EARTH'S SURFACE METEORIC ABRASION. 71 



original mass, by denudation of some kind or another, 

 would seem to have been planed into a flowing, 

 gradually undulating table land, with gently sloping 

 sides. Subsequently, by one or other of the kinds of 

 denudation, or, what is more probable, by two or more 

 combined, valleys and ravines were cut traversing 

 the table land in various directions, and forming the 

 individual hills. When meteoric abrasion began to 

 work by itself, or a short time afterwards, peat must 

 have begun to grow on the hill-tops, which effectually 

 prevented them from being denuded; therefore those 

 powers could only act in the valleys and ravines, on 

 the sides of which they formed steep slopes, crowned 

 by perpendicular escarpments usually a few feet 

 higher than the thickness of the envelope of peat. 

 The deposit under the peat is generally the remains 

 of the original surface drift of the ancient undulating 

 table-land, and as the covering of peat now protects 

 its surface, meteoric abrasion works in sideways, 

 undermining the peat, which consequently topples 

 over in large pieces when its weight opens the joints. 

 As the waste thus proceeds both vertically and hori- 

 zontally, the area of the flat summits must decrease, 

 although most slowly. Favourable circumstances 

 will much accelerate the horizontal denudation, 

 such as in some districts continuous and prevalent 

 winds from certain points of the compass ; these 

 rapidly undermine the peat ; consequently on such 



