588 PROF. T. G. BONNET AND MISS C. A. RAISIN ON [Nov. 1 894, 



made by gradually rising ground. But if the axis of this syncline 

 sloped to the south-west at an angle of about 12 , 1 the lower strata 

 of grit, etc., would spread over a broader tract. Is, however, the pale 

 slate completely absent from the summit of the hill ? It seems to 

 us that it does occur (probably over a limited area), and that 

 although the rocks show varietal differences, there is no strongly 

 marked lithological distinction. 2 



Mr. Blake's hypothesis moreover only seems to substitute another 

 and rather greater difficulty. If the Moel Goronwy felsite overlies 

 in succession the Banded Slates, what has become of the syncline 

 which admittedly has extended from the railway for nearly J mile 

 to Y Bigl, a ad then has died out in less than 300 yards ? Or, if the 

 syncline is continued, then the Moel Goronwy felsite must be 

 separated from the Banded Slates by a fault. Theu what has 

 become of this fault at the railway ? (Compare fig. 5, p. 597, postea.) 



The strata of T Bigl also are said to be horizontal, and this, if 

 true, would be strongly against the view that they belong to the 

 railway succession. 3 The same position is exhibited in Mr. Blake's 

 fig. 5 (op. cit. p. 449 ). 4 As no dips are given on the map (away from 

 the lake), we were unable to verify the particular observations on 

 which the statement was founded, but the amount of the dip was 

 measured at some twenty places 5 lying within Mr. Blake's ' post- 

 Llanberis ' area on Y Bigl. Of these observations, in one the 

 crumpled laminae maintain a roughly horizontal direction, 6 in all 

 others the dips were not less than 35°, and in more than half they 

 were as much as 50° or higher. 7 And these are the beds whose 

 horizontal position proves the unconformity ! 



V. East of Llyn Padarn : Alleged Unconformity on the 

 Slate Railway. 



In the previous instances the evidence is indirect, but at one 

 spot we are told that the hypothesis can be brought to a direct test. 



1 A greater slope than this is indicated at Yr Allt Wen in the Survey 

 Memoir, fig. 64, p. 181. 



2 Specimens have been examined under the microscope and they show a 

 certain amount of family likeness, particularly in the coarser bands. In each, 

 but especially in two specimens from the syncline of the railway, several rather 

 angular fragments of the dark rock (described infra, p. 598) occur, which 

 generally are abundant in Mr. Blake's ' post-Llanberis ' group. 



3 An argument in support of this view may be drawn from the strike which 

 can be traced in certaiu distinguishable layers as roughly to north-eastward ; 

 that is, agreeing in direction with the outcrops indicated in the railway section. 



4 This section is difficult to understand, unless we are to suppose that a fault, 

 throwing down the 'post-Llanberis' conglomerate at the N.W. end, ran oppor- 

 tunely along the nearly vertical junction of the felsite and the banded slates. 

 But even so, the latter must have rested on the former and have been afterwards 

 curiously twisted up just on the southern side of the fault. 



5 These were the clearest exposures. The dips were noticed at many other 

 outcrops. 



6 At a place which we believe may be on the line of the anticlinal from the 

 lake. 



7 Here also, as at Moel Tryfaen, these measurements are sometimes less than 

 the true dips. See p. 580. 



