. TRATIGRAPHIC GEOLOGY. 35 
PLEISTOCENE AND RECENT. 
TRAVERTINE. 
At Alkali Spring, H miles northwest of G'oldfield, is a low dome 
200 feet long formed of platy masses of grayish-brown travertine. 
This was the vent of the spring probably in late Pleistocene time. 
DESERT GRAVELS. 
The desert gravels cover the surface of the broad valleys above the 
playas and fill valleys of erosion in the mountains. They extend over 
about 45 per cent of the area mapped. These detrital deposits are 
composed of bowlders, pebbles, and sand. Many of the angular or 
subangular bowlders near the mountains are G feet in diameter; near 
the playas the bowlders grade into pebbles, measuring one-half inch 
or less. Some bowlders of quartzite show small crescentic fractures, 
and some of fine-grained limestones show white crushed spots; these 
are due to the impact of one bowlder against another during rapid 
transportation such as is characteristic of cloudbursts. Near the 
mountains the gravels are but slightly stratified, and here cross- 
bedding and slight erosional unconformities are common. Qn the 
border of playas layers of small pebbles and sand are regularly in- 
terbedded with lamina? of playa clay. The material of these detrital 
deposits is undecomposed, and if consolidated would form a secondary 
rock of practically the same chemical composition as the rock from 
the debris of which it was derived. Calcium carbonate is a common 
cement particularly adjacent to limestone and basalt outcrops. In a 
number of the arroyos shelves along the sides and tiny scarps in the 
channels mark lime-cemented conglomerates. Gypsum and silica also 
occur as cements at a few places. 
The thickness of the desert gravels is unknown. In some of the in- 
closed valleys lava flows and older alluvium are exposed here and 
there and probably underlie the valley at no great depth. In other' 
valleys, however, the gravels probably reach a thickness of hundreds^ 
and possibly thousands of feet. The Amargosa mine at Bullfrog,, 
which is situated about five-sixteenths of a mile from the rhyolite^ 
ridges, has been sunk 330 feet in detrital deposits. Wells have been 
put down in other valleys to depths of 75 to 275 feet, and, while the* 
records are very imperfect, it is apparent that they have not passed 
hrough the desert gravels. 
These gravels in large part make up alluvial fans and the contigu- 
ous alluvial slopes ; in minor part they form cliff taluses. Where no 
laya exists there is a zone in the center of the valley in which the 
naterial is derived from the mountains on both sides, since clpud- 
rarsts, first from one range and then from the other, push the debris 
cross the median drainage line. 
