and the Palaeozoic System of England. 4i77 



rocks of other localities. One of the species [H. didyma) had 

 not been found lower than the Aymestry limestone ; and Spiri- 

 fera crispa not lower than the Wenlock limestone. A few of the 

 species, as is well known, have a much wider range. 



It follows from the above facts, and especially from the list of 

 fossils, that the sandstone of Presteign is a true May Hill sand- 

 stone : but it does not exhibit its relations to any inferior group ; 

 neither does it exhibit any mixture of the characteristic Cam- 

 brian with the characteristic Silurian types. 



II. Sections through the Llandeilo Flag and the Trappean Hills 

 north of BuiltK (Fig. 2.) 



The rocks here noticed are too well known to need any elabo- 

 rate discussion, as they are excellently described in the ' Silurian 

 System.^ They break out immediately on the west side of the 

 great terrace which runs into Mynydd Epynt, and is composed 

 of the Upper Silurian rocks (Wenlock and Ludlow). These 

 Builth rocks do not, however, dip under the " Upper Silurian " 

 rocks, hvAfrom them towards the west — where they are overlaid 

 by the Norbury limestone and the Wenlock shale. The shale 

 extends several miles further west through a comparatively low 

 country; and beyond it rise the higher hills of Radnorshire, 

 which in the ' Silurian System ' are coloured as Cambrian ; and 

 have always been considered by myself as a part of the Upper 

 Cambrian series. But the author of the " System,^^ in adopting 

 the nomenclature and colours of his map and sections, took it 

 for granted that the Builth flags were geologically superior to 

 these higher hills of Radnorshire ; and if the assumption be in- 

 correct I am not in any way responsible for the mistake. For I 

 never examined, or profecsed to have examined, the physical 

 base of the so-called " Lower Silurian ^' rocks of South Wales ; 

 nor did I ever see a single section of them between the summers 

 of 1834 and 1846. As a matter of fact, the Builth rocks show 

 no unequivocal relations either to the Cambrian or Silurian 

 series ; being surrounded by a sea of Wenlock shale, which 

 abuts against their lower beds on the eastern side and overlaps 

 their upper beds on the western. 



The accompanying section may be shortly described as fol- 

 lows : — 

 Fig. 2, Section about two iniles N. of Builth, nearly E. and W. 



* 5 ^ ^ s 



1. Trap. 4. Norbury limestone, &c. 



2. Alternations of recomposed trap and shale. 5. Wenlock shale, &c. 



3. Tark flagstone ending with a bed of recomposed trap. 



