10 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



Berwyns ; and, consequently, on this line of traverse, the Bala 

 limestone dips under the Berwyns, as Mr. Murchison * has. cor- 

 rectly stated. Further northwards that is not the case. 



[The series of fossils on the line from Llanwddyn to the head of the Pennant 

 valley, and thence to the top of the pass west of Langynog, is the exact counter- 

 part of the list from the limestones of Bala and Glyn Dyffws. ] 



11. The limestone is followed in descending order, 1st. by fossi- 

 liferous slates (I) ; 2dly, by slates without fossils alternating with 

 beds of Porphyry (m). These are supposed to represent a part 

 of the series between the Bala limestone and Arenig at the, 

 western end of the section ; and they are cut off, near Llangynog, 

 by a complicated series of faults. The author here takes occasion 

 to remark on the very great aggregate thickness of the fossiliferous 

 beds which are traversed by the line of section just described ; 

 although, on the one hand, the section has no determinate base, 

 and, on the other, does not reach to the highest of the protozoic 

 rocks ; since it is impossible to tell how many hundred feet may be 

 wanting to connect the highest beds which are traversed in this 

 section, with the base of the Denbighshire flagstones. 



Section II. 

 The Arenigs to Llanwrdyn. 



Horizontal base 20 miles. 

 N.W. S.E. 



Arenig. 



This section, like the former, commences On the west side of 

 Bala lake, and with a ridge of porphyry (a a) ; but the por- 

 phyry appears at a higher geological level than in the former 

 section. The section passes through the grey slates (b) on the 

 west side of the lake, and on the east side, through the Bala 

 limestones (c c' c r/ ), which, on this line, are very much contorted. 

 It then traverses the strike of the Hirnant limestone (d), and ex- 

 hibits in great perfection the beds (e), above that limestone. The 

 synclinal axis (<r) lies here considerably to the east of the moun- 

 tain crest : and, to the west of that axis, the same beds are again 

 repeated ; but they are now much faulted and broken (e'). The 

 beds (/), supposed to represent the Bala limestone, reappear in 

 the hills near the village of Llanwddyn ; from whence they may 

 be followed northwards in the direction of their strike, through 

 the head of the Pennant valley, and thence to the top of the pass 



* See "Proceedings of the Geological Society," vol. iv. parti, p. 11. 



