Correspondence, 137 



difference of level can give a claim to a different origin. Mr. 

 Wynne (Geol. Mag. Jan. 1867, p. 10), admits the marine denuda- 

 tion of plains. _ Mr. Maw goes much further, and agrees with me 

 in attributing all straight and level surfaces, including terraces with 

 adjacent cliffs or escarpments, to the planing action of the sea. I ven- 

 ture to proceed a step still further, and ask why should an extensive 

 level surface he called a plane of marine denudation, while a flat- 

 bottomed vale (not the effect of deposition) is excluded from the 

 designation ? Mr. Maw, in admitting that all, or even the majority, 

 of hollows, with terraced cliffs or uniformly scarped sides, have been 

 modified by the sea, concedes, as could easily be shown by sections, 

 that the sea, in many cases, has had a considerable, if not the 

 greatest, share in the process of denudation. The extreme sub- 

 aerialists, who attribute all inland strike-following cliffs^ and escarp- 

 ments to the atmosphere, are consistent in referring the formation of 

 planes to the same agency. 



Transverse Gorges and Longitudinal Valleys. — Professor Jukes, in 

 a truly philosophical spirit, sets limits to his theory by admitting 

 that undidating surfaces (Geol. Mag., May 1866, p. 234), sea-ward 

 or outside hill-slopes (Exp. to Irish G. S. Maps, sheets 124 and 125, 

 p. 6), and gajjs or passes upon the crests of ranges of hills'^ (Quart. 

 Journ. Geol. Soc, June 1862, p. 391), are the result of marine denu- 

 dation. I have lately been led to agree with this distinguished 

 geologist, in attributing long and narrow river- valleys on the sides 

 of table-lands and mountain ridges, to fluviatile erosion. I likewise 

 think it probable that the transverse gorges, which connect longitu- 

 dinal valleys, may have been partly excavated by streams flowing 

 down once continuous slopes, though the abruptly commencing and 

 fresh-looking sides of these gorges would seem to point to a farther 

 and more recent excavation by marine currents. These longitudiaal 

 valleys and basins, which are not open plains, and which often 

 occur in what must once have been land-locked situations, appear 

 the more mysterious the more frequently they are contemplated. 

 Their outlines are generally so smooth, and horizontally continuous, 

 and their recesses so curvilinear, as to preclude the idea of any pro- 

 cess of rutting down by pluvial runlets which must have conformed 

 to the slightest local variation in the denudability of rocks, while 

 in many instances they appear to have been swept so clear of all 

 detrital traces of the excavating agent,^ as to forbid our referring 

 them to rain and frost, which must have left numerous indications 

 of their slow, intermittent, and irregular mode of action. It must 

 have been a wholesale denudation, and not a denudation by instal- 



1 Mr. Topley (Geol. Mag., Oct. 1866), makes some statements in reference to the 

 escarpments and sea-cliffs of E. Yorkshire, which, if necessary to the support of the 

 subaerial theory, show that this theory is not applicable to many parts of the S.AV. 

 of England and other districts, where the sea, in making cliffs shows a tendency to 

 follow the strike, and where many inland cliffs run obliquely to the strike. Numerous 

 instances might be brought forward did space permit. 



^ Professor Jukes clearly includes ravines and narrow winding valleys crossing- 

 watersheds. 



' This was long ago shown by Sir E. I. Murchison. 



