W. M. Davis — Shaler Memorial Study of Coral Reefs. 243 



Now around an island thus constituted, the sea is supposed 

 to sink to a lower level, and as a first result the streams are 

 impelled to transect the reef and deepen their valleys in the 

 volcanic cone ; then the sea is supposed to rise again and 

 transform the valleys into bays: if several changes of level 

 take place, one in each epoch of the glacial period, the 

 observed embayments will represent the integrated result of all 

 of the changes. The transection of the reef limestones may be 

 a relatively short task, but the incision and still more the 

 widening of valleys in the resistant lavas of volcanic islands or 

 in the crystalline rocks of New Caledonia and Queensland is a 

 task of slower accomplishment. It is moreover questionable 

 whether the greatest possible lowering of the sea surface during 

 the glacial period was sufficient to permit the erosion of valleys to 

 the depth indicated by the form of the spurs that enclose the 

 present embayments of certain islands. 



In judging this matter, it must be borne in mind that the 

 valleys of small streams are steep-sided only during the early 

 or immature stages of their whole cycle of erosion, while 

 downward corrasion is still rather active ; when downward 

 corrasion practically ceases the slower process of lateral corra- 

 sion in the mature stage of the cycle allows the valley sides to 

 weather to gentler slopes. Hence if a maturely open, flat- 

 floored valley be partly submerged, its depth will not be 

 indicated by prolonging its side slopes downward until they 

 meet; but if a steep-sided immature valley be partly sub- 

 merged, its depth may be fairly estimated in that way. Now 

 the existing embayments of certain barrier-reef islands are 

 enclosed by spurs of relatively steep slopes, which must be 

 interpreted as the sides of relatively immature valleys, not so 

 far advanced in their pre-submergence development as then to 

 have had flat floors; hence their depth near the bay mouths 

 may be fairly determined by the depth at which the down- 

 ward prolongation of the bay sides intersect ; and this is often 

 as much as 600, 800 or perhaps 1000 feet. So great a depth of 

 erosion cannot be reasonably ascribed to the revival of the 

 streams during the glacial period. 



Aye of Valleys in Barrier-reef Islands. — But although some 

 of these half-drowned valleys are rather steep-sided, even the 

 narrowest of them are by no means young gorges ; and on 

 many islands the bay-filled valleys are of maturely open form, 

 with well-graded, soil-covered sides. In such cases, the most 

 important matter to be considered is not the deepening of 

 the valleys, but, as above stated, the pre-submergence widening 

 that they have suffered even in resistant rocks, such as the lavas 

 of the Fijis, and the crystalline schists of New Caledonia and of 

 Queensland ; and likewise the pre-submergence reduction of 



