]50 GEOLOGICAL HISTORY OF LAKE LAHO]!fTAN. 



impure from the intermingling of sand and gravel, and finally wedges out 

 and is replaced by water-worn debris like that forming the bars. It seems 

 to form a lenticular mass, filling a local basin, but the section does not 

 give complete proof that such is the case. Our observations would apply 

 equally well to a low oflf-shore embankment built by gentle currents, and 

 subsequently buried by ordinary shore-drift. The gravel bars resting on 

 the marl were formed dui'ing a subsequent rise of the waters and were 

 never afterwards submerged; consequently the marl must have been de- 

 posited previous to the last high-water stage of Lake Lahontan This will 

 be of interest when the oscillations of the lake are more fully described. 



Passing to other localities where white marl has been observed, we 

 find that in sheltered ravines on the sides of the basaltic buttes overlooking 

 the southern shore of the South Carson Lake there are fine, mealy deposits 

 of this nature, 20 or 30 feet thick, and some distances below the highest 

 of the Lahontan terraces, which contain gasteropod shells in abundance. 

 Similar beds were also observed aboiat 2 miles north of Allen's Springs, in 

 the bottom of the ancient channel leading to the Carson Desert. The 

 exposure is here imperfect, and as the beds are but little elevated above 

 the general desert-surface it is with doubt that they ai'e referred to the 

 same jjeriod in the history of the lake as the similar deposits observed at 

 higher levels. White marl may also be seen at a number of indefinite 

 exposures at a uniform hoi'izon, some distance below the Lahontan beach, 

 along the steep bluffs which border the Carson Desert on the south. In 

 Alkali Valley, 2 or 3 miles west of Sand Springs, similar marls filled with 

 gasteropod shells occur in a group of embankments that project into the 

 valley. Another locality of the same nature was observed on the west 

 side of Humboldt Lake at an elevation of four hundred feet above the 

 lake surface. 



In the Truckee Canon, about a nnle below the Truckee Narrows, there 

 are beds of pure, white, chalky marl not less than 50 feet in thickness, that 

 are grouped about a butte of volcanic rock which was formerly completely 

 buried beneath Lahontan sediments, but is now exposed by the erosion of 

 the river. These beds are in part overlain by Lahontan deposits, but the 

 exposure is obscure and the relation of the marls to the associated clays 



