Murchison's Silurian System. 529 



all the intermediate strata, nothing being left to hypothesis 

 or the imagination. 



The carboniferous limestone forming the lowest member 

 of the coal measures, is a " strong regularly bedded mass, 

 partly light coloured, rarely oolitic, and occasionally of so 

 dark a tint as to be termed black marble. It is overlaid by 

 the conglomerates and sandstones of the millstone grit, or 

 base of the coal-field, and rests upon another conglomerate 

 very similar to the millstone grit, but generally of a redder 

 colour, which constitutes the uppermost stratum of the old 

 red sandstone. The limestone in its course through Brecon, 

 Monmouth, and Caermarthen, forms a tortuous girdle con- 

 forming to the shattered outline of the coal measures. 



The thickness of the limestone varies from a mere band 

 to a thousand feet, and it is in those situations where it is 

 most expanded, as in the eastern division of Monmouthshire 

 and adjacent parts of Gloucestershire that the beds of 

 passage are best seen. The passage from the lower beds 

 of this limestone to the old red sandstone is analogous to 

 that previously pointed out between the uppermost part of 

 the coal measures and the new red sandstone, the phe- 

 nomena being always best displayed in those districts 

 where the respective portions of these systems are most 

 developed. The limestone shale, by which the lower lime- 

 stone is accompanied, is another peculiar character of this 

 member of the coal series, and the course of these shales may 

 be discerned even when covered by herbage and detritus, by 

 the surface consisting of boggy ground interspersed with 

 rivulets and springs, the waters of which having been ab- 

 sorbed by the pervious strata of the overlying coal grit and 

 limestones, are thrown out by this argillaceous sand, and fall 

 in rills over the scarped and lofty edges of the old red 

 sandstone. A peculiar glazed appearance of the surface of 

 the limestone, was pointed out to Mr. Murchison by the Rev. 

 H. Lloyd, on detached fragments which appeared to have 



