GEOLOGY AND PALEONTOLOGY OF THE CANAL ZONE. 



243 



He adds, however, 



* * * but as we have seen, the larger groups of atolls in the Pacific and Indian 

 Oceans have not been formed on banks of this nature. ' 



Fig. 6. Reproduction of J. B. Jukes' section across the great barrier reef of Australia, a. 

 Sea outside the barrier, generally unfathomable, b. The actual barrier, c. Clear chan- 

 nel inside the barrier, generally about 15 OR 20 fathoms deep. d. The inner reef. e. Shoal 

 channel bet'wieen the inner reef and the shore. F. The great buttress of calcareous rock, 

 formed of coral and the detritus of corals and shells. O. The mainland, formed of granites 

 and other similar rocks. 



2. The first important protest against the Darwinian explanation 

 was by Carl Semper,^ who, in 1863, after studies in the Pelew Islands, 

 advanced the hypothesis that atolls could be formed in areas of 

 elevation by the solution of the interior of preexistent limestone 

 masses, and that solution, erosion by currents, and wave-cutting 

 could develop platforms behmd fringing reefs, thus transforming a 

 fringing into a barrier reef. 



3. Murray ^ introduced the idea of banks bemg built upward 

 by showers of the remains of pelagic organisms until the bathymetric 

 zone of reef-forming organisms is reached, and he called attention to 

 the cutting of volcanic islands down to wave base. His theory has 

 been briefly summarized by himself in the following words:* 



That when coral plantations build up from submarine banks they assume an atoll 

 form, owing to the more abundant supply of food to the outer margin, and the removal 

 of dead coral rock from the interior portions by currents and by the action of the 

 carbonic acid gas dissolved in sea-water. 



That barrier reefs have been built out from the shore on a foundation of volcanic 

 debris or on a talus of coral blocks, coral sediment, and pelagic shells, and the lagoon 

 channel is formed in the same way as a lagoon. 



That it is not necessary to call in subsidence to explain any of the characteristic 

 features of barrier reefs or atolls, and that all these features would exist alike in areas 

 of slow elevation, of rest, or of slow subsidence. 



4. H. B. Guppy in 1890 published the following important opmion 

 regarding the relations of barrier reefs to submarine plateaus or 

 ledges :^ 



I have now gone far enough to establish the probability, judging from the instance of 

 the Australian Barrier-reef, that reefs of this class are in reality, and not in appearance, 



1 Structure and distribution of coral reefs, ed. 3, pp. 138, 139. 

 ' Semper, Carl, Reisebericht, Zeitsch. fiir wiss. Zoologie, vol. 13, pp. 563-569, 1863. 

 3 Murray, John, On the structure and origin of coral reefs and islands, Roy. Soc. Edinburgh Proc, 

 vol. 10, 1879-80, pp. 505-518, 1880. 

 * Idem, p. 517. 

 6 Guppy, H. B., The origin of coral reefs, Victoria Inst. Joum. Trans., vol. 23, pp. 51-61, 1890. 



