Here then we have a stratum which must have been once uppermost on the surface of the dry land, though one part of it is now immersed under the sea, and another covered with earth, to the depth of sixteen feet. A change of level in the sea itself will not explain these appearances: they can only be explained by supposing the whole tract of land to have subsided, which is the hypothesis adopted by the author of the description in the Transactions, M. CORRIA DE SERRA; the subsidence, however, is not here understood to arise from the mere yielding of some of the strata immediately underneath, but is conceived to be a part of that geological system of alternate depression and elevation of the surface, which probably extends to the whole mineral kingdom. To reconcile all the different facts, I should be tempted to think, that the forest which once covered Lincolnshire, was immersed under the sea by the subsidence of the land to a great depth, and at a period considerably remote; that when so immersed, it was covered over with the bed of clay which now lies on it, by deposition from the sea, and the washing down of earth from the land; that it has emerged from this great depth till a part of it has become dry land; but that it is now sinking again, if the tradition of the country deserves any credit, that the part of it in the sea is deeper under water at present than it was a few years ago. This might also serve to reconcile, in some measure, the phenomena of this submarine forest with the appearances which indicate an extension of the land on the coast of Lincolnshire. Indeed the extension of the land is no direct proof, either of its own elevation, or of the depression of the sea, as we may conclude from the instance of Ravenna already mentioned. 401. We have concluded from the facts stated above, that the level of the sea rises in the Mediterranean, and sinks in the more northern latitudes; and thence some have suspected, that the level of the sea had in general a tendency to rise towards the equator, and to sink towards the poles. This is the notion of Frisi, as has been already remarked, and he suggests, that this rise of the sea may be owing to a slight acceleration in the earth's diurnal motion. But there are facts which show, that between the tropics the relative level of the sea and land has sunk, and is lower at present than it was at some former period, probably not extremely remote. The opinion of Frisi, therefore, is unsupported by observation, and, as has been already shown, cannot be justified from theory.