830 DESCIIIPTIVE GEOLOGY. 



is also notable as being rich in microscopical tridymite and rounded spliaeru- 

 lites. Although a true massive eruption, this rhyolite field frequently shows 

 bedding planes. 



In the bottom of Antoine's Canon, about 2 miles above its mouth, is 

 an isolated body, designated basalt on the geological map. After a careful 

 microscopical analysis. Professor Zirkel has shown that in its mineralogical 

 composition and habit this rock must be placed with the augite-andesites, pos- 

 sessing among the feldspars a preponderance of sanidin. It contains well- 

 crystallized augite, but no hornblende, while half-decomposed olivine again 

 suggests its relation with the basalts. Geologically, it would appear to be 

 one of those augite-andesites more closely allied to basalts than to true ande- 

 sites, inasmuch as the impressions received in the field at the time of our 

 examination were such as to suggest an outburst later than the adjoining 

 rhyolites. 



A short distance farther down Truckee Canon, on the north side, and 

 connected in general with the already described mass of rhyolite, occurs a 

 glass-bearing rhyolite, full of sanidin crystals and small quartz-grains, which 

 is distinctly overlaid by a pure, black basalt, having a rude columnar struct- 

 ure. It appears to be the end of a flow descending from the heights west 

 of Sheep Corral Cai3on, and is jin extremely fine micro-crystalline basalt, 

 one of the most compact in texture of any in the collection. No mineral 

 constituents arc visible to the eye, but the rock is apparently rich in globu- 

 litic glassy base; at least, it stands unusually high in silica, yielding 55.79 

 per cent. On the higher ridges, the texture is more coarsely crystalline, 

 olivine and triclinic feldspars being visible. 



The rhyolites directly underlying the end of this flow are light-colored 

 and somewhat tufaceous, partly the result of accumulation of white rhyolitic 

 rapilli, and partly the flow of :fine tufa. It is excellently bedded, and con- 

 tains more or less angular fragments of the Spanish Peak rhyolite. Mica, 

 irregular grains of quartz, and some large crystals of both fresh and decom- 

 posed sanidin are found in it. 



From this point to the end of the canon, 12 miles to the eastward, there 

 outcrops a continuous line of sanidin-trachytes appearing in low hills on 

 both sides of the river. On the south side they are soon concealed by 



