258 BULLETIlSr 103, UXITED STATES NATIONAL MUSEUM. 



and the shore. In other volcanic islands the sea may not be held 

 back from the harder rocks and may cliff them. 



There are numbers of possibilities which deserve consideration, but 

 the actual explanation of how present conditions were brought about 

 is possible only through detailed field work in each area. 



Some other kinds of shore lines may be mentioned. It is well 

 known that one of the important factors in determinmg the amount 

 of cliffing and the character of the cliffing of some shores is geologic 

 structure. In an uphfted island composed of bedded sediments 

 which have been moderately tilted the highest cliffs will be on the 

 up-dip side along the line of the strike; the cliffs will decrease in 

 height from the up-dip exposure along the line of the dip, and on the 

 side of the island where the rocks pass beneath sea level there may be 

 almost no cliffs. These relations are well illustrated in Anguilla and 

 other islands in the West Indies. After such an earth block has 

 been outlined there may be oscillation of strand line without further 

 local crustal deformation. 



The island of St. Croix is interesting in this connection. Just 

 south of its north shore, which is determined by a fault, are maturely 

 dissected mountains which attain an altitude of about 1,000 feet. 

 Off the south foot of the highland is a sloping, slightly undulating 

 plain, underlain by limestone, which extends to the south coast, 

 (See pi. 70, fig. D). If this island were submerged 120 feet the lime- 

 stone plain would form a submarine flat from one to about three 

 sea-miles wide. Corals might grow on such a flat and form a barrier 

 reef inside which there would be no strongly cliffed spurs along the 

 shore, while the mountains would be in a stage of mature dissection. 



American Tertiary and Pleistocene Reef Corals and Coral Reefs. 



Most investigators of the genesis of coral reefs have considered 

 only the modern; but the ancient, or fossil, reefs in many instances 

 afford better opportunities than the living reefs to determine the 

 geologic character of the basement on which the reefs have been 

 built, the change in the relation between the reef basement and sea 

 level, and the importance of corals as constructional agents. The 

 southeastern United States and near-by West Indian Islands furnish 

 numerous examples of both ancient and modern coral reefs, and 

 these have been the subject of investigation for many years. The 

 location of the Tertiary fossil reefs in the southeastern United States, 

 their associated faunas, the inclosing sediments, including in most 

 instances both the overlying and underlying strata, the stratigraphic 

 relations of the successive geologic formations, the geologic structure, 

 and the geologic history, have been ascertained with a fair degree 

 of accuracy. The coralhferous beds range in age from the base of the 

 Eocene to Kecent, and the coral fauna of each geologic formation is 



