200 SOUTHWESTERN NEVADA AND EASTERN CALIFORNIA. 
and very finely laminated and partly porous. The areas in the mid- 
dle of the valley northwest of these springs are of interest, since Hows 
of the supposed Pleistocene basalt are inter! n-dded with the clay, 
while other layers contain bowlders of basalt. Tn some places the 
clay beneath the basaH ha- been reddened and slightly baked. These 
clay beds, which are well up in the section, are broadly contempora- 
neous with basalt eruption. 
About 2,500 feet of the older alluvium i- exposed in the Amargosa 
Range, and it i> probable that the series was originally over 3,000 
feet thick. The beds are gently Hexed, and dips from the mountains 
to the valley axis predominate, particularly in the northern part of 
the valley. The sediments have been jointed, the principal system 
striking X. 25° W., and along some of these joints occur normal fatdts 
with from '2 to 4 feet displacement. 
GEOLOGIC HISTORY. 
Iii Tertiary, probably early and middle Miocene time, Death Val- 
ley did not exist, Amargosa and Panamint ranges were low, and their 
southern portions'' at least were covered by a lake which extended well 
into the present Death Valley south of Salt ('reek. This lake was at 
leasl partly contemporaneous and may have been connected with that 
in which the Sieberi lake bed- were deposited, and which probably 
it one time covered almost the entire area surveyed. In late Pliocene 
time, however. Death Valley was probably a closed basin, occupied 
by a sheet of water, which appear- to have been a playa. This 
playa varied widely from time to time in extent and was bordered by 
the uplifted Panamint and Amargosa ranges. Before the deposition 
of the older alluvium ceased the inwasb of material from the 
mountains raised the center of the valley hundreds of feet above the 
present level. Alluvial slope-, much like those of to-day, extended 
well up on the Panamint and Amargosa ranges. In the playa great 
thicknesses of clay were deposited, and at periods of unusual desicca- 
tion lime-tone and gypsum were precipitated. Unusual floods from 
time to time throughout the period of deposition spread thin layers 
of bowlders over the playa. Basalt outflows accompanied the deposi- 
tion of the upper playa deposits. 
Death Valley i> believed to have been roughly blocked out in late 
Miocene and early Pliocene time, through erosion and deformation, 
including faulting and possibly folding. The folding of the Amar- 
gosa and Panamint ranges doc- not alone account for the valley, and 
it appears to be a block dropped down between the bounding ranges 
by faults. The earliest structural lines, probably of late Miocene age, 
trend northwest and southeast and include the fault '' on the north- 
" Spurr, J. K.. Bull. U. S. Geol. Survey No. 208, L903, pp. 189, 202. 
6 Campbell, M. It., Bull. U. S. Geol. Survey No. 200, 1902, p. 16. 
