Correspondence. * 235 



Supposing no grass at all to exist, the rate of erosion will be still 

 more rapid, as on the recent volcanic cones mentioned above, or as 

 may be seen on a new railway embankment or cutting where one or 

 two years' storms produce perfect models of mountain glens and 

 ravines in miniature. J. Beete Jukes. 



Dublin, April 6th, 1866. 



OEIGIN OP VALLEYS. 

 To the Editor of the Geological Magazine. 



Sir, — As I have commenced a line of investigation among the 

 valleys, gorges, and drifts of Central Wales, which will require for 

 its completion a series of observations on the sea-coast of Cardigan- 

 shire, I shall not take up your valuable space with a concluding 

 article on the Origin of Valleys for several months to come. Mean- 

 time permit me to add a few lines to my last article.^ The combes 

 behind Malvern Wells are much deeper than they would seem from 

 the woodcut on page 157. The one behind the Holy Well (right 

 side of the woodcut) embraces nearly three-fourths of a circle, and 

 is exceedingly smooth and regular in its outline. All the three 

 combes referred to have been cut back beyond the axial ridge of the 

 Malverns. The rocks in which they have been scooped out would, 

 by Dr. HoU, be classified as Hornblendic and Micaceous Gneiss, 

 with Quartzo-Felspathic and Granitic veins. In reference to the 

 denudation of the Longmynd Yalleys, locally called " gutters," 

 Mr. E. Wilding, of Church Stretton, reminds me of a cwm (English 

 combe) to the north of Carding Mill valley, with no regular stream 

 flowing through it ; and this cwm must have been excavated by the 

 same cause as that to which the valleys owe their origin. Mr. 

 Wilding is convinced that the streams have only furrowed the 

 bottoms of the valleys of the Longmynd. 



I now write from the heart of Siluria ; and on entering this 

 hallowed region, I was struck with its richness, not only in under- 

 gTOund relics of the past, but in the most striking indications of the 

 various modes in which the surface of the earth has been denuded. 

 This is the land, not only of trilobites, but of escarpments, cliffs, 

 cwms, gorges, and all kinds of drifts. Geological tourists, during 

 the coming season, would do well to devote particular attention to 

 the stupendous accumulations of tumultuously-distributed clay, earth, 

 and sand, with enormous rounded boulders, which may now be seen 

 exposed in cuttings on the line of railway running between Here- 

 ford and Llanidloes. The successive tiers of inland sea-cliffs, half 

 wrecked by the weather, but still retaining in sheltered situations 

 their smoothed, grooved, pitted, and caverued forms, near Abereddw, 

 are likewise worthy of minute inspection. Neither ought the tourist 

 to pass by the deep and rocky ravines of the " Great Desert " of 

 Central Wales, to the west and south of Rhyader. — Yours truly, 

 BuiLTH, Breconshire. D. Mackintosh. 



1 Geol. Mag., April, 1866. At page ]60, line 25, for " indication," re«rf " in- 

 duction." 



