102 
ELEMENTS OF GEOLOGY. 
Suppose, for exaruple, part of a continent, comprising with¬ 
in it a large hydrographical basin like that of the MississijDpi, 
to subside several inches or feet in a century, as the west 
coast of Greenland, extending 600 miles north and south, has 
been sinking for three or four centuries, between the lati¬ 
tudes 60° and 69° N.* It will rarely happen that the rate of 
subsidence will be everywhere equal, and in many cases the 
amount of depression in the interior will regularly exceed 
that of the region nearer the sea. Whenever this happens, 
the fall of the waters flowing from the upland country will 
be diminished, and each tributary stream will have less pow¬ 
er to carry its sand and sediment into the main river, and the 
main river less power to convey its annual burden of trans¬ 
ported matter to the sea. All the rivers, therefore, will pro¬ 
ceed to fill up partially their ancient channels, and, during 
frequent inundations, will raise their alluvial plains by new 
deposits. If then the same area of land be again upheaved 
to its former height, the fall, and consequently the velocity, 
of every river will begin to augment. Each of them will be 
less given to overflow its alluvial plain ; and their power of 
carrying earthy matter seaward, and of scouring out and 
deepening their channels, will be sustained till, after a lapse 
of many thousand years, each of them has eroded a new chan¬ 
nel or valley through a fluviatile formation of comparatively 
modern date. The surface of what was once the river-plain 
at the period of greatest depression, will then remain fring¬ 
ing the valley-sides in the form of a terrace apparently flat, 
but in reality sloping down with the general inclination of 
the river. Everywhere this terrace will present clifi*s of grav¬ 
el and sand, facing the river. That such a series of move¬ 
ments has actually taken place in the main valley of the Mis¬ 
sissippi and in its tributary valleys during oscillations of lev¬ 
el, I have endeavored to show in my description of that 
country ;f and the fresh-water shells of existing species and 
bones of land quadrupeds, partly of extinct races, preserved 
in the terraces of fluviatile origin, attest the exclusion of the 
sea during the whole process of filling up and partial re-ex¬ 
cavation. 
Littoral Denudation. —Part of the action of the waves be¬ 
tween high and low water mark must be included in subae¬ 
rial denudation, more especially as the undermining of clifi*s 
by the waves is facilitated by land-springs, and these often 
lead to the sliding down of great masses of land into the 
sea. Along our coasts we find numerous submerged for- 
* Principles of Geology, 7th ed., p. 506 ; 10th ed., vol. ii., p. 196. 
t Second Visit to the United States, vol. i., chap, xxxiv. 
