444 Proceedings of the British Association. 



respectful to the advocates of those sciences, was a legitimate way of 

 advancing geological inquiries, that it appealed to right reason, and its 

 results were as much deserving of confidence as any other branch of 

 knowledge. — Prof. E. Forbes remarked the great value of microscopic 

 investigation to the physiologist and systematic naturalist, but at pre- 

 sent he should hesitate to found conclusions on the isolated facts of a 

 research at present in its infancy. 



Sir H. DelaBeche exhibited the Ordnance Map of South Wales and 

 the South- West of England, and a section through the Silurian rocks 

 in the vicinity of Builth. The vicinity of Builth is one of much geolo- 

 gical interest, as showing the connexion between the Silurian rocks at 

 Ludlow, Wenlock, and other localities on the N. E., with the same 

 deposits in Brecon, Carmarthen, &c, and as affording considerable in- 

 struction relative to the intermixture of sedimentary and igneous rock 

 at this early period. The section described by the author is part of 

 one now making by the Geological Survey between the old red sand- 

 stone of the Black Mountains in Brecon and the sea N. of Aberystwith. 

 Sir H. De la Beche then compared this developmeut of the Silurian 

 rocks with that in Salopia, and observed, that although the Wenlock 

 limestone is but a trace near Builth, and the Aymestrey limestone little 

 else, still there is a general resemblance to the sequence described by 

 Mr. Murchison, at Malvern, Woolhope, &c. It is at the base of the 

 Wenlock shales the greatest modification is found ; instead of the 

 Caradoc limestone and sandstone are the shales and slates with Asaphus 

 Buchii, and beneath these a mixture of conglomerates, sandstones, &c, 

 with similar fossils, so that either the sandstones representing the 

 Caradoc, are included in the Llandeilo flags, and one appellation must 

 be applied to both, or the Caradoc sandstone must be supposed to have 

 thinned off, so as not to occur in the Builth and western sections. 



Mr. Murchison stated that every group of rocks, when traced into a 

 distant region, must be expected to present a difference in its fossils. 

 He considered the upper Llandeilo flag series the complete equivalent 

 of the Caradoc sandstone, and expressed his belief that in no country 

 would be found* fossiliferous rocks older than the lower Silurian sys- 

 tem.— Mr. Sedgwick described the various modifications presented by 

 the carboniferous strata of the north of England, Devonshire, &c, and 

 by the Silurian rocks of North Wales, and the typical region of Shrop- 

 shire, with the same series in Cumberland and Westmoreland. He con- 

 tended that the term Silurian should be confined to the Ludlow and 

 Wenlock rocks, which formed a complete system, with distinct mineral 

 structure and organic types. The Caradoc sandstone and Llandeilo 



