Page:Popular Science Monthly Volume 53.djvu/23

Rh 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 movements 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 locality has been estimated by Mr. Mitchell at only two feet in a century. The depression of land about the mouth of the Mississippi has lately been measured by Mr. E. L. Corthell, who finds that the sinking there is at the rate of five feet in a century. 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. Lawrence Valley, upon the northwestern flanks of the Adirondack Mountains, 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.

.—Numerous 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 Hudson 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, Delaware, 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 suggestions of a still greater elevation which he then hesitated to follow up on account of their startling character. The existence of the