In a letter to Nature for 14th May, 1874, Mr. Belt has expressed his belief that the presence of shells in glacial deposits, at whatever elevation they may be found, does not necessarily constitute a proof that the land has been depressed to that extent relatively to the level of the sea; but that in such cases as those of the drifts of the basin of the Irish Sea the shells occur in their present positions because they were thrust thither out of the bed of the sea by the ice-sheet which was advancing from the North.