258 [Nov. 29, 



§ 4. On the Structure of the Bericyn Chain. 



1. This chain is cansidered as commencing, on the south, in the 

 ridges above Mallwyd, to the east of the i-iver Dyfi, and as 

 stretching from thence in a north-westerly direction to the hills 

 which overhang the Dee below Corwen. 



2. If a line be drawn from the summit of the mountain pass 

 between Llangynog and Bala to the great bend in the valley of the 

 Dee between LlandriUo and Bala, the south-western portion of the 

 chain, extending as far to the north-east as that line, constitutes a 

 great trough. The subordinate groups of this southern, portion of 

 the chain are made up of the fossiliferous rocks of Bala ; but its 

 crests consist of beds far above the Bala limestone. On the 

 eastern side of the trough the beds are partly vertical, and partly 

 inverted; and on the southeastern extremity of the chain, for 

 several miles along the boundary between these disturbed rocks of 

 the lower system and the co-terminous upper Silurians, the in- 

 version affects also the upper system of rocks.* 



3. A longitudinal fault, with a great upcast to the west, ranges, 

 on the eastern side of the chain, from the vertical and inverted 

 beds above-mentioned to the northern end of Cader Ferwyn ; in 

 consequence of which, the Berwyn chain, for the distance of more 

 than 4 miles north of the Llangynog pass, is no longer in a trough 

 of rocks belonging to the Bala series ; but the crest of the chain 

 consists of rocks which are lower than the Bala limestone, but not 

 lower than the fossiliferous slates on the east of Bala lake. 



4. The strike of the higher ridges of the Berwyn chain varies 

 from N. and S. to N. E. and S. W. ; but N. N. E. and S. S. W. is 

 about the mean strike. 



which is actually covered by Upper Silurian rocks, has been pointed out by 

 Mr. Sharpe. The same observation may be applied to a district extending along 

 the south end of the Berwyns as far as the tributaries of the Severn. Professor 

 Sedgwick observes, that since he had never either examined, or professed to have 

 examined, this part of North Wales until the year 1843, he does not hold him- 

 self responsible for the colouring adopted in that part of the map in question. 



* The following diagram has been prepared by the author, in illustration of 

 bis views respecting the structure of the North Berwyns. 



Part of the Berwyns, N. of 

 Pass to Llangynog;- 



'/ //AL/ ' 



/ / / /o/ 





/'///V/7A----iO< 

 / / / / ' / 7 /^\--::Cx ^^ 



A. System of the North Berwyns. C. Line of fault, North Berwyns. 



B. Line of fault, North end of the Berwyns. D. Llanrhiadr, anticlinal. 



