BLACK EOOK MOUNTAINS. 791 



rises in the mountains on the borders of Nevada and Eastern Oregon, and 

 enters this valley through a break in the Mount Vdry Mountains, gradually 

 losing its water by evaporation and absorption, until, where the road crosses 

 it near the Black Rock, it is only about six feet in width. A western arm 

 of the desert extends a short distance north of the limits of the map, between 

 the Black Rock and Forman Mountains, where it is shut in by the hills 

 which connect them, while the western Mud Lake, which lies between the 

 northern end of the Lake Range and the Madelin Mesa, practically forms 

 part of the same plain, being separated by a low, almost imperceptible ridge 

 of Upper Quaternary, at the southern end of the Granite Range. These 

 plains, which during the greater part of the year are so dry and hard that 

 a wagon- wheel or horse's hoof scarcely leaves a perceptible track upon their 

 surface, are frequently during the winter covered to a depth of several 

 inches with water, which must render them practically impassable, since 

 during our explorations in the months of October and November the occa- 

 sional autumn storms had so softened the surface that mules would sink 

 fetlock- and even knee-deep at every step, and the so-called "self-rising" 

 portions, where the proportion of readily soluble salts in the soil is probably 

 greater than usual, had to be carefully avoided, lest the animals might dis- 

 appear altogether. Where the surface has been recently moistened by rain 

 or spring-water, surface incrustations of soluble salts contained in the soil 

 are found forming thin white films. Such were gathered from near the 

 crossing of Quinn's River, and from near the so-called Hardin City, on the 

 western arm of the desert, north of the limits of the map. They were anal- 

 yzed by Prof. 0. D. Allen, of Yale College, with the following result: 



Quinn^s JRiver Salts. 



Chloride of sodium , 85.27 



Carbonate of soda 2.59 



Sulphate of soda 1.75 



Water 8.57 



Residue 1.82 



100.00 



