128 GROVE KARL GILBERT— DAVIS [MEU0,ns [ ^ I, S 



A careful reexamination of the locality has convinced me that I was in error [as to the place of the outlet], 

 and has led me to assign it a position two miles north of Red Rock. Dr. Peale placed it about 45 miles north 

 of Red Rock, so that my new determination is nearer to his than my old was. 3 



Not a few less magnanimous geologists would have phrased this conclusion: 

 My old determination was much nearer to the true location than his was! 

 But perhaps there was a touch of mischievousness in Gilbert's magnanimity. 



TWO HUMID EPOCHS 



It was by these successive advances in the earlier years of Gilbert's western work on the 

 Wheeler and the Powell surveys that approach was made to an explanation of the Bonneville 

 problem which seemed compulsory; but thus far only a single humid period of lake expansion 

 had been recognized. In view of the great interest of the problem and of Gilbert's acknowl- 

 edged mastery of it, its further study was naturally given a leading place in a main division 

 of the national survey as soon as it was organized, as has already been noted. Progress was then 

 made more rapidly. 



Evidence of the occurrence of two humid epochs of lake expansion separated by an arid 

 epoch of lake contraction or extinction was discovered during the first season's work for the 

 national survey in the winter of 1879-80; and was briefly referred to in Gilbert's first formal 

 report to King, dated at Salt Lake City, October 1, 1880, when an account of the lake-floor 

 sediments included the following passage: 



It was already known that they consisted of marls and clays and sands, but no considerable section had 

 been measured, and no constant order of sequence had been observed. It was ascertained last winter that the 

 marls invariably overlie the clays and form a relatively thin deposit. At one locality a beach gravel was found 

 immediately beneath them, and in such relation as to demonstrate that a very low stage of water had inter- 

 vened between two high stages. This is a capital discovery, proving, as it does, that the humid epoch was 

 interrupted by an epoch of dryness. 



The similarity of Bonneville and Lahontan histories, next announced, although not con- 

 firmed in all respects by later investigations, must have furnished pleasant writing to the 

 geologist at the Salt Lake headquarters and agreeable reading for the two-year director in the 

 Washington office: 



The discovery [of a dry epoch between the two humid Bonneville epochs] confirms in a most gratifying man- 

 ner an independent conclusion of Mr. King's. Reasoning entirely from mineralogical facts and the necessary 

 conditions of chemical reaction, that geologist was led to conclude that Lake La Hontan, the contemporary 

 and [western] neighbor of Lake Bonneville, was first flooded for a long period, without overflow, and then, after 

 an interval of desiccation, was refilled for a shorter period during which there was a discharge. The history of 

 Lake Bonneville is based purely on stratigraphic and topographic data, and is identical in every determined 

 particular. The basin was flooded for a long period represented by ninety feet of clay; there was then a des- 

 iccation, shown by intercalated shore deposits; and there was finally a second flood stage, represented by fifteen 

 feet of marl. The fact of overflow is proved by the discovery of the channel of discharge, and it has been shown 

 that the second epoch of flooding was accompanied by overflow. Whether the first epoch was similarly char- 

 acterized has not been ascertained, but it is a significant fact that the deposits thrown down during those two 

 epochs have a marked difference of composition. If a relation can be established between the clay and marl 

 as indicative of continence and overflow respectively, the parallel will be absolutely complete. 4 



The ingenious hypothesis by which the anticipated relation was established in the follow- 

 ing year is noted below. 



In the meantime a letter to King, dated a month and a half after the official report, above 

 cited, announced two important advances. First. "The evidence of a long dry epoch inter- 

 jected near the end of the Bonneville epoch is no longer restricted to a single locality nor to 

 a single phenomenon " ; that is, in addition to the discovery of new low-lying sections in which 

 gravels were found between the lake-floor clays and marls, certain high-level deposits were 

 found, against which the later-formed beaches lay unconformably; and these deposits were 

 therefore taken to represent the littoral phase of the lake-floor clays that were laid down during 

 the earlier humid epoch, just as the apposed beaches represented the littoral phase of the lake- 

 floor marls laid down during the later humid epoch. 



> The Outlet of Lake Bonneville. Amer. Jour. Sei., XIX, 1880, 341-349. 

 ' First Annual Report, U. S. Oeol. Survey, Washington, 1880, 23-26. 



