374 3IR. J. V. ELSDEX OX THE AGE OF [A-llg. I904, 



examined by me, failed to exhibit this character in greater or 

 less degree. It is, in fact, so distinctive, that specimens can often, 

 by the unaided eye, be at once assigned to one or the other of 

 these two regions. 



An interesting exposure exhibiting these conditions has quite 

 recently been opened up at Llanberis, where blasting has taken 

 place in connection with an alteration in the road, about a quarter 

 of a mile to the south-west of Plas Coch. This occurs at the top 

 of the hill a little beyond the smithy, where a small ' greenstone '- 

 intrusion, about 5 yards wide, is to be seen near the base of the 

 Lingula-Flags. (The same rock is visible in the bed of the Afon 

 Goch close at hand, but the course of the dyke is not visible 

 for any great distance.) Apparently the outcrop of this dyke runs 

 nearly parallel to the strike of the Lin gala-Flags, which here 

 dip almost vertically ; yet, whether the intrusion is a dyke or sill 

 is not quite certain, although the evidence seems to favour the 

 former interpretation. It is here manifest that the igneous rock 

 has been powerfully affected by the crush which folded the sedi- 

 mentary rocks. The southern side has been much broken and 

 faulted against the flags, while the northern contact is cleaner and 

 less crushed, a circumstance which might be expected when the 

 southerly direction of the thrust towards the north is borne in 

 mind. The whole mass of the igneous rock is greatly sheared, 

 becoming in places almost schistose, the fissures and shear-planes 

 thus produced being strongly marked by veins and coatings of silkv 

 asbestos, some of which are nearly 2 inches wide, the asbestos-fibres 

 being arranged transversely to the walls of the fissures. The rock 

 itself is of a light greenish-grey colour, spotted with dark patches 

 of a chlorite-mineral. There is also much secondary calcite, with 

 fan-shaped bundles of epidote in the more weathered portions. 

 A quartz- epidote vein about 18 inches wide traverses the rock in 

 its lower portion near the road -level on the northern side. The 

 rock contains a good deal of pyrites, and the flags at the junction 

 are filled with cubes of this mineral, many of which have been 

 weathered out, or replaced by chloritic pseudomorphs. The petro- 

 graphical features of this rock will be referred to later. 



The exposure in the Afon Goch is exactly similar to the foregoing, 

 and need not now be enlarged upon. There can be no sort of doubt 

 with regard to the age of this intrusion, which must have preceded 

 some part of the earth-movements connected with the post-Bala 

 folding. Previous observers have already called attention to the 

 effects of intense pressure upon the rocks on the southern margin 

 of the quartz- porphyry ridge. Sir Archibald Geikie describes basic 

 dykes near Llyn Padarn which have been converted into a slatv 

 rock by pressure. 1 Similar sheared diabases have been noticed by 

 the Rev. J. F. Blake ~ ; consequently, there appears to be cumulative 

 evidence that these 'greenstones," if not actually intruded before the 

 period at which the curvature and compression of the region took 



1 ' Ancient Volcanoes of Great Britain ' vol. i (1897) p. 162. 



2 ' On the Felsites & Conglomerates between Bethesda & Llanllyfni ' Quart. 

 Journ. Geol. Soc. vol. xlix (1893) p. 441. 



