Davis. — Sig/iiffcdnf Ffafiirc.f of Rf tf-hordered Coasts. 29 



Following Gilbert's method of interpreting the terraces of Lake Bonne- 

 ville, and assuming that the reefs rest imconformably on their rock 

 foundation, a series of superposed reefs, one resting on the other, must 

 have been formed during successive pauses in a long submergence and 

 afterwards rapidlv elevated ; while a series of apposed reefs, one in front 

 of the other, must have been formed during pauses in a long emergence, 

 preceded by a rapid submergence. Such a structure as is shown in fig. 10 

 should be interpreted as meaning that reefs A and B were formed during 

 pauses in submergence, while reefs C and D were formed during pauses 

 in emergence. It is manifest that all details of reef-structure such as are 

 here suggested should be critically observed. 



Summary. 



It is singular that the coral-reef problem, which has been so long under 

 discussion, should not have been already so far standardized as to make 

 the suggestions contained in this article unnecessary ; but, as a matter 

 of fact, neither the special reports by various investigators of coral reefs, 

 nor the leading text-books of geology and of physical geography, present 

 the problem in such a form as to emphasize the matters that are of the 

 greatest importance in its solution. Factors so essential as shore-line 

 embayments and unconformable reef contacts often receive no mention 

 whatever. The meaning of unconformable fringing reefs has been 

 almost universally overlooked. The forms of spur-ends on reef-encircled 

 islands are hardly ever described. The disposal of the waste from a 

 deeplv-dissected, reef-encircled island has received no discussion. Elevated 

 reefs, even if unconformable with their foundation, have nearly always 

 been interpreted as having been formed during pauses in the movement 

 of uplift by which they were elevated, and no recognition has been 

 given to the manifest possif)ility of their formation during pauses in a 

 preceding subsidence. 



Several reasons for the neglect of these essential considerations may be 

 suggested. One is that the investigators of coral reefs have often been 

 zoologists, untrained in geological inquiry Another is that the physio- 

 graphic principles which are involved in a critical study of the reef 

 problem are not always familiar even to geological observers. A third 

 and perhaps the most important reason is that Iq-w investigators of coral 

 reefs a]ipear to have taken the time necessary to think out the essential 

 consequences of the several leading theories of reef-origin in order to 

 discover which of the consequences are best supported by the facts. 

 A fourth, as important as the third, is that observers have too often 

 given their chief attention to the reefs, and liave not attended sufficiently 

 to the islands that they encircle. A fifth is that the origin of coral 

 reefs is a very complicated matter, because many different factors 

 may have a share in it, and many different solutions therefore appear 

 possible. 



It is in the hope of overcoming these deficiencies in the methods of 

 reef-investigation that the preceding pages have been written. While it 

 is recognized that the coral reefs constitute a wonderiul field for zoological 

 study, and that such study throws much light on the life-history of reefs in 

 the past, it is urged that the geological and physiographic study of reef- 

 encircled islands is necessary in order to discover the past inorganic 

 conditions under which reefs were developed. 



