60 vaughan: virgin and leeward islands 



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 St. Thomas, and the es- 

 carpment on the 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 out- 

 lying 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 St. 

 John and Tortola. It is scarcely represented on the seaward 

 side of the promontories, viz. : Cockroach and Cricket rocks, 

 and Outer Brass and Little Hans Lollick islands. However, 

 it spreads out on the flanks of the promontories and ranges 

 from half a mile to nearly 1| 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 escarp- 

 ment, 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 

 landward 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 fathoms flat on 

 the promontory tips, while it is so well preserved in protected 

 places, especially off the south sides of St. 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, subsequent to 

 the formation of which, after perhaps a brief interval of still 

 lower stand of sea-level, the entire area has been re-submerged, 

 to an amount about the same as that of the initial submergence. 



