MUSEUxM OF COMPARATIVE ZOOLOGY. 123 



dred and fifty feet, and declining rapidly seaward. These terraces as they 

 recede from the sea become shallower and shallower, having at their ex- 

 tremity a sort of steep talus extending to the one next below it. This 

 is probably due to the action of the surf, as the force of the sea becomes 

 less and less where it is broken towards the shore line, though Couthouy 

 is inclined to see in these terraces the eflfect of subsidence. But as he 

 distinctly says that the outer terraces have only twelve to fifteen fathoms 

 of water on them, about the limit of reef-growing corals, his explanation 

 does not appear satisfactory even in the case of the terrace of Belling- 

 hausen's Island, on which he found twenty-eight fathoms. 



It seems much more natural to look upon the channels left, either in 

 the barrier reefs or in atolls, as due to the original inequalities in the 

 level of the foundation of the reef. The corals would naturally reach the 

 surface soonest at the highest point, thus leaving passages which might 

 at particular parts of the reef, and under certain local conditions, be 

 gradually closed by the active growth of the corals, or might, on the 

 other hand, remain open wherever the tides or currents rushed through 

 them with sufficient force to check their increase, or where the silt was 

 deposited in such quantities as to obstruct it. 



It is interesting to go back to Kotzebue's Voyage, and to find in the 

 chapter on the Coral Islands, by Chamisso, the following : " The corals 

 have founded their buildings on shoals in the sea ; or, to speak more 

 correctly, on the top of mountains lying under water. On the one side, 

 as they increase, they continue to approach the surface of the sea, on the 

 other side they enlarge the extent of their work." ^ He noticed the more 

 rapid growth of the coral where exposed to the action of the surf, and the 

 obstacles to their growth in the middle of a broad reef, due to the amass- 

 ing of shells and accumulation of coral fragments, and also to the forma- 

 tion of coral land by the cementation of the calcareous sand, gradually 

 increasing in thickness till it is covered by the sea only during certain 

 seasons of the year. He also noticed the formation of reefs more or less 

 circular, with an interior sea, having a depth sometimes of thirty to 

 thirty-five fathoms, which he explains from the action of the natural 

 causes above enumerated. 



Dr. Guppy ^ justly says in Nature : " The development of the new 

 theory should be kept in mind. Chamisso seventy years ago advanced 

 the view that an atoll owes its form to the growth of corals at the 

 margin and to the repressive influence of the reef debris in the in- 



1 Kotzebuc, Vol. HI. p. 331, London, 1821. 



2 H. B. Guppy, Nature, March 15, 1888, p. 462. 



