156 GEOLOGICAL HISTOEY OF LAKE LAHONTAN. 



been blown inland by the prevailing westerly winds. It seems more prob- 

 able, however, that they owe their origin to the subaerial disintegration of 

 the o-ranites of the Sierra Nevada. 



Section 4.— ANCIENT STREAM CHANNELS. 



When the waters of Lake Lahontan subsided during inter- and post- 

 Lahontan periods its basin became divided into separate water bodies or 

 independent lakes, some of which were connected by streams that over- 

 flowed from one to another. The channels eroded by these streams are 

 interesting not only as examples of erosion, but because they contribute to 

 the interpretation of the history of the former lake. When a large inclosed 

 lake is reduced so far by evaporation that the inequalities of its bottom 

 divide it into independent areas, it is evident that this fact in itself is a 

 record of an important climatic change; when the ridges or embankments 

 that divide a lake in this manner are cut bv channels of overflow, it is 

 evident that they may furnish some index of the length of the period of 

 desiccation or perhaps of the date at which it occurred. Tlie multiplica- 

 tion of hydrographic basins by desiccation is illustrated by the present 

 condition of the Lahontan region, as shown on Plate XXIX. The ancient 

 lake basin is now divided into six independent drainage areas. 



Old channels now abandoned and dry occur in the Lahontan basin, 

 between the larger areas of the former lake and the neighboring valle^^s 

 that once formed bays along its shore. The Carson Desert is united with 

 the desert valley south of Allen's Springs by a deeply eroded channel of 

 this nature, which appears to have been cut by a stream flowing northward; 

 a moderate rise of the waters of the Carson Desert would flood this pass and 

 reconvert it into a strait This channel is about 5 miles long, and has pre- 

 cipitous walls composed of lacustral sediments, which are lined with the form 

 of tufa we have called dendritic, while in the bottom of the pass there are 

 crags of thinolitic tufa; from these records we learn that the channel was 

 excavated previous to the rise of the lake during which tufa deposits were 

 foi'med. As the sequel will show, these tufas were deposited during that 



