INVERTEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 811 



the other physical — the former, that those species of corals whose 

 skeletons chiefly make up reefs cannot live in depths greater than 

 from 20 to 30 fathoms ; the latter, that the surface of the earth is 

 continually undergoing slow elevation or subsidence. 



The corals commence by growing up from the shallow waters 

 surrounding an island, and form a fringing reef which is closely 

 attached to the shore. The island slowly sinks, but the corals con- 

 tinually grow upwards, and keep the upper surface of the reef at a 

 level with the waves of the ocean. When this has gone on for some 

 time a wide navigable water channel is formed between the reef and 

 the shores of the island, and we have a barrier reef. These processes 

 have but to be continued some stages fui-ther, when the island will 

 disappear beneath the ocean, and be replaced by an atoll with its 

 lagoon where the island once stood. 



According to this simple and beautiful theory, the fringing reef 

 becomes a barrier reef, and the barrier reef an atoll by a continuous 

 process of development. 



Professor Semper,* during his examination of the coral reefs in 

 the Pelew group experienced great difficulties in aj^plying Darwin's 

 theory. Similar difficulties presented themselves to the author of 

 this paper, Mr. John Murray, of the ' Challenger ' Expedition, in those 

 coral regions visited during the cruise of the ' Challenger.' The object 

 of the present paper is to show, first, that while it must be granted as 

 generally true that reef-forming species of coral do not live at a 

 depth greater than 30 or 40 fathoms, yet that there are other agencies 

 at work in the tropical oceanic regions by which submarine elevations 

 can be built up from very great dejiths so as to form a foundation for 

 coral reefs ; second, that while it must be granted that the surface of 

 the earth has undergone many oscillations in recent geological times, 

 yet that all the chief features of coral reefs and islands can be 

 accounted for without calling in the aid of great and general sub- 

 sidences. 



The most recent charts of all coral reef regions have been 

 examined, and it is found possible to explain all the phenomena by 

 the principles advanced in the paper, while on the subsidence theory, 

 it is most difficult to explain the appearance and structures met with 

 in many grou2)s ; for instance, in the Fiji Islands, where fringing 

 reefs, barrier reefs, and atolls all occur in close proximity, and where 

 all the other evidence seems to point to elevation, or at least a long 

 period of rest. In instances like the Gambicr gi'oup the reefs 

 situated on the seaward side of tlic outer islands would grow nioro 

 vigorously than those towards the interior ; they would extend in the 

 direction of the shallower water, and ultimately would form a con- 

 tinuous barrier around the whole group. The distinguishing feature 

 of the views now advanced is that they do away witli the great 

 and general subsidences required by Darwin's theory, and are in 

 harmony with Dana's views of the great antiquity and permanence of 

 the great ocean basin, which all recent deep-sea researches appear to 

 support. 



• ' Zuitsclir. wibs. Z<x^l.,' xiii. p. 50;5. 



