746 DESCRIPTIVE GEOLOGY. 



The silica exists partly in combination and in part as finely divided 

 sand. Along the Carson Eiver from Ragtown southward, the Pliocene for- 

 mation, as seen in the Humboldt and Truckee Valleys, is well developed in 

 broad bench-like tables, frequently standing one or two miles back from the 

 river, which, upon the west side, ar« cut through at regular intervals by 

 narrow ravines. 



Everywhere along the borders of the desert, the La Hontan beach-ter- 

 races are easily traced, the highest one having an altitude, according to 

 barometric measurements made at a number of points, of about 520 feet 

 above the level of Carson Lake, below which elevation the hills and benches 

 are more or less covered by fragments of calcareous tufa. These tufas are 

 well shown in the neighborhood of Hawes Station, on the old Overland 

 Stage Road, just south of the map, in longitude 119° 10', where both rhyo- 

 lites and basalts are encrusted by a thin coating of yellowish-brown car- 

 bonate of lime. 



The Soda Lakes, situated between 2 and 3 miles northeast from Rag- 

 town on the Carson River, near the southern boundary of Map V, form 

 one of the most striking features of the Carson Desert, The photograph 

 reproduced on Plate XXII represents the smaller lake, the larger being 

 shown in Vol. I, Plate XXVI. They lie depressed below the level of the 

 plain, in what are probably ancient craters, and are not observed until just 

 before reaching the brink. Upon the south side of the larger lake, the bank 

 slopes somewhat gradually to the water's edge, with a fall of not more than 

 35 feet, but rising steadily on the east and west sides, until at the north it 

 attains a height of 150 feet in nearly perpendicular walls. The lake has a 

 clear blue color, is of round form, with a diameter estimated at five-eighths 

 of a mile. It has no outlet, and is fed by a spring of cool fresh water on 

 the northwest side, which breaks through a stratum of gravel outcropping 

 just above the water-level. From the lake-border to the abrupt walls, the 

 water is completely'- encircled by a narrow belt of level shore, composed of 

 loose friable material, mainly small rounded basaltic pebbles, and coarse 

 gravels held together by fine sands. A considerable vegetation is found 

 surrounding the lake, but one not specially rich in species. Mr, Sereno 

 Watson collected near the fresh-water spring the following species: Erige- 



