J. R. Baliijm ^ E. Greenly — Felsitic Slates of Snowdon. 543 



it is a cleaved rook of a quite different character, and in connection 

 with this there is some bedded rock. I have not yet made out the 

 relation of these two kinds of felsite to one another. There are 

 probably some undetected faults, for, to mention only one place 

 where such may occur, at the north-east end of the outlier shown on 

 the map the underlying grits are dipping eastward so as to pass 

 over the felstone or abut against it. The junction of the felstone 

 here with the sedimentary rocks is more like the junction of an 

 intrusive rock ; but possibly this appearance may be due to faulting. 

 There are other difficulties about this small outlier and also with 

 the base of the felstone near Hafodydd-brithion, which I pass over, as 

 they have nothing to do with the cleaved felsites which are the 

 subject of Mr. Greenly's paper. 



Part II : By Edwakd Greenly.^ 

 Introduction. 



Every geologist who has visited Snowdon must have observed 

 that very large masses of highly cleaved, yet felsitic, rocks enter 

 into the structure of the mountain, in addition to the unmistakable 

 felsitic lavas and bedded calcareous ashes. 



These rocks, which for conciseness may be called the ' Felsitic 

 Slates,' are conspicuous along most of the ordinary approaches, 

 except from Llanberis. The path from Pen-y-gwryd passes across 

 them, and so does that from Beddgelert : they form the sharp ridge 

 of Llechog, south of Cwm Ologwyn, and the great dark precipices 

 of Lliwedd overlooking Llyn Llydaw are largely composed of them. 



They are dense, hard, splintery rocks ; intensely cleaved, and yet 

 with the light weathering of a felsite, and also on fresh fractures 

 their pale bluish-green matrix has, in spite of the strong cleavage, 

 a decidedly felsitic aspect, while their cleavage laminee are per- 

 ceptibly translucent at the edges. Small squarish crystals of felspar, 

 from 1-0 to 1-5 mm. long, are generally present and often abundant. 



Most writers on Snowdon have, discreetly perhaps, said very 

 little about them, for they are very unpromising-looking rocks. 

 Sir A. C. Eamsay and Mr. Selwyn, however, having to survey the 

 country, were obliged to deal with them in some way, and they 

 have included them on the maps with the lower or principal felsitic 

 lavas, with which they are coloured and lettered ' F.' Moreover, 

 in the memoir (Geol. North Wales, 2nd ed., pp. 150-1) the intense 

 cleavage is thus referred to : " On the ridge of Llechog, west of the 

 peak of Snowdon, the felspathic porphyry is curiously cleaved, or at 

 least very closely jointed ; and so marked is this above Cwm-y- 

 Clogwyn that it is difficult at first to believe that it is part of the 

 very same slaggy-looking masses previously described. So com- 

 pletely is the appearance of the felstone of Llechog, and of the cliff 

 below the peak of Snowdon, modified by this structure, that there 



^ All the field-work upon wMch this paper is founded is by Mr. DakjTis. I have 

 only visited the ground a few times. — E. G. 



