124 BULLETIN OF THE 



terior ; but this view gave no satisfactory explanation of the foundation 

 of such a coral reef, and Darwin was driven to his theory of subsidence. 

 The great defect in the way of Chamisso was, however, removed by Mur- 

 ray, who supplied the foundation of an atoll without employing subsid- 

 ence, and investigation in the Florida Sea (Agassiz) and in the Western 

 Pacific (Guppy) have confirmed his conclusions. The forms of reefs he 

 attributed to well known physical causes. Both Semper and Agassiz 

 have dwelt upon the importance of other agencies, and in our present 

 state of knowledge it will be wisest to combine in one view the several 

 agencies enumerated by them as producing the different forms of coral 

 reefs. On the outer side of the reef we have the directing influence of 

 the currents, the increased food supply, the action of the breakers, etc. 

 In the interior of a reef we have the repressive influence of sand and 

 sediment, the boring of the numerous organisms that find a home on 

 each coral block, the growth of Nullipores, the solvent action of the 

 carbonic acid in the sea-water, and the tidal scour. These are all real 

 agencies, and we only diff'er as to the relative importance we attach to 

 each. Future investigations will probably add others to the list, besides 

 ascertaining the mode and degree of action of each case." 



According to the theory of subsidence, there is no limit to the thick- 

 ness which a coral reef, rising out of deep water, may attain. It may 

 rest upon rocks of any material situated in a region of subsidence, while 

 naturally it would extend mainly vertically, its horizontal width being 

 comparatively slight. According to the other theory, which does not 

 call upon subsidence to explain the formation of barrier reefs or of atolls, 

 the base of coral reefs, whether atolls or barrier reefs, may be any pla- 

 teau or eminence, either volcanic or the product of accumulations of lime- 

 stone banks, which have reached the requisite height for the growth of 

 corals. The presence of such limestone banks and of eminences, either 

 volcanic or other, which have reached the level at which corals will 

 flourish is nowhere better exemplified than in the West Indian region, 

 where we find atolls, barrier reefs, and fringing reefs in a region which 

 is eminently one of elevation. In an area of elevation we may have 

 comparatively thin reefs, forming a mere casing to the core upon which 

 they have grown, as Guppy has shown in the case of one of the Solomon 

 Islands.^ 



Mr. J. J. Lister, the naturalist of H. M. S. Egeria, Commander Al- 

 drich, describes Christmas Island as being a succession of horizontal 

 terraces, marking the pauses of its gradual elevation during which a 



1 Nature, December 29, 1887, p. 203. 



