Wynne — Denudation and its Causes. 5 



features along more exposed external portions of their neighbour- 

 hood, thus reversing the order of things so often observable upon 

 existing coasts.^ See Plate II. Fig. 6 (Coomassig Cliff, 1100 feet 

 high, forming one side of a glen or coom, near Sneem, Co. Keny, 

 Ireland). 



With regard to the possibly subasrial origin of these cliffs, we 

 may remark, that, through the joints by vphich rocks are traversed, 

 water finds its way more or less vertically downwards, tending to 

 separate masses in this direction which — when deprived of support 

 by springs, streams, ice, or it may be in some cases even by sea 

 waves — naturally fall away, leaving vertical, or approximately ver- 

 tical, faces of rock behind.^ 



In limestone rocks this action may be specially well observed. 

 The illustration, Plate I. Fig. 4, represents one of many fissures be- 

 coming gradually enlarged along the brow of Benbulben mountain 

 in Sligo. At A, a smaller one is beginning to open, another has 

 separated a pinnacle of rock at B, and others have caused landslips 

 and vertical cliffs in many places about this locality. Even clay, 

 drift, or alluvial detritus contains joints or lines of division [see 

 Plate II. Fig. 8, Lines of dislocation or division in drift (sand, clay, 

 and gravel), at Goldengrove, Co. Tipperary, Ireland (see explanation 

 of Sheet 126, Mem. Geol. Surv. Ireland)] ; and although it might be 

 supposed in such homogeneous materials rain channels would always 

 commence gradually, one has only to observe how frequently nullahs 

 at their sources issue from abrupt vertical banks, which sometimes 

 overhang, to see that rain not only acts upon the surface of the 

 ground, but also in some cases at considerable depths, to which it 

 finds its way in the manner above described. 



Long lines of inland cliffs, forming terraces above one another, 

 are assumed to have been caused by successive actions of coast 

 denudation. Against this natural conclusion it would be diffi- 

 cult to argue, particularly in the case of countries otherwise known 

 to have been submerged, while the reasoning in favour is strength- 

 ened by parallelism of the lines to each other, and sometimes to the 

 plane of the horizon. 



Nevertheless some long cliff terraces can be pointed out, like those 

 in the County of Sligo, on Benbulben mountain, (part of which is 

 seen sketched in Plate I. Fig. 5), presenting such an appearance as the 

 cliff-forming strata, being bent into an anticlinal curve, which is 

 followed instead of being intersected by the line of cliffs, the contrary 

 to what might have been expected if their base had remained for a 

 long period at sea level. 



Other peculiarities in limestone cliffs occur, among the Burren hills 

 in Clare, for instance, being equally well marked on all sides of 

 narrow valleys and around nearly circular hills, instead of presenting 

 stronger evidence of exposure to marine erosion on the exteriors of 

 the hill groups, or like many existing islands, at one side than another. 



' See Mr. Maw's paper. — Geo. Mag., No. 28. 



2 Since the above was written a very similar observation has appeared in a paper 

 upon five types of Earth's surface, in the United States, by Mr. Lesley. 



