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



On this view, the rocks of the Carnarvon promontory are the pro- 

 longation of the rocks of the great Carnarvon chain ; in confirmation 

 of which I may state that (with some local exceptions) they have a 

 prevailing strike nearly parallel to the direction of the promontory, 

 and therefore not far from the mean strike of the Carnarvon chain. 

 I must here terminate (for the present) my notice of the sections of 

 North Wales, and of their mutual relations, considered as parts of 

 one great system. 



System of South Wales. 



For the present I use these v/ords in a mere geographical sense, 

 including under them all the slate groups of South Wales (and of a 

 part also of Montgomeryshire) which are expanded through the 

 Principality between the Silurian rocks of Sir R. I. Murchison's 

 map and the sea-coast. That rocks of an enormous thickness, as- 

 sociated with the chain of Cader Idris, plunge under these groups 

 at their northern limit, cannot admit of doubt. Again, at their south- 

 western limit, they are bounded by the slates and porphyries of 

 Pembrokeshire, which are so nearly identical in structure with the 

 older slates and porphyries of North Wales, that I concluded them 

 in 1836, the only time I ever touched on the county of Pembroke, to 

 be of one epoch. This also was the conclusion Mr, Greenough had 

 arrived at when he published the first edition of his Geological Map. 

 This conclusion is now put out of all doubt by the admirable de- 

 tails of the Ordnance Geological Survey ; and it may be at once 

 assumed as a fact, that the principal South Welsh slate groups I am 

 here noticing, occupy a great irregular trough — bounded to the 

 south-west by the older Cambrian slates and porphyries of Pem- 

 brokeshire — to the north by the chain of Cader Idris — and on their 

 other sides by the sea-coast and the rocks of the Silurian system. 

 What then are these rocks ; and where are they to be placed ? To 

 receive a full reply to this question, we must wait for the details of 

 the Ordnance Survey. I profess not to know well this most con- 

 torted and perplexing country, for I only made two hasty traverses 

 through it between the Upper Silurian groups and the sea — one 

 from Aberystwyth to Builth, the other from Llandovery to Aber 

 Aeron by Llampeter, and thence back by the road from Llampeter 

 to Carmarthen. 



I chose these lines because they were the very lines by which I 

 made hasty traverses through this country in the year 1832, with a 

 view to subsequent operations. My object during the past summer 

 was, not to make out the details of this most diflftcult country (that 

 would have been a vain attempt, considering the narrow limits of 

 my time), but to form, as far as I could, a general notion of the re- 

 lative position of the principal mineral groups ; or at least to obtain 

 such an insight into the structure of the country as to be enabled to 

 comprehend the descriptions of others, and the admirable sections 

 of the Ordnance Survey, now in progress of publication under the 

 direction of Sir H. T. De la Beche*. 



* An unpublished section by Mr. Ramsay, along a line from the Upper Silurian 

 rocks to the sea-coast between Aberystwyth and Aber Aeron, was kindly commu- 



