1845.] 



DAVIS ON THE GEOLOGY OF TREMADOC. 



73 



tion from the usual north-east dip of the slates, produced by the in- 

 trusion of the clink-stone. A folding over of a few feet of the slates, 

 the result perhaps of lateral pressure, exhibited by the cuttings of a 

 mining level at the spot last mentioned, was the effect, and the only 

 effect of this nature which came under my observation. These 

 irruptions of clink-stone are associated with only one and the same 

 sedimentary rock, consisting of imperfect shivery slates, totally devoid 

 of organic remains. 



It has already been mentioned that the lower lands of the district 

 present the appearance of having been covered by the sea at no 

 very remote period ; and repeated observations, and the evidence of 

 the inhabitants, all tend to the inference that the land has gradually 

 emerged from the sea, and that a movement of this kind is still in 

 operation. The evidence also, that the higher ridges derive their 

 present configuration from the action of water, is very striking. 

 The curved line between one ridge and another (as seen in the 



Section in the neighbourhood of Tremadoc. 



8. Flagstone. 

 7. Sandstone. 

 6. Flags. 



5. Slates, 

 4. Flagstones with 

 Lingula, &c. 



section) is evidently owing to the denudation of the stratified, and 

 therefore softer rocks, which have escaped denudation only in those 

 parts where they have been protected by the superincumbent por- 

 phyry (I ), or where they have been hardened by immediate contact 

 with it, and have thus resisted the action of the waves. 



Along the sides of the porphyritic ridges, a vast talus has accu- 

 mulated, the effect of the long-continued disintegrating action of 

 rain and frost. A striking example of this talus occurs on the left 

 of the road leading from Tremadoc to Beddgelert, columns of the 

 porphyry of immense size being there piled one above another from 

 a great depth, reaching two-thirds of the height of the perpendicular 

 cliff. Large masses are falling every year, and will inevitably con- 

 tinue to do so until the process of destruction is put an end to by 

 the talus attaining the level of the cliff. This is already the case in 

 the ridges furthest removed from the present sea-level. 



In witnessing the slow but certain destruction of the porphyritic 

 ridges by atmospheric action, it is impossible not to be struck with 

 the change which has taken place in this respect between the por- 

 phyry and the accompanying slates. When both were exposed to 

 the action of the superincumbent waters, the denudation of the slates 

 and sandstones proceeded rapidly, and was only impeded by the 



