310 



DISLOCATIONS ALONG THE CAMBRIAN FRONTIER. 



north-west, and have thus the appearance of being in a position overlying the younger 

 deposits 1 . 



This inverted position, which is traceable at intervals for a great distance, has doubt- 

 less arisen from the intensity with which the Welsh Mountains have been dislocated. 

 Surprising as this phenomenon may appear to those who have not studied geology, it is 

 not without parallel in other mountain chains. I have myself observed it on the 

 northern face of the Eastern Alps, where the newest secondary deposits are broken off 

 by enormous faults and dip under the oldest rocks in the centre of the chain ; and even 

 in this country, inversions similar to these will be specially adverted to and their causes 

 explained in a subsequent chapter on the Malvern and Abberley Hills 2 . 



1 The dip at Moel-ben-tyrch is N.N.W. (See chapter 31, on the Abberley and Malvern Hills.) 



2 The sheets of the Ordnance Survey relating to it were not published when I first examined this tract, and 

 therefore I do not profess to have laid down with minute precision the boundary line between the Silurian and 

 Cambrian Systems throughout Denbighshire and Montgomeryshire. In the northern part, indeed, I have 

 depended chiefly on the assistance of Professor Sedgwick. At Moel-ben-tyrch, however, and at the Berwyns, 

 the demarcation is, I trust, accurate. To render the outlines more accurate, I now avail myself of the issue of 

 two new quarter sheets of the Ordnance Survey to visit the tract again, while these pages are passing through 

 the press. — August 19, 1837. 



