Ch. V. OF CORAL-KEEFS. 153 



as indicated by fringing-reefs, bear any determinate 

 relation to each other ? Is there any relation between 

 the areas of recent subsidence or elevation, and the 

 presence of active volcanic vents? These several 

 questions will be considered in the following chapter. 1 



1 1 may take this opportunity of briefly considering the appearance 

 which would probably be presented by a vertical and deep section 

 across a coral formation (referring chiefly to an atoll) formed by the 

 upward growth of coral during successive subsidences. This is a 

 subject worthy of attention, as a means of comparison with ancient 

 coral strata. The circumferential parts would consist of massive spe- 

 cies in a vertical position, with their interstices filled up with detritus; 

 but this would be the part most subject to subsequent denudation and 

 removal. It is useless to speculate how large a proportion of the 

 exterior annular reef would consist of upright coral, and how much of 

 fragmentary rock, for this would depend on many contingencies,— such 

 as on the rate of subsidence occasionally allowing a fresh growth of 

 coral to cover the whole surface, and on the breakers having force suffi- 

 cient to throw fragments over this same space. The conglomerate which 

 composes the base of the islets, would (if not removed by denudation 

 together with the exterior reef on which it rests) be conspicuous from 

 the size of the fragments, — the different degrees in which they have 

 been rounded, — the presence of fragments of conglomerate torn up 

 rounded and re-cemented, — and from the oblique stratification. The 

 corals which lived in the lagoon-reefs at each successive level, would 

 be preserved upright, and they would consist of many kinds, generally 

 much branched. In this part, however, a very large proportion of 

 the rock, and in some cases nearly all of it, would be formed of sedi- 

 mentary matter, being in an excessively fine or moderately coarse 

 state, with the particles almost blended together. The conglome- 

 rate which was formed of rounded pieces of the branched corals on 

 the shores of the lagoon, would differ from that formed on the islets 

 and derived from the outer coast ; although both might have been ac- 

 cumulated very near each other. The stratification, taken as a whole, 

 would be horizontal : but the conglomerate beds resting on the exterior 

 reef, and the beds of sandstone on the shores of the lagoon and on 

 the external flanks of the reef, would probably be divided (as at Keeling 

 atoll and at Mauritius) by numerous layers dipping at considerable angles 

 in different directions. The calcareous sandstone and coral rock would 

 almost necessarily contain innumerable shells, echini, and the bones of 



