ACTINOZOA. 257 



growing in place ; and a large portion of all reefs is formed by cal- 

 careous deposits of a fragmental character, which have been produced 

 by the action of the sea upon the reef, and which are therefore essen- 

 tially made up of larger or smaller pieces of coral mixed with the 

 entire or broken skeletons of other lime-secreting organisms. Lastly, 

 the materials composing a coral-reef are liable to undergo more or 

 less extensive secondary changes caused by the continued percolation 

 through them of water, these changes being in the direction of a more 

 or less complete crystallisation and of a consequent obliteration of 

 the original organic structure of the rock. 



Essentially, therefore, a modern coral-reef is composed, on the one 

 hand, of more or less extensive calcareous masses formed by the 

 growth in place of corals associated with other lime-secreting organ- 

 isms, and, on the other hand, of purely mechanical deposits produced 

 by the wear and tear of the preceding. As it is known that the reef- 

 building corals do not thrive at depths greater than about thirty 

 fathoms, it is certain that the portions of the reef actually formed of 

 corals which have grown in place must have originated in quite 

 shallow water. On the other hand, the detrital deposits formed by 

 the degradation of the original reef form a belt on the flanks of the 

 latter, and may extend into comparatively deep water. Moreover, the 

 portions of the reef which are of mechanical origin may considerably 

 exceed in amount those which are due to the direct growth of the 

 corals themselves. 



The following are the more important kinds of calcareous rocks which 

 occur in existing coral-reefs : — 



1. " Coral-rock? properly so called, produced by the reef-corals grow- 

 ing in place. Though essentially formed by the growing corals them- 

 selves, this rock is also largely made up of calcareous mud and the 

 entire or broken skeletons of various lime-producing organisms other 

 than corals, the interstices between the corals being thus completely 

 filled up. This is more especially the case near the surface of the reef, 

 where the branching and reticulated types of corals (such as the Madre- 

 pores) chiefly abound. " Coral-rock," produced as above, is liable to 

 undergo secondary changes as the result of the percolation through it of 

 water, the original coralline structure being thus more or less extensively 

 obliterated, and the rock being converted into a compact and crystalline 

 limestone. 



2. " Reef-rock? produced by the action of the sea upon the reef, and 

 consisting" of fragments of broken coral mixed with the entire or broken 

 remains of all sorts of calcareous organisms other than corals (Molluscs, 

 Echinoderms, Foraminifera, Nullipores, &c), the whole being cemented 

 together by a matrix of crystalline calcite. This matrix is formed by the 

 percolation of water through the mass, this leading to partial solution of 

 the fragments, and to the subsequent deposition in the crystalline form of 

 the carbonate of lime thus obtained. " Reef-rock " is always stratified, 

 and varies in texture, according to the size of the fragments of which it is 

 composed. Usually it is more or less compact and homogeneous ; and 

 its component organic fragments may be readily recognisable, or may 



vol. 1. R 



