12 Transactions. 



modern coastal plain to-day ; for otherwise it is difficult to understand 

 why coral reefs should not have been formed there and have prevented 

 the cutting of the high cliffs. 



cuffed Volcanic Islands. — It has been suggested above that the shore- 

 lines of volcanic islands may be regarded as shore-lines of emergence, 

 particularly if the island is largely composed of loosely compacted volcanic 

 ash ; for such a shore-line will be of comparatively simple outline, without 

 pronounced salients or embayments, and the detritus washed down its 

 slopes by its streams, added to that cut by the waves along the shore, 

 v.'iU soon form a continuous beach, extending seaward in a sheet of loose 

 sediments, on which reef-building corals cannot attach themselves (Davis, 

 1916b). Under such conditions the island will be continuously attacked 

 by the waves, cliffs will be cut around its shore while valleys are eroded 

 in its slopes, and if the island stand still long enough it will be com- 

 pletely truncated. Even then it may be difficult for corals to find a 

 firm foundation for their growth until nearly all the loose detritus is 

 swept off the surface of truncation. 



According to Admiral AVharton, atolls were supposed to have been 

 built up around the margin of truncated volcanic islands, no change of 

 sea-level and no subsidence of the island being postulated. Darwin had 

 previously considered this possibility and rejected it, because the resulting 

 lagoons would be too shallow. Ac rding to Daly, atolls are supposed to 

 have been built up on volcanic platforms that were abraded while the 

 ocean was lowered and reef-building corals were killed, during the Glacial 

 period. The best test of these suppositions involves a series of borings 

 along the diameter of an atoll to a depth of 50 or more fathoms below 

 present sea-level : the elevated atolls of the Loyalty Islands are to be 

 recommended for such examination. 



If it be true, as above suggested, that still-standing volcanic islands 

 may, in the absence of protecting reefs, be cut away by the sea, a number 

 of examples in different stages of abrasion should be found in the coral 

 seas of to-day. Reunion is the best example of the kind, stiU in process 

 of abrasion, that has come to my attention. Tahiti is an equally good 

 example, but it has been somewhat submerged, and its shores are now 

 defended by coral reefs, as will be more fully described below. Tutuila, 

 in Samoa, and the Marquesas Islands probably, as noted above, belong 

 to this series, biit their place cannot be safely determined at present. 

 Most volcanic islands in the coral seas are surrounded by barrier reefs, 

 and their shore-lines are not cliffed. It is very desirable that aU islands of 

 the coral seas should be examined with the points here set forth in mind. 

 The brief accounts now available of many such islands do not suffice to 

 determine what stage of erosional and abrasional evolution they have reached. 



Time since Submergence. — The headlands of coasts of submergence in 

 temperate latitudes, not being defended by coral reefs, are vigorously 

 attacked by storm waves ; thus a cliff is formed rising high above sea- 

 level, and a platform lying a little below sea-level. Coasts of submergence 

 in the coral seas are as a rule fronted by barrier reefs or bordered by 

 fringing reefs ; hence they do not generally show the strongly cliffed spur- 

 ends that characterize similar coasts in temperate latitudes. True, the 

 spur-ends of such coasts are often cut off in low bluffs, B (fig. 4), 10 ft. to 

 50 ft. in height, forward from which one may see low-tide rock platforms 

 30 ft. to 100 ft. in breadth before one reaches the fringing reef, F, that is 

 ordinarily found in such situations. 



