GLASSTFICATION OF DEPOSITS. 125 



some attention. In the present division the otF-shore sediments, or lake-beds 

 proper, together with certain interstratified gravels, will be described. The 

 mineral substances contributed to the lake in solution will be studied in 

 connection with its chemical history. 



The sedimentary deposits of Lake Lahontan exhibit three definite divis- 

 ions, viz.: 



Upper lacustral clays. 



Medial gravels. 



Lower lacustral clays. 



Wherever any considerable section of Lahontan sediments is exposed 

 these three divisions appear in unvarying sequence. 



The upper and lower members of the series are composed of marly 

 clays, which show by their fineness and the evenness of their lamination 

 that they were deposited in deep still water. The middle member, on the 

 other hand, usually consists of well-rounded gravel and sand, in some in- 

 stances becoming coarse and including bowlders a foot or more in diameter. 

 This deposit is current-bedded, and exhibits many variations, indicating that 

 it was deposited in shallow water. 



It is apparent, therefore, that the evenly stratified beds at the base and 

 summit of the series are the records of a deep lake of broad extent, and 

 mark periods of comparatively abundant precipitation or of greatly de- 

 creased evaporation. It is also evident that the medial gravels were 

 deposited when the lake was sufficiently lowered to allow stream and cur- 

 rent-borne debris to be carried far out over the previously formed lake- 

 beds, recording an interval of low water in the history of the lake. The 

 significance of these deposits in reference to Quaternary climatic changes 

 will appear more clearly in the sequel. 



Sections of Lahontan sediments are well exposed in those portions of 

 the caiaons of the Humboldt, Truckee, Carson, and Walker rivers that are 

 below the highest water-line of the former lake; and as the sections observed 

 present facts of interest in tracing the Quaternary history of the region, 

 together with many illustrations of geological structure, we shall give some- 

 what detailed descriptions of the principal exposures. 



