SUCCESSION OF TUFA DEPOSITS. 205 



the dendritic tufa in the Humboldt and Truckee canons. During this last 

 rise the water surface reached a horizon about 30 feet above the lithoid 

 terrace, and carved the Lahontan beach — the highest water-line in the 

 basin. From this level tiie lake evaporated away until the basin reached 

 at least its present condition, and probably a much greater degree of des- 

 iccation. We should expect that other deposits of tufa would have been 

 formed during this final evaporation. Thus far, however, there are but few 

 observations to sustain this hypothesis. Smoke Creek Desert and the val- 

 ley of Pyramid Lake are separated by a low divide, which at a certain 

 stage in the lowering of the lake must have parted the waters inithe two 

 valleys. The basin now floored by the desert underwent complete desic- 

 cation, and on its surface we find the mineral matter precipitated from the 

 waters as they evaporated. The desert where not concealed beneath recent 

 playa deposits, is covered with an abundance of thinolite crystals, mostly 

 scattered and broken, which, from their position, must have been deposited 

 during the last recession of the Avaters. The highest point at which this 

 tufa was found may be taken at about 50 feet above the surface of the 

 desert, or a little below the level of the divide at the southern end of the 

 basin. The crystals scattered over the surface of the desert are somewhat 

 different in appearance from the thinolite found in such abundance about 

 Pyramid Lake; and, at their upper limit, pass into a dense, compact, and 

 usually botryoidal mass which closely resembles gray stone-ware. See Fig, 

 18, Plate XXXIV. As is common with tufa deposits, these crystals are 

 usually gi'ouped about solid nuclei, and frequently form rosette-shaped 

 masses 4 or 5 inches in diameter. 



Direct superposition of this variety of tufa upon the dendritic has not 

 been observed; but the sediments on which it rests are probably of the 

 same date as those covering the dendritic tufa in the Truckee and Hum- 

 boldt canons. Thinolite crystals of the same character as those scattered 

 over the surface of the Smoke Creek Desert, have been observed at the 

 southern end of Winnemucca Lake, and on the shore of Walker Lake; at 

 the time of their formation, each of these basins must have contained an 

 independent water-body. 



