G. H. Kinahan — On Denudation. 109 



IV. — Suggestions about Denudation. 

 By G. Henry Kinahan, M.R.I.A., F.K.G.S.I., etc. 



DENUDATION has been divided into only two kinds — viz., 

 Suboerial and Marine ; nevertheless, naturally there seem to 

 be three marked denuding forces — I. Subcerial or Atmos]ilieric ; II. 

 Marine ; and, III. Glacial} 



Marine denudation has so often been described, and by such com- 

 petent writers, that little has to be said about it. However, it may 

 be mentioned that it seems rarely, if at all, to act except on shore ■ 

 lines, and in such places generally above low-water-mark. Some 

 writers seem to consider that its normal work is to cut " Marine 

 plains ;" nevertheless, this seems to be the exception, not the rule. 

 It might possibly happen, if the sea was acting on a homogeneous 

 rock, but when it is considered that rocks in general are rarely 

 homogeneous or, as is often the case, many different varieties are 

 interstratified, it is not surprising that off so many coasts are islands 

 and sea-rocks. These are the remains of compact portions of a rock 

 or tracts of rock or hard beds which the sea in its advance left on 

 account of their firm nature, while it carried away their frail asso- 

 ciates. Chalk apparently ought to be one of the most homogeneous 

 substances in nature ; and if marine action usually planes the ground 

 evenly away, in Chalk it might be expected to cut a "marine plain." 

 Nevertheless, the coast-line of a Chalk country is usually irregular, 

 and lying off it are islands and rocks. If the sea acts on horizontal 

 or nearly horizontal strata, naturally it should cut a plain, except 

 where dykes of intrusive or other rocks occur, when portions of the 

 borizontal strata may be protected and thus form islands, which sub- 

 sequently, after the land rose, would appear as hills. When the sea 

 has retreated and a plain is left, may not this be as much due to 

 atmospheric as marine denudation ? for rain and rivers supply the 

 materials to fill up the irregularities in the sea-bottom. 



Glacial denudation, seemingly, ought to be considered a different 



1 Although suhmrial or atmospheric denudation are the received names for the first 

 class of deinulatiou, yet neither seems unobjectionable ; for, strictly spenking, all denu- 

 dation is subcerial and all is caused by atmospheric influences, as all mechanical denu- 

 dation, without these influences, Avould be nil. Difference of temperature causes the 

 ■wind, the oceanic currents, and the glaciers, also the evaporation that eventually forms 

 the rain and rivers. When it is said " Ice is one of the subirrial agents," the state- 

 ment is quite correct ; but if " Subaerialists" claim its action, they ought also to claim 

 marine denudation ; for, without tbe atmospheric influences, there would be no oceanic 

 currents, no tidal currents, no storms— in fact, no denuding action at all. On the 

 other hand, " Marinists" might claim all work done by rain and rivers, as they were 

 originally sea water, which, previous to leaving, had taken " a return ticket" to go 

 by the atmospheric way and cnme back by the river. 



In the first of the above divisions (Subau-ial or Atmospheric denudation) is included 

 all the ivorh done by the chemical action of the atmosphere and the mechanical work by 

 rain and rivers. This seems to have been the original doctrine of the " Sub- 

 aerialists," although of late tliey would wish to include all glacial work. Seemingly 

 a less objectionable name than either Suban-ial or Atmospheric would be chemical 

 denudation, but, perhaps, a better still would be Clicmico-Jluvial denudation, as this 

 latter term wotdd include not only the chemical action due to Atmospheric in- 

 fluences, but also the mechanical work done by raiu and rivers. 



