THE SLOPE OF REEF FOUNDATIONS 



499 



THE SUBMARINE SLOPE OF REEF -EN CIRC LED ISLANDS 



The above argument is good as far as it goes, but it is deficient in not 

 taking sufficient account of the form of the coast that barrier reefs ordi- 

 narily front. Most barrier reefs encircle volcanic islands, and the islands, 

 as a rule, exhibit maturely carved slopes of fairly strong declivity, AB, 

 figure 4, the product of subaerial erosion, not of marine abrasion. Such 

 slopes may be reasonably prolonged for a considerable distance below sea- 

 level as a rough means of estimating the depth, RD, of the rock founda- 

 tion beneath an offshore barrier reef, and of thus estimating the total 

 thickness of the reef. This is especially permissible in the case of small 



Figure 4. — Submarine Slope of a Volcanic Island 



islands, like Tahaa, figure 1, that consist of single volcanic cones, more 

 or less dissected. As the thickness thus determined may in many cases 

 be 1,000 feet or more, it testifies much more strongly for subsidence than 

 a lagoon depth of 40 or 60 fathoms does. 



However, if it be assumed that an initial cone of resistant lavas wras 

 cloaked with a thousand feet of loosely compacted ash beds, then while 

 the island stood a little higher than now, the ash beds might have been 

 worn down to a lowland rim, leaving the resistant lava slopes with a rela- 

 tively steep inclination; a slight submergence of such an island would 

 provide a shallow submarine platform from which an offshore barrier reef 

 might grow up; and the small depth of such a foundation would not be 

 indicated by prolonging the lava slopes beneath sealevel. But there is no 



