42 p. B. KING 



eastward, where it became an Important component of contem- 

 poraneous sedimentary deposits. 



Over much of the Great Basin east of the eruptive area a succes- 

 sion of andesitic vitric tuff, reworked ash, bentonitic mudstone, 

 sandstone, Hmestone, and diatomite was deposited, which has been 

 variously termed the Truckee, Esmeralda, or Humboldt formation, 

 depending on locality. The deposits were laid down in many sep- 

 arate but probably confluent basins, partly in lakes and swamps. 

 They not only covered the earlier Tertiary rocks but overlapped 

 widely onto low inter\^ening highlands of the Mesozoic and Paleozoic 

 rocks. In southern Nevada faulting was more active at the time, and 

 the Muddy Creek formation of that area consists of coarse alluvial 

 fan deposits along the faulted basin margins, and of finer-grained 

 elastics and evaporites in the basin centers. 



Comparison of floras in a traverse eastward across the area is 

 instructive as to the late Miocene and early Pliocene environments 

 (Axelrod, 1957, pp. 34-38). Conifer forests like those of the modern 

 Sierra Nevada were not well developed on its western slope probably 

 because of low altitude and warm climate. At Carson Pass on the 

 crest of the range, at a modern altitude of more than 9,000 feet, is a 

 flora of deciduous trees which could not have lived at altitudes 

 higher than 2,500 feet. Farther Inland, In the Great Basin, were 

 conifer forests of a type now found at the margins of woodland and 

 chaparral country. Annual rainfall at the western base of the Sierra 

 Nevada must have been about 25 or 30 inches, increasing to 40 or 

 45 inches on the upper slopes, and thence decreasing to 25 inches 

 over the lowlands of the Great Basin. The summit level of the central 

 and northern Sierra Nevada must have stood at an altitude of less 

 than 3,000 feet, and projected about 1,000 feet above the Great 

 Basin to the east ; it created no more than an ineff^ective rain shadow 

 over that area. Evidence of fossil fishes suggests that the Great Basin 

 at this time stood at altitudes well below 2,000 feet, to allow the 

 ingress of lowland coastal faunas (C. L. Hubbs, personal com- 

 munication). 



The andesitic eruptions along the Sierra crest, although spread 

 over a surface of low altitude and low relief, foreshadowed later 

 uplifts along that axis. By late Pliocene time the floras of the Great 

 Basin changed from a woodland and forest fades to a savanna and 

 grassland fades, adapted to less than 15 inches of rainfall. Evidently 



