524 STEVENSON— FORMATION OF COAL BEDS. [November 3, 



is subject to abrupt rise in the early summer, when hot days cause 

 rapid mehing of the snow on those peaks. The upper streams pass 

 over hard rocks and among huge masses which choke the channels, 

 so that, below the union, the creek, following a rocky, mostly narrow 

 gorge and with rapid fall, carries even in great flood comparatively 

 little sand or silt. A rise of 10 feet within 3 or 4 hours is of fre- 

 quent occurrence. The low narrow " bottom," though so often 

 overflowed by rapidly moving water, is grassy and bears some shrubs. 

 In some places, where the canyon widens and the stream, under ordi- 

 nary conditions, flows less rapidly, one finds petty islands of detritus 

 covered by shrubs, some of them more than 15 feet high. Clearly 

 these plants are torrent-proof, they have maintained their place from 

 birth. Similar conditions were observed on many other streams of 

 like character. 



A Cover of Fcgctat-ioii Protects against Erosion. — Within any 

 given district of moderate size, some streams are turbid while others 

 arc limpid even in flood time. The water may be limpid or turbid 

 in dilTerent portions of the same stream. As the rainfall is the same 

 throughout there must be local causes for the variation. 



The process of erosion begins with the impact of a raindrop ; 

 but that impact is incft'ective if the drop fall on more or less elastic 

 material. Thus it is that a cover of decayed vegetable matter, a 

 coating of hunuis, protects a slope against the erosive power of rain- 

 fall ; it protects existing vegetation from removal and it may enable 

 plants to regain hold on spaces l)are(l by fire or other agents, even 

 though the slope he abrupt. 



Ashe* studied an area which has great variety of soils, as the 

 section extends from tlie Archean to the Quaternary. Soils from 

 partly mctaniorpltosed sandy shales or from mica schist oft'er little 

 resistance to erosion : in some cases they cannot nourish a sod but 

 each supports its own ly])e of trees. Denuded of forest, the surface 

 is gashed quickly 1)\ rains, but when forested it loses little. The 

 accunuilation of litter or lumuis within the Potomac area is small 



' W. W. Ashe, " Relations of Soils and Forest Cover to Quality and 

 Quantitj' of Surface Water in the Potomac I'.asin,"' l'. S. Gcul. Sufvcy, 

 Jl'aler Supply and Irrigation Paper, 192, 1907, pp. 29<>-335. 



122 



