Dr. C. Callaway — On Plagioclinal Mountains. 219 



on the N.E. ; passes through the Wrekin and Caer Caradoc chain ; is 

 continued to Stanner Eock and Hanter Hill, W. of Kington ; passes 

 under an anticlinal N.W. of Brecon ; appears to sweep round to the 

 W., and to be connected with the Precambrian ridge of St. Davids. 

 In Shropshire it is a complex system of faults. There appear to be 

 in all about six primary dislocations parallel to each other and to 

 the mountain ridges. For our present purpose, we shall not require 

 more than a pair of these. 



Commencing with Lilleshall Hill, we find that it is bounded by 

 faults on both sides. On the S.E., the Hollybush Sandstone (Caradoc 

 in the Survey) has been thrown down ; on the N.W., the lower beds 

 of the Trias. The structure of the ground is the same in the 

 Wrekin range, save that faulted bands of quartzite are interposed 

 between the axial rocks and the Hollybush Sandstone and the Trias 

 on either side. The Caer Caradoc chain is in like manner bounded 

 by two nearly parallel faults. On the N.W., the lower slates of the 

 Longmynd series rest against the axis ; on the S.E., the Shineton 

 Shales, the Hollybush Sandstone, and the Caradoc Sandstone are 

 successively thrown down. 



It is evident that in each case a wedge of the solid crust has been 

 separated from the main mass by two nearly parallel faults, and 

 thrust up through overlying Cambrian and Silurian strata. Or it is 

 probable, in the case of the Wrekin at least, that the wedge was 

 first pushed up in Pre-Cambrian times. 



As the rocks of this chain consist, not of eruptive greenstones 

 (according to the late Sir E. Murchison and the Geological Survey), 

 but of bedded lavas, tuffs, grits, conglomerates, and other stratified 

 material ; and as they are flanked ou each side by strata, some of 

 which are at least as old as the Lower Cambrian ; it is clear that 

 these mountain ridges are of Precambrian age. 



The writer has been greatly interested to find that cei'tain other 

 Precambrian elevations, described in recent times by Prof. Phillips, 

 Dr. Holl, and Dr. Hicks, display a similar abnormal structure. 



The Malvern Hills trend N. and S., but the schists and granitoid 

 rocks of the chain strike across the axis in a S.E. direction. A great 

 dislocation throws down Triassic sandstones against the eastern 

 flank, and the author is convinced from a personal examination that 

 the ridge is also bounded by a fault on the W. side. These rocks are 

 still more ancient than those of the Wrekin chain. The range has 

 evidently been formed in the same way. A solid slice seven miles 

 long, composed of strata striking to the S.E., has been isolated by 

 two parallel N. and S. faults, and pushed up as the original ridge 

 out of which the Malvern Hills have been sculptured by frost and 

 rain. 



A third Precambrian ridge exhibiting a similar structure occurs 

 at St. Davids, and has been described by Dr. Hicks. It extends for 

 about two miles from the city to the S.W., and has an average 

 width of rather less than a mile. It consists of quartziferous and 

 granitoid rocks distinctly bedded, and striking across the axis to the 

 S.E., that is, at about a right angle with the strike of the ridge. 



