Results of Gravity Measurements in Southern California

The various types of instruments for gravity measurements are briefly discussed. The theory of an inverted pendulum of the "Holweck-Lejay" type with an elastic mounting at its base is given, and an instrument of this design, ninety times as sensitive to differences in gravity as an ordinary pendulum, is described. The theory of the interpretation of gravity data is discussed. The results of gravity observations along a profile across the Los Angeles Basin and San Gabriel Mountains are given. From this data together with geologic data and that obtained from a reflection seismograph survey, a hypothetical structure section along the profile is constructed. The outstanding features of this section are the presence of a basin of Tertiary sediments about thirty thousand feet deep, and the increasing predominance of relatively lighter rocks under the San Gabriel Mountains and Mojave Desert. This last feature is in accord with the concept of "regional isostatic compensation."