528 W. M. DAVIS SUBSIDEXCE OF REEF-ENCIKCLED ISLANDS 



greater width or greater rock-bottom depth than can have been eroded 

 while the ocean was lowered in the Glacial period ; and that the geological 

 history of the islands shall, as a rule, indicate recent stability and shall 

 not discountenance too strongly a long period of stability for neighboring 

 atolls and banks. 



The other theory does not set any small or uniform limit to the depth 

 of lagoons and banks or to the depth of platforms along island borders ; it 

 demands that such platforms shall be, as a rule, backed by an embayed 

 and not clift coast, but the embayments may be of any dimensions, and 

 the geological history of the islands may indicate an}^ degree of stability 

 or disturbance. The chief features of this theory are that when a coast 

 stands still reefs shall grow outward from it; that when subsidence does 

 not take place too rapidly it shall be accompanied by a corresponding 

 reef upgrowth; and finally that when subsidence takes place more rap- 

 idly it shall for a time at least cause the submergence of preexistent reefs. 

 Rapid subsidence should therefore produce drowned atolls or submarine 

 banks in the open sea, while along island coasts it would change surface 

 reefs into submerged platforms fronted with incomplete barriers and 

 backed with fringing reefs of a new generation on an embayed and little 

 clift shoreline. The submarine banks of the China Sea and the fringing 

 reefs and submerged platforms of the Philippines give unexpectedly 

 strong corroborating testimony for the second theory. 



SUBMARINE BANKS IN THE PACIFIC AND INDIAN OCEANS 



Fifteen or more submarine banks of moderate or small size occur north 

 of the Fiji group, as will be more fully stated below. Several larger 

 banks occur around the Tonga Islands ; their variations in depth strongly 

 suggest slanting subsidence. Apart from these examples, few others are 

 known in the vast area of the central Pacific. On the other hand, the 

 central Indian Ocean, much less provided with atolls than the central 

 Pacific, contains a good number of extensive submarine banks, brief ac- 

 count of which will be given in a later section on the unequal depths 

 of banks and lagoons. Submarine banks are therefore not distributed 

 equally through the coral seas; they occur in groups, as if controlled by 

 some local process. When all factors of the problem are considered, the 

 most probable process of the kind is accelerated regional subsidence. 

 Submarine banks may therefore be regarded with good reason as drowned 

 atolls. 



The group of banks north of the Fiji Islands gives, to my reading, 

 remarkable support to this view. It occupies a region measuring about 

 200 miles nortli-south by 800 miles east-west, between the Samoa Islands 



