vaughan: virgin and leeward islands 61 



There is doubt as to the interpretation of the 7 to 10 or 12 

 fathoms 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 sea-level. 



According to the physiography of the sea-bottom, the Vir- 

 gin 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 betrachians found in 

 St. Thomas and St. John form only a herpetological appendix 

 to Porto Rico." Doctor Bartsch informs me that the testi- 

 mony 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; (b) above 

 the outer edge of the 14 to 20 fathoms flat; (c) above the outer 

 edge of the 28 to 34 fathoms 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 

 must have developed during subsequent submergence. The 

 flat therefore cannot be due to the growth of the reef. 



The members of the St. Martin Group have indented shore- 

 lines, seacliffs, and an unusually fine development of bay bars. 

 The relations on the windward side of the St. Martin plateau 

 are similar to those north of St. Thomas (see fig. 3) . The outer, 

 deeper flat, from 26 to 36 fathoms in depth, has a maximum 

 length east and west of over 30 miles. It seems composed of 

 two terraces. The scarp on its landward side is distinct and 

 in places is about 50 feet high, in depths between 20 and 28 fath- 

 oms, as off the east end of Scrub Island, east of Anguilla Island. 



As some of the submerged valleys on the east side of the St. 

 Martin plateau resemble valleys in the Upper Cretaceous Ana- 

 cacho limestone of Texas, it appears that not only must the 

 scarp line which has been pointed out be interpreted as a former 



