248 GEOLOGICAL HISTORY OF LAKE LAHONTAN. 



No leaves or vegetable stems of any kind, excepting the fossils men- 

 tioned in the last paragraph, liave been found among the records of the old 

 lake, and no drift timbers seem to have been deposited in the bars and 

 embankments that have been examined. The absence of such fossils appar- 

 ently indicates that the shores of the former lake Avere not heavily wooded. 

 The borders of our Northern lakes at the present day are thickly clothed 

 with forests and their shores encumbered with stranded logs and stumps in 

 such quantities as to have considerable influence on the character of the 

 shore phenomena resulting from wave action. Had the shores of Lake 

 Lahontan been as densely wooded as are those of Lake Michigan, for 

 example, it seems impossible that abundant records of the fact should not 

 have been discovered during our sojourn in the basin. 



A superficial microscopical examination of the sediments of the ancient 

 lake has shown that they are richly charged with the silicious skeletons of 

 infusoria, and sometimes abound in sponge spiculse. A detailed study of 

 these fossils was not practicable, and, as these forms of life are so widely 

 distributed and live under such diverse conditions, it seems doubtful in the 

 present instance, if a more critical examination would greatly assist in solv- 

 ing the chemical and climatic problems with which the student of the Qua- 

 ternary geology of the Great Basin is principally concerned. 



No fossils have been found in the thinolitic tufa, although careful search 

 has been made at many localities. At times shells may be seen in the open 

 spaces between the crystals, but these are believed in all cases to have been 

 accidentally introduced at a recent date. The absence of all life records 

 in this deposit strengthens the hypothesis that the thinolite was crystallized 

 from waters highly charged with mineral matter. 



In correlating the various Lahontan deposits we have considered the 

 thinolite as stratigraphically intermediate between the lower and upper 

 lacustral clays, and, at least in part, contemporaneous with the medial 

 gravels. The fossils obtained from the medial gravels are of fresh-water 

 species, but were collected near the borders of the basin and at a greater 

 elevation than the upper limit of the thinolite. The fossils may thus rep- 

 resent a stage in the recession or in the refilling of the lake when its waters 

 were not so dense as when the thinolite was crystallized. In the Humboldt 



