CHAPTER IX. 

 GEOLOGICAL AGE OF LAKE LAHONTAN. 



A review of the facts bearing on the age of Lake Lahontau necessi- 

 tates some repetition, but seems desirable in order to present the evidence 

 in a connected form. 



The reader is ah-eady aware that Bonneville and Lahontan were the 

 largest of an extensive series of lakes which formerl}^ occupied the valleys 

 of the Great Basin. That the lakes here indicated — represented for conve- 

 nience of reference on Plate I — ^were contemporaneous seems too positive 

 to be questioned. The records in the various basins are identical, consist- 

 ing of terraces, gravel embankments, sedimentary deposits, fossils, etc., in 

 which no difference of age can be detected. Moreover, the existence of 

 lakes in inclosed basins is dependent on climatic changes too broad in their 

 effect to have been felt in a single valley without producing similar results 

 in others near at hand That the lakes now under discussion, not only 

 existed at the same time, but were also of a very recent date, is considered 

 as abundantly proven by the fact that they left the very latest of all the 

 completed geological records to be observed in the Great Basin. 



The fossil shells obtained from the sediments and tufas of Bonneville 

 and Lahontan, and a few of the smaller sister lakes, all belong to living 

 species. The mammalian remains discovered in the sediments of Lake La- 

 hontan are the same as occur elsewhere in Tertiary and Quaternary strata. 

 The spear head of chipped obsidian obtained in the upper Lahontan sedi- 

 ments is considered good evidence — although as yet unsustained by other 

 finds of a similar character — that man inhabited this continent during the 

 last great rise of the former lake. 



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