Vol. 65.] ORDOVICIAN AND SILURIAN ROCKS OF CONWAY. 187 



obvious that they represent the Zone of Cyrtograptus murehisoni, 

 the characteristic zone of the base of the Wenlock. They are 

 succeeded by sandy shales with Monograptus riccartonensis, Lapw. 

 as the most abundant fossil. A small thrust separates these from 

 the next highest beds, flags with few grits, which also contain 

 M. riccartonensis ; cone-in-cone structure is shown in places, and 

 the same beds are continued up to the Point, where massive grits 

 overlie them. 



Beyond the Point where the massive grits come on, and where the 

 Hendre-Gyffin-Benarth line of disturbance affects the succession, 

 the beds are sharply bent into a series of isoclinal folds of various 

 dimensions, causing a deceptive appearance of thickness at first 

 sight. It is worthy of notice that the minor faults (? thrusts) 

 affecting the beds, which are all much compressed, invariably 

 occupy the crests of the anticlines. 



The massive grits beyond the Point are sharply folded and broken, 

 while between the Point and the Old Limekiln the section shows 

 a series of flags and grits which still contain Monograptus riccar- 

 tonensis ; in places, however, higher beds comprising fine shales 

 which weather to a bright orange may be seen nipped into the folds. 

 These beds contain Monograptus dubius, Suess, as the characteristic 

 fossil. M. dubius is known in other areas to come in abundantly in 

 the Zone of Cyrtograptus symmetricus, immediately above the Zone 

 of Monograptus riccartonensis ; it is therefore extremely likely that 

 these beds, together with the flags and thin grits overlying them 

 (which have only yielded a graptolite very like M. basilicus, Lapw.), 

 represent the horizon of the Zone of Cyrtograptus symmetricus. 



Beyond the Old Limekiln (where the ' Orange Shales ' are not very 

 well seen, but appear to be sharply bent) a series of flags with 

 thin grits comes on, representing the highest Salopian beds of the 

 district. They are exposed in a broad syncline the axis of which 

 lies approximate^' beneath the Cottage, and are seen once again 

 in a smaller fold near the termination of the section ; apparently 

 the intervening portion is chiefly occupied by folds of the Shales 

 containing Monograptus dubius. (See section on PL VIII.) 



VI. Comparison wrra other Aeeas. 



In its broad outlines the succession of beds at Conway agrees 

 well with that of other British areas. There are, however, two 

 outstanding difficulties : — 



(1) Where to draw the Hue between the Llancleilian and the Caradocian. 



(2) Where to draw the line between the Ashgillian and the Valentian. 



Difficulties such as these naturally arise when beds pass gradually 

 into each other, as they do in the Conway District ; but, in this 

 case, the pakeontological evidence seems to demand that the 

 junction between the Llandeilian and the Caradocian should be 



