxene, while the rest of the 

 serpentine is derived from olivine. 



The material of the veins is similar to that forming the bulk 

 of the rock, and has the same felted structure. The serpentine 

 here is more nearly pure, however, and occurs in larger indi- 

 viduals, so that it forms more satisfactory material for study of 

 the optical properties. The mineral is transparent, has a low 

 index of refraction and varies in color from pale olive-green to 

 colorless. In the colored sections their is a distinct pleochroism . 

 Rays vibrating parallel to the elongation are the more absorbed, 

 and are colored a more or less intense olive-green, while the rays 

 vibrating in the perpendicular direction are pale yellowish green. 

 Between crossed nicols the interference colors are low, and do 

 not rise above yellow of the first order. The elongation is 

 positive. 



As in other serpentines, magnetite is formed as a secondary 

 product, and the centre of the veins is always marked by a line 

 of magnetite dust. A few grains of chromite were also seen. 



The red grains observed in the hand specimen form perhaps 

 the most unusual feature of the rock. They are composed of 

 quartz, as determined by a low refractive index, weak double 



