394 Mackintosh— On the Cliff's and Valleys of Wales. 



all the inequalities produced hij subaerial denudation ; the majority of 

 the inequalities below the sea-level must, therefore, at any given time, he 

 the result of marine denudation} 



Bemarlcs on Cader Idris. — From the sea on the west, and the 

 valley of the Mawddach on the north, a succession of cliffs and plat- 

 forms, in some places regular, in others very much interrupted, rises 

 up to the summit of the narrow table-land of Cader Idris. The 

 northern rocky escarpment of this moimtain looks more like an old 

 line of sea-cliffs than any form with which subaerial agents are 

 capable of investing rocks. On this and other escarpments of the 

 mountain there are extraordinary cwms or semicircular indentations. 

 One of these is occupied by Llyn-y-Gader. Its excavation must 

 have been accompKshed in a very wholesale fashion, if we are to 

 judge from the immense number of stones choking up its entrance 

 and distributed towards the west — stones which merge into, but in 

 their immediate derivation are distinct from, the debris of the adjacent 

 cliffs. Lyn-Cae, on the other side of the peak, is situated in a still 

 more striking cwm. Ice may have had a share in the excavation of 

 both ; but to attribute to land ice. any grea^t amount of denudation in 

 this district would be inconsistent with its surface-configuration, 

 which could never have admitted of an ice-shed worthy of the name. 

 There are several cwms towards the northern end of Cader Idris 

 (two of them represented in Plate XV. Fig. 8) which appear in very 

 unlikely situations to have been excavated by streams of either ice or 

 water. From the summit of Pen-y-Gader peak (the highest point 

 of the mountain) many cwms, quite distinct in form from any pos- 

 sible modification of a river channel, may be descried. Not a few 

 of them would seem to have been hollowed out in the gable-ends of 

 old headlands. The beholder may see a cwm of this kind, at a high 

 altitude, staring him in the face, as he looks towards the northern 

 side of the valley of the Mawddach. 



Sea-worn Summits of Hills. — On Mynydd Gader, between Cader 

 Idris and Dolgelley, the hard, rocky surface has retained the peculiar 

 characteristics of an ocean-floor. The general form of certain parts 

 is more or less rounded, but in detail the surface is very uneven. 

 Both hollows and projections, however, are here and there smoothed 

 in a way indicative of the backward and forward motion of stones 

 in water. Indeed they present fac-similes, oiily a little roughened 

 by rain, of rock-surfaces now under high-water on the neighbouring 

 Welsh coast. The granular disintegration of rocks is here almost 

 imknown. The mountain streams, after long-continued rains, are as 

 clear as crystal, excepting when they flow through an exceptional 

 superficial deposit of clay or mud. The river Mawddach itself, at 

 Dolgelley bridge, I observed to be quite transparent during and after 

 heavy rains. We have no a priori reason therefore to attribute the 

 inequalities of the rocky hill-summits to rain. The ridges of the 



1 The shores of the intricate channels and inlets on the Pacific coast of British 

 North America, if elevated from the sea, would present but slight difference from sides 

 of the narrow valleys iu the Eocky Mountains at an altitude of 3,500 feet." — Dr. 

 Hector, Quart. Journ. Geol. Soc, Vol. svii. part 1. 



