an uneven foundation of subaerial erosion. This point has been too generally overlooked, although it is of the highest theoretical importance. Its converse is of practical value: reefs that rest unconformably on surfaces of subaerial erosion must have been initiated by submergence. Hence the nature of the contact of a reef and its foundation should be carefully observed, whether the reef be at sea-level or elevated above it. Fig. 1. Amount of Submergence.—The amount of submergence that an embayed coast has suffered is not well indicated by the depth of its embayments, for they may be much filled with sediments; the amount is better inferred by drawing a true-scale cross profile, as at P, fig. 2, of the spurs that enclose a bay-mouth, and continuing their slopes with decreasing declivity below sea-level until they meet. The visible cross-section of the valley above the Fig. 2. bay-head at Q should be taken as indicating the pattern of the submerged cross-section at the bay-mouth, P. The measure of submergence thus gained is only a minimum value, for, as shown in fig. 2, the depth of the submerged valley near the bay-mouth may be only about half the depth of the original valley-mouth, V. Pre-submergence Period.—The duration of the pre-submergence period of subaerial erosion should be estimated as short, long, or very long, by comparing the actual form of the visible land-surface with its inferred initial form, due allowance being made for rock-resistance. In the case of dissected and embayed volcanic islands this comparison may often be made without much difficulty. On the coasts of continents and of continental islands the comparison may not be so easily instituted, but an attentive examination of the form of the coastal slopes will usually suffice to determine whether the cycle of erosion was in an early, middle, or late stage of its progress when it was interrupted by submergence.