any Evidences of former Glaciers in North Wales ? 477 



A slight notice of a different kind of striae on the Welsh 

 rocks, arising from another cause and capable of easy expla- 

 nation, will be sufficient. At the north end of Trefriw, in the 

 vale of Llanrwst, by the side of the road to Conway, large sur- 

 faces of the true beds of Lower Silurian rocks are laid bare in 

 several different places; they have an average dip of about 

 18° to the E.S.E., and form smooth and regular inclined 

 planes. This arises from wayboards of softer shale, or from 

 slight adhesion of the beds. On one of these inclined planes 

 to the north of the little quay, are many nearly parallel lines 

 or striae, running in the direction of the valley, which are very 

 liable to be mistaken for those produced by glaciers grating 

 upon their bed. But on close examination I found them to 

 be only the ends or sections of one of the series of joints which 

 pervaded the bed and divided it nearly at right angles to its 

 surface. The very general occurrence of joints symmetrically 

 arranged in the Silurian rocks, makes it necessary to guard 

 against deception arising from this cause. I will only add, 

 under this head, a short notice of one or two patches of rocks 

 by the side of the Holyhead road, where it overlooks the great 

 chasm opening into Glyn Lledr, a mile east of the Waterloo 

 Bridge, which have the appearance of being somewhat striated 

 and polished. But the surfaces are also irregularly undulated, 

 and the dip is to the south, or diagonal to the direction of the 

 trough of the Conway; and there is a very steep hill imme- 

 diately above them, down which, if these striae were produced 

 by glaciers, the icy mass must have descended, a supposition 

 which inspection shows to be impossible. Besides, the slopes 

 of the great opening from Glyn Lledr are very favourable for 

 the formation and preservation of lateral moraines, of which 

 I could observe no traces. And it may be remarked gene- 

 rally, that as both lateral and terminal moraines are more 

 frequently formed on the outskirts of glaciers than stria? on 

 the rocks beneath, and are from their situation and the nature 

 of the material, more likely to be preserved as monuments to 

 after ages, their occurrence in North Wales, either accom- 

 panied by striated rocks, or alone in other parts of the coun- 

 try, would have added to the probability of their common ori- 

 gin ; while their entire absence from localities so favourable 

 for their production, makes it necessary to exercise the greater 

 caution, in referring to glacial action the other phaenomena 

 described in this paper. 



I met with but one example of rock with a rounded or 

 domed surface, to which Saussure has given the name of" roches 

 moutonnees" and this was in a locality where it seemed so 

 much more natural to refer it to a cause still in operation, 



