WORK IN PHYSIOGRAPHY 359 



vaguest nature. It was Ramsay's merit that he based 

 his discussion upon the results of careful surveying. 

 He had traced out the structure of a complicated 

 geological region, and was able to show what should 

 have been the form of the surface had it depended 

 entirely on geological structure. He was thus in a 

 position to demonstrate how much material had been 

 removed by denudation, and how far the process of 

 removal had been guided by geological structure. It 

 is true, as he himself afterwards confessed, that at that 

 time he assigned too much power to the sea, and too 

 little to the subaerial agents, in the lowering of a mass 

 of land. But his exposition of the old base-level of 

 ancient erosion, or 'plain of marine denudation,' as he 

 called it, will ever be a classical study in geological 

 literature. 



Subsequently, as he realised more and more how 

 mighty had been the action of rain, frost, rivers, 

 glaciers, and other subaerial forces in carving the sur- 

 face of the land, he came boldly forward to take the 

 lead among British geologists in enforcing this 

 doctrine. 



(2) During the last ten years of his official life the 

 physical history of river-valleys exercised a peculiar 

 fascination on Sir Andrew's mind. The subject had 

 for many years engaged his attention, but not until the 

 appearance in 1862 of his friend Jukes's remarkable 

 memoir on the river- valleys of the south of Ireland 

 did he realise how the problem might be satisfactorily 

 attacked. He was led to the conclusion that the 

 denudation of the Weald had been effected by subaerial 

 waste, and that the cause of the flow of the rivers, 

 from that central low tract through the encircling rim 

 of chalk downs, was to be sought in the ancient topo- 



