Although there are in existence numberless books, many of them of great excellence, dealing with the effects of denudation (Geomorphology) nevertheless long experience and much reading seem to show that the more descriptive side of the subject, the result of the processes, has received more attention than the actual mechanism by which the results have been brought about. This applies more particularly to river erosion. In most books there is at the outset a tacit assumption that rivers do erode and the matter is left at that. Now rivers and running water generally certainly do perform the work of denudation somehow, as is proved beyond doubt by the large amount of material sometimes carried in suspension. But a careful consideration seems to show that the subject is not quite so simple as would appear from most textbooks.