SUB-AEKIAL AND SUB-LACUSTilAL TOPOGEAPHY. HI 



structure complex and their bearing on Lahontan history, i. e., their value 

 as records of Quaternary climate, exceedingly difficult to trace. From the 

 occurrence of Lithoid tufa**' — the oldest variety found in the basin — on the 

 western slope of the main bar, it is evident that the deposit of gravel at 

 this locality was commenced early in the existence of the old lake, perhaps 

 at the very first rise of its waters; and, as we have seen, enlargement and 

 reconstruction have taken place at many subsequent periods and at many 

 stages in the vertical range of the lake's surface. 



The trifling changes that have occurred in the area embraced b}* the 

 accompanying maps, since the Avithdrawal of the waters of Lake Lahontan, 

 are indicated by the arroyos or small water-courses cutting the embank- 

 ments, and by the gap eroded hj the overflowing waters of Humboldt 

 Lake; Avith these exceptions, the structures are as fresh in appearance and 

 as perfect in form as if they had been exposed to subaerial degradation for 

 only a few years. 



The contrast in the topography above and below the Lahontan beach, 

 in the region about Humboldt Lake, is so pronounced that it at once attracts 

 the attention of the observer. The mountains above the level of the for- 

 mer lake have the rugged and angular aspect characteristic of the subaerial 

 erosion of arid climates, while below that horizon the topography is remark- 

 able for its sweeping curves and flowing outlines. In the former instance 

 the direction of the lines of erosion is controlled by the flow of rills and 

 rivulets, and approaches the perpendicular; in the latter the predominating 

 lines in the landscape are modeled by the waves and currents of a level 

 water surface, and are therefore horizontal. The characteristics of the 

 topography in one instance are due to subaerial, and in the other to sub- 

 aqueous, conditions. 



Orographic movement has taken place on a grand scale in the region 

 represented on Plate XVHI. This is illustrated by the great fault, witli a 

 throw of several thousand feet, which determines the northwestern face of the 

 west Humboldt Range. This fault jjasses between the Niter Buttes and the 

 main range to the southward, and a curving branch determines the western 

 face of the promontory formed b}' these buttes. The nearl}^ perpendicular 



■•^ Thinolite and dendritic tufa also occur in abundance at this locality. 



