346 University of California Publications in Geology [Vol. 8 



those above described are reported by Russell to be forming at the 

 present time around lake-bottom springs in Mono and Pyramid lakes, 

 the waters of both of which are quite concentrated. As it appears, 

 however, that the deposition of calcareous tufa might occur by liber- 

 ation of carbon dioxide previously in solution under pressure in the 

 spring water, or by other means, it would not seem that the presence 

 of tufa domes in the lake-beds could by itself be legitimately consid- 

 ered positive evidence that the waters of the Miocene lake were highly 

 concentrated. The rather common occurrence of the domes, however, 

 and the considerable volume of water which presumably issued from 

 each dome as inferred from the size of the openings in them, do imply 

 that the region in upper Miocene time was blessed with a supply of 

 spring water in sharp contrast with the present aridity and paucity 

 of springs of even the smallest size. 



Thinolite tufa of gray color, and similar in general appearance to 

 that described by Russell from the Lahontan Basin, was noted at a 

 few points, occurring either as a coating on lithoid tufa domes or as 

 a strata between sediments. Only one occurrence of dendritic tufa 

 was observed. 



The cherts of the lacustral series occur at many localities. The 

 greatest single thickness noted, about thirty-five feet, was in the middle 

 of Stewart Valley. These rocks apparently occur at more than one 

 horizon in the lacustral strata. They vary in thickness from place 

 to place, but are quite extensive horizontally. This rock takes on a 

 great variety of colors, of which the more common are white, yellow, 

 red, brown, blue, and green. It is in general not distinctly bedded, 

 but shows regular banding of different colors, the thickness of the 

 bands varying from extreme thinness to several inches. The chert 

 is harder than steel and very brittle, giving sharp edges and smooth 

 faces on fracture. On the areas of badland topography which present- 

 day erosion is developing on the lake-beds the cherts stand out promi- 

 nently because of their superior resistance and often cap mounds of 

 softer beds. While no chemical analysis of these rocks was made, 

 the absence of visible particles of foreign material, as also their hard- 

 ness and constant texture, indicate that they are nearly pure siliceous 

 rocks. 



Besides the considerable quantities of fine ash in the shales and 

 of fine ash and pumice in the sandstones, there are some strata in the 

 lake-bed series which consist almost entirely of fine ash. others almost 

 entirely of pumice, and yet other beds which are made up of both in 



