1846.] SEDGWICK ON THE FOSSIL SLATES OF N.WALES, ETC. 151 



I may first remark, that to the south of Radnor Forest (so far as 

 I know the country from personal observation) there are no Upper 

 Silurian groups brought in among the undulations of the South 

 Welsh system. To the north of Radnor Forest the Upper Silurian 

 rocks pass (in many undulations) so far to the west of their line of 

 demarcation, as laid down on our maps, that I once supposed, as 

 above mentioned, that they might be included among the undula- 

 tions of Plynlimmon and its associated ridges. This extension was 

 hypothetical; and I now withdraw it, as I believe the true demar- 

 cation of the Upper Silurian rocks passes in an irregular line from 

 the neighbourhood of Mallwyd to the hills near Llanidloes, leaving 

 the Plynlimmon ridges far to the west. North of Radnor Forest 

 there is a great overlap of the Upper Silurian rocks ; so that in the 

 neighbourhood of Mallwyd, and thence round the southern end of 

 the Berwyn chain, they are brought into the anomalous position de- 

 scribed in a former paper*. 



The coast between Aber Dyfi and Aberystwyth I did not exa- 

 mine ; I am therefore not able to estimate (even approximately) the 

 probable thickness of the slate groups between the upper beds of 

 the Cader Idris system and the group which breaks out a few miles 

 north of Aberystwyth, and is thence extended for many miles along 

 the Cardigan coast. But these groups, whatever may be their thick- 

 ness, are above the whole Cader Idris chain. 



Section through the Older Rocks of South Wales. 



Commencing at Aberystwyth or Aber Aeron, and taking a section 

 across the older rocks of South Wales as far as the base of the 

 Upper Silurian groups, on either of the lines above specified, it ap- 

 pears that these older rocks may be conveniently subdivided into at 

 least four principal groups : viz. 1. The Aberystwyth group. 2. The 

 Plynlimmon group. 3. The Upper South Welsh slate group. 4. The 

 Cambro-Silurian group. The lower part of this fourth group is 

 composed of slates and conglomerates which can hardly be separated 

 from No. 3. The upper part of the fourth group represents the 

 Llandeilo flag series, at the base of the Silurian system, as published 

 by its author, and stands in the place both of the Llandeilo flag and 

 of the Caradoc sandstone, forming a single and inseparable group, 

 which at its superior limit passes (by almost insensible gradations) 

 into the Wenlock shale, and contains (mixed with its own character- 

 istic fossils) many of those most characteristic of the Wenlock shale. 



In 1832 I separated the great series above enumerated into five 

 groups, by striking off the conglomerates of No. 4- from the group of 

 the Llandeilo flag, and enumerating them as a distinct group ; for at 

 that time I was partially acquainted with the scheme of arrangement 

 (afterwards adopted in the Silurian system) whereby the Llandeilo 

 flag was cut off from the slates and conglomerates of No. 4. At the 



nicated to me by Sir H. T. De la Beche before I visited South Wales during the 

 past summer. 



* See Journal, vol. i. p. 5. 



M 2 



