124 GEOLOGICAL niSTOEY OF LAKE LAHOi^TAN. 



nation to their having' been deposited by currents on tlie steep slopes 

 of orravel embankments. That much of tlie material now fillini^- the Hum- 

 boldt Valley Avas brought down by the river, and is in reality of the nature 

 of a delta deposit, there can be little doubt, but it has mostly been re- 

 arranged by currents and the topographic form of a delta is wanting. The 

 same is true also of the accumiUations at the points where the Truckee and 

 Carson rivers entered the lake; about the ancient mouths of these streams 

 there is a thickening of the river-borne debris, but no distinct delta forms 

 are visible. The canons of Buffalo and Smoke creeks were excavated to 

 their present depth before the existence of the lake, and during the time the 

 basin was flooded each of these channels formed a long narrow inlet which 

 became deeply filled with sediments. When the lake's surface was lowered, 

 the greater part of this material was removed by stream erosion ; and so far 

 as the history of their deltas is concerned, there is no more to be said than 

 in the case of the larger rivers. 



Nowhere in the Lahontan basin are deltas to be found that are com- 

 parable with those formed along the bold eastern shore of Lake Bonne- 

 ville, or those deposited in the Mono Lake basin by streams that descended 

 the eastern slope of the Sierra Nevada. 



Section 3.— SEDIMENTS OE LAKE LAHONTAN. 



The tributaries of lakes — disregarding organic substances — contain two 

 classes of impurities, (a) mineral matter in susj^ension, and (b) mineral 

 matter in solution. 



Besides holding fine silt in suspension, streams also roll pebbles and 

 stones along their beds. On entering a lake all this material subsides more 

 or less quickly, forming lake-beds, gi'avel-deposifs, etc. In the sedimenta- 

 tion of lakes ihe coarser and heavier debris is invariably dropped near 

 shore, while the finer and lighter substances are floated to a greater distance 

 before subsiding. In this manner coarse shore and fine ofi-shore deposits 

 original e. The shove deposits of Lake Lahontan liave aheady received 



