W. Gunn — Suhaerial Denudation v. Glacial Erosion. 101 



correspond to those of the present stream, for those who hold that 

 the river formed its valley cannot suppose that it always ran exactly 

 along its present line. 



The general absence on these limestone terraces of much debris 

 from the higher ground may perhaps be explained in this way : — 

 The bed immediately above the limestone is shale, generally forming 

 a slope above the limestone terrace, and the decomposition of this 

 produces a clay pretty easily carried off in minute portions by heavy 

 rains. That we do not find much of this on the terrace may be 

 owing to the fact that the clay is carried down with the rain through 

 the numerous joints of the limestone. I remember a distinguished 

 geologist once remarked to me on the comparative freedom from 

 drift of limestone areas when compared with areas of other rocks 

 similarly situated. May not drift disappear more rapidly from 

 limestone on account of the easy passage of muddy water down- 

 wards, and leave but a few scattered boulders here and there to pose 

 the geological surveyor who does not know in some cases if he 

 should consider them the remains of a drift covering or the remnants 

 of beds which have been denuded off the limestone ? Of course I 

 may admit that these terraces were probably swept bare of loose 

 rock by the ice-sheet, and yet be unable to accept the view that the 

 terraces themselves were formed by that force. 



Next with respect to the general weathering of the terrace and 

 the formation of swallow-holes. We are told that the limestone of 

 the terrace is uniformly weathered (p. 324) ; that the part nearest 

 the marginal scar usually exhibits no greater amount of weathering 

 than does th« innermost part close to where the next bed above 

 comes on (p. 358). I do not think I have ever seen the whole 

 known thickness of a limestone bed in one of the scars unless where 

 the shale bed came on immediately above, so that there was no percep- 

 tible terrace. Where the terrace is wide, there is generally nothing 

 like the whole thickness to be got near the marginal scar ; so that if 

 that part is not more weathered, at least it has been more denuded 

 than the part nearer the inner margin of the terrace. 



Mr. Goodchild supposes that by Glacial Erosion all swallow-holes 

 were removed from the low ground, while many were left on the 

 high ground. I think another explanation may be given of the 

 fewness of swallow-holes on the low limestone terraces. It is well 

 known that ordinary rain water has but a slight dissolving power on 

 limestone, which is much increased if the water passes through peat 

 or other decaying vegetation, and thus absorbs much carbonic acid.' 

 Now there is little or no peat on the low ground, while there is 

 abundance on the high ground, much of the surface of the highest 

 limestone terraces being covered with it ; and I can testify that the 

 peat-covered limestone has swallow-holes sown broadcast over it, 

 and that they are not confined to' the inner margin of the terrace in 

 such cases. Most of these swallow-holes could not have been formed 

 by streams, but by the dissolving action of the water that drains 

 from a limited area like those mentioned by Mr. Bonney.^ 



1 Lyell, Elements, sixth ed., pp. 42, 363 ; Jukes and Geikie, p. 397. 



2 Geol. Mag. Sept. 1875, p. 427. 



