CENTRAL OR CORNSTONE FORMATION. 



175 



There are many other situations, particularly in the proximity of coal-fields, (see Map,) 

 where this subdivision exists ; but I have only alluded to some of the best-defined in- 

 stances. With the exception of a large fish scale detected near Crickhowell, no animal 

 organic remains have yet been observed in this upper division; and although a few frag- 

 ments of carbonized vegetables have been collected, none of these, as before said, are iden- 

 tifiable with the plants of the coal measures. In their imperfect state of preservation, 

 the botanist can only decide that they belong to land plants. I have, however, little doubt 

 that future observers will detect vegetable forms sufficiently well defined to be described; 

 and I would recommend those ravines near Cusop to be well examined in which deep 

 sections are exposed, as well as other parts of the escarpment of the Black Forest 1 . 



Central or Cornstone Formation. (/. of wood-cut and PL 31. fig. 1.) 



The central masses of this system are chiefly composed of alternations of red and 

 green argillaceous spotted marls, affording, on decomposition, the soil of the richest 

 tracts of Brecknockshire, Monmouthshire, and Herefordshire ; a large position of Salop, 

 and small parts of Gloucestershire and Worcestershire. These argillaceous beds some- 

 times alternate with sandstone, but more frequently with irregular courses of concre- 

 tionary impure limestone, mottled also red and green. When compact, the calcareous 

 beds are termed simply limestones by the quarrymen, but when mixed with sand and 

 marl, giving them a brecciated or conglomerate aspect, they constitute the well-known 

 cornstone of these counties. The dull-green colour on the Map indicates the chief lines 

 where these concretionary courses have been observed, though it is not pretended that 

 all the points where calcareous rocks appear at the surface, are laid down; for the corn- 

 stone formation of the Old Red System contains so many inosculating and slightly per- 

 sistent bands of concretionary limestone, in some cases mixed with marl, shale, and 

 sand, in others assuming a sub crystalline form, that it is seldom practicable to separate 

 the calcareous matter, even in any one district, into a given number of bands. 



We may commence the detailed description of this formation in Caermarthenshire, 

 because, although the cornstones in that county are of very impure quality, their relations 

 are clearly exposed in a section exhibited in descending from the Caermarthenshire fans 

 towards the bridge called Pont-ar-lleche. (See PI. 34. fig. 5.) The river Sowdde here 

 runs in a narrow cleft or channel which cuts the strata at right angles to their strike. 

 The inclination of the beds is towards the south-east, and the dip varies from 65° near the 

 superior limits of the formation, to 75° towards its base, or junction with the tilestones. 

 In the short distance, therefore, of about three quarters of a mile, we obtain by means 



1 The shells and fishes of the Old Red System are described in subsequent chapters, followed by descriptions 

 of the Silurian fossils. 



Y 2 



