SEDGWICK ON NORTH WALES. 11 



between Llangynog and Bala, dipping westward beneath the chain 

 of the southern Berwyns. 



Below the village of Llanwddyn there continues a prevailing 

 Westerly dip ; but the derangements are enormous, and, at the 

 great bend in the Fyrnwy river, the lower Silurian rocks (g) are 

 seen resting upon the upper (K) in a reversed position. 



In this section, as in the former, the protozoic series is of great 

 thickness. 



Section III. 



Arran Mowddy to Llaxlihangel. 



Direction of the Section, W. 10° N. to E. 10° S. 

 Horizontal base 1 7 miles. 



Carves 

 y big. 



i i,i 



b" c d d' e e' f 



This section commences with cotemporaneous porphyries 

 {a a') ; but they break out at a still higher geological level than 

 in Section II. The porphyry is succeeded by grey slates (b b") 

 containing the Bala series (b'), which may be followed southward 

 in the direction of the strike, down the western bank of the Dyfi. 

 The beds dip east by south, except to the extent of a faulted in- 

 terval (c) on the east side of Carreg-y-big. We have in this line 

 of section a great thickness of the fossiliferous portions of the 

 protozoic series, but not the whole thickness ; since these beds are 

 succeeded in the line of section by a trough of overlying and un- 

 conformable Upper Silurian rocks, dd! . These rocks accord with 

 the type, not of the Denbigh or Montgomery flagstones, but of the 

 coarse-grained greywacke and flagstone which form the base of 

 the upper system near Cernioge. 



Beyond this trough, the older beds (e e') again rise out, but with 

 a reversed or northwesterly dip, and at a very high angle of incli- 

 nation. At the east end of this, as of the former section, the Upper 

 Silurians {/) pass under the Lower (e), owing to inversion. 



In following the fossiliferous beds of the southern Berwyns to 

 the neighbourhood of Mallwycld, the author found those beds over- 

 laid by Upper Silurian rocks of the Cernioge type ; a fact which 

 had previously been noticed by Mr. Sharpe. Mr. Sharpe, how- 

 ever, considers that these Upper Silurians rest conformably on the 

 Lower, and that the entire upper part of the Lower system is here 

 displayed.* The author considers that the upper system wraps 

 round the southern end of the Berwyns unconformably ; and that 

 the upper part of the lower system is incomplete, f 



* Proceedings of the Geological Society, vol. iv. p. 13 



t The errors committed in certain parts of Mr. Murchison's map, in the 

 neighbourhood of Mallwydd, by spreading the Cambrian colour over an area 



