498 W. M. DAVIS SUBSIDENCE OF REEF-ENCIRCLED ISLANDS 



of these assumed processes is, as I have elsewhere shown more in detail 

 (1916, c, 563), that if the ocean were lowered for a long enough time to 

 permit the opening of many embayed valleys to the observed width of 

 half a mile or a mile by the slow processes of subaerial weathering under 

 the leadership of comparatively small streams, then the powerful waves 

 of the trade-wind seas must have in the same time cut back broad plat- 

 forms and high cliifs on islands where the preglacial reefs had small 

 breadth, and the cliffs thus cut should still be partly visible above present 

 sealevel. 



The greater rapidity of platform and cliff, cutting by ocean waves than 

 of valley deepening and widening by small streams is attested by the form 

 of various oceanic islands, such as Tristan da Cunha, in the South At- 

 lantic. If while the ocean was lowered small-stream valleys v^ere so much 

 deepened and widened that they would hold embayments half a mile or 

 more in width after the ocean rose to its present level, the waves must 

 have, in the same period of lower ocean level, cut a platform one or two 

 miles wide in the same rocks; the cliffs at the back of such a platform 

 would be 1,000 or 2,000 feet high, and hence would be still in great part 

 visible above normal ocean level. The absence of such cliffs along the 

 southeastern coast of Ngau, Fiji, v^here the present reef is a fringe about 

 half a mile wide, and along the northeastern side of Samar, Philippines, 

 where fringing reefs are very narrow or wanting, and along many similar 

 coasts is strong proof that the reefs of those coasts were not killed during 

 the Glacial period, and that abrasion did not then act as it is assumed in 

 the Glacial-control theory to have acted. 



The Slope of Eeep Foundations 



DEPTH OF LAGOONS AS A MEASURE OF THICKNESS OF REEFS 



In view of the fact that reef -building corals can not thrive at greater 

 depths than 25 or 30 fathoms, it has been urged by Darwin and others 

 that barrier-reef lagoons which have a depth of 40 or more fathoms give 

 strong evidence for the theory of subsidence. Two authors, both of whom 

 had at one time accepted, but had later given up the subsidence theory, 

 may be here quoted: Sir A. Geikie noted that this theory, although in 

 general unproved, might still hold good for those reefs which inclbsed 

 lagoons 40 fathoms or more in depth (1883, 27), and Guppy wrote: "It 

 is the depths of from 35 to 60 fathoms which are found occasionally 

 within both barrier reefs and atolls that lend the greatest support to the 

 theory of subsidence" (1886, 887). 



V 



