110 GEOLOGICAL HISTORY OF LAKE LAHONTAN. 



level, and derived from the same source. As the rock composing the buttes 

 has a bright reddish tint, the position of this terrace is rendered conspicuous 

 by contrast in color. At the southern side of the buttes this terrace leaves 

 the steep slope and crosses an alluvial cone, changing at the same time 

 from a terrace to a barrier bar with a rounded crest. In cross-section, like 

 nearly all gravel bars, it exhibits an oblique stratification which is most 

 pronounced on the lake ward slope. 







Pig. 18. — SectiOD of b.ir on the Niter Buttes. 



The angular alluvium on wliicli tlie bar rests is large.y composea of 

 brightly-colored rhyolite, derived from the cliffs above, while the bar, like 

 the terrace of which it is a continuation, is almost wholly formed of pebbles 

 of black slate, which indicate by their rounded forms and polished sur- 

 faces that they have traveled a long way since leaving their parent ledges 

 in the cliffs on the southern border of Humboldt Lake. On following 

 these gravels still farther southward, we find that they again form a terrace, 

 which soon loses its identity, however, on approaching the Mopung Hills. 



The group of bars on the north side of the valley, like the correspond- 

 ing structures at the base of the Niter Buttes, are level-topped ridges of 

 clean, well-worn gravel, forming graceful curves; but the gravel in this 

 instance was ver}' largely contributed by currents from the west. The 

 spaces inclosed by these ridges are in almost all cases floored with light- 

 colored mud, forming pla3^as, which are converted into sliallow lake- 

 lets by every storm. The relative age of these bars may be determined in 

 some instances by the superposition of one upon another, and by the fact 

 tliat some are partially covered with tufa, wliile others, of later date, are 

 free from that deposit. 



The complexity of these embankments, arising from the fact that they 

 were formed at many horizons and at various stages in the former lake, 

 together with the erosion and rebuilding that has taken place, renders their 



