342 



BOUNDARY OF CAMBRIAN ROCKS— BRECKNOCKSHIRE. 



analogy to the north-western uplands of Radnorshire, where it has been shown there 

 are no traces of the Carad'oc sandstone and Llandeilo nags. 



" Cambrian Rocks." 



The Upper Cambrian or slaty greywacke group in Brecknockshire is simply a pro- 

 longation of those rocks which have been described as advancing in Radnorshire to the 

 left bank of the Wye, in the hills of Dolevan, Rhiw-graid, and Gwastaden. Their 

 south-eastern frontier runs along the hills of Pen cefn-ty-mawr and Alt-y-clych, their 

 summits being marked by quartzose conglomerates like those of Dolevan. As they 

 trend to the south-west, the quartzose grits thin out, but reappear in the high hills of 

 Pentwyn and Esgair-davydd, south-west of Llanwrtyd, whence they are prolonged at 

 intervals into Caermarthenshire. These quartzose grits are subordinate to a thick 

 assemblage of coarse and imperfectly formed slates, which are displayed on the sides 

 of the transverse and parallel gorges by which the streams of the Chwefru, the Dulas, 

 the Cammarch, the Cnyffiad, and the Yrfon flow from the higher and western regions 

 of Brecknockshire. A slaty cleavage, the planes of which dip at high angles to 

 the north-west, is, for the most part, distinctly impressed upon these old rocks, and 

 further to the north-west they graduate into true roofing slates. Though organic 

 remains are very rare, I have discovered imperfect casts of them in the grits near 

 Llanwrtyd. 



An important feature in the arrangement of the rock masses of this country is, that 

 the Cambrian System and older slates are thrown over with an inclination to the north- 

 west, and sometimes at high angles. This phenomenon of the older or Cambrian rocks 

 being found in a position discordant to those of the Silurian System, has been before 

 alluded to, p. 309, and is observable along the line of junction in the counties of Salop, 

 Montgomery, Radnor, and Brecon. It is, indeed, precisely what must have resulted 

 from those elevations by which the Lower Silurian Rocks have been developed in an- 

 ticlinal ridges, and have thus thrown off on both sides the Upper Silurian Rocks, which 

 on one flank, are thus necessarily brought into abrupt contact with the strata of the 

 older system. In Caermarthenshire and Pembrokeshire, however, we find, that where 

 other parallel lines of elevation affecting the Lower Silurian Rocks have taken place 

 near the frontier of the Cambrian rocks, there are, as in the example of the Berwyns, 

 p. 308, indisputable evidences of a passage from the one system into the other; and 

 hence the great break between the Silurian and Cambrian Systems, marked in many 

 previous sections like that between the New Red and Carboniferous Systems, is to 

 be only classed among limited phenomena. (See also remarks on the passage from the 

 New Red into the Coal Measures, pp. 54, et seq.) 



