GEOLOGICAL SURVEY OF THE TERRITORIES. 173 



miles wide and ten miles in length. How much farther it extended northward I could 

 not tell, but if it covered the plain in that direction as it did where we crossed, its 

 extent must have been very much greater. The salt, which was very pure and white, 

 averaged from one-half to three-fourths of an inch in thickness, and was equal in all 

 respects to our finest specimens for table use. Assuming these data, the quantity that 

 here lay upon the ground in one body, exclusive of that in a deliquescent state, 

 amounted to over four and a half millions of cubic yards, or about one hundred 

 millions of bushels. 



Areas of greater or less extent, covered with this saline incrustation, 

 occur in numerous localities, so that we may infer that in all probability, 

 at no very distant period in the past the salt lake extended either con- 

 nectedly, or in isolated portions, over the greater part of the Great 

 Basin. 



It would be a most interesting subject to trace the history of this 

 wonderful lake far back in the geological past ; from the records which 

 have been left in the sediments, I have obtained comparatively few facts 

 as yet, but they seem to be quite conclusive, and I believe that each 

 successive step in the changes which this great region has undergone can 

 be interpreted with accuracy from the records left in the surface deposits, 

 if they could be studied in detail. 



One of the most conspicuous features in this basin, is the system of 

 terraces or benches which borders the valleys as well as the streams. 

 These terraces seem to form an independent system in this basin, discon- 

 nected, both in regard to time and the causes that produce them, from 

 those so well known along the Missouri and Columbia Rivers. 



Not only do they seem to be universal over this great basin, but they 

 are all of about the same level. I have never observed more than two or 

 three of these benches well defined, but Captain Stansbury speaks of 

 counting thirteen successive terraces at the northern end of the lake, the 

 highest about two hundred feet above the valley. 



In volume II of the Pacific Railroad Reports, page 97, there is a most 

 interesting note in regard to these remarkable shore lines, which I am 

 sure will be as instructive to those who may read this volume as it has 

 been to me : 



The old shore Hues existing in the vicinity of the Great Salt Lake present an interest- 

 ing study. Some of them are elevated but a few feet (from live to twenty) above the 

 preseut level of the lake, and are as distinct and well defined as its present beaches, whilst 

 their magnitude and smoothly-worn forms as unmistakably indicate the levels which 

 the waters maintained at their respective formations for very considerable periods. 

 In the Tuilla Valley, at the south end of the lake, they are so remarkably distinct 

 and peculiar in form and position that they attracted the attention of the least-informed 

 teamsters of my party, to whom they appeared artificial. From these beaches the Tuilla 

 Valley ascends gradually toward the south, and in a few miles becomes blocked up by 

 a cross range of mountains, with passages at either side, leading, however, over quite 

 as remarkable beaches, into what is known to the Mormons as Rush Valley, in which 

 there are still small lakes or ponds, once doubtless forming part of the Great Salt Lake. 



The recessions of the waters of the lake from the beaches at these comparatively 

 slight elevations must have taken place within a very modern geological period, and 

 the volume of the water of the lake at each subsidence — by whatever cause produced, 

 whether gradual or spasmodic — seems as plainly to have been diminished ; for its pres- 

 ent volume is not sufficient to form a lake of even two or three feet in depth over the 

 area indicated by these shores, and, if existing, would be annually dried up during the 

 summer. 



These banks are not peculiar to the vicinity of this lake of the basin, but were ob- 

 served near the lakes in Franklin Valley, and will probably be found near other lakes, 

 and in the numerous small basins which, united, form the Great Basin. They clearly 

 seem to have been formed and left dry within a period so recent that it would seem im- 

 possible for the waters Avhich formed them to have escaped into the sea, either by great 

 convulsions opening passages for them, or by the gradual breaking up of the distant 

 shore, (rim of the basin,) thus draining them off, without leaving abundant records of 

 the escaping waters, as legible at least as the old shores they formed. 



But high above these diminutive banks of recent date are seen, on the mountains to 

 the east, south, and west, and on the islands of Great Salt Lake, formations preserving 



