270 J. R. Bahjns — Geology of Snowdon. 



the thrust plane, but it cannot coincide throughout with the junction 

 of A and B, because, as stated above, that junction is at Clogwyn 

 Person a natural one. 



In the figure, which is merely a diagram, the curved lines in A 

 represent calcareous seams in the felstone ; and in B the gently 

 sloping lines represent bedding, and the vertical lines cleavage. 



9. I had long years ago noticed some peculiar vesicular rocks in 

 the calcareo-ashy series, which puzzled me. It was Sir Archibald 

 Geikie who first pointed out that there were andesites on Snowdon. 

 After reading his account of these, I examined the rocks in the 

 places on Snowdon mentioned by him, and then found that these 

 andesites were one and the same with my peculiar vesicular rocks, 

 so I set about mapping them, and very useful I have found them 

 in showing where faults do and do not occur. In doing this work 

 I have found and partly mapped several rocks that bear a family 

 likeness to one another; some of them are not vesicular, and 

 I fancy may be fine-grained andesitic tuffs ; but whether tuffs or 

 lavas, they form a group of similar rocks distinct from the rocks amid 

 which they occur. They are characterized by a splintery fracture, 

 smooth texture, dull grey colour, rusty weathering, and are often 

 highly cleaved. Most of the specimens examined show under the 

 hand lens the characteristic " trachytic mesh work of minute felspar 

 laths." At present I look upon them as contemporaneous inter- 

 bedded rocks : there are, however, places where they might be 

 supposed to be intrusive, but perhaps this is a deceptive appearance 

 due to faulting. I do not know yet about this. 



The regions where I have mapped, or commenced mapping, them 

 are on the flanks of Crib-y-ddysgl and beneath the peak of Snowdon, 

 both above Glaslyn on one side and at the head of Cwra Clogwyn 

 on the other, and on the ridge leading from the summit of the 

 mountain to Bwlch-y-maen. I suspect from what I have lately 

 seen (in unfavourable weather) that similar rocks also occur, as is 

 but natural they should, in the calcareous series south-east of 

 Lliwedd, and I know that a rock of this description forms Clogwyn- 

 y-gysgfa on the east side of the road from Pen-y-gwryd to 

 Beddgelert. 



The andesitic rocks lie chiefly in the calcareous series (B), but 

 one, a well-marked lava, which I had mapped as such before I knew 

 anything about andesites, occurs amid the lower felsitic rocks (C) 

 near the head of the rocky spur called the Gribbin or Criman, 

 which runs down to the foot of Glaslyn from the south. I see that 

 the tracing of these beds will necessitate a modification of some of 

 the lines on the Survey map. 



10. As far as I know, all the intrusive rocks coloured as Green- 

 stones on the Geological Survey map are Diabases; but there are 

 others of a different character, which are not shown on the Survey 

 map, to wit, a boss of igneous rock of an apparently intermediate 

 but variable character (as I am informed by Mr. E. Greenly) on the 

 south side of Cwm Dyli, a mass of Diorite below Caer-gors (rather 

 more than a mile from Rhyd-dhu station), and several sills and 



