Davis. — Signifiaaii Fta/iires of Rft f-lioi(h i-rd ('mists. 'J 



Thus the submergence of the Queensland coast, in association with 

 wliich the Great Barrier Reef of Australia and the discontinuous fring- 

 ing reef in the broad lagoon were formed, did not occur until the rather 

 resistant rocks which there prevail had been reduced to subdued forms of 

 late maturitv or even to the low relief of old age. The same may be said of 

 much of the south-western coast of New Caledonia, except that the rocks 

 there present near the shore are for the most part weaker than those of 

 Queensland. In both these examples the pre-submergence period of sub- 

 aerial erosion must have been of long duration. 



In view of these various considerations it is evident that careful obserxa- 

 tion should be made of reef-bordered coasts from a physiographic as well as 

 from a geological ])oint of view, in order to determine whether the reefs 

 have been formed in association with the submergence or the emergence of 

 their foundation. It is also important that reef-free coasts in the coral seas 

 should be similarly observed, in order to discover the conditions that do 

 not favour reef-forniation. 



Rate of Submergence. — The ordinary statement of Darwin's theory of 

 coral reefs implies that the rate at which reef-foundations have been sub- 

 merged as a result of their own subsidence must not be greater, but may 

 be less, than the rate of reef-upgrowth ; and this has been held to be an 

 improbable condition. Darwin's own statement of the problem made no 

 such limitation as to the rate of subsidence, except where barrier reefs and 

 atolls are actually found. For those reefs he stated that relatively rapid 

 subsidences of small amount alternating with long stationary pauses pro- 

 babl}' represent the ordinary succession of events, and he believed that the 

 average rate of submergence thus determined was not in such cases faster 

 than the rate of reef-upgrowth. 



This seems to hold true for the greater part of the open Pacific, where 

 atolls and barrier reefs prevail, even though the submergence due to insular 

 subsidence there has been accelerated by a sub-recent rise of ocean-level 

 during the melting of the Pleistocene ice-sheets — a matter which has come 

 into importance in recent years, as will be shown in more detail below. 

 But exception to this statement is needed for an area to the north of the 

 Fiji Group, where fifteen or more submarine banks, apparently submerged 

 reefs or " drowned atolls,"' have been discovered since Darwin's time ; and 

 also for the region of the Tonga Islands, where extensive submarine banks 

 occur. In both these regions of the mid-Pacific, and in a few others, sub- 

 mergence appears to have taken place at a faster rate than reef-upgrowth. 

 They thus corresjjond to a large part of the Indian Ocean, where submarine 

 banks, ap})arently " drowned atolls,'' prevail, as Darwin clearly understood. 



Darwin on Fringing Reefs. — Furthermore, although Darwin regarded 

 most fringing reefs as having been formed on stationary or on rising coasts, 

 he clearly understood that rapid subsidence might drown earlier-formed 

 reefs, whereujjon the reefs that would grow on the new shore-line would 

 be of the fringing class, as noted above. The statement of this point on 

 page 124 of his Coral Reefs (1842) deserves attentive reading. True, inas- 

 much as Darwin did not understand that embayed shore-lines and uncon- 

 formable reef contacts around spur-ends are sure signs of submergence, he 

 discovered no examples of fringing reefs of this kind in the records that he 

 studied, and all the fringing reefs on his chart are classed as occurring on 

 stationary or rising coasts. 



But his deductive expectation may now be confirmed, for the Austral- 

 asian and other archipelagoes contain numeroiis examples of fringing reefs 



