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.

Field studies of the Cambrian rocks in northwestern Montana were begun in 1931 when C. H. Clapp and the writer were mapping the areal geology of the Coopers Lake quadrangle for the Montana Bureau of Mines and Geology. At that time only two sections of these Cambrian rocks had been published, one from the Dearborn River (Walcott, 1908c, p. 200–203), and the other from Gordon Mountain (Walcott, 1917b, p. 16–19). It soon became clear that, with the possible exception of the Flathead and Wolsey, the Cambrian formations in the Three Forks (Peale, 1896), Little Belt Mountains (Weed, 1899), and Philipsburg (Calkins and Emmons, 1915) quadrangles had no readily recognizable lithologic equivalents in northwestern Montana. Fossils had not been found, and detailed sections of these rocks were unmeasured. Consequently, further mapping could not be done accurately until at least the major details of the Cambrian stratigraphy were understood. Data for the solution of at least six important problems were needed. (1) Which Cambrian series are represented in northwestern Montana? (2) What is the total thickness of the Cambrian rocks, and what is the thickness of those in each series represented? (3) Are unconformities or disconformities present, and if so, what is the magnitude of each? (4) What are the important lithologic characteristics of the Cambrian sediments within the area? (5) What are the persistent lithologic units which can be used as mappable formations? (6) What faunas do the sediments contain? During the summer of 1932, nine detailed sections were. . .