16 Transactions. 



in the succession and the vahie of the movements and pauses recorded 

 by the physiographic features of the islands within the reefs, as has already 

 been briefly noted in an earlier paragraph, and as will be set forth more 

 fully in a later one. / 



Origin op Lagoons. / 



According to Darwin's theory of intermittent subsidence, the lagoon 

 occupying the depression or " moat " between an island slope and an 

 upgrowing barrier reef is more or less completely filled by the outwash of 

 detritus from the central island, by the in wash of debris from the reef- 

 face, and by the accumulation of locally formed organic material. Recent 

 observations warrant the assignment of a large value to the last-named 

 process, which is further important because it is just as effective in 

 aggrading a large lagoon as a small lagoon. If subsidence cease for a long 

 period, the lagoon may be converted into a reef plain, as suggested in a 

 preceding section. 



This view of the relation between barrier reefs and their lagoon is the 

 very opposite of that implied in Murray's theory of outgrowing reefs on 

 non - subsiding foundations, for it is there postulated that lagoons are 

 formed by the solution of the outgrowing reef along its inner border. 

 There can be no question that sea-water flows into lagoons in sufficient 

 quantity to dissolve away a large volume of limestone ; but, as far as 

 observational evidence goes, the loss thus occasioned is far overbalanced 

 by the supply of new detritus from the various sources above mentioned. 



According to Murray's solution theory, the inner slope of a barrier reef 

 should consist of ragged and decaying limestone, and the lagoon-fioor should 

 be covered with insoluble silts (Davis, 1914, p. 641) ; but as a matter of 

 fact the inner slope of barrier reefs usually consists of white coral sand, 

 washed in from the outer reef-face ; and the lagoon-floor is covered with 

 accumulating calcareous deposits, except that near the deltas of large 

 streams inorganic deposits preponderate. Detailed observations should be 

 made by dredging in lagoons in order to test the generality of the above 

 statements. Atoll lagoons deserve as much attention as barrier-reef lagoons 

 in this phase of the problem. 



Attention may here be called to Vaughan's view that many barrier reefs 

 are built upon platforms which were produced by other than coral-reef 

 agencies. Inasmuch as the supposed platforms beneath sea-level reefs are 

 not open to direct observation, their existence as structures independent of 

 reef-forming agencies is for the present only an inference. Any observable 

 facts that bear on this aspect of the problem should be carefully noted. 

 Among such facts pointed out by Vaughan, three may be noted : the first 

 is that the exterior profile of most reefs shows a change from a moderate 

 slope to a steep pitch at a depth of about 40 fathoms ; the second is that 

 reefs ocasionally stand a short distance back from the outer margin of a 

 40-fathom bench ; the third is that where reefs are breached, as frequently 

 happens on their leeward side, the lagoon-floor or " platform " continues. 



It may, however, be reasonably urged that none of these facts neces- 

 sarily leads to the conclusion that the production of a platform by some 

 agency independent of reef-formation preceded the formation of the present 

 reefs. As to the change from a gentle slope to a steep pitch in the exterior 

 profile, many observers, including Darwin, Murray, and Gardiner, are agreed 

 that this is the result of wave-action on reef detritus at present sea-level : 

 the exterior slope of a reef is, in effect, a small " reef shelf," corresponding 



