G. H. Kinahan — On Denudation. 113 



able circumstances will much accelerate this horizontal denudation, 

 such as in some districts continuous and prevalent winds from cer- 

 tain points of the compass, these rapidly undermine the peat, conse- 

 quently on such sides of the hills the slopes are much more gradual 

 than on those sheltered from the wind, where the chemico -fluvial 

 agencies have, unaided, to work in under the peat. 



In cultivated districts, where the valleys are inhabited and tilled, 

 the protected area of the whole surface must gradually diminish ; for, 

 as the valleys widen, the quantity of tillage is increased. But in wild 

 districts, where nature still reigns, what is taken from the protected 

 summit, has an equivalent returned in the vallej'^s ; for when the 

 latter have become wide enough, there peat also forms, which, once 

 it has established itself, rapidly grows not only on the flat, but also 

 creeps up the slopes ; so that eventually there is only a small fringe 

 round the edge of the table-topped summit which can be acted on 

 by the denuding forces. 



In moist climates, not only on mountains, but also on low lands, 

 (if uncultivated,) unless drained naturally or artificially, peat will 

 grow and defy denudation. Even in cultivated tracts, when the land is 

 under grass and not in tillage, denudation can have very little effect. 

 This is well illustrated when a section is opened down a hill-slope 

 that has for a long time been grass land, more especially if it 

 merges at the bottom into a boggy flat. In this instance it will be 

 found that there is scarcely any difference between the thickness of 

 the surface soil at the summit and base of the hill, while the peat 

 from the flat has been gradually encroaching on the grass land and 

 creeping up the hill. On the other hand, if a section is cut down a 

 similarly circumstanced hill, except that it has been tilled for a con- 

 siderable time, the results will be different, and the denudation quite 

 apparent ; for the surface soil will be much reduced in thickness 

 on the slope and summit, it being carried down the hill and even 

 found overlapping and lying on the peat in the flat below. Such a 

 section will also give a record of the number of times and periods the 

 hill has been in tillage and under grass ; for the peat and surface soil at 

 the base of the slope will be found interlaced according to the number 

 and length of times the hill was exposed to the denuding influences. 

 It may be said, that although the chemico-fluvial agencies do not de- 

 nude the surface of the grass land, yet the water percolating through 

 the soil increases its depth by gradually rotting the subsoiL This 

 also, seems to be disproved, as it is not of uncommon occurrence 

 when draining grass land, that formerly was tilled, to find lying on 

 the surface of the subsoil the old sods, and this could not be the case 

 if the atmospheric influences had the power of increasing the depth 

 of the surface soil in grass land. Against the protecting power of 

 grass land, it may be put forward that in many places " stones grow 

 on the surface," or, in other words, that the surface of some grass 

 land gradually becomes covered with stones. This cannot be contro- 

 verted ; however, it is only an exception to the general rule, and in 

 moist climes can never occur except in places that naturally are siib- 

 jected to extreme drainage ; such as Chalk hills, or a country over the 



VOL. VI. — NO. LVII. 8 



