GEOLOGY. 371 



brought about under which the living coral reefs have formed." 

 Evidence has been presented showing conchisively that iVntigua, the 

 islands standing above the St. Martin Plateau, St. Croix, and the Virgin 

 group have undergone geologically Recent submergence.^ Should the 

 sea-level have remained stationary for a period of appreciable length 

 antecedent to this submergence, there should be a submerged scarp 

 indicating its former stand; should there have been a succession of 

 temporary stands there should be a series of submarine terrace flats, sep- 

 arated by scarps. In other words, there should be a submarine physi- 

 ographic record, which, if it could be recovered, should be as readily 

 deciphered as a subserial record. The only available sources of infor- 

 mation were the charts of the U. S. Hydrographic Office and of the 

 British Admiralty. Two areas were selected for special study, the 

 Virgin Bank, east of Porto Rico, and the St. Martin Plateau. The 

 charts of the former area are on a scale of slightly more than 1 mile to 

 an inch; that of the latter is on a scale of 2.5 miles to 1.06 inches. 

 These charts were contoured on a 2-fathom (12 feet) interval from the 

 shore to a depth of 40 fathoms, and on an interval of 10 fathoms (60 

 feet) from 40 to 100 fathoms. This method of treatment brought out 

 well-developed submarine terraces, as well as other submarine physio- 

 graphic features, especially on the windward side of the banks. 

 Although the charts need further study, certain features are clear and 

 some of them will be briefly characterized. 



On the north side of St. Thomas there is an extensive outer flat, 

 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 8.5 miles the range in relief of the surface is as small as 2 

 fathoms, in depths between 29 and 31 fathoms. Its outer margin is 

 cut by reentrants which have bottoms about 40 fathoms deep and which 

 simulate hanging valleys. There are also, near the outer margin of 

 this terrace, 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 plantation and then submerged, as indicated by other evidence. 

 As all the other shoals with one exception are truncated at nearly the 

 same level, it seems that they 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 is a submarine plain, which several lines of evidence 



iBuU. Amer. Geograph. Soc, vol. 46, pp. 426-429, 1914. 



