EXPECTABLE FORM OP ABRADED PLATFORMS 551 



the spur ends are not clift, although the reef on the east and north is a 

 narrow fringe or close-set barrier, only a quarter or half a mile in width — 

 be overlooked, and the floors of barrier reefs and atoll lagoons and the 

 floors of submarine banks be regarded platforms of abrasion, now more 

 or less covered by postglacial reef growth and sediments. The problem 

 is to determine the features by which the existence of the abraded plat- 

 forms beneath the aggraded floors can be discovered. A general treat- 

 ment of this problem has been given by Barrell (1915). I have discussed 

 it in some detail in an article on "Submarine banks and the coral-reef 

 problem" (1918, a), and will therefore present here only its leading con- 

 clusions. 



First, abraded platforms must have a gentle seaward slope; hence a 

 large platform which completely truncates a preglacial island 50 miles 

 or more in diameter should be 20 or 30 fathoms deeper around its margin 

 than at its center; as a whole such a platform should have the form of a 

 very flat cone. Second, the center of a platform of complete truncation 

 and the inner cliff-base margin of an island-benching platform should 

 give the best indication of the level of the sea at the time when abrasion 

 took place. True, the center of a truncating platform might, if abrasion 

 continue long enough, be worn down to a moderate depth below the level 

 of the abrading waves, but the inner margin of an island-benching plat- 

 form ought to lie close to the mean scale vel at the time of abrasion. 

 Third, the level of the abrading sea thus recorded ought to be everywhere 

 at about the same dejDth below present sealevel. Fourth, postglacial aggra- 

 dation will make the depths of lagoons and banks less than that of the 

 underlying platforms, and the decrease of depth thus caused will be, as 

 Daly has shown, "in indirect proportion to the width of the platform" 

 (1915, 192)^; for tlie smaller the platform the greater the aggrading 

 effect of waste inwashed from the upgrowing reef around its margin. 

 The central area of very large lagoons and banks will be aggraded chiefly 

 by locally formed organic deposits. Fifth, a further consequence of plat- 

 form abrasion to a fairly uniform depth followed by marginal upgrowth 

 of reefs in a uniformly rising ocean is that barrier and atoll reefs today 

 should be, as a rule, of similar dimensions in cross-section. 



It follows that large lagoons and banks will give the best indication of 

 the depth of the (supposed) underlying abraded platforms; that the 

 depths thus found should be less, by reason of their aggradation, than the 

 30 or 40 fathoms by which the ocean is supposed to have been lowered in 

 the Glacial period ; and that depths of barrier-reef lagoons should always 



2 The original statement is misprinted "in direct proportion," and is liere corrootod 

 witli tlie approval of the author. 



XLI— Bull, Geol. Soc. Am., Vol. 20, 1917 



