7 8 CORAL-REEFS. 



smooth surface, strewed with worn fragments of coral. They 

 consisted in their lower part of hard calcareous sandstone, 

 and in their upper of great blocks of several species 

 of Astraea and Madrepora, loosely aggregated; they were 

 divided into irregular beds, dipping seaward, in one hillock 

 at an angle of 8°, and in the other at i8°. I suspect that 

 the superficial parts of the reefs, which have been upraised 

 together with the islands they fringe, have generally been 

 much more modified by the wearing action of the sea, than 

 those of Mauritius. 



Many islands 1 are fringed by reefs quite similar to those 

 of Mauritius : but on coasts where the sea deepens very 

 suddenly the reefs are much narrower, and their limited 

 extension seems evidently to depend on the high inclination 

 of the submarine slope ; — a relation, which, as we have 

 seen, does not exist in reefs of the barrier class. The 

 fringing-reefs on steep coasts are frequently not more than 

 from 50 to 100 yards in width; they have a nearly smooth, 

 hard surface, scarcely uncovered at low water, and without 

 any interior shoal channel, like that within those fringing- 

 reefs, which lie at a greater distance from the land. The 

 fragments torn up during gales from the outer margin 

 are thrown over the reef on the shores of the island. I may 

 give as instances, Wateeo, where the reef is described by 

 Cook as being a hundred yards wide; and Mauti and 



1 I may give Cuba, as another instance ; Mr. Taylor (Loudo?is 

 Mag. of Nat. Htst., vol. ix. p. 449) has described a reef several 

 miles in length between Gibara and Vjaro, which extends parallel 

 to the shore at the distance of between half and the third part of a 

 mile, and encloses a space of shallow water, with a sandy bottom 

 and tufts of coral. Outside the edge of the reef, which is formed of 

 great branching corals, the depth is six and seven fathoms. This coast 

 has been upheaved at no very distant geological period. 



