J. D. Dana — Origin of Coral Reefs and Islands. 103 



local displacements by either method would not necessarily in- 

 terfere with any secular change of level in progress. 



17. The shore-platform of an atoll, or the " flat" as called by 

 Darwin, situated just above low-tide level, consists usually of 

 the true reef rock, or the rock made by under-water consolida- 

 tion ; and its height is determined chiefly by the height of 

 wave action, its general surface being produced by the chisel- 

 ing effect of the inflowing waters. When found above its normal 

 level it is probable evidence of an elevation ; and on this kind 

 of evidence the conclusion rests in several of the cases of sup- 

 posed elevation which I mention in my Eeport. The width 

 of the platform and its evenness of surface vary with the height 

 of the tides. When the tides are five to six feet, the platform is 

 narrow, more cut up by channels and less even in surface. 

 After an elevation, if but a foot or two in amount, the surface 

 of the platform becomes restored finally for a large part of its 

 surface to its normal level and gentle slopes may connect the 

 newer and older portions. But if the rise of an atoll is ten 

 feet, great degradation takes place along the lifted edge of the 

 reef, which may end in reducing the elevated coral barrier to a 

 wall with numerous channels and broad spaces opening through 

 to the lagoon, as observed by the writer (from ship-board) on 

 the south side of Dean's Island.* 



18. The differences in the kinds of coral rocks should be under- 

 stood (as the recent discussions of Darwin's theory have shown) 

 in order to appreciate the structural facts that bear on changes of 

 level. The beach-made rock is of above-water consolidation, 

 (through calcareous deposition about the grains as evapora- 

 tion takes place), and is porous, often oolitic ; and if a con- 

 glomerate it consists mostly of worn masses. The rock made 

 of drifted sands is similar. But the true coral-reef rock is of 

 wider-water consolidation, and is usually very compact, like an 

 ordinary limestone ; and if a conglomerate it is commonly a 



* Our cruise took us from the Paumotu atolls to Australia, and there, the sand- 

 stone bluffs making the capes of Port Jackson gave me my first understanding 

 of the atoll's "shore platform." This bluff had its "shore platform," 50 to 150 

 yards wide, bare at low tide ; it was the lower layer of the sandstone, a regularly 

 jointed rock, lying like a loosely laid pavement. It seemed strange that it was 

 able to keep its place in the face of the breakers. But the first waters of the 

 incoming tide swelled quietly over it and served to shield it from the plunging 

 waters of the later part of the flow ; the waves, therefore, found nothing to batter 

 against short of the base of the bluff. 



A view of Dean's Island from the south is given in Wilkes's Narrative, i, 342 ; 

 it fails only in not giving a nearly even top line to the columns. The view on p. 

 334 looks as if representing another example of similar erosion. But, as the 

 text implies, the group of masses of coral rock was made by the artist by bringing 

 into a single view the blocks that had bean observed in an isolated way over the 

 platforms of atolls. The size and shapes of the blocks are exaggerated. But, 

 although isolated, such blocks are often so united to the coral platform that they 

 appear to be a constituent part of it (my Report, p. 61), and suggest the question 

 whether they may not be remnants of an overlying layer elsewhere removed. 



