The mapping of sub-drift topography is an important problem in this country and one which, under suitable circumstances, lends itself readily to solution by the method of gravitational survey with the Eötvös Torsion Balance. It has been pointed out in previous papers that structures such as anticlines, synelines, faults, and dykes may be determined by a linear traverse with the balance transverse to the strike; but in the case of buried topographic features a network of observations with subsequent plotting of the gradients and isogams, i.e. lines of equal gravity, is necessary to determine even approximately the shape and dimensions of rock-masses underlying a thick covering of drift.