(jQ GEOLOGICAL HISTOKY OF LAKE LAHONTaX. 



the lake, September 9, 188'4?, was as follows: x, 21.0 feet; y, 9.8 feet; z, 

 23.7 feet. 



This record may" be increased by adding the following elevations above 

 the lake level as determined in September and October, 1882:-^ 



Summit of Anaho Island 517 feet 



Summit of "Mushroom Rock," ou the north shore of Anaho Island 



(see Plate XIV) 17 feet 3 inches 



Rock to the south of Mushroom Rock (beneath bird on Plate XIV) . 8 feet 5 inches 



Summit of Pyramid Island (Plate XI) 289 feet 



Highest spire among The Needles (Plate XIII) 300 feet 



HUMBOLDT I.AKE. 



Humboldt Lake is but an expansion of the river that supplies it, and 

 is held in check by an immense gravel embankment that was thrown com- 

 pletely across the valley by the currents of the former lake, at one time 500 

 feet deep at this point. An accurate map of this structure is given on Plate 

 XVIII, and a detailed description on page — . As there described, the em- 

 bankment has been cut across by the overflow of the lake and the breach 

 partially filled during the past few years by an artificial dam, which has 

 greatly increased the area of the lake. During the dry season the lake 

 seldom overflows and is then the limit of the great drainage system of the 

 Humboldt River, but in winter and spring the waters escape southward, 

 and spreading out on the desert form Mirage Lake. Farther southward on 

 the northern part of the Carson Desert they again expand and contribiite 

 to the formation of North Carson Lake. 



In the summer of 1 882, Humboldt Lake covered an area of about 20 

 square miles, did not overflow, and although somewhat alkaline was inhab- 

 ited by both fish and mollusks, and was sufiiciently pure for human use. 

 The following analysis of its waters by Prof 0. D. Allen, of Yale College, 

 is taken from the reports of the United States Geological Exploration of the 

 Fortieth Parallel, Vol. II, p 743. 



^ All these measurements were made with an engineer's level. 



