J. H. Daki/ns — Geology of Snowdon, 271 



dykes of an intermediate cliai'acter on Mynydd Drws-y-coed. I may 

 as well say at once that tlie mineralogical character of all the 

 above-mentioned rocks, as well as of others not mentioned, were 

 determined ^ for me by my friend Mr. E. Greenly, to whose great 

 kindness in naming my specimens my warmest thanks are due. 



There ai'e also some intrusive felstones, bosses, and dykes in the 

 Snowdon area. One of these I have already mentioned in a note to 

 the British Association at Bristol. It is an oblong mass of felstone, 

 showing lines of viscous flow parallel to its north-western face, and 

 rudely columnar perpendicular to that face, exactly like the felstone 

 of Crib Goch, and not like any part of the lower felsitic rocks of the 

 immediate neighbourhood. I take it to be a plug and root of the 

 Crib Gocli lava. It occurs between Glaslyn and Bwlch Goch. 



On the ridge called Llechog, that bounds Cwm Clogwyn on the 

 south, a band of compact uncleaved felstone. running N.W. and S.E., 

 stands up as a dyke amid the cleaved felsitic rocks (C). In the 

 Cwm also a felstone dyke running W.N.W. and E.S.E. is seen 

 rising up the cliff. This may possibly be the lower part of the 

 dyke mentioned above. I have not decided about this yet. 



I may remark that the existence of felstone dykes amid the 

 Snowdonian rocks is a discovery, for none such have been hitherto 

 recognized. 



11. The great masses of diabase that occur so plentifully in the 

 neighbourhood of Snowdon are generally massive rocks quite 

 uncleaved, but they are often cleaved or sheared along their 

 margins. Eamsay remarks on this feature, and says, on p. 157 of 

 "The Geology of North Wales" : "At its junction with the felspathic 

 porphyry the greenstone is often partially decomposed ; its crystal- 

 line character is gone (probably from rapid cooling at its sides), and 

 it possesses a flaky structure to so great an extent that it sometimes 

 looks like some of the cleaved dark-green calcareous ashes, or in 

 other cases assumes an appearance of a more slaty description." 

 This is an excellent description of the diabase along its margin, but 

 there is evidently some confusion between the phenomenon of 

 a chilled edge and that of marginal cleavage or schistosity, which 

 must (as it seems to me) be due to movement or pressure. The 

 same sort of thing occurs in the Scottish Highlands, where masses 

 of basic rock among the schists are sheared along their margins, 

 though in the Highlands the rock is generally reconstructed. 

 Besides the above-noted deformation along their margins, the 

 diabases are in some localities deformed and rendered schistose, or 

 something very like it, throughout nearly their whole extent. 

 I first noticed this in Cwm Llan, on the south side of Snowdon, 

 and took Mr. Greenly to see it, so different in appearance was 

 the rock from that of the ordinary run of diabases. A precisely 

 similar rock occurs on the west side of Snowdon near Llyn Nadroedd. 

 On the Geological Survey map a large mass of greenstone is 

 rejoresented there, but it really consists of two separate masses 



^ Only a few, however, liave yet been sliced. 



