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



Each of these topics will be considered only in its relation to the 

 main problem. 



The Effect of Floods upon a Cover of Vegetation. 



A torrent dashing through a narrow gorge is the poet's symbol 

 of resistless force ; a great river in flood, bearing on its surface 

 houses, trees and other floating materials in prodigious quantity, 

 seems possessed of almost illimitable power for destruction. These 

 conditions, so familiar to all, have led to the conception that, in the 

 eroding and transporting power of streams in flood, one can find 

 explanation for the origin not only of sandstone and other inorganic 

 deposits but also for that of coal and lignite beds interstratified with 

 them. So much importance has been assigned to this explanation 

 bv several authors, that the phenomena must be considered in detail. 



The Work of Torrents. — The torrent in full flood is an interest- 

 ing spectacle but its importance in this connection is confined chiefly 

 to its bearing on the origin of inorganic sediments. 



Long ago De Luc^ described deposits made by torrents as resem- 

 bling a much flattened loaf of sugar and he employed the term 

 " cone " to distinguish them from the talus at foot of clifl^s. When 

 a stream ceases the cutting down of its bed. the formation of a cone 

 ends and vegetation takes possession of the surface, even of steep 

 slopes alongside of the stream. Eventually the surface is covered 

 with a thin coat of soil and men settle upon it. Such a cone was 

 seen by De Luc on the right bank of the river Arc, en route from 

 Mount Cenis into Italy. It extends from Aigue-belle to Saint-Jean 

 de Maurienne and the line around its base is nearly three miles. It 

 is high against the rocky wall and the surface is comparatively steep. 

 Its history is the same with that of many others. The stream cuts 

 a channel in the cone ; occasionally, during a great flood, the banks 

 are overflowed, but the injury is not enough to drive away the inhabi- 

 tants. A fall from the walls of the gorge forms a talus against 

 which the stream flows; water finds its way through the sands and 



' J. A. DeLuc, " Lettres physiques et morales sur I'histoire de la terre e*^ 

 de I'honime," Vol. II., 1780, pp. 67-68. 



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