Vol. 56.] 



SHALES OF THE WELSH BORDERLAND. 



397 



persistent elsewhere, that it is not unlikely that it occurs in proper 

 sequence in this district. 



It is evident from the facts collected and noted in the foregoing 

 pages, that the sections in the Long Mountain, so far as they go, are 

 in full accord with the succession made out in the Builth District, 

 both as regards the nature of the rocks and the sequence of the 

 fossils. 



The presence of the Purple Shales in the north, lying so close to 

 the beds of Middle Wenlock-Shale age (M.-Linnarssoni zone) seems 

 to point to considerable overlap in this part of the Long-Mountain 

 area ; though, as in the case of the earlier beds, it is hard to detect it 

 apart from the palaBontological evidence. 



In the south the succession is probably complete, at any rate 

 down to the bottom of the M.-riccartonensis zone, though the 

 Middle Wenlock zones are all concealed by alluvial deposits. 



Y. The Dee Yalley. 



In the Llangollen Basin the lower zones of the Wenlock Shales 

 are exposed in several localities, where their relation to the under- 

 lying Tarannon Shales is clearly shown. The general structure of 

 the district is a syncline, complicated by many minor folds and by 

 faults. 



(a) Ruthin Road-Section. 



The relationship to the Tarannon Shales is perhaps best seen on 



the road from 



Fig. 10. 



M. riccartonensis? 



Llan- 

 gollen to Ruthin, about 

 1 mile north of Pen- 

 tredwfr ; here is an ex- 

 cellent section exposed 

 for some little distance, 

 the basal Wenlock 

 Shales being easily 

 recognized and dis- 

 tinguished from the 

 Tarannon Shales, on 

 account of their very 

 different lithological 

 character. 



The beds which lie 

 immediately above the 

 ' Pale Slates ' are soft 

 and weather deeply : 

 they are succeeded, 

 however, by others 

 which are harder and much more slaty in character. At the junction 

 with the Tarannon s a few pale bands appear to alternate with the 

 darker Wenlock Shales, but these soon disappear. 



Q. J. G. S. No. 222. 2 e 



Murchisoni 



