84 ALLUVIUM. [Ch. VH 



tombed in the strata, -which, enable us to determine their age and 

 mode of origin, we regard them as part of the regular series of fos» 

 siliferous formations, whereas, if there are no fossils, we have frequently 

 no. power of separating them from the general mass of superficial al- 

 luvium. 



The usual rarity of organic remains in beds of loose gravel is partly 

 owing to the friction which originally ground down rocks into pebbles or 

 sand, and organic bodies into small fragments, and it is partly owing to 

 the porous nature of alluvium when it has emerged, which allows the free 

 percolation through it of rain-water, and promotes the decomposition and 

 solution of fossil remains. 



It has long been a matter of common observation that most rivers 

 are now cutting their channels through alluvial deposits of greater depth 

 and extent than could ever have been formed by the present streams. 

 From this fact a rash inference has sometimes been drawn, that rivers in 

 general have grown smaller, or become less liable to be flooded than for- 

 merly. But such phenomena would be a natural result of considerable 

 oscillations in the level of the land experienced since the existing valleys 

 originated. 



Suppose part of a continent, comprising within it a large hydrographical 

 basin like that of the Mississippi, 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 latitudes 

 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 power to carry its 

 sand and sediment into the main river, and the main river less power to 

 convey its annual burden of transported matter to the sea. All the rivers, 

 therefore, will proceed 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 would begin to aug- 

 ment. Each of them would 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 channel 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 fringing 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 cliffs of gravel and sand, facing 

 the river. That such a series of movements has actually taken place in 

 the main valley of the Mississippi and in its tributary valleys during 



* Principles of Geology, 7th ed. p. 506, 8th ed. p. 509. 



