CHAPTER XLII. 



CONCLUSION OF PART I.— GENERAL VIEW OF THE FORMER 

 CHANGES OF THE SURFACE. 



Parallelism and divergence of mountain chains. — Ancient dislocations, and their 

 intensity.— Vastness of scale of former depository action.— Central heat. — 

 Ancient dislocations and modern changes reconciled. 



IT has been repeatedly explained in the preceding chapters, and a mere glance over 

 the accompanying map will show, that the prevalent direction of the stratified deposits 

 in Wales and the adjoining borders of England, is from north-east to south-west. The 

 map further shows, that this direction is coincident with the lines of fissure along which 

 the rocks of igneous origin have been erupted ; and thus we connect the effect with a 

 cause, and make it probable that volcanic matter, bursting out in that allinement, has 

 been instrumental in producing the leading direction of the strata. 



This strike prevails throughout all the rocks of the Cambrian and Silurian systems, 

 from Lilleshall and Haughmond Hills in Shropshire, to the mouth of the Towy in Caer- 

 marthenshire, a distance of upwards of one hundred and twenty miles. Even in Pem- 

 brokeshire, where, after many violent dislocations, the formations are partially cast into 

 an east and west direction, as explained in the 28th and 29th chapters, the south- 

 westerly course of the Cambrian and Silurian rocks is maintained in the coast cliffs of 

 St. David's and in Marloes Bay, more than one hundred and fifty miles distant from 

 their types in Shropshire. 



When, however, we examine these phenomena more in detail, we find, that although 

 there is a general coincidence in direction between each ridge of stratified deposits and 

 that of the intrusive rocks, connected with or adjacent to it, the same direction does not 

 always prevail in the separate chains at certain distances from each other. In the tract 

 selected as the type of the Silurian System, the strike is true north-east and south-west, 

 or parallel to the ridges of the Wrekin and Caradoc, and the same is on the whole per- 

 sistent into Caermarthenshire ; but, in the country situated to the west and north-west 

 of the mineral axis of Shropshire, where the Silurian deposits undulate between the Cam- 

 brian Rocks of the Longmynd and the Berwyn mountains, there are marked discre- 

 pancies in the direction of the strata. Thus, in the singular mining tract of Shelve and 

 Cornden, the lines of trap, and the stratified rocks trend N.N.E. and S.S.W.; in the 



