About a year ago my friend Professor Dobbie, of the University College of North Wales, drew my attention to the remarkable experiments of Sir W. Roberts-Austen (whose premature death we must lament as a very great loss to science) on the diffusion of solid metals, suggesting that they might have some geological application. The phenomena referred to in this paper, in which I have been very keenly interested ever since my work as a Geological Surveyor in Eastern Sutherland, occurred to my mind at the time as a probable case; but after some reflection certain difficulties began to appear, and I put the subject aside for a while. The very suggestive address of General McMahon to the Geological Section of the British Association at Belfast has reawakened my interest; and it seems to me worth while to put forward some considerations on the matter, somewhat speculative indeed, but which may perhaps be of service in stimulating research on a fascinating though difficult subject.