144 GEOLOGICAL HISTORY OF LAKE LAHONTAN. 



large quantities of pumiceous dust, which must have been thrown out by 

 some volcano in the state of violent eruption. The second rise of the lake 

 was followed by the present period of desiccation, which witnessed the 

 evaporation of its waters and the exposure of its sediments to subaerial ero- 

 sion. The rivers in flowing across the exposed lake-beds carved the deep 

 channels we have described, and are now spreading stream and current 

 borne gravels far out in the central portions of the valleys, thus in many 

 ways repeating the conditions that characterized the time during which the 

 medial gravels were deposited. 



In order to represent the sediments of Lake Lahontan on a geological 

 map of the region one has but to color the area once occupied by the lake 

 with the appropriate tint. The older rocks throughout the area are not com- 

 pletely concealed by the sediments of the lake, however, the exceptions 

 occurring along the borders of the basin and about isolated buttes; but these 

 portions being usually precipitous, the belt left unconcealed is so narrow 

 that it would be scarcely possible to represent it on a geological map of the 

 scales ordinarily used. 



To prevent confusion it seems appropriate to indicate at this time some 

 discrepancies that exist between the published reports of the United States 

 Geological Exploration of the Fortieth Parallel and the conclusions pre- 

 sented in the present volume. On map V of the atlas issued for that explo- 

 ration a large portion of Lahontan basin is included. The area covered by 

 the sediments of Lake Lahontan, as determined by the present survey, are 

 there indicated in four different ways. Some portions are represented as 

 Truckee Miocene, others as Humboldt Pliocene, while the greater part is 

 divided between Upper and Lower Quaternary. 



The areas colored as Truckee Miocene are situated at the lower end of 

 Humboldt Lake and at the southern end of Winnenuicca Lake. The de- 

 l)osits at these localities are similar, consisting, if the writer's determinations 

 are correct, of gravels that were accumulated in the form of bars or embank- 

 ments through the action of the curi'ents of the Quaternary lake. The 

 some deposits about the south shore of Humboldt Lake have been described at 

 length in the preceding pages (105 to 112), and a detailed map of the area 



