272 GEOLOGICAL HISTORY OF LAKE LAHONTAN. 



manent of topographic forms; more constant, in fact, than the rocky mount- 

 ain tops. It is not surprising, therefore, that the gravel structures fail to 

 give evidence as to their age. 



We might consult the canons carved through Lahontao sediments since 

 the recession of the lake for a time measure ; but the amount of erosion here 

 apparent could have been performed by the existing streams in a few years, 

 owing to the unconsolidated character of the strata and the high grade of 

 the streams caused by the lowering of their base level upon the withdrawal 

 of the lake waters. Moreover the streams have meandered but little within 

 their canons, tlius indicating that these trenches have not been long finished. 

 On the whole the canons indicate that but a brief period has elapsed since 

 their excavation began. 



The tufa deposits of the basin have been exposed to erosion since the 

 withdrawal of the lake waters, and might be expected to present some indi- 

 cation of the time they had been subjected to subaerial erosion. These 

 deposits are porous and open in structure and favor the absorption and 

 retention of moisture. They are thus especially liable to the destructive 

 effects incident to the freezing of water in the interspaces of rocks, espe- 

 cially as the rains and frosts of the Great Basin occur together. We may, 

 therefore, expect that the subaerial erosion of the tufa deposits would be 

 rapid, and that if they had been exposed for a long period they would 

 exhibit marked evidence of waste and decay. The fact is, on the contrary, 

 tliat these deposits are remarkably well preserved. The greater amount of 

 fracture and displacement that has been observed has evidently resulted 

 from the weight of the deposits when left unsupported by the waters in 

 which they were formed. The only conclusion to be drawn from the tufa 

 deposits in reference to the date of the last desiccation of Lake Lahontan 

 is that their time of exposure has been short. 



Again, in reference to the shells strewn over many portions of the 

 deserts which, in man}' cases, must have been left by the evaporation of the 

 former lake, we find that these fossils, or semi-fossils, as they have been 

 termed, are bleached white and have lost their epidermis, but are otherwise 

 frequently as perfect as when inhabited by the mollusk to which they 

 belonged. That these fragile bodies have been drifting about at the caprice 



