Ch. VI. RECAPITULATION. 191 



We may therefore conclude that the subterranean 

 changes which cause some areas to rise and others to 

 sink, have generally acted in a closely similar manner. 



Recapitulation. — In the three first chapters, the 

 principal kinds of coral-reefs were described in detail, 

 and they were found to differ little, as far as relates to 

 the actual surface of the reef. An atoll differs from 

 an encircling barrier-reef only in the absence of land 

 within its central expanse; and a barrier -reef differs 

 from a fringing-reef only in being placed, relatively 

 to the probable inclination of its submarine foundation, 

 at a much greater distance from the land, and in 

 the presence of a deep lagoon-like space within the 

 reef. In the fourth chapter the growing powers of the 

 reef-constructing polypifers were discussed ; and it was 

 shown that they cannot flourish beneath a very limited 

 depth. In accordance with this limit, there is no diffi- 

 culty respecting the foundation on which a fringing- 

 reef is based ; whereas, with barrier-reefs and atolls, 

 there is the greatest difficulty on this head;— in bar- 

 rier-reefs from the improbability of rock or banks of 

 sediment having extended, in every instance, so far 

 seaward within the required depth ; — and in atolls, 

 from the immensity of the spaces over which they are 

 interspersed, and the apparent necessity for believing 

 that they are all based on mountain-summits, which, 

 although rising very near to the surface of the sea, in 

 no one instance rise above it. To escape this latter 

 admission, which implies the existence of submarine 



