THINOLITIC TUFA. 193 



stated in the preceding paragraph. In a few instances, however, a dendritic 

 structure maybe detected; as is the case, also, in the lithoid deposits of the 

 lake we are now studying. 



In the Lahontan basin thinolite crystals are only found in the lowest 

 depressions, as, for example, about the shores of Pyramid and Winnemucca 

 lakes and on the borders of the Carson Desert. Its geographical extent, 

 as shown on the accompanying map, embraces the Carson Desert, the val- 

 leys of Pyramid and Winnemucca lakes, together with the Black Rock and 

 Smoke Creek deserts. The divide between Carson Desert and the valley 

 in which Pyramid Lake is situated, is higher than the upj^er limit of the 

 thinolite tufa; we must conclude, therefore, that when Lake Lahontan 

 evaporated away sufficiently to admit of the formation of thinolite — or of 

 the crystals after which it is a pseudomorph — the water-surface was below 

 the level of the pass to the eastward of Wadsworth, and the lake conse- 

 quently, divided into at least two water-bodies. On the preliminary map 

 of Lake Lahontan, published by the writer in tlie Third Annual Report of 

 the U. S. Geological Survey, the lake at its thinolite stage is represented as 

 extending through from the Carson Desert to Pyramid Lake; this error has 

 been corrected on the accompanying map. It was also indicated on the 

 previous map, that Smoke Creek and Black Rock deserts were without 

 thinolite deposits; later observations have shown that they do occur in 

 large masses at certain localities on the borders of these basins, with the 

 same general relations to the other tufa deposits that they have in the 

 most typical localities. 



Tlie valley of Walker Lake must also have formed an independent 

 warter-body during the thinolite stage of Lake Lahontan, but no crystals 

 belonging to this period of the lake's history have been found in that basin. 

 As will be described later, there are masses of thinolite about Walker Lake 

 corresponding to quite recent deposits of the same mineral in the Black 

 Rock Desert. 



In its vertical range the thinolite is limited by the broad shelf, named 

 the thinolite terrace, which occurs at an elevation of 110 feet above the 

 1882 level of Pyramid Lake. In only a single instance has the thinolite 

 been observed to extend above this horizon. On Anaho Island the terrace 



MoN. XI 13 



