STRUCTURE OF CORAL REEFS 



401 



indicate subsidence from about 25 fathoms to the present depth of 

 these points. (Figs. 79, 80.) 



Cavernous character of reefs. Observation on modern reef 

 knolls shows that the mass as a whole is often cavernous, large, 

 empty spaces remaining where branching corals reach across to 

 form a solid canopy. These caverns are the favorite haunts of the 

 echinoderms, Mollusca and Crustacea which inhabit the reefs, and 

 through the openings by which they communicate with the open sea 

 the pelagic fish of the reefs find constant in- and egress. Such 



Noll 



Sketch ot MasflMARHU I 

 Shotf//}^ appronmale 

 BOS/ i/ on or Sections 



Fig. 80. 



Section 2 of Masamarhii Island in the Red Sea. The small map 

 shows the location of the sections. (After Saville-Kent.) 



caverns may be preserved after the coral rock is uplifted into dry 

 land, and Walther (90:9/^') has argued for such an initial origin of 

 many limestone caverns in earlier geological formations. 



In general the structure and form of barrier reefs around 

 oceanic islands differ in no essential from those of the atolls, so that 

 the designation of such a barrier as an atoll with high land rising 

 within the lagoon (Balbi) is quite applicable. Such are the reefs 

 of Tahiti and others in the Society Archipelago, of Vanikoro and 

 of others. The distance from shore of the reef is from one to one 

 and a half, and occasionally even more than three miles, in the 

 Society Archipelago (Ellis). In the few cases where encircled 

 highlands exist in the Caroline Archipelago greater distances are ob- 



