1843.] ■ 259 



5. The beds, on the two opposite sides of the great upcast 

 fault, are in a most" anomalous position. On the west side, they 

 strike about N. by E. ; but on the east side, nearly E. and W. 



6. At Bwlch Maengwynnedd, above a mile north of Cader Fer- 

 wyn, is another great fault or flexure. To the north of that point 

 all the beds, to the further extremity of the chain, dip either N., 

 or N. by E., and strike either E. and "W"., or E. by S., and W. by 

 N. This strike is continued towards the east, as far as the 

 mountain limestone on the confines of Shropshire ; and, towards 

 the west, to the hills north-west of Llandrillo, on the left bank of 

 the Dee, between that river and the brook, Nant Ffrauan. 



This position of the strata on the eastern side of the Berwjm 

 chain gives a regular ascending section from the lower series to 

 the upper, in advancing towards the Dee from south to north, 

 along a meridian passing to the east of Llangynog. 



§ 5. Sections East of the Berwyns. 



Section IV. 

 Welch-pool on the Severn to Llansaintfraid on the Ceiriog. 



Mean Direction of the Jine of Section, S. to N. 

 Horizontal base 24 miles. 



N.W. S.E. 



e d 



1. At the southern extremity of Section IV., we have the Upper 

 Silurian flagstone of the Severn (a), which formation, after two 

 intervening portions of lower Silurian rocks {b b'), re-appears in 

 the Broniarth hills {a"). 



2. Then occurs a great undulating series of Caradoc sandstone 

 {b", 6'"), with innumerable fossils ; but among these the author dis- 

 covered no trace of Asaphiis Buchii, nor of some of the other cha- 

 racteristic species of the lower rocks in the Bala sections. These 

 beds extend as far as the Tanat river, where the strike is nearly 

 east and west. 



3. From beneath the Caradoc sandstone, there rises, north of the 



* Those points where, either in the author's sections or coloured copy of the 

 Ordnance Map, calcareous beds are marked as occurring, are denoted by the 

 letter A ; those points where porphyry is marked as occurring, are denoted by 

 the letter ir. 



