I. Introduction. The object of this paper is to describe a case of unconformity, accompanied by a certain amount of horizontal thrusting, which occurs in the Coal-Measures exposed—chiefly at low water—on the coast of Northumberland at the foot of the Whitley cliff's, and is traceable for upwards of a mile. The section studied stretches from the bold headland formed by the Table Rocks, northwards almost as far as the Brier-Dene Burn. At the southernmost end—that is, at the Table Rocks—the plane of erosion in question dips and disappears beneath the sea. Near the mouth of Brier Dene it is permanently covered and concealed by Drift and beach-deposits. The Table Rocks consist of a series of ledges of sandstone, bedded almost horizontally, and presenting vertical cliff-faces to the sea. A little north of this the cliff becomes overhanging, owing to the undercutting by the waves of the soft beds which come to light at its base. Near the spot marked M on the sketch-map (fig. 1, p. 532), the sandstone-cliff, which so far varies from 30 to 40 feet, gradually lessens in height, and is overlain by a considerable thickness of Boulder-Clay. Up to this point also the foreshore is free from cover, though strewn with boulders, and in places obscured by talus-heaps burying the cliff-foot, and the strata comprised in it can be well seen at low tide. Farther north, however, the boulder-strewn outcrops give place to a broad sandy beach (Whitley Sands proper), which is broken, near low-water-mark only,