I9I3.] STEVENSON— FORMATION OF COAL BEDS. 139 



tree trunks — a method employed in South America, Austraha and on 

 the Pacific coast for semi-torrential as well as for lowland floods. 

 The overflow flood, that portion outside of the channel, moves slowly 

 at the bottom and does not scour ; instead, it deposits inorganic mate- 

 rials. If forced aside into a narrow space, it may cut a channel; 

 but in that case it has ceased to be a flood and has become a local 

 current. These are characteristics of lowland floods everywhere ; 

 the movements of water are governed by the same law throughout 

 the world ; there is no reason to suppose that other laws prevailed 

 during earlier periods, to be repealed abruptly at the beginning of 

 the Quaternary. 



The floods of torrents can hardly be regarded as supporting the 

 doctrine of allochthony. In some features they resemble those on 

 lowlands, but in many ways the phenomena are different. Ordina- 

 rily, torrents flow in narrow valleys, more or less gorge-like with 

 here and there a petty flood-plain, on which trees grow. Some large 

 rivers, such as the Potomac, Monongahela and others rising in the 

 Appalachian chain, are torrential during flood in the greater part of 

 their length, but dififer from the ordinary torrent in the width of 

 their valleys and of the wooded flood-plains. In all, the rapidity of 

 flow suffices to carry ofl: the water, with, at most, trifling overflow of 

 the plain, the chief change being in the channel which may be 

 widened or deepened. At ordinary stages, torrents, in areas of con- 

 solidated rocks, transport very little mineral matter and the water is 

 the plain, the chief change being in the channel which may be 

 very great. But the coarse material is pushed along the bottom, 

 except in extraordinary instances, and comparatively' little is carried 

 over to the flood-plain. The rushing water does insignificant injury 

 to trees or plants on that plain, in spite of great speed, as was shown 

 well during the great flood of the Potomac. One can see this for 

 himself along mountain torrents, where trees grow to within a foot 

 of the ordinary water line. There are many such torrents in the 

 central plateau of France, whose fierce floods have done no more 

 injury to trees on their rocky walls than is done to trees by the low- 

 land floods of the Seine area ; only fallen stems and other unattached 



