332. In contemplating such coasts as these, when we go back to the time when the rivers ran upon a level as high as the highest of the cliffs on the sea shore, we must suppose, that the land then extended many miles farther into what is now occupied by the sea. When at Plymouth, for instance, the Tamer and the Plym flowed on the level of Mount Edgecombe or of Staten Heights, if the rivers ran with a moderate declivity into the sea, the coast must have advanced many miles beyond its present line. Thus the land, when higher, was also more extended, and the limits of our island in that ancient state, were doubtless very different from these by which it is at present circumscribed. If with the same views we consider any other of the bold coasts which the map of the world presents us with, we shall quickly remark, that wherever a deep intersection of the sea is made into the land, as on the western shores of our own island, or on those of Norway, a river runs in at the head of it, and points out by what means such inlets are formed, viz. by the united powers of the sea and of the land, the waters of the latter having opened the way by which those of the former have penetrated so far into the country. 333. It is not meant assuredly to deny the irregularities of the sea coast, as it may have originally existed; these irregularities no doubt determined the initial operations of that waste and decay, by which, in process of time, they were themselves entirely effaced. The line of our coasts may be compared to one of those curves, which are sometimes treated of in the higher geometry, where the ordinates are functions, not only of, their abscissæ, but also of the time elapsed since a certain epocha. The form of the curve at that epocha, or when the time began to flow, corresponds to the original form of the sea coast, on its emerging from the ocean, and before the powers of wasting and decay had begun to act upon it. To speak strictly, the original figure, in both cases, influences all the subsequent; but the farther removed from it in point of time, the less is that influence; so that, in physical questions, and for the purpose of such approximations as suit the imperfection of our knowledge, the consideration of the original figure may be wholly left out. NOTE XVII. § 105. _Remains of Decomposed Rocks._ 334. THE plain of Crau was the _Campus Lapideus_ of the ancients; and, as mythology always seeks to connect itself with the extraordinary facts in natural history, it was said to be the spot where Hercules, fighting with the sons of Neptune, and being in want of weapons, was supplied from heaven by a shower of stones: hence it was called _Campus Herculeus_.