144? PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [DeC. 16, 



4. A great series of beds of flagstone, occup3nng all the country 

 near Festiniog and Maentvvrog, alternating ^ith bands of contempo- 

 raneous porphyry, and intersected in several places by dykes of fel- 

 stoiie ; but 'never, so far as I have observed, by any dykes of augitie 

 trap like those of Anglesea and the Menai Straits. The average dip 

 along this part of the line of section is from 20° to 25°; but towards 

 Festiniog these beds are affected by the flexures at the northern end 

 of the great Merioneth anticlinal, so that the average dips are con- 

 siderably less. The whole thickness of this group must be very 

 great, and in portions of it are innumerable impressions of Fucoids 

 and Lingulce. 



5. Over the preceding group are the great slate bands of the 

 Festiniog mountains, alternating with trappean shales and great ribs 

 and contemporaneous beds of porphyry ; in structure perfectly iden- 

 tical vv^ith the rugged porphyritic crests of the Snowdonian chain, 

 and partaking of a common system of undulations, a common strike, 

 and conditions which (independently of fossil evidence) seem to 

 offer sufficient proof that the Carnarvon mountains and the Fes- 

 tiniog mountains belong to one great physical and inseparable group. 

 This conclusion is, however, now also supported by fossil evidence ; 

 for the slates and porphyries of the crests of Snowdon and Moel 

 Hebog are about the same geological elevation above the Lingula 

 and iron ore beds of Tremadoc, that the beds of Moel Wyn and 

 Cynicht (at the extremity of this section) are above the Li7igula beds 

 of Festiniog. 



Section V 



From the coast south of Harlech, across the Merioneth Anticlinal, 

 to the Hills near Dinas Mowddwy, 



W.N.W. . >. E.S.E. 



a h c d e f ^ 



a. Very coarse greywacke. e. Slate and porphyry. 



b. Slate and porphyry. /. Bala series. 



c. Hard grits and porphyry. g. Upper Silurian. 



d. Supposed Lingula beds. 



If we examine the Merioneth coast between Maentwrog and Bar- 

 mouth, we first meet with the Lingula flags and afterwards a series 

 of superior beds, which come down, with a regular strike and a north- 

 western dip, from the high Festiniog hills. Near the mouth of the 

 estuary (Traeth Bach) the beds above-mentioned are met, on a line 

 of fault, by a great system of disturbed strata, which strike about 

 N.N.W. and dip E.N.E. These disturbed strata may be followed 

 several miles towards the south. They appear to be intersected by 

 a second great fault, ranging*about N.E., and entering the sea near 

 the Llandanwg estuary. The north-western strike and north-eastern 

 dip are however continued considerably to the south of that estuary, 

 and also extend into the interior as far as the western skirts of the 



