Wynne — Denudation and its Causes. 7 



latter have been, comparatively recently, removed by the action of rain. 

 Still this atmospheric action is sometimes slighter upon limestone 

 than upon other rocks, as may be witnessed in ruins and old build- 

 ings, the masonry of which includes both limestone and sandstone.^ 



Trappean rocks frequently yield to this action more readily than 

 limestone ; and even where silicious ones form large masses, local 

 prevalence of joints or alternations of strata may favour the denuding 

 power of the atmosphere. 



Almost every rock fragment we pick up is found to be weathered, 

 and if we observe a mountain slope covered with bare shingle, the 

 result of subserial action, we cannot doubt its power to carry on the 

 reduction of the fragments to their ultimate disiategration, or that 

 the vast quantities of alluvium thus produced and left upon lower 

 portions of the surface indicate but partially more extensive oper- 

 ations of this nature. The power of this agency to denude the land 

 being admitted, it follows that the excavation of valleys and forma- 

 tion of hills is only a matter of time.^ 



Coast Lines. — Although the sea coast has a certain relation to the 

 form of the land, it may be doubted whether it has any to show that 

 the surface of the latter has depended for its shape upon marine 

 denudation. To remove such doubt it must be proved that all parts 

 of the existing surface have been successively acted upon by the 

 sea, and consequently that atmospheric influences have not since 

 materially altered its configuration ; but this is just the point in dis- 

 pute among the advocates of marine versus subeerial denudation, and 

 is certainly far from being proved. 



Where deep vaUeys in a mountainous coimtry open down to the 

 coast the sea enters them, but shows its denuding power most upon 

 the projecting lands between^ — as might have been expected to occur 

 if the Yalleys were produced by atmospheric agencies. 



It is hardly necessary to allude to the evidence that the British 

 Isles at no distant geological period had a greater extent ; while 

 around the South of Ireland the frequent occurrence of peat below 

 high water mark, even close to the bolder portions of the coast, serves 

 to show that some of the present sea-cliffs can hardly be entirely due 



^ Numerous instances of the slow rate at whicli even limestone weathers, and there- 

 fore the enormous time required to produce results so evident, are familiar to most 

 observers. An inscribed slab of this rock in the interior face of the battlement of a 

 bridge, a couple of miles west of Athlone, although somewhat weathered, distinctly 

 showed in the year 1862, a date 100 years previous. Another limestone slab in a very 

 similar situation, facing the E.S.E., (The Liberty Stone), at "Whitehall Bridge, near 

 Limerick, has an inscription, in raised characters, about one-eighth of an inch relieved. 

 The stone is but slightly weathered ; the inscription, except in the last figure of the 

 date, is perfect, and the date is 1635. The preservation of glacial striae upon some 

 smooth surfaces of limestone, indicates a certain variability in amount of subaerial 

 action. 



* The absence of references prevents further allusion here to the subject of valleys 

 which are lower than sea-level or those which being valleys of denudation discharge 

 water over rock barriers at a greater height than that of their interior parts. The 

 glacial origin of these will be found discussed in Professor Ramsay's paper to the Geol. 

 Soc. Lond., Vol. xviii., p. 185. 



3 See "On Watersheds, by Geo. Maw, F.G.S. etc.— Geol. Mag. Vol. III. No. 8, 

 p. 344." ajid the outline of the S. "W. Coast of Ireland. 



