DARWIN ON CORAL REEFS. 387 



that the- reef-building coral animal does not generally live at a 

 greater depth than about thirty fathoms, and this is not invalidated 

 by the fact that living specimens have been found occasionally at 

 much greater depth. The greatest depths at which the ordinary 

 species grow appear to be those of the more northern parts of the 

 Red Sea ; but it is well known and has been already mentioned, 

 that coral reefs occasionally exhibit a much greater thickness than 

 this depth, although the lower portion is in such cases dead. 



Having thus described the chief phenomena of coral reefs, and 

 the circumstances of their growth, the author enters on the subject 

 of the theory of the formation of the different classes of them, and 

 shows the unsound nature of the theory most generally received, 

 namely, that they are based on submarine craters, and also of the 

 better theory of Chamisso, that the outer portions of a reef, being 

 of the most vigorous growth, would first reach the surface. Neither 

 of these theories, however, account for barrier reefs. 



The cause that has given to atolls and barrier reefs their cha- 

 racteristic forms, is supposed by Mr. Darwin to have been the 

 gradual subsidence of portions of the bed of the ocean over large 

 areas, and is partly deduced from the consideration of these two 

 circumstances, viz. : — first, that reef-building corals flourish only at 

 limited depths, and, secondly, that vast areas are interspersed with 

 coral reefs and coral islets, none of which rise to a greater height 

 above the level of the sea than that attained by matter thrown 

 up by the waves and winds. The foundation of each reef is as- 

 sumed to have been rocky, but it cannot be thought probable that 

 the broad summit of a mountain lies buried at the depth of a few 

 fathoms beneath every atoll, with no one point of rock projecting 

 above the surface over such a wide extent, and it is known that 

 neither the flat disk of an abraded point, nor any other rocky 

 foundation, is ever at the top of the reef. Besides the evidence 

 in favour of a subsidence having taken place, derived from these 

 and other considerations connected with the general appearance of 

 the rocks, other evidence is adduced showing the probability 

 of this view, which is then proved to explain in a very simple 

 manner the ordinary phenomena of fringing reefs, barriers, and 

 atolls, and some of the less regular forms of the two great classes 

 of reefs, and it only remains to consider whether the actual dis- 

 tribution of coral reefs and their relative condition, agrees with 

 such a theory of their formation. A chapter is added on this sub- 

 ject, and is accompanied by a coloured chart of all known coral 

 reefs and islands, one colour being used to mark barrier reefs and 

 atolls, and another distinguishing the fringing reefs which form a 

 distinct type of structure. In this chart the difference between 

 the atolls and barrier reefs is indicated by the tint ; so that the 

 one colour marks those districts in which it is supposed that sub- 

 sidence has taken place at a slow but steady rate, allowing of the 

 gradual accumulation of extensive coral reefs, while the other 

 denotes that no such change of level to any sensible extent has 

 taken place. 



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