The geometry of the Blatchford Lake Intrusive Suite, a complex of alkaline igneous rocks on the north shore of Hearne Channel, Great Slave Lake, Northwest Territories, was investigated by a detailed gravity survey (1-2 km station spacing) along two east-west traverses. Modelling of the resulting gravity profiles indicates that the granitic and syenitic rocks in the eastern part of the complex have the form of a thin tabular body; the maximum thickness of these rocks is modelled as 1 km. The gravity data suggest that mafic rocks in the western part have a deep root, and that a related, sill-like branch may extend under the granitic and syenitic rocks for as much as one-half of the area of the complex.