GENERALIZED SECTION. 143 



which our conclusions rest with as much accuracy as could be desired, and 

 also because the subject will claim further attention in connection with other 

 orographic disturbances that have aflPected the Lahontan basin. 



GENERALIZED SECTION OF LAHONTAN SEDIMENTS. 



On grouping the numerous sections of Lahontan sediments observed 

 iu the Humboldt, Truckee, Carson, and Walker canons, we have the fol- 

 lowing generalized section of sedimentary deposits formed in the ancient 

 lake: 



Average thickness, 

 ^_ in feet 



fUpper lacustral clays : 



I Evenly lamiuated marly clays, fine and homogeneous, nsnally saline ; with interstiatified 



bands of dendritic tufa near the top ; in places containing intercalated layers of vol- 

 canic dust. In some places this member is divisible into three parts, the upper and 

 lower being normal clays, while the ii;termediate member is more sanily, and usually 



contains iron-stained lyse, that are frequently contorted 50 to 75 



Fossils: Cypris, Anodonta, Margariiana, Sphwrimn, Pindium, Heluoma, Gi/rauZttS, etc., 

 together with mastodon or elephant, horse, and camel. 



Contact uncomformable. 

 Medial gravels : 



Cross-stratifled sand, gravel, aud loam, in beds that are irregular both in thickness and 

 inclination, frequently forming arches of deposition. At times exhibiting two plainly 

 marked divisions; the upper being a compact, earthy, homogeneous, flood-plain de- 

 posit; the lower clean, well-rounded sand and gravel, at times strongly cross-bedded.. 50 to 200 

 Fossils: Anodonta, Gyraulus, Lt/mnophysa, Pompholyx. 



Contact unconformable. 

 Lower lacustral clays : 



Laminated marly clays, very similar to the clays at the summit of the section. The clays 

 throughout the section frequently exhibit two systems of joints at nearly right angles 



to each other (full thickness not exposed) 100 



Fossils : Pompholyx. 



The interpretation of this section gives an outline of the later Quater- 

 nary history of the Lahontan basin ; but as the base of the lower clays is 

 nowhere exposed, all the changes that may be recorded by the lower strata 

 remain unknown. From the sedimentary deposits observed we learn that 

 there have been two high- water periods in the history of the Lahontan 

 basin, during which fine clays were deposited. Separating these two periods 

 was a time when the lake was low and allowed current-borne gravels to be 

 carried far out over the previously formed lake-beds. During the second 

 flooding the waters underwent long concentration, and at a certain period 

 deposited a vast quantity of tufa ; the lake during this stage also received 



