Gravity studies have been useful in volcanic regions where buried geologic features are partly concealed by overlying lava flows. In the Snake River Plain, Idaho, recent geophysical investigations by the U. S. Geological Survey have revealed a marked contrast between the structures of the western and eastern parts of the plain. The western plain is a deep graben, whereas the eastern plain is a downwarp or shallow graben. Depth-estimation formulas and numerical integration techniques have been used to interpret the gravity anomalies. The gravity pattern in the east implies relatively shallow subsurface elongate mass distributions whose long axes seem to trend parallel to the axes of the adjoining mountains. Residual anomalies suggest that the basalt may be only a few hundr d feet thick at places in the eastern plain, and that the total accumulation of lava has been much less in the east than in the west. The U. S. Geological Survey is continuing its study of this region with the aid of seismic-refraction data obtained from both chemical and nuclear explosions.

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