The geometry of the Saint George Batholith has been investigated along three detailed (1–2 km station spacing) north–south gravity traverses surveyed in conjunction with a geothermal study. Modelling of corresponding gravity profiles indicates that the batholith is 6.6, 4.3, and 7.0 km thick along the western, central, and eastern traverses, respectively. The western profile crosses the buried western granitic extension of the batholith and the exposed mafic Bocabec complex, modelled as a sheet 200–400 m in thickness. The central profile crosses the western half of the exposed batholith, and the eastern profile crosses the eastern half near a borehole used for heat-flow measurements.Two other gravity profiles, defined by detailed and regional (average station spacing about 5 km) observations, were also modelled. One trends northeast across the eastern margin of the batholith and passes directly through the borehole site, at which the batholith is modelled as 6.5 km thick. Here, geothermal calculations suggest that the thickness of radiogenic granite lies in the range 1.4–3.3 km. The large discrepancy between gravity and geothermal estimates supports suggestions that radiogenic elements have been concentrated in the upper part of the batholith, probably by circulating groundwater. The second profile crosses the Pleasant Ridge and Sorrel Ridge granitic intrusions, located northwest of the batholith. Modelling indicates that these intrusions are rooted in the buried roof of the Saint George Batholith at depths of about 1.1 km.Relatively high levels of the gravity field south of the batholith are attributed to dense Precambrian rocks, which are modelled as forming the wall rocks along much of the southern margin of the batholith. The walls in places are steep, suggesting control by major faults that step the Precambrian rocks down to the north, where they apparently floor the batholith and (or) lower Palaeozoic rocks at a depth of about 7.6 km. This picture of block faulting is supported by interpretation of a regional gravity profile located northeast of the Saint George Batholith and running northwest from the Bay of Fundy to the Miramichi Massif.