FEESH-WATEB STREAMS. 647 



Extent of Erosion. — The outlining of mour. tain-ridges and val- 

 leys has been in part produced by subterranean forces, uplifting and 

 fracturing the strata ; but the final shaping of the heights is due to 

 erosion, and mostly, as has been stated, to erosion by fresh waters. 

 This cause has been in action ever since continents began to be ; and 

 it has been thus making earth and gravel for stratified rocks, as well 

 as gorging hills and mountains. The Appalachians have lost by 

 denudation much more material than they now contain. Mention has 

 been made of faults of ten thousand feet or more, along the course of 

 the chain, from Canada to Alabama. In such a fault, one side was left 

 standing ten thousand feet above the other, enough to make alone a 

 lofty mountain ; and yet now the whole is. so levelled off that there is 

 no evidence of the fault in the surface-features of the country. The 

 whole Appalachian region consists of ridges of strata isolated by long 

 distances from others with which they were once continuous. Fig. 

 103, page 96, represents a common case of this kind. The anthracite 

 coal-fields of central Pennsylvania were once a part of the great 

 bituminous coal-field of western Pennsylvania and Virginia (Fig. 613, 

 p. 310). They now form isolated patches ; and formations of great 

 extent have been removed from over the intervening country. The 

 coal-region of Great Britain is broken into many patches, in con- 

 sequence of similar denudation and uplifts. 



In New England, there is evidence of erosion on a scale of vast 

 magnitude, since the crystallization of its rocks. On the summit-level 

 between the head-waters of the Merrimac and Connecticut, there are 

 several pot-holes in hard granyte ; one, as described by Professor Hub- 

 bard, is ten feet deep and eight feet in diameter, and another twelve 

 feet deep. They indicate the flow of a torrent for a long time, where 

 now it is impossible ; and the period may not be earlier than the 

 Quaternary. Many other similar cases are described by Hitchcock. 



These examples of denudation are sufficient for illustration. The 

 other continents furnish cases that are no less remarkable. Scotch val- 

 leys and mountains gave to Hutton the first right ideas on the subject. 



In the work the ocean has taken some part, as explained beyond. 



2. Transportation by Rivers. 



The materials transported by running water are (1) stones, pebbles, 

 sand, and clay; (2) logs and leaves from the forests, and sometimes 

 trees that have been torn up or dislodged by the current ; (3) mollusks 

 or their shells, worms, insects, etc., attached to the logs or leaves ; 

 (4) occasionally larger animals, that have been surprised and drowned 

 by freshets, or bones that have been exhumed by the waters. 



The fine earthy material deposited by streams, or their sediment, is 



