W. M. Davis — Shaler Memorial Study oj Coral Reefs. 245 



sea level, as indicated by the breadth of the embayments and 

 the slope of their sides in the less maturely dissected islands, 

 although the depth thus indicated is, as above noted, frequently 

 greater than the estimated depression of the sea surface during 

 the glacial period ; but the graded side-slopes of the open val- 

 leys that admit broad bays far in toward the center of the 

 more maturely dissected volcanic masses ; for in view of the 

 moderate amount of general subaerial degradation accom- 



Fig. 6. 



Fig. G. Types of submature, L, mature, M, and late mature, N, dissection 

 of a volcanic island within a barrier reef. 



plished in resistant rocks since the deposition of the earliest gla- 

 cial deposits, it must be inferred that the excavation of late- 

 mature valleys in volcanic masses demands a longer period of 

 time than the whole of the glacial period. 



The detailed form of certain spur-ends is significant in this 

 connection. Basset-edge outcrops, as in M, fig. 6, are of 

 fairly common occurrence near spur ends, because the terminal 

 slope of the spur, although by no means resembling a sea-cliff, 

 is not infrequently steeper than the dip of the volcanic beds 

 that may be traced along the spur sides : excellent examples 

 of such forms are found in Huaheine, Raiatea and Tahaa. 

 From the best determinations that I could make, such spur 

 ends have been slowly fashioned by long enduring subaerial 

 degradation, unaided by the active erosion of streams or by 

 the still more active attack of unimpeded sea waves. The 

 glacial period does not appear to have been long enough for 

 the accomplishment of so large a measure of sculpture by gen- 

 eral weathering. Yet spur ends of this kind are often 

 wrapped around by fringing reefs within the lagoon of a bar- 

 rier-reef, and this indicates that erosion with respect to a lower 

 baselevel than the sea surface of to-day has taken place for a 

 longer time than the glacial period. 



The partly submerged coastal lowlands within the Great Bar- 

 rier reef of Queensland, as interpreted by Andrews, as well as 



