The author, having observed in Iceland some remarkable intermixtures of lava with volcanic tuff; and, more recently, on the coast of East Lothian, a somewhat similar intermixture of masses of greenstone in trap-tuff, conceived the idea that cavities existed underneath the crust of the earth, or deeply seated within it, containing water exposed to heat, and the agitation of which, occasioned by the production of steam, had reduced the materials of trap-tuff to the state in which we find them, which had been afterwards erupted at the bottom of the sea, or forced between the beds of superincumbent rocks.