SHEEP COERAL OA^ON. 837 



relation to the neighboring basalts is not clearly made out, but it would seem 

 most probable that they appear as dikes penetrating the genuine basalts. 



Some idea of the immense volume of the volcanic series is obtained 

 from the study of so long and deep a canon as this transverse valley of the 

 Truckee. Upon either side, the hills rise from 1,200 to 3,000 feet. Much 

 of this canon-depth cannot be attributed to erosion, but certainly throughout 

 the lower portion it would seem as if a very considerable part must have 

 •been produced in this manner. The canon therefore exhibits a deep cut 

 across the Virginia Range, and it is somewhat remarkable that so little of 

 the earlier formations have been exposed. A cut through the range in the 

 region of Washoe must have exposed a very considerable amount of strati- 

 fied rocks, and a large body of diorite. Here at the Truckee Canon, there 

 are but two small patches of diorite and a band of porphyry to represent 

 the older rocks, all the rest being of Tertiary volcanic age, conveying some 

 idea of the immense development of these comparatively recent outbursts. 



At the bend of the river, near Wadsworth, the Truckee having followed 

 a course a little north of east through the canon, suddenly describes a 

 semicircle having a diameter of 8 miles, and thence flows in a northwest 

 direction until it pours into Pyramid Lake. North from the intricate group 

 of eruptive rocks along the Truckee Canon, the Virginia Range is over- 

 whelmed and masked by an immense flood of trachyte, which occupies 

 nearly the entire distance to Mullen's Grap, with the exception of abelt 

 along the east flank extending from Sheep Corral Canon for 10 miles to the 

 northward. 



Sheep Cokral Canon. — The walls of this canon expose mainly heavy 

 beds of dark-brown trachyte. Scattered through a glass-base are occasional 

 crystals of sanidin and biotite, the microscope also revealing the presence 

 of plagioclase, while within the glass-base various forms of devitrification 

 have taken place, resulting in a variety of microscopic products, largely 

 microlitic. Upon the summit-ridges, there is an unusual development of 

 hyalite, covering many of the volcanic blocks to the thickness of one- 

 quarter .of an inch. Where the normal botryoidal hyalite is wanting, the 

 blocks are incrusted with a thin film of siliceous material. The rock is also 

 characterized by frequent cavities lined with crystalline sinter, which rises 



