282 Correspondence. 



to 150 feet above the river, and either gradually or suddenly suc- 

 ceeded by the angular drift of the neighbouring mountains, and that 

 vsdth little or no change in the inclination of the acclivity. 



Mr. Jukes, in attacking the great seeming objection to the fluvial 

 origin of valleys, namely, the necessity for believing that a river 

 must have wandered over, and excavated a large plain during the 

 time that its action, in a contiguous area, was limited to the wearing 

 down of a narrow gorge, endeavours to explain the disparity by 

 reference to the more easily eroded rocks composing the area of the 

 plain. In Ireland he believes that the Carboniferous limestone was 

 the easily denuded rock, and the Old Eed Sandstone, or some other 

 silicious formation, the comparatively resisting rock. But, I think, 

 in many districts, this explanation would not hold good. In the case 

 of the plain of Herefordshire, and the narrow gorge of the Wye 

 between Eoss and Chepstow, it would require to be reversed ; for 

 there the plain is Old Eed Sandstone, and the sides of the gorge Carbo- 

 niferous limestone. Farther up the Wye, I do not think Mr. Jukes' 

 explanation would apply ; though on this point I would wish to 

 speak with deference, and with the gi-eatest willingness to be cor- 

 rected. 



The gorges or passes connecting the vales of Central Wales look 

 as if they were more recently excavated than the vales themselves. 

 They cut abruptly, and without any warning, through the ridges by 

 which the vales are separated. Their commencement is as sharply 

 defined as if they had been sliced out of the ridges, and I cannot 

 help thinking that they have been widened, and their sides rendered 

 more precipitous, by the action of the sea during the glacial period 

 of submergence. At the same time, probably, the cliff's of Abereddw 

 (which, in many respects, are perfect fac-similes of cliffs now 

 washed by the sea on the Cardiganshire coast), were formed and up- 

 heaved in- succession. It is quite true that all this implies the pre- 

 vious existence of the valleys on a smaller scale ; but on this subject 

 I cannot enter farther at present. Its elucidation would require a re- 

 examination of the nature and distribution of the various kinds of 

 drift by which a great part of Central Wales is covered from the 

 mountain top to the lowest depression.' — ^I am. Sir, yours truly, 



D. Mackintosh. 



DoLGELLY, Wi May, 1866. 



LBPIBOSTROBUS BROWNII^ 

 To the Editor of the Geological Magazine. 



Sir, — In the interesting paper by Mr. Carruthers, which appeared 

 in the October number of your Magazine, I was glad to see that he 

 distinguished the beautiful specimen of Dr. Eobert Brown (who had 

 shown it to me during his lifetime,) from the Lepidostrobus described 



' There are several very important points in Mr. Jukes' letter, the consideration of 

 ■which I must reserve for a future, and more systematic communication. 



2 See ante, p. 271. Report of the Manchester Geological Society. — This letter was 

 accidentaUy omitted from our last number. — Edit. 



