42 PROCEEDINGS OP THE WASHINGTON MEETING 



the United States Hydrographic Office and of the British Admiralty. The 

 Virgin Bank and the Saint Martin Plateau were selected for special study. 

 The charts of the former, on a scale of slightly more than 1 mile to an inch, 

 and that of the latter, on a scale of about 2y 2 miles to an inch, were contoured 

 on a 2-fathom interval from the shore to a depth of 40 fathoms, and on an 

 interval of 10 fathoms in depths between 40 and 100 fathoms. 



The shoreline of the Virgin group shows indentations indicative of sub- 

 mergence, and that the sea has stood at its present level long enough for 

 alluvial filling of the heads of harbor digitations, while sea-cliffs occur at the 

 ends of promontories. The chart of the near-by sea-bottom shows that south- 

 of Saint John, Tortola, and Virgin Gorda there are two distinct submerged 

 terraces and a less definite third terrace. The outer terrace fiat lies at depths 

 between 26 and 28 fathoms on its landward and between 28 and 30 fathoms 

 on its seaward margin, and it ranges in width from a half mile to 3 miles. On 

 its sea front is a ridge which is inferred to be a submerged barrier coral reef. 

 On its landward side a scarp rises from a depth of 26 or 28 fathoms to about 

 17 fathoms. Above this scarp is a second terrace flat, which has a depth of 

 14 to 15 fathoms on its landward and a depth of 14 to 20 fathoms on its 

 seaward face, and ranges in width from one-third of a mile to 2 miles. Appar- 

 ently the outer margin of this flat also bears a coral reef. These are the two 

 principal terrace flats. The scarp separating them is indicated by crowded 

 contours, and chart number 1832, United States Hydrographic Office, shows 

 its continuity for 36 nautical miles, or about 1% land miles farther than from 

 Washington to Baltimore. A third still higher terrace flat is suggested be- 

 tween depths of 6 and 10 fathoms, above which a fourth terrace may now be 

 in process of formation, but the information regarding these is at present not 

 definite enough to warrant a positive statement. The continuity of the upper 

 one of the two well marked flats needs to be emphasized. It should be noted 

 that east of Virgin Gorda there has been an uptilt. 



On the windward side of Saint Thomas there is an extensive outer fiat, 

 bounded on its landward side by a steep escarpment which in places is nearly 

 160 feet high. The landward margin of the plain is between 26 and 28 fathoms 

 in depth; the seaward margin has a depth between 30 and 34 fathoms;, the 

 width is as great as 10 miles and for distances as great as 8% miles, in depths 

 between 29 and 31 fathoms, the range in relief of the surface may be as small 

 as 2 fathoms. Its outer margin is cut by reentrants which have bottoms about 

 40 fathoms deep and simulate hanging valleys. There are also near the outer 

 margin of this flat banks or ridges, the upper surfaces of which are relatively 

 flat, between 17 and 20 fathoms in depth. One of these banks has a total 

 basal width of about 4 miles and a length of more than 5 miles. As its form 

 is not that of a coral reef, it can only be the base of what was an island, 

 which had been reduced almost to a smooth surface by marine planation and 

 then, as indicated by other evidence, submerged. As all the other shoals, with 

 one exception, are truncated at nearly the same level, it seems that most of 

 them should be ascribed to a similar origin. These shoals usually show 

 escarpments between 20 and 30 fathoms on their windward sides and more 

 gradual slopes on the leeward sides. The outer flat on the north side of Saint 

 Thomas corresponds to the lower flat on the south side of Saint John, Tortola, 

 and Virgin Gorda. Both are submarine plains, which several lines of evidence 



