300 Proceedings of the Royal Physical Society. 



" run in.'' It appears to be a law attending the travelling of 

 river-curves, that, other things being equal, they shall flow 

 by preference in parts of their valley where the curves can 

 travel rapidly ; for, if a river seek to thrust a curve into a 

 rocky bank when it has the choice of a course through al- 

 luvium near at hand, other curves supersede the eccentric 

 one, and the water is drawn away from it. Eivers, in a word, 

 cannot but concentrate their channels as they excavate them, 

 unless the amount of planation is out of all proportion to the 

 rate of deepening. 



5. The division of valleys into gorges and basins has tended 

 to regulate and equalise the effects of spasmodic movements 

 of elevation at the coast. It seems to be quite impossible, 

 with that arrangement, that ^' as the sea fell from one level to 

 another, so must also the rivers have fallen from one level to 

 another." The cutting through of a rock-barrier in a river- 

 course is not accompanied by any such spurts and stoppages as 

 can produce marked stages of planation and repose in the 

 basin above it. Pulsations and fluctuations there doubtless 

 are. There are cycles in the meteorological causes that pro- 

 duce floods ; there may be alternations of harder and softer 

 rock in the barrier ; and there are other variations, such, for 

 instance, as when a waterfall has worked back to the edge of 

 an alluvial flat, a circumstance which is doubtless productive 

 of some leisurely acceleration in its excavation. The un- 

 stable equilibrium maintained by a river in the various parts 

 of its channel, is, without doubt, liable to slight derangements 

 (or re-adjustments) of many different kinds. But when the 

 relations of a river to its bed are considered, the erosive im- 

 petus of the one seldom in two places the same, the varia- 

 tions of harder and softer in the other seldom equally resistant 

 for many yards together, it is hard to escape the conclusion 

 that the effects of these pulsations are lost, — like pulsations 

 of all kinds, by travelling through a succession of varying 

 media. Successive uprisings of the coast in these circum- 

 stances will have so blent their effects as to have in the in- 

 terior the results of equable elevation. 



When, therefore, it is assumed that " as the sea fell from 

 one level to another, so must the rivers also have fallen from 



