88 SEA AND LAND 



features are best known alon^' tlie coasts of the North Atlan- 

 tic, perhaps for the reason that there alone is the form of the 

 bottom ascertained. In going- from New York to Liverpool, 

 the traveller for the first hundred miles of his eastward course 

 passes over a portion of the sea where the water rarely 

 exceeds five hundred feet in depth ; the bottom slopes grad- 

 ually toward the central portion of the ocean, at the rate of 

 about five feet to the mile. There are occasional broad 

 swales on the nearly level floor, but as a whole it is much 

 more nearly a plain than an)- similarh' extensive part of the 

 prairie land of the Mississippi Valley, This gradual descent 

 toward the deep sea is terminated by an abrupt slope where 

 the bottom declines at the rate of from ten to one hundred 

 feet to the mile, from the crest of the submerged plain to 

 the abysmal depths of the ocean. 



Unlike most of the submarine topography it is possible for 

 us to orct a clear idea as to the "eneral character of this con- 

 tinental shelf from certain portions of the dry land which 

 have recently been elevated above the watery envelope. 

 While it is true that nearly if not quite all parts of the 

 continents have been formed on the ocean-floor, the greater 

 portion of the land surface has been so much warped by 

 mountain building, and made uneven by stream action, that 

 the original impress of the marine conditions has been 

 entirely lost. In the southern portion of North America, 

 from Virginia through the lowlands of the Carolinas, Georgia, 

 Alabama, Mississippi and a part of Texas and Louisiana, we 

 have a portion of this shelf, which was formed when the shore 

 was farther inland and when the area in question was below 

 the level of the sea, constituting a portion of the continental 

 deposits such as is now submerged along our shores. Like 



