The theory of the origin of continents and ocean-basins developed during the last third of a century, chiefly by Prof. Dana, and commonly known as Prof. Dana’s theory, is now accepted by many geologists. The main points in this theory, as gathered from the latest expression of Prof. Dana’s views, are the following:1— The earth, superficially at least, is, and was originally, before it had a solid crust, of unlike composition on different sides. This heterogeneity caused a corresponding difference in heat-conductivity. The more rapidly conducting areas cooled fastest and were the first to become covered with a solid crust. Solidification is attended by contraction; and therefore the newly formed crust must have been heavier than the liquid immediately beneath it. As a consequence, it broke up and sank until it reached a liquid stratum of the same specific gravity as itself; and afterwards the process of crusting and sinking went on until a solid crust was built up from this point to the surface. Through the continued escape of heat this primitive crust is thickened, and is still thickening by additions to its lower surface. These first formed portions of the crust became, and will always continue to be, the continents. The remainder of the earth’s surface was still liquid, after the solidification of the continental areas was well advanced; and, of course, as long as it continued liquid, its surface was level with that of the crust-areas. Finally, it became the theatre of a similar process of crusting and sinking, and at last permanently froze over. Now the main point is that the contraction of this inter-continental crust during its formation caused its surface to sink below that of the continents; and the depressions thus developed became the future ocean-basins, which, like the continents, are necessarily of a permanent character. Indeed, it is a plain deduction from Prof. Dana’s theory that the existing continents and oceans are as old as the earth’s crust; and that during the course of geological time the continents have become constantly wider and the oceans deeper.