The position of the coast-line of Egypt has been known for much more than two thousand years. In that time it has made, by reason of the detritus brought down by the Nile, a distinctly-marked encroachment on the Mediterranean. But all Lower Egypt has had a similar origin. The coast-line near the mouth of the Mississippi has been well known for three hundred years, and during that time has scarcely made a perceptible advance on the Gulf of Mexico; but there was a time when the delta of that river was at St. Louis, more than seven hundred miles from its present position. In Egypt and in America--in fact, in all countries--the rivers have been inch by inch prolonging the land into the sea; the slowness of their work and the vastness of its extent satisfy us that we must concede for the operation enormous periods of time. To the same conclusion we are brought if we consider the filling of lakes, the deposit of travertines, the denudation of hills, the cutting action of the sea on its shores, the undermining of cliffs, the weathering of rocks by atmospheric water and carbonic acid. Sedimentary strata must have been originally deposited in planes nearly horizontal. Vast numbers of them have been forced, either by paroxysms at intervals or by gradual movement, into all manner of angular inclinations. Whatever explanations we may offer of these innumerable and immense tilts and fractures, they would seem to demand for their completion an inconceivable length of time. The coal-bearing strata in Wales, by their gradual submergence, have attained a thickness of 12,000 feet; in Nova Scotia of 14,570 feet. So slow and so steady was this submergence, that erect trees stand one above another on successive levels; seventeen such repetitions may be counted in a thickness of 4,515 feet. The age of the trees is proved by their size, some being four feet in diameter. Round them, as they gradually went down with the subsiding soil, calamites grew, at one level after another. In the Sydney coal-field fifty-nine fossil forests occur in superposition. Marine shells, found on mountain-tops far in the interior of continents, were regarded by theological writers as an indisputable illustration of the Deluge. But when, as geological studies became more exact, it was proved that in the crust of the earth vast fresh-water formations are repeatedly intercalated with vast marine ones, like the leaves of a book, it became evident that no single cataclysm was sufficient to account for such results; that the same region, through gradual variations of its level and changes in its topographical surroundings, had sometimes been dry land, sometimes covered with fresh and sometimes with sea water. It became evident also that, for the completion of these changes, tens of thousands of years were required.