﻿320 BULLETIN 103, UNITED STATES NATIONAL MUSEUM. 



in depth, and is separated by an escarpment from a shallower plain 

 that now ranges between 17 and 20 fathoms in depth. Wliat appear 

 to be marginal hanging valleys north of St. Thomas and on the St. 

 Martin Plateau, and solution wells, in the Bahamas, 33 to 38 fathoms 

 deep, suggest that there may have been a short stand of sea level 

 about 40 fathoms below its present stand. 



7. The fact that the terrace flat between 17 and 20 fathoms in 

 depth is cut away on promontory tips on tlie windward side of St. 

 Thomas, while it is preserved in protected areas, indicates that the 

 higher flat is older than the lower, and that it has been resubmerged 

 after the development of the lower flat. The general similarity of 

 the submarine profiles off Antigua, on the St. Martin Plateau, and on 

 Mosquito Bank favors the inference that there was in those areas a 

 similar lowering and subsequent rise of sea level. The submerged 

 channel within the channel at the mouth of Habana Harbor, and 

 simdar phenomena at other localities around the Cuban coast, show 

 that during later Pleistocene time Cuba stood more than 100 feet 

 higher than immediately previous to the cutting of these valleys 

 within older vaUej^s, and that after the valle^^s-wi thin -valleys were 

 formed there was submergence to an amount of about 100 feet. Fall 

 of sea level during Pleistocene time and rise auiing Recent time is 

 indicated for the Bermudas, the Bahamas, Florida, Central America, 

 and the mouth of the Amazon, as well as for the areas just mentioned. 

 These phenomena are in essential accord with the demands of the 

 Glacial-control hypothesis. 



8. The principal living West Indian and Central American reefs are 

 superposed on submarine flats or plateaus of pre-Pleistocene age, that 

 were dry-land areas during at least a part of Pleistocene time, and 

 while they were dry land they were wave cut and remodeled around 

 their margins by submarine planation. 



9. There are two kinds of atolls, namely, (a) those that rise above 

 relatively shoal-water platforms and were shaped by the prevalent 

 currents, which are largely wind induced ; (h) those that more or less 

 completely encircle the flat summits of eminences that rise from 

 ocean depths. These rings are formed by constructional geologic 

 agencies, because, as submarine solution by sea water in such areas 

 and at such depths is chemically impossible, a lower, flat area, sur- 

 rounded by a higher rim can not be formed by submarine solution 

 or by any other known destructional agencies. The depths on such 

 banks as Saba, Pedro, Rosalind, etc., indicate that they were in 

 large part, at least, above water during part of Pleistocene time, 

 and that the flat summits are largely due to processes operative in 

 pre-Pleistocene time. What the processes were that caused the 

 leveling of the summits is a matter of pure speculation, but it seems 

 probable that they were subaerial erosion and submarine planation. 



