EMBANKMENTS AT HUMBOLDT LAKE. 109 



the Humboldt, where the homogeneous black gravel composing the end 

 bars rests directly on the more heterogeneous material forming the main 

 embankment. Its greater age is also shown by its being incrusted on its 

 western slope by the three main varieties of Lahontan tufa hereafter to be 

 described, while the end bars are almost entirely free from these deposits. 



In a terrace of black gravel on the northern slope of the Niter Buttes, 

 dendritic tufa between heavy beds of gravel indicates that there were 

 periods of bar building both before and after the formation of the tufa. On 

 the north side of the valley, northeast of the north end of the main embank- 

 ment, an arroyo has cut the gravel deposits so as to reveal an interbedded 

 stratum of white marl; this again marks a division between two periods 

 during which embankments were formed. These interruptions in the 

 gravel deposits are noted now as a part of the facts observed in the region 

 we are describing, but their connection with the history of the lake will be 

 described in a future chapter. 



The embankments on the south side of the valley, as shown on Plate 

 XVIII, appear, topographically, to be branches of the main structure, but 

 in reality they were formed at a later date by currents sweeping west- 

 ward along the southern border of the valley. This is shown not only by 

 their being tangent to the projection formed by the Niter Buttes, but is 

 also evident from the nature of the material of which they are composed. 

 The Niter Buttes and Mopung Hills are rhyolite, while the mountains skirt- 

 ing the southern shore of Humboldt Lake are largely composed of black 

 slate. The gravel forming the symmetric embankments at the base of the 

 Niter Buttes is composed almost wholly of water-worn pebbles of black 

 slate, and could only have been derived from cliffs of the same material to 

 the eastward; the gravel of which the bars are composed may, in fact, be 

 traced continuously to the quarries from which it was obtained. 



On the steep western slope of the Niter Buttes are a number of terraces, 

 each of which records a pause in the fluctuation of the lake in which they 

 were formed. The most conspicuous of these is a broad shelf of black 

 gravel which girdles the promontory at an elevation of 220 feet aljove 

 Humboldt Lake. The gravel forming this shelf consists of well-rounded 

 fragments of black slate identical with those composing the bars at a lower 



