Littoral Denudation.—Part of the action of the waves between high and low watermark must be included in subaërial denudation, more especially as the undermining of cliffs by the waves is facilitated by land-springs, and these often lead to the sliding down of great masses of land into the sea. Along our coasts we find numerous submerged forests, only visible at low water, having the trunks of the trees erect and their roots attached to them and still spreading through the ancient soil as when they were living. They occur in too many places, and sometimes at too great a depth, to be explained by a mere change in the level of the tides, although as the coasts waste away and alter in shape, the height to which the tides rise and fall is always varying, and the level of high tide at any given point may, in the course of many ages, differ by several feet or even fathoms. It is this fluctuation in the height of the tides, and the erosion and destruction of the sea-coast by the waves, that makes it exceedingly difficult for us in a few centuries, or even perhaps in a few thousand years, to determine whether there is a change by subterranean movement in the relative level of sea and land. We often behold, as on the coasts of Devonshire and Pembrokeshire, facts which appear to lead to opposite conclusions. In one place a raised beach with marine littoral shells, and in another immediately adjoining a submerged forest. These phenomena indicate oscillations of level, and as the movements are very gradual, they must give repeated opportunities to the breakers to denude the land which is thus again and again exposed to their fury, although it is evident that the submergence is sometimes effected in such a manner as to allow the trees which border the coast not to be carried away. Inland Sea-cliffs.—In countries where hard limestone rocks abound, inland cliffs have often retained faithfully for ages the characters which they acquired when they constituted the boundary of land and sea. Thus, in the Morea, no less than three or even four ranges of cliffs are well-preserved, rising one above the other at different distances from the actual shore, the summit of the highest and oldest occasionally attaining 1000 feet in elevation. A consolidated beach with marine shells is usually found at the base of each cliff, and a line of littoral caverns. These ranges of cliff probably imply pauses in the process of upheaval when the waves and currents had time to undermine and clear away considerable masses of rock.