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



enter among the undulations of Moel Hebog and Moel Ddu, and 

 hence I believe them nearly of the same age. 



I have not gone round the promontory of St. Tudwal's Head since 

 1831, when I knew little of the structure of North Wales. From 

 what I remember of the structure of this promontory, and from the 

 fact that dark earthy slates occur to the north of it, with pisolitic 

 iron ore like that of Tremadoc, I should place it provisionally on 

 the same geological parallel with the slates and quartzose flags of 

 the promontory south of Tremadoc. 



In my former papers I have described the rocks of Carnarvon- 

 shire and Merionethshire as one connected system, repeated in a suc- 

 cession of great and nearly parallel undulations. In making a section 

 from the Menai to the chain of the BervA^yns, we meet, however, 

 with considerable difficulties, especially from three causes : 1st, from 

 the enormous masses of igneous rocks, which may not always be of 

 one epoch, though the greater part of them alternate with, and pass 

 into, the associated roofing slates ; 2ndly, from the manner in which 

 the great Merioneth anticlinal dies away towards the north, so that 

 the beds which cross its extreme northern limit are thrown into a 

 kind of arch, and dip, at a very low angle, towards the north-west, 

 the north, and the north-east ; Srdly, from the confusion introduced 

 by great disturbing forces (marked by the two great estuaries 

 Traeth Mawr and Traeth Bach) which have thrown the beds above 

 noticed near Tremadoc, and the beds of the Harlech coast, into 

 positions which are altogether anomalous. Judging however from 

 the best comparisons I could make of the several sections, I con- 

 cluded that the rocks along the Menai were among the oldest por- 

 tions of the Cambrian slates ; and that the beds along the Merioneth 

 anticlinal were perhaps the very oldest visible beds of the whole 

 series I am describing. The conclusion was chiefly based on this 

 fact, — that we have a regular ascending section from the Merioneth 

 anticlinal through Great Arrenig to the top of the Berwyn chain 

 (a distance of about twelve miles on a straight line) without any 

 material change of dip*. I can now fortify this conclusion by 

 better evidence than I offered before, by help of sections drawn from 

 this anticlinal line through the hills of Festiniog on its north-west 

 side, and through the chain of Cader Idris and other hills on its 

 south-east side. 



Passing by the line of the road from Beddgelert to Maentwrog 

 we soon get beyond the disturbed beds connected with Traeth 

 Mawr, and cross a long succession of slates and contemporaneous 

 beds of porphyry, trappean shales, &c. &c., which enter into the 

 structure of the high mountains north-west of Festiniog. For 

 several miles there is a steady dip. and at a considerable angle, 

 about north-west by north ; and the same dip is continued on the 

 south side of the Festiniog valley, where a well-known flaky pyritous 

 flagstone rises in very large and well-defined beds, and alternates with 

 a few bands of contemporaneous porphyry. From their mineral 

 character, I concluded them to be of the same age with the flags and 



* The same conclusion is indicated by Section 5. See also Journal, vol. i. p. 10, 

 Section 2. 



