THE CORAL-REEF PROBLEM. 67 



ing cliapter, there is nothing in the present land-mass to indi- 

 cate that it bears any direct relation to an atoll ring. 



An objection that has been frequently urged against the sub- 

 sidence theory, and one that has been more particularly in- 

 sisted upon by Guppy as the result of extended observations 

 made in the Solomon Islands, is that where fringing reefs are 

 exposed they usually exhibit only a moderate thickness of 

 true coral-rock, the basement or sub-structure being mainly of 

 a pelagic character — that is, built up of the remains of 

 pelagic animals (Foraminifera, etc). Hence, it is argued that 

 in the so-called subsidence reefs — atolls and barrier-reefs, — the 

 actual thickness of coral is very limited, or barely more than 

 that whicli would fall within the regular zone of coral growth. 

 The few observations that have been made on this point, can- 

 not be considered to throw much light upon the question, the 

 more especially as the evidence obtained is far from corrobora- 

 tive. Furthermore, it is just in such elevated reefs that in ac- 

 cordance with the Darwinian theory we should frequently 

 look for a thin deposit of coral-rock, for if there has been eleva- 

 tion instead of subsidence the thickness must necessarily be 

 slight; when, however, subsidence had preceded elevation the 

 result would be the opposite. No weight should be attached to 

 the oft-repeated as.sertion that in the older geological forma- 

 tions there are no really massive reef-structures. This asser- 

 tion is entirelj- opposed to the facts, to cite but a single instance 

 presented by the Dolomites of the Tyrol, the reef-structure of 

 which has been so ably worked out by Mojsisovics and others. 

 Furthermore, it is practically impossible in the case of a large 

 number of the altered limestones to state whether they are of 

 coral origin or not. 



One objection against the sub'eidence theory has still to be 

 considered. It is the association of fringing reefs with atolls. 

 This commingling of two distinct types of structure, implying 

 movements in opposite directions, has been much commented 

 upon, and placed under strong emphasis by the adherents of 

 the new views regarding the formation of coral islands. But 

 the occurrence appears to be entirely without significance. 



