554 W. M. DAVIS SUBSIDENCE OF REEF-ENCIRCLED ISi^ANDS 



through a considerable range; but the in wash and the local supply of 

 secliments will tend to lessen the depth that uncounteracted subsidence 

 would bring about. Furthermore, the inwash of sediments from encir- 

 cling reefs will be, as above noted, more effective in aggrading small 

 lagoons than large ones, in whatever way the reefs are formed. More- 

 over, the eifect of a rapid subsidence in deepening a lagoon will be lessened 

 by the more active inwash over the reef that will then be for a time partly 

 submerged; and converse^ the effect of a stationary period in shoaling 

 the lagoon will be lessened by the obstruction to inwash caused by the 

 sand islands which are formed on the broadened reef during such a period. 

 Hence it is reasonable to expect that the average depths of a good number 

 of lagoons, which are divided into classes according to their breadth, 

 should vary in indirect proportion to the lagoon breadth, whether the 

 reefs have been formed by upgrowth during the subsidence of their foun- 

 dations or by upgrowth in a rising ocean over non-subsiding foundations ; 

 but it should also be expected that individual lagoons of the same breadth 

 should, according to the Glacial-control theory, have closely similar 

 depths, while, according to the subsidence theor}', their depths might vary 

 through a considerable range. 



It may be stated at once that the facts agree better with the latter than 

 with the former expectation : thus Ringgold and l^orth Argo atolls, in the 

 Fiji group, have the same breadth of about 5 nautical miles, although the 

 first is much longer than the second; but the maximum lagoon depth of 

 the first is 48 fathoms, although the breadth of its reef, nearly a mile, 

 suggests a considerable aggradation of the lagoon fioor, while the maxi- 

 mum depth of the second is only 21 fathoms. Again, Budd reef, west of 

 Ringgold atoll, in northeastern Fiji, an almost-atoll inclosed by an unu- 

 sually narrow reef rim, measuring 12 by 6 miles, has a maximum depth 

 of 47 fathoms ; two lagoons imperfectly inclosed by barrier reefs near the 

 islands of Rambi and Taviuni, not far to the west of Budd reef, have 

 depths of 47 and 49 fathoms, thus strongly suggesting recent and rapid 

 subsidence; while Ngele Levu, to the northeast, measuring 13 by 7 miles, 

 and therefore a little larger than Budd reef, has a maximum depth of 

 only 10 fathoms; and the maximum depth of the much larger Great Argo 

 atoll, farther south, is no more than 36 fathoms, although it measures 22 

 by 8 miles in diameter. Other examples of the same nature might be 

 adduced, but lack of space forbids their citation here. 



In addition to the examples of atolls of similar breadth and unlike 

 depth, instanced above, the depths of the Maldive lagoons in the northern 

 Indian Ocean west of India have an interest in the present connection. 

 They vary from 20 or 30 fathoms in the northern members to 40 or 48 



