GEOLOGY AND PALEONTOLOGY OF THE CANAL ZONE. 279 



less than three-quarters of a mile on the west end; and on the north 

 side west of Sugar Bay the distance ranges from one-quarter to one- 

 haK mile. Off the south shore the distance to the 100-fathom curve 

 in places slightly exceeds 3 miles; off the east end for 7 miles the 

 water is less than 40 fathoms deep, while off the north coast the 

 platform gradually narrows westward until near Salt River Point its 

 width is less than one-half mile. 



There is a long, disconnected barrier reef off most of the south 

 coast, and barrier reefs are present off the north coast to a short dis- 

 tance west of Christiansted. The indented, ragged coast hne and 

 the depth of water on the platform so clearly point to the same con- 

 clusion as that already drawn from a study of Antigua, St. Bar- 

 tholomew, etc., that reiteration is not necessary. 



VIRGIN BANK. 



The Virgin group of islands consists of about 100 small islands and 

 keys (text fig. 13). The bank above which they rise is an eastward 

 prolongation of that on which Porto Rico stands . The chart shows the 

 indented coast line and the extensive, relatively shoal platform above 

 the surface of which the islands project. The maximum depth of 

 water between the islands is about 17 fathoms. St. Thomas well 

 exhibits the coastal phenomena to which attention has already been 

 so often directed — reentrants with alluvial fillings at their heads, 

 unterraced alluvial bottoms along streamways, and wave-cut cliffs on 

 the unterraced promontories (pi. 70, figs. A, B, C). 



In my paper on some littoral and sublittoral physiographic features 

 of the Virgin and northern Leeward Islands, abeady referred to, it 

 has been pointed out that there are three terrace flats under the sea 

 off St. Thomas, St. John, Tortola, and Virgin Gorda (see text fig. 11, 

 p. 275). On the leeward side the deepest lies between 26 and 30 

 fathoms in depth and is separated by a scarp or steep slope on its 

 landward side from a flat ranging from 14 to 20 fathoms in depth, 

 which in turn is separated by a steep slope from a flat ranging from 

 6 to 10 fathoms in depth. On the windward side the respective 

 depths are 26 to 34 for the deepest flat, 14 to 20 fathoms for the 

 intermediate flat, and 7 to 10 fathoms for the shallowest one. The 

 intermediate flat is narrow or absent on the promontory tips on the 

 windward side, while it is preserved on the leeward side, strongly 

 suggesting, if not actually proving, that the intermediate flat is older 

 than the deeper one and was cut away in exposed places while the 

 deeper one was forming. This evidence necessitates the deduction 

 that in recent geologic time the Virgin Islands, except minor differ- 

 ential crustal movement in the vicinity of Anegada, have been sub- 

 merged to a depth of about 20 fathoms, and that they were previously 

 joined to Porto Rico, a deduction completely corroborated by bio- 

 37149— 19— BuU. 103- 7 



