Axelrod: The Pliocene Verdi Flora of Western Nevada 101 



feet, and grades upward into large cobble conglomerates of the lower member which 

 overlap Alta andesite north of the basin fault against which the breccia is but- 

 tressed. 



Basalt member.— In most of the area where it is exposed, basalt separates ap- 

 proximately the lower dark-colored member from the upper light-colored part of 

 the formation, though in the northern part much of it is in the lower member. In 

 that area it is separated usually from Alta andesite by only a thin regolith or by 

 a few feet of the lower member, but locally it may rest on the volcanics. It is not 

 exposed in most of the Truckee River valley owing to structural complications. To 

 judge from its relations along the Truckee River near Mogul, and in the low hills 

 south of Lawton, it lies well above the base of the section in the valley area. This 

 variable stratigraphic position is explainable on the basis of extravasation on the 

 margins of the basin, as well as within the basin proper. 



The thickest section crops out on the bluff bordering the Truckee River on High- 

 way 40 a mile east of Verdi, where it is approximately 400 feet thick, and com- 

 prises three well-defined flows. The lowest one is buttressed against basalt ag- 

 glomerate at the highway level ; the second one lies on a lenticular 8- to 10-foot 

 tuffaceous sand and gravel section ; and the upper one rests on a thin, discontinuous 

 tuffaeeous sand. The lower flow is black, massive, exhibits columnar jointing, and 

 is an augite olivine basalt. The upper augite olivine basalt flows are typically 

 flow-jointed, reddish brown on weathered surfaces, but a fresh surface reveals a 

 gray -brown color and a subvitreous surface. Louderback (1906, p. 669) interpreted 

 this basalt as a Quaternary lava that had poured off the slopes of Peavine Mountain 

 to northward. Actually, it has been gently folded, and is faulted. To the northeast 

 it dips under the Coal Valley sandstones and conglomerates of the basal part of the 

 upper member. The basalt appears to rest on Kate Peak andesite to judge from 

 the relations near the middle of the basalt outcrop a hundred yards north of the 

 highway. 



Basalt is much thinner elsewhere in the region. The numerous patches in the 

 area to the northwest range from 10 to 30 feet thick. At the east margin of the map 

 south of Lawton, the lower and upper flows are separated by 25 to 30 feet of sedi- 

 ment. The thick river bluff section presumably represents accumulation in a valley, 

 as compared with the more widely distributed upper flow-jointed section that 

 appears to have poured out over a low plain adjacent to, and within, the basin. 



Upper member. — The dark-colored lower conglomerate-breccia member grades 

 upward into a lighter, gray to brown section typified by coarse sandstone, pebble 

 conglomerate, diatomite, and sandy shale. The elastics north of the river, having 

 been derived from areas of bleached Alta rocks, are often varicolored, commonly 

 mustard brown, yellow-brown, and occasionally rusty, as well as gray when derived 

 from the less-altered section. The sandstones frequently show rapid changes in 

 color, and are especially conspicuous as thinner beds from 1 to 3 or 4 feet thick. The 

 succession of different-colored sandstones and pebble beds derived from Alta 

 andesite is suggestive of flood deposits coming from relatively local drainage areas 

 near at hand. A similar diversity of sediment could readily be derived from the 

 different exposures of Alta andesite that crop out now in valleys on the slopes of 

 Peavine Mountain. Summer showers in the drainage basins bordering the Verdi 



