Campbell.] GEOLOGY ALONG ROUTE TRAVERSED. 15 
the middle of the lake also, for a small island shows the uppermost 
lake beds at a lower altitude than they appear on the western side of 
the basin. 
The lake basin was apparently silted up to the level of the outlet, 
and since then Amargosa River has cut its way through the rocky 
barrier on the south and trenched the sediments to a depth of 150 feet. 
Evidently drainage conditions changed about the time the basin was 
filled, giving to Amargosa River sufficient fall or sufficient volume 
to trench its barrier at least 150 feet. It seems possible that the 
change was due to the sinking of Death Valley to its present position 
below sea level and 1,200 feet below the level of the lake near Resting 
Spring. The tilting of the sediments of this lake toward the west pre- 
sumably was effected by the subsidence of Death Valley, and since 
this downward movement evidently postdates the period in which the 
lake beds of Death Valley were folded, it seems probable that the lake 
in Amargosa Valley was of late Tertiary age, and that it constituted 
one of the chain of lakes to which reference has been made. 
The sediments deposited in this lake are strongly alkaline, but no 
deposit of borax was seen, nor are any reported from this locality. 
The borax works which flourished for a time in this basin depended, 
according to report, upon the marsh of Amargosa River for their 
supply of crude material. The borax on the marsh is probably derived 
from erosion of beds of colemanite in the northern end of Funeral 
Mountain, and since this erosion was presumably in progress during 
the deposition of the sediments about Resting Spring, it is altogether 
probable that beds of this mineral occur intercalated with the sedi- 
ments of the lake. The occurrence of the borax in regular beds or its 
dissemination through the clay doubtless depends upon the degree of 
aridity of the climate during the existence of the lake. From the 
large amount of alkaline salts now visible in these sediments, it seems 
probable that the climate was sufficiently arid at times to cause tin- 
water to be largely evaporated, leaving its mineral contents as a crust 
on the bottom of its basin. It would not, therefore, be surprising to 
find in these sediments beds of borax derived from the great deposits 
of Funeral Mountain. Before the discovery of the bed of colemanite 
near the railroad the Amargosa works were of considerable impor- 
tance. A wagon road was built between this point and Daggett and 
wells were dug and relay stations were established along the route. 
but the opening of mines on the bedded colemanite put an end to oper- 
ations in Amargosa Valley. 
So far as known, lake sediments do not occur east of this point. If 
a lake ever existed in Pahrump Valley its sediments have never been 
exposed by crustal movements, and they lie undisturbed in the bottom 
of the valley. 
