154 GUIDEBOOK OF THE WESTERN UNITED STATES. 
post 659 there are extensive pits 6 to 8 feet deep in which the gypsum 
is obtained. It is carried by a small railway to the plaster mill at 
Amboy, where it is heated to expel the water and ground to the fine 
powder known as plaster of Paris. 
Amboy is dependent on the plaster mill and a few mines in the 
mountains. A stage line which runs to Dale, a mining camp 45 
miles to the south, crosses the lowest part of the 
Amboy. basin near the salt deposit and goes through a pass or 
Pastner tay peek depression in the Sheep Hole Mountains, which are 
fs ‘about 11 miles distant. These mountains consist of 
granite and schist of pre-Cambrian age and form a part of the topo- 
graphic barrier of granite and other igneous rocks which borders the 
south side of the valleys traversed by the railway as far as Barstow. 
The Marble Mountains, constituting the north rim of the basin a 
few miles north of Amboy, consist mainly of coarse-grained granites, 
mostly light gray, penetrated by large bodies of dark coarse-grained 
quartz monzonite. Many masses of limestone altered to white 
marble occur north and northeast of Amboy. At one locality 4 miles — 
north of Amboy considerable iron ore has replaced the limestone near 
the igneous contact. In the foothills of this range 24 miles north of 
Amboy there is a low ridge of light-colored lava (rhyolite), and a 
short distance northwest of this a large rounded hill of fine-grained, 
dark-colored rock (diorite), probably much older than the lavas. 
A mile east of these knobs is a small pit in light-colored clay which 
has been used for mixing with the plaster at the Amboy mill. This 
clay is probably part of the mass of earlier sediments that underlie 
the valley and are here upturned along the foot of the mountain rim. 
A 700-foot well in Amboy is reported to be entirely in sand and 
gravel and to have yielded only salt water, which occurred in con- 
siderable volumes, especially near the surface. At a depth of 70 
feet it penetrated a deposit of gypsum. 
Summer temperatures in this part of the desert are very high, those 
at Amboy often exceeding 120° F. However, the mean annual tem- 
perature at Amboy is much less than that of places at lower altitudes’ 
farther south within our borders, a maximum of 130° having been 
recorded at Salton, in the Colorado Desert. The rainfall in the 
desert region of southeastern California to Barstow and beyond is 
very small, averaging only about 5 inches a year. West of Amboy the 
train passes a series of rather recent voleanic cones and lava flows. 
In the center of the basin, not far southwest of Amboy, there is @ 
fine cinder cone on an extensive sheet of black lava (basalt). This 
lava is, geologically, very recent and may have flowed out over the 
bottom of the basin within the last thousand years. It covers @ 
nearly circular area about 5 miles in diameter. Its surface js remark- - 
ably rough, being covered with large blisters, most of them broken, __ 
