134 GEOLOGICAL HLSTORY OF LAKE LAHONTAN. 



at the same time increasing in thickness in the west bank of the canon, 

 until it finally composes the entire section of more than a hundred feet. 



Just before the Truckee Cafion opens out into the valley occupied by 

 Pyramid and Winnemucca lakes, it becomes quite narrow, and is bounded 

 on either side by rocky walls; for convenience of reference we have called 

 this the "Truckee Narrows." From this point to the Agency Bridge, a 

 distance of about four and a half miles, the walls of the canon exhibit a 

 continuous section in which the tripartite character of the Lahontan sedi- 

 ments is strikingly displayed. The exposures actually observed on the east 

 side of the stream have been sketched by Mr. McGee and form Plate XXIV. 

 The most instructive feature illustrated by this section, as is the case of the 

 exposures along the Humboldt, is the fact that it consists of two series of 

 fine homogeneous strata, separated by a heavy deposit of heterogeneous, 

 current-bedded gravels. A generalized section of the beds here exposed 

 agrees in a remarkable way with the similar sections observed in the Hum- 

 boldt Canon. The upper and lower lacustral clays occurring in the Truckee 

 section, like those exposed in the banks of the Humboldt, show but little 

 variation. They are composed of fine, evenly laminated, drab-colored, 

 marly clays, that are somewhat saline and alkaline as indicated by chemical 

 tests. On the west side of the river near the Agency Bridge, however, the 

 upper clays show some variation, especially near their contact with the 

 underlying gravels, as is exhibited with considerable detail in the section 

 forming Plate XXV. 



One of the most instructive portions of the Truckee section is a stratum 

 of dendritic tufa interbedded with the upper clays. At the northern end of 

 the section, i. e., towards the deeper portion of the lake in which the sedi- 

 ments of tufa were deposited, the tufa-stratum is but 3 inches thick and is 

 buried beneath 25 or 30 feet of laminated clay; when followed shoreward, 

 or up stream in reference to the present drainage, the tufa gradually increases 

 in thickness, at the same time approaching nearer the surface of the section, 

 until at the Narrows of the Truckee it forms a sheet of huge mushroom-shaped 

 masses at the top of the bank, which are from 10 to 15 feet in diameter and 

 so thickly planted that they form a continuous pavement fully 10 feet thick. 

 The rocks at the Narrows above the level of the lacustrine deposits are 



