50 PALAEOZOIC TIME. [CHAP. V. 



Since, however, we cannot be sure that the strata which 

 conformably succeed the Silurian are represented to any 

 great extent in the Lower Devonian group, and as it is 

 possible the mass of them may be older, the safest plan 

 will be to treat the rock- groups above mentioned as three 

 separate series, viz. : (1) Lower Old Eed Sandstone, and 

 its homotaxial equivalents in Scotland and Ireland; (2) 

 Devonian rocks ; (3) Upper Old Eed Sandstone. This is 

 the plan adopted by Mr. H. B. Woodward in his new 

 edition of the " G-eology of England and Wales." 



1. Stratigraphical Evidence. 



1. Lower Old Red Series. In England this series is 

 only found on the borders of Wales, in the counties of 

 Brecknock, Monmouth, Hereford, and Shropshire, where 

 it covers a considerable area. The rocks consist of sand- 

 stones, flagstones, and marls, the sandstones generally red, 

 the marls red, grey, or green, and including lenticular 

 masses or nodules of limestone, which are locally known as 

 " cornstones ; " hence the whole series is sometimes called 

 the " Cornstone Series," and its thickness is supposed to 

 be from 2,000 to 2,500 feet. It is succeeded by the Middle 

 Old Eed Sandstone, or "Brownstone Series," but Mr. 

 Symonds believes that there is a break in the succession 

 at the summit of the Cornstone group. 1 



The limits of this group are not yet accurately known, 

 but its western outcrop is believed to extend from the 

 head-waters of the river Usk in Brecknock to Much Wen- 

 lock in Shropshire, and as it also flanks the Silurians of 

 Woolhope and Malvern, &c., it would appear to lie in a 

 broad synclinal which has a general north-east to south- 

 west strike. The area becomes narrower towards the north - 



1 " Records of the Eocks," p. 234. 



