154 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [DeC. 16, 



but this opinion I abandoned ; and the position I have here given 

 them appears to be confirmed by some sections taken farther south. 



4. Cambro- Silurian group. — This group includes, along the line 

 of section, the slates and conglomerates of Dol Fan, and all the Lower 

 Silurian rocks of Builth ; and is overlaid by the fine mountain ridge 

 of the Upper Silurian rocks of Mynydd Epynt, &c. Details are 

 here unnecessary, as they appear in the ample descriptions of the 

 " Silurian System," and in the published sections of the Ordnance 

 Geological Survey, under the direction of Sir H. De la Beche. But I 

 may remark, that under this name (Cambro-Silurian) I include rocks 

 of the thickness of several thousand feet, which may be, perhaps, 

 conveniently separated into four sub-groups. 



(1.) Conglomerates and slates, both sometimes affected by slaty 

 cleavage, and here and there containing fossils. They are carried 

 by undulations far to the west of the demarcation of the Lower Silu- 

 rian rocks, as published in the Map of the " Silurian System." 



(2.) Lower Llandeilo flag. 



(3.) Slates and griis (Caradoc sandstone of Noedd Grug), &c. 



(4.) Upper Llandeilo flag, passing by insensible gradations into 

 Wenlock shale, and containing several very characteristic Wenlock 

 shale fossils. In proof of this I may state, that I have seen slabs of the 

 Upper Llandeilo sub-group containing Phacops caudatus, Ogygia 

 Buchiiy Calymene Blumenhachii^ associated with Leptcena depressa 

 and euglypha ; — an association unlike what I have ever seen in any 

 of the lower groups, but not to be regarded as anomalous among 

 what may be considered (on the evidence both of sections and fos- 

 sils) as passage-beds between the older Cambrian slates and the Silu- 

 rian system. And to these I may add defences of fishes^ also found 

 in the Upper Llandeilo flag. 



Second Transverse Section through the Older Groups of South Wales, 



In like manner, commencing at Aber Aeron, and making a com- 

 plete traverse by Llampeter, Pump Saint, and the old road to Llan- 

 dovery, we meet with a nearly corresponding succession of deposits 

 in the following ascending order : — 



1. The Aberystwyth group — of great thickness and in its most 

 characteristic form. 



2. A contorted series of earthy slates, flags, and bands of grey- 

 wacke, which I would arrange nearly on the parallel of the groups 

 west of Plynlimmon. These extend to Llampeter; and among them 

 are the Annelides discovered by Dr. Olivant, and figured in the 

 " Silurian System." I may remark, by the way, that Annelides, 

 though perhaps of a different species, are also found in the lower 

 beds of the " Cambro-Silurian group," and also among the tilestones 

 at the very top of the " Silurian System ;" e.g. at the Sugar-loaf, a 

 hill between Llandovery and Llanwrtyd Wells, and in the " Tile- 

 stone " ridge south of Llandeilo. 



3. A long series of contorted slates and grits, undulating through 

 the hills between Llampeter and Pump Saint, and extending to the 



