Coral Reefs and Islands. 159 



of the true reef-rock, or the rock made by under-water con- 

 solidation ; and its height is determined chiefly by the height 

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

 chiseling-effect of the in-flowing w T aters. 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 supposed elevation which I mention in my Report. 

 The width of the platform and its evenness of surface vary 

 with the height of the tides. When the tides are 5 to 6 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 10 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 

 understood (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- 

 ivater consolidation (through calcareous deposition about the 

 grains as evaporation takes place), and is porous, often oolitic; 

 and if a conglomerate, it consists mostly of worn masses. 

 The rock made of drifted sands is similar. But the true 



* Our cruise took us from the Paimiotu atolls to Australia, and there, 

 the sandstone bluffs making the capes of Port Jackson gave me my first 

 understanding of the atoll's " shore-platform." This bluff has 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 in-coming tide swelled 

 quietly over it, and served to shield it from the plunging waters of the 

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

 against short of the base of the bluff. 



A view of Deans Island from the south is given in Wilkes's Narrative, 

 i. p. 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 

 been 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. 



