THE CONTINENTAL SHELF. 109 



America it reappears, having a width of from 150 to 300 miles. An 

 elevation of only 50 feet would turn the Bahama l)anks into a broad 

 land area, and a rise of 100 to 300 feet would extend the continent to 

 the edge of the upper shelf in front of the southern states, coastwise of 

 which the lower terrace, or Blake plateau (so named by Professor A. 

 Agassiz), is de})ressed from 2,500 to 3,500 feet. A considerable portion 

 of this drowned plain has an average depression of 2,700 feet. East 

 of the continental shelf (both the normal and the Blake plateau) the 

 continental margin descends rapidly to a depth of 12,000 feet or more. 

 Glancing at the accompanying map (section B B'), the drowned flats, 

 seaward of the coastal plain of the southern states, behind which are the 

 Appalachian mountains, and the Bahama banks in front of the plains 

 and mountains of Cuba and Haiti, may be seen without any epeirogenic 

 break and with only the interruptions of a few great valleys, the ana- 

 logues of which occur on the continent, indicating that the whole conti- 

 nental shelf is a geologic unit and must be so treated. The Blake ]:>lateau 

 may be somewhat modified by the Gulf Stream, the bottom of which in 

 passing over the highest ridges of the straits of Florida is only about 

 2,100 feet below the surface, or several hundred feet above the mean depth 

 of the water over the Blake plateau ; but it is to be remembered that the 

 force of the current, which diminishes with its depth, is very much re- 

 duced except on the more elevated surfaces of the submerged terrace. 

 It might be noted here that in such places the current keeps the surface 

 of the rocks free from loose deposits (Agassiz). The depressed plateau 

 has all the api)earances of the modern coastal plain of the continent, as 

 if formed in the same way, which would indicate a long continued ele- 

 vation of the region as great as is the present submergence. This shelf is 

 traversed by great valle3\s and fjords, and its margin is indented l)y cor- 

 resi)onding embay ments. 



The western side of the Florida mass shows the continental shelf sub- 

 merged to a depth of about 300 feet, or somewhat more, beyond which 

 there is a steep descent to 10,000 feet. The northern slope of the Mexi- 

 can basin is not so precipitous as that of the Florida shelf (section A A' 

 on map). Moreover, in the vicinity of the ]\Iississipi)i fjord there is a 

 terrace like the JUake plateau su1)nierge(l to a])out 4,000 feet. Nortli of 

 the Yucatan lianks there is a i)recipitous descent (section A A' on ma})) 

 from the shelf (which is there submerged from 50 to 300 feet) to the floor 

 of the (iulf ])asin, aljout 12,000 feet below the surface. This trough con- 

 tinues to the isthmus of Tehuantepec. 



The continental shelves also occur in the Caril)bcan sea and the sea of 

 Honduras. Those basins are separated by the Honduras and Bosalind 

 l)anks and .Taniaica. Ow the northern side the sea of Honduras is sei)a- 



