170 GEOLOGICAL SURVEY OF THE TERRITORIES. 



The sediments reach a thickness of eight hundred to one thousand 

 two hundred feet at least. It not only occupied a vast area in the great 

 basin, but extended up the valleys of the numerous rivers that flow 

 therein. In the Weber Valley above the Devil's Gate, or on the east 

 side of the Wasatch range, this group occupies an oval area of twelve 

 miles long and eight miles wide, or about one hundred square miles. It 

 forms one of the series of mountain lakes which occupied hundreds of 

 the oval areas, or parks as they are now called, in the great mountain 

 system which extends from the Arctic on the north to the Isthmus of 

 Darien on the south, and I presume also to Patagonia, in South America. 



The Salt Lake Group I regard as of pliocene age and contemporane- 

 ous with the Niobrara, Arkansas, and Santa Fe Groups, with numerous 

 other small accumulations of marls and sands in the Middle Park, and 

 among the mountains far to the northward and throughout the Hum- 

 boldt Valley, Oregon, &c. All the proofs we can secure, up to the pres- 

 ent time, indicate their fresh- water origin. What geographical changes 

 have occurred in this long period of time we will not now attempt to de- 

 termine; we wish simply to express our belief that, at least since the 

 middle tertiary period, the salt ocean has not had access to this great 

 basin. If now we pass to what may be called for convenience the quar- 

 tenary period, or the one that gradually merges into the present, we 

 shall find that it presents geological features of no ordinary interest. 

 In descending the Weber Valley, after we emerge from the canon of the 

 Wasatch range into the open valley of Salt Lake, we observe on either 

 side thick beds of sands and arenaceous clays, which must have been de- 

 posited in the quiet waters of a lake. 



In the valley of Salt Lake, and especially in that of the Weber Eiver, 

 these drift deposits possess a thickness of several hundred feet, and of 

 these materials the terraces are formed. Near Salt Lake City, in dig- 

 ging a well, fresh-water shells were found in these deposits, forty feet 

 below the surface, and on the north side of the lake, where these deposits 

 are very largely exhibited, the cuts in the railroad, through the gravel 

 and sands, reveal the greatest abundance of fresh- water shells, showing 

 that at this time the physical conditions were unusually favorable for 

 the existence of fresh-water molluscous life. So far as I could ascer- 

 tain, these conditions do not exist at the present time, or if they do, it 

 must be only to a limited extent. 



I am indebted to the kindness of Mr. George W. Tryon, jr., for the 

 identification of the species obtained from this drift : 



1. Fluminicola fusca. This species seems to have been very abundant ; 

 it exists at the present time in the mountain streams. 



2. Pomatiopsis Cincinnatiensis. 



3. Amnicola limosa. 



4. Valvata sincera. 



5. Limnea desidiosa. 



6. Limnea catiscopnim. 



Prom these observations I infer that a vast fresh-water lake once oc- 

 cupied all this immense basin ; that the smaller ranges of mountains 

 were scattered over it as isolated islands, their summits projecting aboA^e 

 the surface ; that the waters have gradually and slowly passed away 

 by evaporation, and the terraces are left to reveal certain oscillations of 

 level and the steps of progress toward the present order of things; and 

 that the briny waters have concentrated in those lake basins, which have 

 no outlet. The entire country seems to be full of salt springs, which 

 have, in all probability, contributed a great share to the saline charac- 

 ter of the waters. 



