academy of 8CIENCH] n s GEOLOGICAL SURVEY 129 



Second. " The idea that the accented beaches lying between the Prove- and Bonneville 

 levels were formed by the lingering of the water during its fall from the Bonneville level to the 

 Provo is completely exploded. Those intermediate beaches were formed in ascending order — 

 the lowest first, the highest last — and all of them are older than the Bonneville beach." This 

 important conclusion was based on sections in which it was seen that the higher members of 

 the intermediate beaches were successively superposed upon the lower ones, while the lower- 

 lying Provo beach was apposed upon or built forward from the sublacustrine slope of the lowest 

 intermediate beach; and all of these beaches except the highest ones were found, at one place 

 or another, to rest upon previously formed deposits which were taken, as above noted, to rep- 

 resent the littoral phase of the lake-floor clays formed during the earlier humid epoch. 



A third topic of equal interest was not carried to so satisfactory a conclusion; this was 

 the relation of the Lake Bonneville to the glacial period, concerning which Gilbert's theoretical 

 surmise of August, 1872, has already been quoted. Analogy pointed strongly to the syn- 

 chronism of these two similarly complex manifestations of past climatic changes, but the only 

 locality at which glacial and lacustrine deposits were found in contact — namely, where the 

 beautiful lateral and terminal moraines of Little Cottonwood Canyon in the Wasatch Range 

 a few miles south of Salt Lake City, advances across the belt of lacustrine shore lines — 

 "failed ... to yield crucial evidence for which search was made, and practically afforded no 

 contribution to the subject." 5 A more confident opinion was reached later, when the evidenca 

 furnished by moraines in the Mono Lake basin at the eastern base of the Sierra Nevada, and 

 that furnished by the depauperization of molluscan fossils confirmed "the presumption derived 

 from the recency and exceptional nature of the lakes and glaciers, that the two phenomena 

 were coordinate and synchronous results of the same climatic change." " 



BONNEVILLE CLAYS AND MARLS 



Another matter which remains to be considered may be regarded as having received one 

 of the most venturesome and most purely hypothetical interpretations that Gilbert ever pub- 

 lished. It concerns the relation already alluded to between the lacustrine clays and marls as 

 indicative of lake continence and lake overflow, respectively, a relation which Gilbert felt would, 

 if it were established, render the parallel between Bonneville and Lahontan "absolutely com- 

 plete." To appreciate the offered interpretation it must be understood that the Bonneville 

 clays and marls did not differ greatly in composition, but that one merely contained more argil- 

 laceous and lees calcareous material than the other; and that both deposits might therefore be 

 regarded as having been supplied by land-derived detritus of the same constitution, provided 

 that a reason could be found for distributing the different constituents of the detritus in dif- 

 ferent proportions in the off-shore lake waters of the two humid epochs. It must also be noted 

 that sedimentation experiments were made with the lake-floor clays, which indicated a five- 

 fold more rapid settling in fresh water taken from an inflowing stream of to-day — City Creek 

 at Salt Lake City — than in brine taken from the present lake. Whether the settling was due 

 to a chemical reaction between the salts of the brine and certain salts dissolved in the streams 

 that washed the sediments into the ancient lake — like the reaction employed for the clarifica- 

 tion of certain turbid rivers for city water supply, the Mississippi water at St. Louis, for ex- 

 ample — does not appear; but the result is peculiar in view of the generally accepted experimental 

 conclusion that the salt of sea water- accelerates sedimentation. In this connection reference 

 may be made at once to the final monograph, in which it is said that the water of Bear Creek, 

 which enters the basin farther north than City Creek, precipitated clayey sediments as rapidly 

 as the brine of Great Salt Lake; thus rendering the conclusion previously reached somewhat 

 uncertain. Citation may also be made here of a footnote in the final monograph which states, 

 in view of sedimentation experiments that had been made by various observers and that came 

 to Gilbert's attention in Washington: "It is not to be supposed that the sodium chloride and 

 other constituents of the Salt Lake brine retard the precipitation of sediments," but " that they 

 promote it less than the mineral constituents of City creek water." 



* Second Ann. Kept. U. S. Geol Survey, 1881, 189. 

 » Monogr. I, 1890, 315. 



