PASSAGE FROM LOWER SILURIAN TO CAMBRIAN ROCKS. 



309 



parts of Caermarthenshire, the colours indicative of these formations are not very dis- 

 similar ; while in all the tracts where there is abrupt collocation of masses of different 

 age, the intermediate strata being omitted, the boundary is rigorously defined by strongly 

 contrasted colours. 



Dislocations along the Boundary between the Silurian and Cambrian Systems. 



I have said that a transition from the Silurian into the Cambrian System is rarely to 

 be detected. This is clearly proved by passing from the Berwyns northwards into Den- 

 bighshire, or southwards into Radnorshire, for we then see that all simplicity and 

 clearness of succession vanish. Powerful lines of disturbance mark the frontier ; on 

 one side of which are certain members of the Silurian System (seldom even the lower 

 group), on the other old slaty rocks of the Cambrian System. Examples of such rela- 

 tions may be observed along the flexuous demarcation extending from the Berwyns to 

 Moel-ben-tyrch. (see Map.) A traverse to this last-mentioned mountain from Llanfair, 

 five miles east of it, exhibits a space watered by the river Einion and occupied by grey, 

 graptolite schist, in vertical and undulating beds, with a strike varying from north-east 

 and south-west to N.N.E. and S.S.W. Large portions of this tract are denuded and 

 covered with coarse detritus derived from the mountains on the west. I consider those 

 schists to belong to the lowest members of the Upper Silurian rocks which have been 

 mentioned as undulating throughout a wide area between Welch Pool and Newtown. 

 This region of mudstone is bounded on the west by a chain of rocks of very different 

 composition, which ranges from Moel-ben-tyrch to near Llanwnnog, six miles west of 

 Newtown, and is thence prolonged into Radnorshire in the mountains south of Pen- 

 strowed. The prevailing rocks in these wild tracts is a hard, quartzose, slaty sandstone, 

 passing into a coarse grit (the greywacke of foreign geologists) usually of grey, but 

 sometimes of purplish, brown, and ferruginous colours. In Moel-ben-tyrch I detected 

 casts of encrinites and corals in this rock, but in general it is void of fossils, and litho- 

 logically unlike all the strata described, except certain grits and sandstones of the 

 Longmynd in Shropshire. There is, therefore, no doubt that it belongs to the Cambrian 

 System, and that it is of higher antiquity than the fossiliferous strata on the eastern flank 

 of the Berwyns. Along this line of separation the absolute contact of the Silurian and 

 Cambrian rocks is rarely discernible, owing to the vast accumulations of detritus above 

 alluded to, but on ascending the older chain a startling phenomenon presents itself. 

 Instead of rising to the north-west in their regular order, these older rocks usually dip 



2 Q 



