152 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [DeC. 16) 



same time I saw that this plan of breaking up the fourth group into 

 two parts was not confirmed, but was apparently invalidated, by the 

 evidence of the only two sections I had examined. In making the 

 present attempt at subdivision, I must however state, that the sec- 

 tions are singularly contorted, that the groups are ill-defined, and 

 that the actual order of superposition is obscure ; hence I think it 

 not improbable, that rocks, newer than any above enumerated, may 

 in some places have been brought in among the great folds and 

 undulations of South Wales. On the other hand, I may state, that 

 the geographical distribution of the formations of North and South 

 Wales favours the above arrangement ; for the newer groups (Car- 

 boniferous, Old Red Sandstone, and Silurian) are arranged on 

 the south-eastern limits of the great transverse sections, while the 

 older groups succeed on the north-western limits of any complete 

 lines of traverse ; so that the intervening groups are arranged in an 

 ascending order as we traverse from the western to the south-eastern 

 limits of the Principality, along any of the lines of section above in- 

 dicated. From this remark we must however except the groups in 

 Pembrokeshire and the country immediately adjoining it, where the 

 east and west strike supersedes the more prevailing strike of North 

 and South Wales. 



1. The Aberystwyth group. — Commencing a section from the 

 coast near Aberystwyth, we find a great group composed of hard, 

 close-grained gritstone, generally of a dark colour, much broken 

 and jointed ; and sometimes looking as if made up of distinct con- 

 cretions rudely placed side by side, and blended one into another. 

 Associated with it are many small quartz veins and strings, and 

 more rarely small veins of carbonate of lime. These gritty beds 

 (which are seldom of great thickness) alternate indefinitely with 

 bands of indurated shale and flagstone ; the latter of which some- 

 times form thick beds, and are used as a building-stone. No shells 

 or corals have (so far as I know) been found in this group ; but 

 the flagstones are in some places covered with innumerable fucoids, 

 and with small cylindrical stems, supposed to be of vegetable origin. 

 This group ranges along the coast from a point a few miles north of 

 Aberystwyth to a point several miles south of Aber Aeron, through 

 a distance of about thirty miles ; and ranges several miles (five or six 

 on the average) into the interior of the country. The whole group 

 is astonishingly contorted and shattered, yet it must be of great 

 thickness. 



2. The Plynlimmon group. — This group (which in a more de- 

 tailed and accurate description might be divided into two or three 

 sub-groups) is also of very great thickness and extent. I include in 

 it provisionally the groups of flags, grits and slates which rise into 

 the hills on the west side of the Aberystwyth group forming the 

 ridges of the Devil's Bridge, ranging thence on the west side of 

 Plynlimmon, and continued to the east of Plynlimmon in repeated 

 undulations (still however giving the indications of an ascending 

 section) to the neighbourhood of Llangurig, on the road from Aber- 

 ystwyth to Rhayader. In this great group I would therefore include 



