H. 8. Jevons — The Breidden and Berwyn Rocks. 15 



to be seen on the cliff above Llyn Llync-caws, were found to have 

 the composition of a keratophyre ;' that is to say, they are essentially 

 composed of alhite and diopside. The proportion of diopside to 

 albite was found to vary somewhat, and to be generally greater 

 than in the case of the keratophyre of the Breiddens. Other 

 minerals rarely make up as much as 10 per cent, of the rock. The 

 texture is never porphyritic, and is generally that characteristic of 

 the dolerites (diabases), from which these keratophyres are only to 

 be distinguished by the refractive index or extinction angles of their 

 felspars. As I believe this to be the first description of keratophyre 

 as an intrusive rock, I may state that in this case there can be no 

 question as to its intrusive character. The slates are distinctly 

 metamorphosed above and below each sheet, the spotted slate so 

 well exposed above having been mistaken for tuffs in 1850 by 

 officers of the Geological Survey. The crag on Carnedd-y-Ci shows 

 the hanging wall rent and penetrated b}"^ minute tongues of the 

 igneous rock, and fragments of the sedimentary rock have been 

 floated off into the magma. 



No dolerites of the keratophyre facies (i.e. containing diopside) 

 occur in the Berwyn district so far as I have been able to discover. 

 The only basic rock in the neighbourhood is an olivine-dolerite with 

 titaniferous augite forming a dyke trending north-west and south- 

 east in Nant Llwyn Grwern, near Craig Wen. This is erroneously 

 mapped as a triangular patch on the Survey Map, but is simply 

 a coarse-grained dyke, probably to be connected with the Post- 

 Carboniferous dykes of Anglesey and Carnarvonshire. 



General. 



The superficial resemblance of the intrusive keratophyres here 

 -described to the dolerites (diabases) so common in Carnarvonshire 

 may be regarded as a significant fact. The felspars of the latter 

 rocks have in a few instances been determined,- and were found 

 to belong generally to the andesine-labradorite series. Albite has 

 not been recorded. I would suggest that an interesting field of 

 research lies open to some one more favourably situated than myself 

 in determining the felspars of a large number of the Welsh pre- 

 Carboniferous dolerite intrusions. Should albite be proved present 

 in Carnarvonshire, we should have the association of keratophyres 

 and dolerites confirmed, and it would be interesting to discover 

 whether there was a passage between the two rocks, and, if so, 

 whether horizontally or vertically. On the other hand, should 

 albite be absent in Carnarvonshire, the existence of a series of 

 rocks all of the same facies, basic in the north-west but acid in the 

 south-east, would have to be explained. 



A complete petrographical description of the rocks mentioned in 

 this note is in hand, and will be published in another place as soon 



1 Thiu sections of some 60 specimens taken from various parts of these masses 

 have heen examined. 



2 Harker: Q.J.G.S., vol. xliv (1888), p. 449; and "Bala Volcanic Series of 

 CarnarTonshire," p. 81, Cambridge, 1889. 



