76 GEOLOGICAL HISTORY OF LAKE LAHONTAN. 



shore to shore, thus indicating that two craters are combined in the forma- 

 tion of the present depression. 



The bottom, as shown by the samples obtained by the cup at the end 

 of our sounding lead, is a fine tenacious black mud having a strong odor of 

 sulphuretted hydrogen. When exposed to the air for some time this mate- 

 rial loses its inky color and shows itself to be of the same nature as the fine 

 dust-like lapilli that form a large part of the crater walls. The organic 

 matter impregnating these sediments is evidently derived from the millions 

 of brine shrimps (Artemia gracilis) and the larvae of black files that swai'm in 

 the dense alkaline waters. 



Near the shore the rock and pebbles, as well as bits of organic matter 

 are coated with beautiful crystals of gaylussite which form about any solid 

 nucleus that chances to be available. The crystals are white, with trans- 

 parent edges, mouoclinic in form, and thin in the direction of the orthodiag- 

 onal, as illustrated by Figure 607, Dana's S5'^stem of Mineralogy, 5th edition. 

 The small island in the northern part of the lake is completely coated with 

 gaylussite crystals and trona. An analysis of a crystal of gay-lussite from 

 this locality by Prof. 0. D. Allen gave the following composition :^^ 



Lime i!>. 10 



Soda 1!>. 95 



Carbonic acid -".'. 55 



Water 31.05 



Sulphuric acid Trace. 



Cliloiiue Trace. 



lusoluble residue 0. 20 



90. 94 



Trona also occurs along the shore of the lake up to an elevation of 1 or 

 12 feet, and not unfrequently contains casts of the larval cases of a fly which 

 now lives in the lake in immense numbers. An analysis of a sample of 

 trona from this locality, by Prof O. D Allen is here copied." 



38 



^' U. S. Geological Exploration of the Fortieth Parallel, Vol. II, p. 749. 

 3*11. S. Geological Exploration of the Fortieth Parallel, Vol. II. p. 748. 



