ANTIQUITY OP MAN. 235 



carry ourselves in imagination to the time when a 

 plain was raised out of the sea, bare and bald, and 

 a river began to run in it, we at once see our 

 error. The river so running and beginning to cut a 

 channel, and cutting back the falls which would occur 

 in its course, must in a few years execute a stupen- 

 dous work o erosion almost diluvial in its character ; 

 but in the course of centuries its work becomes com- 

 pleted, a state of equilibrium succeeds, and its banks, 

 protected by vegetation, scarcely experience any modi- 

 fication. An elevation to a higher level, or a new 

 depression succeeded by re-elevation, or forest fires or 

 other causes laying bare the isurface, would at once 

 initiate a new series of erosions ; but until this occurs 

 all things continue as they were. 



It must also be observed that in the period No. 4 

 (p. 233), there were not only oscillations of level, but 

 apparently a somewhat extreme climate, in which 

 alternate frosts and thaws and violent river floods must 

 have greatly aided the work of denudation ; and also 

 that in a wooded condition of the country, its streams, 

 as we know from sad experience of the effects of clear- 

 ings in America, are large in volume but equable in 

 flow, and that the removal of the forest leads to great 

 floods alternating with periods of desiccation, remark- 

 ably increasing and modifying the denuding power of 

 the streams. Sir Charles Lyell gives some striking 

 illustrations of this in his " Principles of Geology." 



It is, perhaps, necessary here to refer to the conclu- 

 sion recently developed at great length by Dr. James 



