Vol. 51.] SERIES OF SOUTH DENBIGHSHIRE. 15 



and northern foot of the hill east of JNant-y-gro, and are then 

 followed by a considerable thickness of much more siliceous beds, 

 in which no fossils have yet been discovered. 



These siliceous or gritty beds form the whole of the top of this 

 hill and dip gently to the east, occupying most of the steep slope 

 south of the Holyhead road. As we approach Pen-y-vivod they 

 pass up into less gritty and more slaty beds ; but no very definite 

 boundary can be drawn between the two series. 



On the Wriddiog spur, where the Holyhead road makes an acute- 

 angled V, what appear to be the same gritty beds are well ex- 

 posed. They here form an anticlinal or part of an elliptical dome, and 

 on the southern side of this anticlinal they are shown in a quarry 

 by the side of the road, where they dip 37° E. of S. at an angle of 

 15°. Following the path which leads from the road up the side of 

 "Wriddiog to Pen-y-vivod they are succeeded by more slaty beds 

 which split up into long prisms, and in which Monograptus leintwar- 

 dinensis occurs in large numbers. Between these Leintivardinensis- 

 beds and the slaty beds close to Pen-y-vivod runs a small dyke, 1 which 

 has been worked apparently under the impression that it was a 

 mineral lode. There is no evidence of a fault along the line of this 

 dyke, and therefore, though Monograptus leintwardinensis has not 

 yet been found south of the dyke, it must be concluded that the 

 slaty beds near Pen-y-vivod belong to the same horizon. They 

 certainly overlie gritty beds of much the same character as those 

 of "Wriddiog. 



It is clear, then, that the Denbighshire series, so far as it is seen 

 in this part of the basin, may be divided, in descending order, 

 into : — 



Slaty beds with Monograptus leintwardinensis. 



Gritty beds. 



Flags with M. colonus, Cardiola interrupt a, Orthoceras primmvum, etc. 



Moel Ferna Slates, with M. Flemingi, M. pripdon. 



Pen-y-glog Grit. 



Pen-y-glog Slates, with M. personatus, M. priodon, etc. 



Dinas Bran Beds. 



The Leintwai-dinensis-beds are the highest in the area described 

 in the present paper, but higher beds than these occur within the 

 basin. East of Wriddiog there is a certain amount of disturbance 

 and a small fault, so that the sequence is no longer quite continuous 

 and requires further examination. But the disturbance is not 

 great ; the rocks are of the same character on the two sides of the 

 fault, and the discontinuity must be small. The general dip, as is 

 well shown in the bed of the Dee, is easterly, and the beds near 

 Llangollen must therefore lie above the Leintwardinensis-sl&tes. 

 The most interesting of them are those of Dinas Bran, the striking 

 conical hill north-east of that town. These have already been 



1 Fragments of what appears to be the same dyke are to be found on the 

 hill immediately north of the Dee at Glyndyfrdwy, in the little valley which 

 runs up from Coed-ial. 



