242 Rev . J AMES Y ATEs on the Structure of the 



wards the mountainous country of Wales, it terminates in perpendicular pre- 

 cipices ; it contains beds of soft crumbling shale, and is evidently to be re- 

 garded as a continuation of the interrupted chain of the limestone of Eglvv-y- 

 seg. Among the corallines of these strata is a madrepore, bunches of which 

 are seen on the upper surfaces of the beds of limestone along this ridge, evi- 

 dently in the same position in which they originally grew. The limestone, 

 which now penetrates and envelops them, has been deposited without remov- 

 ing them from their places, or even materially altering their form. I have ob- 

 served the same curious phaenomenon in the ridge of mountain limestone 

 which rises over the strata of Brecon Forest in South Wales ; and I am in- 

 formed by an accurate observer (Mr. Underwood), that he has seen it in the 

 same formation at Plasnewydd in Anglesea, and on the opposite side of the 

 Menai. 



On advancing westward, so as to surmount the ridge of limestone, the eye 

 is impressed by the immediate change in the character of the scenery, from 

 that of long ridges of moderate elevation to the appearance of huge mountains 

 divided by valleys of corresponding dimensions. Between these two kinds of 

 scenery the wall of limestone forms a line of demarcation. The strata, which 

 dip under the limestone, and constitute mountains 1500 feet in height, consist 

 of numerous beds of a slaty micaceous sandstone, dividing by natural separa- 

 tions into rhomboids of a micaceous clay-slate, and of a soft shale. These 

 beds are very numerous, and various in their thickness ; but in general the 

 rhomboidal kind is conspicuous among the others, in consequence of the even 

 vertical surfaces presented by its broken strata. It seems to be the same rock 

 with the rhomboidal killas of Devonshire and Cornwall, which Dr. Berger has 

 denominated Common Grauwacke*. 



Besides the stratified rocks now described as lying under the limestone, a 

 mass of porphyritic greenstone presents itself at Glascoed near Llansilin in five 

 or six considerable hills, which are distinguished by their abrupt rounded forms, 

 rising like irregular knobs among the stratified rocks, and covered with gorse 

 and coarse grass. The strata of limestone appear more inclined where they 

 lie upon the greenstone, but do not show marks of disturbance. 



3. I shall now attempt some account of the rocks which occur in the direc- 

 tion of the road from Welchpool to Ludlow, containing the country interme- 

 diate between the limestone ridge already described and that of Wenlock 

 Edge. The whole of this tract is principally occupied by strata of the slaty 

 micaceous sandstone, dividing into rhomboidal pieces and alternating with 



* Geol. Trans, vol. i. p. 112. 



