146 



CARBONIFEROUS LIMESTONE OF LLANYMYNECH, ETC. 



appears, proceeding in a northerly direction at Orsedd-wen 1 on the higher part of the 

 Sallattyn Hill, 1300 feet above the sea, and is worked on each side of the little stream 

 at Craig-nant five miles north-west of Oswestry 2 . From Craig-nant its course is north- 

 north-east by Bronyarth and Vron-frian, passing to the west of Chirk, whence it is 

 more or less continuous along the edge of the carboniferous tracts of North Wales. 

 In this range the inclination of the strata varies from 5° to 45°, and they everywhere 

 dip beneath the millstone grit. Some of the upper members of the limestone at Llany- 

 mynech are of yellowish and reddish colours, and are more or less charged with mag- 

 nesia, and have occasionally cells containing crystals of pearl spar. 



These beds pass down into the great body of the limestone, which is subcrystalline, and of excel- 

 lent quality. In the lofty vertical face of the quarries at Llanymyncch, one of the principal beds, 

 called "the Upper Red/' has a thickness of twenty feet without a divisional way-board ; beneath this 

 are certain yellowish impure beds called "Delf/' probably containing magnesia: the lowest stratum 

 in work is of a grey colour, and so veined and mottled with deep red, that it is known as the " bloody 

 red bed." The whole succession of the strata cannot be seen in any one escarpment, and the litho- 

 logical characters vary so very much in the different ridges above alluded to, that the description of 

 the yellowish upper beds near Llanymynech would little agree with the dark grey beds with shale at 

 Trefonen, though the latter are also upper beds. The best section, however, of the lowest beds with 

 which I am acquainted is at Craig-nant, five miles west of Oswestry. The thick or central masses 

 of pure limestone are there seen passing down into dark brown and yellowish, impure, sandy lime- 

 stone and chert, alternating in thin beds with dark-coloured shale. These beds, called tc malk" by 

 the workmen, represent the lower limestone strata, and they rest unconformably on Silurian 

 Rocks. 



It may be here remarked, that like other solid rocks hereafter to be described, this limestone is 

 in most parts symmetrically divided by a number of joints, some of which are vertical; others vary 

 in their inclination, but usually their planes form right angles with the surfaces of the beds. The 

 greater number of these joints run in directions diagonal to the strike, though some proceed directly 

 in the lines of the dip, and others coincide with the strike ; but whatever these relations may be, 

 they invariably change with each variation in the direction of the mass of rock examined. (See ob- 

 servations on joints in a subsequent chapter on the Silurian Rocks in the environs of Ludlow.) 



At Llanymynech the limestone was in ancient times the seat of mines, as is proved by the galleries 

 attributed to the Romans; but now there are few indications of any metallic veins worthy of notice, 

 although the sides of the joints are occasionally tinged green by carbonate of copper. 



In a range of low hills north of Llanymynech, and near the canal, some of the upper beds, con- 

 sisting of reddish and yellowish hard limestone, contain thin veins of green and grey carbonate of 

 copper, running transverse and diagonally to the general strike of the limestone ; but though trials 

 have been attempted, no body of ore has been detected. Small veins of lead are said to occur north 

 of Craig-nant. 



The prevailing fossils are the large Froductiis hemisphtericus, P. Martini, and other 

 mollusks, with a vast abundance of corals and crinoidea peculiar to the limestone of 

 this age. 



1 Orsedd-wen : Angl., The white palace. 



2 At the spot where Mr. West has recently erected a tower to mark the course of OfFa's Dyke. 



