GEOLOGY AND PALEONTOLOGY OF THE CANAL ZONE. 277 



is indented, and a number of instances, Road Bay, for example, of 

 the separation by beaches of an inner lagoon from an outer bay are 

 present. Three instances of inclosed basins having underground 

 communication with the sea were noted (pi. 69, fig. C). No definite 

 terraces are present and wave-cut cliffs are greatly developed. 



That the last important change of sea level Avas by submergence 

 of the land is evident from the character of the shore hne in St. Bar- 

 tholomew, St. Martin, and Anguilla; and in Anguilla additional 

 evidence is afforded by the underground communication between 

 inclosed basins in the limestone and the sea. Stable condition of the 

 shore line for a considerable time is attested by the wave-cut chfTs, 

 the development of the beaches, the alluvial fillings at the heads of 

 reentrants into the landmass, and in St. Martin by the presence of 

 unterraced flood plains along the streamways. 



In my paper on the littoral and sublittoral physiographic features 

 of the Virgin and northern Leeward Islands, referred to in the foot- 

 note on page 272, 1 have shown that on the windward side of the St. 

 Martin plateau there is an outer deeper fiat, 26 to 36 fathoms below 

 sea level, with a maximum length east and west of over 30 miles, and 

 that this flat may be subdivisible into two subordinate terrace flats. 

 The scarp on the landward side of the deeper flat in places is about 

 50 feet high, in depths between 20 and 28 fathoms ; above the deeper 

 flat is a shallower one, whose outer edge is about 20 fathoms under 

 the sea (see text-fig. 11, p. 275). Other submarine evidence of sub- 

 mergence in this area is given in my paper cited. At the time the 

 shore line around the St. Martin Plateau was about 20 fathoms 

 lower than at present, Anguilla, St. Martin, and St. Bartholomew 

 must have been united. The biologic evidence at present available 

 is not sufficient to be decisive, but all that is known accords with 

 this interpretation. Notches on the outer edge of the plateau sim- 

 ulate hanging vaUeys and may represent the outer ends of vaUeys 

 cut while the sea stood about 40 fathoms lower than now; but the 

 information on these is too scant to justify more than a suggestion. 



The hydrographic chart does not show weU the reefs of these islands, 

 nor does the British Admiralty West India Pilot give a good descrip- 

 tion of them. Because of rough weather most of my own observa- 

 tions were made from the shore. Coral reefs occur across the en- 

 trances to most of the bays on the northeast and southeast sides of 

 St. Bartholomew: reefs are well developed on the east side of St 

 Martin, of? North Point, and on the southeast side of Tintamarre 

 Island; and there are dangerous reefs off the southeast coast of 

 AnguiUa and on the north coast of the east end of the island. Seal 

 Island reefs occur on a ridge extending westward from the northeast 

 end of Anguilla. Some of these reefs are of the barrier type, as 

 navigable channels lie between them and the shore, one at Forest 

 Point is an instance. 



