FORM OF ISLAND SPUR ENDS 



563 



though they appear to afford good evidence for relatively rapid subsi- 

 dence followed by a stationary period, as Darwin thought was probable, 

 rather than for the slow and gradually ceasing rise of ocean level that 

 inheres in the Glacial-control theory. The features referred to are the 

 small rock platforms backed with low bluffs, the work of lagoon waves 

 at present sealevel on the spur ends of dissected islands within barrier 

 reefs, as already illustrated in figure 8 and in the lower views of figure 

 15. They are here shown on a larger scale in figure 17 (fringing reefs 

 are omitted from the submerged slope to make the diagram more legible). 

 The significant point is that the outer margin, B, of the visible rock 

 platform, AB, usually lies in the prolongation of the spur profile, FL; 



Figure 17. — Effects of intermittent Suhsklcnc 



this means that the bluff, DC, of any similar platform, ED, cut at a lower 

 level, was not worn so far back as B. Hence a submergence of greater 

 measure than the height of the former bluff, DC, must have brought the 

 island to its present position with respect to sealevel ; and the submergence 

 must have been so rapid that, while it was in progress, no considerable 

 amount of abrasion was accomplished on the slope CB. For if the meas- 

 ure of submergence, KJ, were less than the height of a previously abraded 

 bluff, KM, then the platform, JH, cut after such a submergence, could 

 not extend forward to N" in the line CGM, prolonged from FL; and if 

 the submergence that caused the change of level from QR to KO were 

 slow and ended gradually, then instead of liaving a part of the, normal 

 slope, OP, preserved between a submerged clift' top, P, and the outer 



