AGGRADATION OF LAGOON FLOORS 553 



kinds of change of level, the theory of subsidence appears to me more 

 probably correct than the Glacial-control theory, in which, although ele- 

 vation is accepted wherever high-standing reefs occur, subsidence is re- 

 garded as very exceptional. Changes of ocean level vs^ere mentioned very 

 briefly by Darwin : the need of combining the oscillations of ocean level 

 during the Glacial period with submergence due to subsidence of reef 

 foundations is, to my mind, the most important contribution of the Gla- 

 cial-control theory ; but, as far as I have been able to interpret the history 

 of coral reefs, oscillations of ocean level have been of less importance than 

 island subsidence. Certain special consequences of intermittent subsi- 

 dence must next be considered. 



THE SMOOTHNESS AND DEPTH OF LAGOON FLOORS 



The smoothness of lagoon floors, whatever the form of their buried rock 

 foundation, was ascribed by Darwin to aqueous deposition (26). This 

 finds confirmation in Gardiner's detailed studies of the Maldive lagoons, 

 of which he says : "It is only in a few protected situations, where the depth 

 is as great as 40 fathoms or more, that the lagoon bottom appears not to 

 be churned up by the currents and waves. In heavy weather the lagoon 

 water is almost milky, and floating surface nets are almost useless on 

 account of the enormous amount of mud in suspension. The total amount 

 of mud that passes out of the lagoon in the water is enormous" (1903, 

 210). My own limited experience in the Pacific included two examples 

 of rough weather — one in Fiji, one ojff the Queensland coast in the lagoon 

 of the Great Barrier reef — during which the water of inclosed lagoons 

 was turbid with suspended sediments. 



It therefore seems reasonable to follow Darwin in this matter and not 

 Wharton, who ascribed the smoothness of lagoon floors, some of which 

 have very uniform depths at 25 fathoms, to abrasion by waves, at present 

 sealevel (1897, 392) ; still less can I follow Daly in saying: "Wharton's 

 choice of the agency which produced the flatness of lagoon floors and of 

 banks seems irresistible. He rightly regarded this flatness as no less than 

 fatal to the Darwin-Dana theory" (1915, 196). The lagoon floors to 

 which Wharton especially referred have, as above noted, depths of about 

 25 fathoms, and these must, even according to the Glacial-control theory, 

 have been aggraded by postglacial deposits 15 or 20 fathoms in thickness : 

 hence their flatness must be due not to abrasion, but to the even distribu- 

 tion and deposition of sediments by the agitation of the lagoon waters; 

 for this process would, as Darwin perceived, produce a smooth floor, what- 

 ever the form of the rock foundation. 



The depths of lagoons may vary, according to the subsidence theory, 



