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Transactions. 



an uneven foundation of subaerial erosion. This point has been too gene- 

 rally 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 

 natu.re of the contact of a reef and its foundation should be carefully 

 observed, whether the reef be at sea-level or elevated above it. 



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Fia. 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 w^ith 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 bav-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, \ . 



Pre-submeiyeuce Period. — The duration of the pre-submergence period of 

 subaerial erosion should be estimated as short, long, or very long, by com- 

 paring the actual form of the visible land-surface with its inferred initial 

 form, due allowance being made for rock-resistance. In the case of dis- 

 sected 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 deter- 

 mine whether the cycle of erosion was in an early, middle, or late stage of 

 its progress when it was interrupted by submergence. 



