810 DESCEIPTIVE GEOLOGY. 



less feldspar, forming the summit of a prominent ridge, whose lower 

 slopes are almost completely buried by basalt. The beds strike north 

 25" to 30" west and dip to the southward, apparently agreeing in direc- 

 tion with the beds bordering on Winnemucca Lake. Indeed, their chief 

 interest consists in showing the extent of the Triassic formations, and' 

 their development to the southwestward from the great Triassic region 

 to the north, since they resemble the metamorphic series of the West 

 Humboldt Range. These beds are traversed by numerous dikes of pre- 

 Tertiary rocks, but most of them are so fine-grained or so altered, and 

 present such a variety of physical habit, as to render it difficult to deter- 

 mine their species. Many of them are dense, tough rocks, with compara- 

 tively undecomposed mineral constituents ; others break readily, possess an 

 earthy appearance, and, to the unaided eye, have no well-defined character- 

 istics. In thin sections, under the microscope, ZirkeP has shown that most 

 of them are diabase, and composed of plagioclase, augite, quartz, magnetite, 

 and apatite. In nearly all of them the augite is much decomposed, and an 

 earthy, yellow base occupies the space between the crystalline minerals. 



At the entrance to Miner's Canon occurs a somewhat singular rock, 

 and, unlike those just described, mainly consists of orthoclase and horn- 

 blende, with some small quartz-grains, both the principal minerals being 

 well developed in a greenish-gray crystalline groundmass. It bears a close 

 resemblance to the propylite of Kaspar Pass, just to the northward, and 

 has been classed as a quartz-propylite, although field-observations as to its 

 true position and age are wanting. Natrolite and stilbite associated with 

 quartz are found in veins crossing Miner's Canon. 



Except in one or two localities, including the more elevated sum- 

 mits and some low hills exposed by the inequalities of the flows, all the 

 rest of the Truckee Eange south of "Nache's Pass is formed of black basalt. 

 When observed in detail, the whole region is seen to consist of slightly 

 rounded ridges separated by narrow ravines, the ridges being formed of 

 successive curved beds of basalt, which seem to flow down from the middle 

 on each side. The entire country has the appearance of resulting from a 

 number of powerful dikes, which were erupted with great force along the 



^ Microscopical Petrography, vol. vi, 97. 



