190 



OUTLIERS OF OLD RED SANDSTONE. 



cannot have any one prevailing direction. Minor dislocations abound in this, as in 

 all other old rocks, but they are less known than in the carboniferous system, because 

 the entire absence of mineral productions has precluded mining operations, by which 

 the extent of such faults is usually ascertained. The construction of new roads, how- 

 ever, occasionally lays them open, and a very instructive example is to be seen on 

 the sides of the road south of Lydney near the Severn (see PL 30. fig. 13.), marking 

 one of the dislocations which surround the coal basin of the Forest of Dean 1 . Other 

 dislocations of the Old Red Sandstone are exposed on the edges of the Abberley and 

 Malvern Hills (see PI. 36. figs. 1, 2, &c), and analogous phenomena will be described 

 in the chapters upon the Pembroke and Tortworth districts. In the mean time, I will 

 now direct attention to certain great outliers of the system which have escaped the 

 notice of previous observers. 



Outliers of Old Red Sandstone— <" Forest of dun." (PL 33. fig. 1.) 



The outliers of Old Red Sandstone, which I proceed to describe, are separated 

 from the great mass of that system by wide intervening tracts composed of Silurian 

 rocks. The largest of these outliers is a considerable district of nearly one hundred 

 square miles in superficial extent, and of which Clun Forest forms the principal part, 

 surrounded by the towns of Newtown, Montgomery, Bishop Castle, Knighton, and Clun. 

 The eastern boundary is upwards of ten miles and its western limits more than twenty 

 miles from the edge of the great area of Old Red Sandstone. (See Map.) The whole 

 of the soil of this detached district is red, and the beds beneath it are similar to those 

 previously described as members of the Old Red System, whilst numerous natural 

 sections show, that the grey-coloured masses which surround and support it at low angles 

 of inclination, are the true Ludlow rocks. (See PL 33. fig. 1.) Clun Forest itself, is 

 principally composed of the lower group of the Old Red, consisting of hard, thin- 

 bedded, micaceous sandstones with argillaceous marls. These rocks are here known as 

 " firestones " from their power of resisting heat. They are the only good building ma- 

 terials over an extensive tract, and are distinguished from the grey-coloured strata of the 

 Silurian System on which they rest, to which the term of " greenstone 2 " is there uni- 

 versally applied. By traverses from Newton on the west to Knighton on the east, and 

 across Kerry Hill to Bishop's Castle, and to Clun, I have ascertained that most of 

 this " firestone " of Clun Forest represents the tilestone or lowest member of the Old 

 Red ; because it occupies the bottom of a large basin and, reposing upon, passes 



1 This and other faults of that region have been examined in great detail by Mr. Bragge Bathurst of Lydney 

 Park, who has laid them down with much precision on a sheet of the Ordnance Survey. 



3 " Greenstone," i. e. a perishable stone. (See note in the ensuing chapter upon the Ludlow rocks.) 



