A gravity survey and a survey of rock densities have been carried out over an area of two thousand square miles near Parry Sound. A closed positive Bouguer gravity anomaly of thirty milligals was delineated, and a considerable variation was found to exist in the densities of pre-Cambrian gneisses. In most previous interpretations of gravity over the Canadian Shield the gneisses have been assumed to possess a uniform density and anomalies have been attributed to changes in the thickness of horizontal crustal layers. In this paper it is shown that the Parry Sound anomaly can be explained in terms of structures within the crust taking the form of projections downward of the density variations found at the surface. The postulated structure is a nearly circular basin of dense gneisses. The shape is reasonable and agrees with such geological evidence as is available.

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