ABSTRACTS AND DISCUSSIONS OF PAPERS 43 



show were developed when sealevel was about 20 fathoms, or slightly more, 

 lower than now. The escarpment extending from the islands north of Culebra 

 Island, east of Porto Rico, across the Virgin Passage, and along the north side 

 of Saint Thomas, and the escarpment on the seaward face of the outlying- 

 shoals apparently can be explained in no other way. 



The indentations on the outer margin of the outer flat may have been caused 

 by emergence and stream cutting after its formation, or they may be due to 

 initial marginal irregularities which have not been obliterated. 



The approximate accordance in level of the tops of the outlying shoals at 

 depths between 17 and 20 fathoms has been mentioned. These summits accord 

 in height with a flat or gently sloping zone, which lies above and nearer shore 

 than the deeper flat and represents the 14 to 20 fathom flat south of Saint 

 John and Tortola. It is scarcely represented on the seaward side of the 

 promontories, namely, Cockroach and Cricket rocks and Outer Brass and Little 

 Hans Lollick Islands. However, it spreads out on the flanks of the promon- 

 tories and ranges from half a mile to nearly iy 2 miles in width ; it is separated 

 on its seaward side by a steep slope or escarpment from the deeper flat and 

 on its landward side by a less distinct escarpment, in places about 26 feet in 

 height, from a less developed flat, which has a depth of 7 to 10 fathoms. The 

 descent is sudden from the shore to about 6 fathoms, which is near the land- 

 ward margin of the highest submarine flat. This flat also is narrow on the 

 tips of the promontories mentioned, but widens on their flanks and along the 

 shores of the main island. The submerged valley in Charlotte Amalia Harbor 

 has a depth of 10 fathoms. 



The narrowness or absence of the 14 to 20 fathom flat on the promontory 

 tips, while it is so well preserved in protected places, especially off the south 

 sides of Saint John and Tortola, shows that it is older than the deeper flat, 

 and in exposed places was cut away during the formation of the latter, sub- 

 sequent to the formation of which, after perhaps a brief interval of still lower 

 stand of sealevel, the entire area has been resubmerged to an amount about 

 the same as that of the initial submergence. 



There is doubt as to the interpretation of the 7 to 10 or 12 fathom flat. In 

 places it seems to be distinct and older than the one next lower, but it may 

 represent the submarine terrace being formed at present sealevel. 



According to the physiography of the sea-bottom, the Virgin Islands were 

 joined to Porto Rico during the cutting of the scarp separating the deepest 

 from the next higher flat. The biogeographic evidence shows conclusively 

 that the two were united and have been severed in Recent time by submergence. 

 Stejneger says in his Herpetology of Porto Rico : "It is then plain that the 

 16 species of reptiles and batrachians found in Saint Thomas and Saint John 

 form only a herpetological appendix to Porto Rico." Doctor Bartsch informs 

 me that the testimony of the land Mollusca is the same as that of the reptiles 

 and batrachians. The biogeographic evidence substantiates the deductions 

 based on the purely physiographic study. 



There are three tiers of coral reefs in the Virgin Islands. They rise above 

 (a) basements 10 fathoms or less in depth; (6) above the outer edge of the 

 14 to 20 fathom flat; (c) above the outer edge of the 28 to 34 fathom flat. 

 As the escarpment within the outermost reef could not have been cut during 

 the presence of such a reef, the flat must be older than the reef and the reef 



