742 BESCEIPTIVE GEOLOGY. 



comparatively recent volcanic action. It is quite unlike any other basalt 

 region in Nevada, and resembles more the immediate base of many large 

 basaltic volcanoes. Directly connected with these basaltic outbreaks, but 

 south of flumboldt Station, is a narrow vertical fissure, which has been 

 filled with native yellow sulphur by solfataric action, long since extinct. It 

 had been mined to some extent, but, at the time of our visit, had been aban- 

 doned, the yield being too small to be of practical importance. It may 

 be well to mention here the occurrence of a somewhat peculiar sand- 

 stone, which appears to have been hardened and baked by the basaltic 

 eruptions, forming an exceedingly tough rock with a vitreous lustre, com- 

 posed of quartz-grains and particles of feldspar, and carrying secretions of 

 jasper. 



At the southern end of Humboldt Lake, there occurs the narrow crest 

 of an anticlinal ridge, which strikes directly across the Humboldt Valley, 

 and forms a dam, backing up the waters of the river into a broad lake, 

 two or three miles in width. This anticlinal is perhaps 5 miles in length, 

 and rises about 100 feet above Mirage Lake. The formation offers no 

 good exposure of beds, but lies inclined at a considerable angle, and has 

 therefore been referred to the Miocene age. The Lake La Hontau beach-lines 

 are seen along the foot-hills on both sides of the valley, where large blocks 

 and pebbles of basalt are cemented together by calcareous tufa. An outlying 

 hill must have been an island during the period of the La Hontan Lake, as it 

 is entirely surrounded by the old shore deposits. 



Nowhere along the valley, from Fairbank's Point to Mirage Lake, 

 have the Miocene and Pliocene beds been observed in direct contact, and 

 nowhere on the river-bottoms are the Pliocene beds cut through to the 

 older series. At one point, nearly west of Oreana, the two formations 

 approach each other, but the direct contact is concealed by the Quaternary 

 drifts. 



Humboldt Lake is probably a shallow sheet of water, and, as it drains 

 off through the Mopung Marshes into Mirage Lake, must have nearly the 

 same saline ingredients as the lower portion of the river. It has a strongly 

 brackish alkaline taste, but, owing to its free drainage, is less highly charged 

 with soluble salts than most of the other interior lakes. A portion of the 



