Vol. 65.] GLACIAL EROSION IN NORTH WALES. 285 



such larger structural features as the great Clogwyn synclinal fold 

 (Ramsay, 1866, p. 121: 1881, p. 151) in felsites, ash-beds, and slates, 

 finely displayed in the precipitous walls of Llanberis ' pass ' (valley) 

 about half way between the col and the first lake ; the fold was 

 first seen from the valley-bottom, and afterwards to better advantage 

 from the mountain-railway, where the general structure of the 

 huge series of rocks involved was well exposed in the over-steepened 

 cliff that rises a full thousand feet on the north-east side of the 

 valley to the gentler highland slopes of Girder Fawr. We 

 occasionally made out such smaller features as the double belt of 

 Li i uj ula-grits streaked with quartz-veins (Ramsay, 1866, p. 135; 

 1881, p. 167), which so curiously follows the bottom of Pantile 

 Valley for a time near its head, and then obliquely ascends its 

 southern slope. 



V. Indifference of Form to Stkucture. 



Distinct as the chief structural features are in the side-cliffs of 

 the valleys and in the head-walls and front steps of the cwms, 

 a great part of the slopes is so well graded and grass-covered as 

 to conceal structural details. Wp were repeatedly impressed, not 

 only by the invisibility of the rocks in the well-rounded slopes of 

 the bare dome-like hills and mountains, but also by the relative 

 indifference of form to structure, as exhibited, for instance, in the 

 highland north of Llanberis valley, where the gently undulating 

 crest-line trends north-westwards between the summits of Glyder 

 Fawr and Elvdir Fawr, transverse to a series of steeply-dipping 

 formations which strike north-eastwards and include felsites, ash- 

 beds, Llandeilo Beds, greenstones, 'massive intrusives,'and Cambrian 

 beds, as recorded on the geological map. Many other examples 

 of the same nature that might be instanced are of importance, 

 inasmuch as they prove that over large areas no great difference of 

 resistance to subaerial erosion exists between the strongest and the 

 weakest formations ; that all the formations may be regarded as 

 resistant, although some are more resistant than others. The 

 finely-shaded sheets of the 1-inch Ordnance Survey map (third 

 edition, sheets 106, Bangor, and 119, Snowdon) tell the same story. 

 On the maps, and still more in the field, one must be impressed by 

 the arbitrary distribution of the mountain-summits, the extremely 

 irregular pattern of the spurs, and the apparently insequent 

 arrangement of the valleys and streams : — ' insequent ' meaning 

 that the streams seem to be of arbitrarily irregular courses, and 

 neither prevailingly ' consequent ' upon the initial slope of the 

 strata (for they pay little attention to the dip of the beds or to 

 the pitch of the synclinal troughs), nor ' subsequent ' in the sense 

 of having grown by headward erosion along belts of weak rocks ; 

 nor ' obsequent,' nor ' resequent ' ; but that whatever have been 

 the controls of stream-location, they are for the most part so 

 indefinite or so obscure as to be beyond recognition by the present 



