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Regional gravity studies in southwestern Montana show a –40 mgal anomaly associated with the Boulder batholith. The anomaly pattern indicates that the batholith is approximately symmetrical about a northeast-trending axis which is parallel to the length of the present intrusive outcrop. The axis of the gravity low, however, lies approximately 25 km northwest of the present outcrop axis of the Boulder batholith but would coincide with the center of the intrusive area if the surficial covering of volcanics were removed.

Calculations made to determine the subsurface shape and lateral extent of the batholith indicate that the bottom of the anomalous mass is probably located at depths less than 15 km and is concave upward. Six configurations are postulated for the shape of the intrusive.

The eastern, northern, and northeastern limits of the Boulder batholith are well defined by steep gravity gradients which parallel the margins of the batholith outcrop. The western extent of the gravity low is indefinite. Therefore, it is postulated that intrusive rocks underlie a considerable area to the west of the outcrop of the Boulder batholith, in the vicinity of Philipsburg, Montana. There is no geophysical evidence for a sharp structural delineation between the Idaho and Boulder batholiths. The relation of the outlying stocks to the Boulder batholith is discussed on the basis of the geophysical evidence.

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