JOINTS AND DISLOCATIONS OF THE BRECON ANTICLINAL. 



339 



the great mass of calcareous matter and some of the characteristic shells, as Pentamerus 

 Knightii, have disappeared. Again, beneath these calcareous beds, are fissile and flaglike 

 strata, perfectly resembling those Lower Ludlow Rocks which form the base of the 

 mountains of Radnor Forest. In short, nearly all the well-known types of the forma- 

 tion may be here detected, the only essential differences being a less proportion of cal- 

 careous matter, and the superior hardness, as well as more slaty structure of the stone. 



Let those who doubt of the practicability of drawing a well-defined boundary line, 

 between the Silurian and Old Red Systems, place themselves upon this narrow ridge of 

 grey coloured rocks, and contrasting their colour and external features with the surround- 

 ing expanse of red lands, they will, at once, admit that no two formations can be more 

 neatly separated. (See PI. 33. f. 6.) This observation is indeed of general application 

 and admits of no modification, for it may truly be said that the physical demarcation is 

 clear in all districts where the surface is free from drifted materials 1 . 



The Brecon anticlinal exhibits in its sharply inclined, undulating, and dislocated 

 ridges, many beautiful examples of symmetrical joints, the direction of which changes 

 (as in the Ludlow promontory) with every variation in the strike of the strata. As the 

 rock is here well exposed and more compact than usual, there is perhaps no situation 

 where symmetrical jointed structure can be better studied or more clearly distinguished 

 from those cracks and fissures which have resulted from disturbance. On this point, 

 however, I must refer the reader to Chapter 20, in which he will find a general expla- 

 nation of my views ; some apposite remarks are also thrown out in various other places, 

 particularly in the ensuing chapter, in describing Noeth-griig, Caermarthenshire ; while 

 those who wish to go into the details must consult the Appendix 2 . 



The great dislocations, however, which occur throughout this promontory form its 

 chief attraction, since the magnitude of the cross fractures which have dissevered it 

 into a number of separate masses, prove that the upheaving force must have operated 

 with intense violence. 



1 For an account of the drifted materials superficially strewed over parts of this region the reader is referred 

 to the concluding chapters ; but I cannot avoid remarking that the phenomena which I have found to prevail 

 in all elevated tracts are here most strikingly displayed. Not a single boulder is to be found amid the dome- 

 shaped or sharp ridged hills of this axis of elevation, though there are within their range numerous hollows and 

 deep cavities ready formed for the reception of such drift, whilst broken materials, derived from these very hills, 

 are piled up, sometimes in enormous masses, on the exterior slopes of the elevated chain. See many of the 

 hills between Lower Chapel and Brecon, &c. 



9 Anxious to examine this ridge with one who has thrown so much light on jointed structure, I was delighted 

 when Professor Phillips resolved to look at this anticlinal on his way from Bristol to North Wales (1836), and 

 I met him at Brecon. The portion we examined together, lies to the west of the river Honddhu. I owe much to 

 my friend's suggestions on that occasion, and had great reason to admire the precision with which he observed 

 the phenomena. I have only to regret that he had not leisure to examine the north-eastern range of the pro- 

 montory, extending from Castel Madoc to the Wye, where the best and most symmetrical joints are developed. 

 Geologists look forward with anxiety to the enlarged publication of his views on this difficult subject, knowing 

 that their conclusions must be regulated by a multitude of correct observations. 



