the Middle Atlantic Slope. 4:bl 



made out in the Great Basin. In 1878 Gilbert described the 

 sediments of the extinct lake Bonneville, recorded the infer- 

 ence that the prevailing arid climate of the Great Basin was in- 

 terrupted by a period of humid climate during which its moun- 

 tain-enclosed basins were flooded and the lacustral sediments of 

 Bonneville deposited, and correlated the humid epoch with 

 that of northern glaciation ;* and he subsequently pointed out 

 that the humid epoch was brief and apparently " an episode 

 occurring in the later part of a long period of aridity. "f In 

 1878, King called attention to the sediments and chemical pre- 

 cipitates of the ancient lake Lahontan, and, avoiding detailed 

 discussion of the former, conceived the latter phenomena to 

 record (1) flooded and free-drained condition of the lake- basin, 



(2) shrinking and concentration of the waters and final desicca- 

 tion of the basin with formation of the precipitate gaylussite, 



(3) re-flooding of the basin for a long period during which the 

 soluble salts were washed away and the gaylussite changed into 

 thinolite by pseudomorphosis, and (4) partial desiccation pro- 

 ducing present conditions. This sequence was regarded as 

 partly coincident with and partly supplementary to that 

 deduced by Gilbert from the Bonneville phenomena ; it was 

 inferred that there was " a period of humidity anterior to Gil- 

 bert's earliest age of dryness" which was "enormously longer 

 than [the period of humidity] in the second age of desicca- 

 tion;" and the first of these humid periods was correlated with 

 " the earliest and greatest Glacier period," and the second with 

 "the later Reindeer Glacier period."^: Gilbert later found 

 evidence in the sediments of the Bonneville basin not only of 

 a long humid period antedating that previously recognized but 

 also of a much longer arid period preceding it, and concluded 

 that the sequence of deposits represents a climatic sequence of 

 " two humid maxima separated by an interval of extreme aridity," 

 the second humid maximum being the more pronounced and 

 the first the longer. § Still later and after extended investiga- 

 tion, Russell found the sediments and precipitates of lake La- 

 hontan to yield alike a record coincident with that of the Bon- 

 neville deposits,! save that the intermediate epoch of aridity 

 was lengthened and some minor vicissitudes were introduced. 

 He ascertained from the continuity of shore-lines and other 

 evidence, however, that Lahontan did not overflow during 



*Bull. Philos. Soc. of Wash., vol. i, 1874, 84-85; Progress Rep. Geog. and 

 Geol. Surveys West of 100th Merid., for 18*72, 1874. 49-50. 



f Rep. Geog. and Geol. Surveys West of 100th Merid., vol. iii, Geology, 1875, 

 96-97. 



± TJ. S. Geol. Expl. 40th Parallel, vol. i, Systematic Geology, 1878, 522-4. 



8 Second Annual Rep. U. S. Geol. Survey, 1880-1, 1882, 186-200. 



I Third Ann. Rep. U. S. Geol. Survey, 1881-2, 1883, 221-231 : and Monograph 

 U. S. Geol Survey, vol. x, Geol. History of Lake Lahontan, 1885, 261-263. 



