GLACIAL-CONTROL THEORY OF CORAL REEFS. 237 



to maximum depths; it is not likely to be shaken by additional sound- 

 ing. Yet, if the great atolls are due to the sinking of correspondingly 

 extensive islands, it is truly incredible that the lagoon depth should 

 nowhere greatly exceed 300 feet (91 m.). 



The subsidence in such a case has been estimated in terms of thou- 

 sands of feet. As the initial fringing reef was converted into a barrier 

 and that finally into an atoll, the broadening lagoon waters must 

 cover a kind of encircling moat between the central island or shoal 

 and the upgrowing exterior reef. The reef speedily reaching the sea 

 surface through most of its length, the waves and currents of the open 

 ocean can do very little toward aggrading the lagoon floor, which is 

 little or not at all disturbed by any waves or currents generated in the 

 lagoon itself. Some of the fragmental material flooring each lagoon 

 is derived from the rimming reef. From the outer edge of that reef 

 the heavy surf is constantly forming mud, sand, and loose blocks. 

 Some of the detritus is thrown forward by the breakers, to compose 

 reef islets or reef awash. A much greater proportion is dragged out 

 into deep water. ^^ One often meets the statement that much material 

 is washed over the reef into the lagoon, but obviously the amount so 

 transported can be only a small part of the whole detritus. That 

 fraction, together with material locally brought in through reef 

 channels, must be deposited near the reef, for waves and currents 

 inside the lagoon are usually powerless to move sand in water 40 m. 

 or more in depth. Abrasion by the lagoon waves is very low. The 

 resulting debris, of grain coarser than that of fine mud, is likewise 

 deposited close to the reef. As stated in the Royal Society report on 

 Funafuti (page 375) Sollas observed that coral debris forms " but an 

 insignificant part" of the "sand" (loose material) flooring that atoll 

 lagoon, the chief constituents being foraminiferal shells and calcareous 

 algae. In the extensive interior of the lagoon, any clastic material 

 derived from the main reef is mud, with a little sand distributed by 

 occasional great storms. The filling and smoothing-out of the hypo- 

 thetical "moat" about a subsiding island is evidently little aided by 

 this mud. The coarser detritus should form a well-defined terrace 

 slowly growing inward from the reef. Such terraces would also be 

 expected as a consequence of the Glacial-control theory; they are, in 

 fact, conspicuous, though narrow, in Curtis's wonderful model of 

 Funafuti at the Harvard University Museum. Their volumes are 

 exactly of the order demanded if the reefs are of modern origin. 



69 Cf. V. Cornish, Geog. Jour., 11, 530 (1898). 



