THE WEST INDIAN BRIDGE. 13 



areas, but they do not belong to the phenomena of thrusts which 

 give rise to mountain structures, nor are they the effects of volcanic 

 forces. The land rises or sinks so gently that usually these move- 

 ments do not disturb the courses of the drainage of the rivers, 

 although barriers sometimes do appear across valleys. The modern 

 changes of level illustrate these gentle oscillations. Islands and 

 farms, which were located upon the low coast of New Jersey in the 

 early settlement of the country, are now reduced in size, and are 

 partly converted into salt marshes. The rate of sinking in this local- 

 ity has been estimated by Mr. Mitchell * at only two feet in a cen- 

 tury. The depression of land about the mouth of the Mississippi has 

 lately been measured by Mr. E. L. Corthell, who finds that the sink- 

 ing there is at the rate of five feet in a century. f For these low lands 

 this subsidence promises to become a serious economic question in 

 the not distant future. On the other hand, certain northern regions 

 are rising. Thus, in the district of Niagara Falls, the rise is a foot 

 and a quarter in a century, or perhaps a little more. In the St. Law- 

 rence Valley, upon the northwestern flanks of the Adirondack Moun- 

 tains, the upward movement, for the last fifteen hundred years at 

 least, has been from four to five feet per century. Such gentle 

 changes, accelerated or retarded, and continuing sufficiently long, 

 are capable of transposing the ocean floors and mountain heights. 



Recent Elevation of the Eastern Coast of America. — jSTu- 

 merous soundings, chiefly for the use of navigators, but occasionally 

 taken for scientific purposes, have been made off the American coast 

 and in the West Indian seas. In order to insure mariners against 

 the occurrence of sunken rocks, the surveys have often been carried 

 from the coast all the way to oceanic depths. Upon the submarine 

 coastal plains the extensions of some of the great rivers have long 

 been known. From data thus collected, Lindenkohl traced the Hud- 

 son River across the submerged banks off the New York and New 

 Jersey coasts to a depth of nearly three thousand feet.:|: Later, the 

 writer * gathered evidence from the drowned St. Lawrence, Dela- 

 ware, Susquehanna, and other rivers as far as the Mississippi, and 

 it became apparent that the whole of the eastern coast of America 

 in recent times stood three thousand feet higher than now, with sug- 

 gestions of a still greater elevation which he then hesitated to follow 

 up on account of*their startling character. The existence of the 



* Of the United States Coast Survey. 



\ Geographical Development of the Lower Mississippi. Read before the Toronto meeting 

 of the British Association. 



X Appendix XIII, Report of the United States Coast Survey for 188*7 (1889), pp. 270-2*73. 



* High Continental Elevation preceding the Pleistocene Period. Bulletin of the Geo- 

 logical Society of America, vol. i, 1889, p. 65. 



