WEST END OF DETAILED SECTION 329 



greatly exaggerated vertical scale.* It has seemed desirable therefore 

 to present a carefully surveyed profile, on which are plotted the observed 

 geological features, so that the relative heights of mountains and valleys, 

 the angles of slopes, the details of sculpture, etcetera, may be seen in 

 true relationship and on natural scale. Pursuant of this purpose, the 

 valleys have been surveyed and included, for no true idea of the Basin 

 region can be obtained without an understanding of the broad and floor- 

 like valleys which, quantitatively, are often more important than the 

 intervening ranges. A brief description of the special features of the 

 section, not otherwise noted, will here be given, passing from west to 

 east. The position of the section, which follows a line broken at several 

 points so as to strike the exposed lowland basalt areas, is shown in the 

 map, plate 21. 



THE WEST END 



The section begins in the foothills of the Trinity mountains, 1,400 feet 

 above the town of Lovelock, in the middle of Humboldt valley. From 

 this point west the slope gradually rises, bearing the complete volcanic 

 series toward the summit. The hill from which the section starts is 

 capped by rhyolite lava, but just north of it the basalt is found occupy- 

 ing the top of the series — the surface layer. The rhyolite is somewhat 

 over 200 feet thick, and is underlain by about 500 feet of tuff, which 

 appears to dip southeast at a moderate angle. At the base of the slope 

 is a stream channel. Coming from the south, it joins one from the west 

 a few hundred yards beyond the section. The section, then, to the 

 margin of the range follows the side of a stream valley from which the 

 rhyolite and tuff have been largely removed. Wherever removed to its 

 base, the tuff is seen to have been deposited on a granite or hornfels 

 floor. The hornfels is dark colored and compact and is broken through 

 by coarse tourmaline-bearing pegmatite dikes. 



Between the margin of the range and Lone mountain extends a 2-mile 

 gentle slope, covered along its upper part with alluvium. In some of 

 the rain gullies near Lone mountain the friable, fine grained, deposit 

 looks very much like Lahontan sediment, though no deep cut exposes 

 the beds. That the lake occupied this stretch there is abundant evidence 

 in shorelines and tufa deposits, and the long line of low tufa domes ex- 

 tending north parallel to the margin of the range is proof that only a 

 very slight amount of deposition or erosion has taken place since the 

 disappearance of the lake. 



LONE MOUNTAIN 



The structure of Lone mountain has already been described and illus- 



* Gilbert's original sections in Wheeler survey are, however, drawn to true scale. 

 XLIII— Bull. Geol. Soc. Am., Vol. 15, 1906 



