The Mojave Desert in southeastern California is a wedge-shaped region having its apex toward the west. This report deals with the western part of the desert - a part that is characterized by interior drainage and is sharply delineated by mountain ranges on both the southwest and northwest. The part mapped geologically inc1udes about 7,200 square miles between latitudes 34 34' and 35 30' N. and longitudes 117 and 119 W., or parts of Kern, Los Angeles, and San Bernardino Counties. The center of the region lies about 70 miles airline north-northeast of Los Angeles. Fieldwork was done during the spring, autumn, and winter months from 1952 to 1955 and during short intervals from 1956 to 1959. This report and map represent part of a geologic investigation by the U.S. Geological Survey of the known and potential deposits of borate minerals in the southern California desert regions. The primary purpose of this report and the geologic map is to provide a geologic background for the exploration for possible concealed borate deposits in the western Mojave Desert, based upon (1) the geology of the known deposits of these minerals within the region, (2) the areal geology of the region, and (3) the subsurface geology as determined insofar as possible from logs of exploratory test holes, mine workings, and geophysical data. Available logs of test holes so used, including those drilled for petroleum and some for water, and of borate mine shafts are summarized in tabulated form; the gravity geophysical data are published separately. The resulting geologic data are of use for many secondary purposes, such as exploration for other commodities, including petroleum and gas, and in evaluation of ground-water conditions."