■268 J. R, Dakyns — Geology of Snowclon. 



occasional volcanic bands, D ; and below D the great mass of 

 volcanic rocks (E) which form Glyder Fach and Y Tryfan. 



1. The lower Snowdonian felstones (C) are coloured on the map 

 as lavas, and are always spoken of as such in the memoir on North 

 Wales. In many places, however, they are not lavas, but frag- 

 mentary rocks. Their fragmentary character may be seen (to 

 mention a few localities) in Cvvin Llan, especially west of the copper 

 vein not far from the stone erected iu honour of Gladstone, and also 

 near the footpath leading from Rhyd-dhu to Snowdon some five 

 hundred yards or so beyond the half-way house. These rocks, C, in 

 fact, consist of three kinds : — First, massive uncleaved rocks showing 

 fluxion structure and in some places columnar; these are un- 

 doubtedly lavas : secondly, compact, often close-jointed, rocks of 

 a smooth fracture, somewhat like lava, but without fluxion structure; 

 these are probably tuffs or masses of consolidated felsitic dust : 

 thirdly, highly cleaved felsitic rocks. These last are in some places 

 obviously of a fragmentary character, but more often not so ; and 

 in such cases it is difficult to say whether they are so-called volcanic 

 ash or cleaved lavas. As the obviously fragmentary rocks are part 

 and parcel of them, my own opinion is that they consist mainly of 

 ash ; but to settle this question definitely microscopic examination is 

 needed. A slice cut from one of these fine and highly cleaved 

 felsitic rocks shows (as Mr. Greenly tells me) certain small aggre- 

 gates with igneous structure, which look much more like fragments 

 than surviving augen from such rocks as the Bala felstones. The 

 rock also contains a considerable quantity of calcite, much more than 

 <!ould be expected from the felsitic lavas in question, none of which 

 have yielded so much as -5 per cent, of lime (Harker, "Bala 

 Volcanic Series," p. 13). It is probable, therefore, that the rock is 

 a felsitic tuff or dust. 



2. Eamsay has pointed out on p. 150 of " The Geology of 

 North Wales " how difficult it is to fix on a boundary-line between 

 these lower felsites and the overlying series (B) on the ridge of 

 Llechog, west of the peak of Snowdon, because both are felspathic 

 and both cleaved. In my opinion the difficulty is due to the fact 

 that both are ashes ; but I have not yet had any specimen from 

 this locality sliced and examined. In many places, the lower rocks 

 look to me like the ' altered ashes ' of the Lake Country. 



3. Not only are the lower felsites often intensely cleaved, but 

 a very striking peculiarity about them in many places is their 

 sheared appearance ; they are often more than cleaved, they are 

 sheared (or even to a slight extent foliated *), so much so as in 

 some places to remind me of the quartz-schist of the Western 

 Highlands. This quasi-foliated appearance is (among other places) 

 remarkable alongside the road from Rhyd-ddu to Beddgelert. 



4. The great mass of these rocks (C) is quite devoid of bedding. 

 In fact, their unbedded character is the chief distinction between 

 them and the overlying series. At the same time, it is not always 

 easy to fix upon a good line of demarcation between the two, as 



' The divisional planes are coated with thin sheets of ' sericitic ' mica. 



