INFERRED PLATFORMS AS REEF FOUNDATIONS 501 



'^the width of a submerged platform [assumed to underlie a barrier-reef 

 lagoon] bordering a land area is indicative not of the amount of sub- 

 mergence, but .of the stage attained by planation processes" (1914, 60) ; 

 for without fuller evidence that a platform on which planation processes 

 have acted underlies a reef, this assertion is open to misapprehension, as 

 will now be shown. 



REEFS DO NOT USUALLY REST ON SHALLOW PLATFORMS 



If it be urged, in opposition to the view expressed above as to the sub- 

 marine extension of supermarine slopes, that marine deposition and 

 abrasion have, with or without the aid of subsidence, fashioned a plat- 

 form-Hke foundation for barrier reefs at a small depth, certain principles 

 of subaerial and of marine erosion must be recalled in order to test the 

 correctness of this view. First, reef-encircled islands are so well pro- 

 tected from wave attack that their form is almost wholly carved by sub- 

 aerial erosion, the work of the lagoon waves being nearly negligible. 

 Second, volcanic islands that are exposed to wave attack in the open ocean 

 are cut back in cliffs somewhat faster than their small-stream valleys are 

 cut down by streams and much faster than the valleys are widened by 

 weathering; abrasion then takes place near shore, and deposition offshore. 

 Third, the height of wave-cut cliffs on volcanic islands will ordinarily 

 be about a quarter of the breadth of the abraded and aggraded platform 

 from which they rise. To these three theoretical considerations, a fact 

 of observation must be added : the spur ends of reef -encircled islands are, 

 as already noted, rarely cut back in cliffs; when cliffs occur, they are in 

 nearly all cases of small height and they rise from narrow rock-platforms 

 that have been cut by lagoon waves at present sealevel. 



Hence, if a barrier reef be supposed to have grown up from the outer 

 margin of an abraded and aggraded platform, the spur ends of the island 

 should, as above noted, be cut off in cliffs having a height equal to about 

 a quarter of the lagoon breadth, and the cliffs should still be partly visible 

 after moderate submergence; bilt such cliffs are almost unknown. If a 

 platform has really been abraded it must have been strongly submerged 

 by subsidence since it was abraded, in order that the spur-end cliffs back 

 of it shall no longer be visible; the measure of the subsidence must be 

 about a quarter of the lagoon breadth at least. It thus appears probable 

 that, even if abrasion produced a platform before reef growth began (as 

 will be later shown to be probable ) , the thickness of the reef built up on 

 the platform margin must be much greater than the depth of the aggraded 

 lagoon that it incloses ; and reef growth must therefore have been accom- 

 panied by a rather strong subsidence of the reef foundation. 



