148 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Feb. 25, 



crossed the Rhinog Fawr chain since 1832. Lastly, the Harlech 

 grits form the extreme point of the promontory south of Tremadoc ; 

 and they may be traced round the great southern headland of Caer- 

 narvonshire from St. Tudwal's Island to Hell's Mouth. I have 

 thought this short explanation necessary, in order to shovr what is 

 here meant by the lowest Cambrian group of the Tabular View. 



The Festiniog group (No. 2), taken collectively, and where it is well 

 developed, is not, 1 think, less than 9000 or 10,000 feet in thick- 

 ness. Its lowest sub-group (Lingula-flags) is best seen to the south 

 of Festiniog and Tremadoc ; but for details respecting it, I must refer 

 to my former papers. The mineral structure of the Tremadoc slates 

 is very peculiar. It is sometimes penetrated by metalliferous veins, 

 and it contains beds or large concretionary masses of magnetic and 

 pisolitic iron ore. This iron-ore is a good finder for the group, as I 

 can assert on personal experience. It exists, for example, in the 

 country east of St. Tudwal's Road, in the black slates between Clynog 

 and the Rivals, at Tremadoc, on the east side of the Merioneth an- 

 ticlinal, and on the N.W. flank of Cader Idris ; and in all these places 

 it defines the position of the sub-group, whatever other mineral mo- 

 difications it may have undergone. 



The third sub-group, Arenig-slates and porphyries, is of vast thick- 

 ness, and in general structure is almost the exact counterpart of 

 the green-slates and porphyries of Cumberland. The whole mass is 

 stratified very regularly, and in its upper portion are irregular con- 

 cretionary beds of dolomitic altered limestone, without fossils. The 

 trappean beds, whether erupted or recomposed (such as trappean 

 conglomerates, trap-shale, &c.), are of very variable thickness ; and 

 where they are degenerate, the regular slates expand, and sometimes 

 contain fossils. Arenig and Cader Idris may, perhaps, be near the 

 centres of plutonic eruption ; but they are regularly stratified, and 

 I never found among them any which I thought true subaerial pro- 

 ducts. 



The Bala group (No. 3) is also of great thickness. It may be 

 divided into two sub-groups, the lowest member of which is finely 

 developed in a mountain-ridge of dark pyritous and rather earthy 

 slates (in some places, however, forming a good roofing-slate), which 

 overlies the S.E. flank of Cader Idris : the same dark slates appear 

 on the east side of Arenig. But I must not here describe the great 

 succession of earthy and arenaceous deposits, slates, flag-stones, &c., 

 often highly fossiliferous, and more or less calcareous, which form the 

 lower Bala group, and conduct us to the Bala-limestone. For details 

 I must refer to my published papers and abstracts. 



The Upper Bala group (No. 3, b), in North Wales, cannot, I think, 

 be less than 4000 feet in thickness ; it begins with the Bala-lime- 

 stone, to the east of Bala, and includes the Hirnant limestone and 

 shelly sandstone; and it includes, near its upper surface, some are- 

 naceous flagstones ; and (if I have not misinterpreted some obscure 

 sections) ends with dark indurated shales, here and there passing into 

 a bad pyritous roofing-slate. 



Over the group last-noticed is a series of beds of considerable 



