LOWER LIMESTONE SHALE AND OLD RED SANDSTONE. 



453 



Caradoc Sandstone, that without a clear order of superposition and close examination 

 of the organic remains, even practised observers might be misled. These beds consist 

 of yellowish and dull brownish red sandstone, alternating with shale and impure lime- 

 stone, the surfaces of the former being frequently impressed with the circular forms 

 of crinoidal stems, resembling the casts so common in the Caradoc formation. In 

 Monmouthshire, particularly to the west of Chepstow, I have already noticed a large 

 development of these beds. (p. 160, and PL 36. f. 24.) A small boss of them has 

 recently been exposed near the Ship Inn, at Abbey Land, on the sides of the high road 

 from Bristol to Gloucester, the organic remains from whence having been examined by 

 Mr. Stutchbury, prove to consist of the palates of the same species of fishes which 

 characterize the bottom beds of the carboniferous limestone in other places; viz. Psam- 

 modus linearis, P. Icevissimus, &c, together with bodies supposed to be Coprolites, and 

 the Pileopsis angustus, PhilL, a shell of the carboniferous system. These beds are not, 

 however, peculiar to Gloucester or Monmouth, for they are exposed at various points 

 around the great South Welsh coal-field and in Pembrokeshire 1 . (See p. 384.) 



Old Red Sandstone. 



Succeeding to the carboniferous limestone, the Old Red Sandstone of Tortworth 

 presents peculiarities of structure, which are not found in the chief range of this 

 system. The upper beds, instead of being conglomerates, consist of finely-grained, 

 thin flagstones of white and whitish grey colours. These are seen in the elevated 

 platform of Tortworth Green, where they dip distinctly under the carboniferous lime- 

 stone, and pass down into red sandstone and argillaceous red and green marls of consi- 

 derable thickness. This upper division is underlaid by coarse, quartzose conglomerate 

 and red sandstone. Sections made in remote parts of the district, six and seven miles 

 from each other, either in ascending from Falfield to Tortworth, from Thornbury to 

 Milbury Heath (where some of the best quarries are seen), or from Kington west of 

 Thornbury to Mumbleys Plat, exhibit the same succession and persistency of the con- 

 glomerate in the central and even low part of the formation. 



An enormous thickness of red and green argillaceous marl is clearly indicated by 

 the sections at Gatcombe and Purton, on the right bank of the Severn. We there see, 

 red, green, and white-spotted marls, quite undistinguishable from those of the New 

 Red Sandstone, with a few strong bands of dark red micaceous sandstone. The large 

 development of these argillaceous marls of the Old Red, renders it most difficult to 

 draw a line of separation, in any geological map, between them and beds of similar 

 composition in the New Red, wherever the marls of the two systems are brought 



1 See also the detailed sections in the memoir of Messrs. Buckland and Conybeare, by Mr. De la Beche, 

 Mr. Cumberland, Dr. Bright, &c., Geol. Trans., vol. i. p, 240. 



3 L 



