INTRODUCTION. 



characters over wide tracts, their colour and composition in parts of Caermarthenshire 

 and Pembrokeshire, and also at Tortworth, being very different from those of the same 

 rocks in Shropshire and Herefordshire. And thus the lesson which has been already 

 taught us by an examination of the younger deposits is repeated,— that the zoological 

 contents of rocks, when coupled with their order of superposition, are the only safe 

 criteria of their age. By such proofs we are enabled to distinguish the Silurian de- 

 posits from all others previously described, and through every lithological change we 

 can thereby separate the system into upper and lower divisions. 



Quitting the Silurian rocks, and taking an ascending prospect, I may observe, that 

 although the stratigraphical position of the Old Red Sandstone and its general rela- 

 tions, both in England and Scotland, were well known, no complete account of its 

 range, succession, and zoological contents, in Herefordshire and the adjacent counties, 

 had yet appeared, though it there forms one of the largest of the stratified British groups 1 . 

 Perceiving that besides this great thickness, the Old Red Sandstone had an individuality 

 of mineral character, and peculiar organic remains, I ventured for the first time to 

 designate it a system. This step was taken, not only to mark distinctly the vastness 

 of the strata by which the Silurian deposits are separated from those of the carboniferous 

 sera, but also to indicate at one glance, that the coal measures of our country are in- 

 cluded between two great red coloured systems of rock — the New Red Sandstone above, 

 the Old Red Sandstone below. 



Had my labours terminated at this point, the Silurian Rocks, and their immediate 

 relations to the overlying and underlying deposits, might have been long ago deve- 

 loped ; but I felt that unless the structure of the whole region in question was made 

 known, the map would have been imperfect, and the work would have borne an un- 

 finished character. I therefore resolved to give sketches of all the coal fields included 

 in the area examined, and further to explain their relations to the surrounding deposits. 

 In adopting this resolution, however, I could scarcely estimate the additional exertions 

 required ; for, with the exception of the basins of the Forest of Dean, and of South 

 Wales, none of these coal tracts, so important in the mineral economy of the nation, 

 had been described. 



But these subjects did not exclusively engross my attention. To render their history 

 complete, it was necessary to examine minutely all the rocks of igneous or volcanic 

 origin, which are interlaminated with, or penetrate the strata at so many points, as well 

 as to follow out the lines of elevation and dislocation. Further, to arrive at clear 



i The highest mountain in South Wales, called the " Fans" or points of Brecon, from its double summit, is 

 2862 feet above the sea, and is entirely composed of nearly horizontal beds of Old Red Sandstone. Through 

 an error of the press, (p. 170) this mountain is stated to be 2500 feet high. The height of the Caermarthen 

 Fans is accurately given as 2590 feet. Both these mountains are seen in the view facing p. 346, and the 

 heights of both are correctly marked in that lithograph and on the map. 



A* 



