216 Rev. J. F. Blake — The Llanberis Unconformity. 



Next, they cannot believe that the conglomerate passes over to the 

 other side of the purple slate, because the rock there found seems to 

 them to be less squeezed, more purple, with a few additional varieties 

 of pebbles, aud thinner. These differences (excluding the first, 

 which one cannot deal with seriously) are slight at the best, am] 

 just what we should expect in a shore deposit, the change in colour 

 being related to its position over purple slate rather than over felsite. 



Next, in relation to the synclinal in the railway cutting and my 

 belief in its unconformity, Miss Eaisin says that the purple slate 

 " turns up again " on the south-east side of it, and thus behaves as 

 the overlying strata do. But the phrase quoted is ambiguous ; it may 

 be intended as a colloquial expression for " occurs," or it may mean 

 that the dip is changed. It is quite to be expected that slate would 

 occur where she has marked it, and, indeed, I have it so in my note- 

 book section, here copied (Fig. 2), though it is omitted in my general 

 section of this cutting, but I could find no proof of dip in it, while 

 in the nearest visible purple slate I noted a high dip to the E.S.E. 

 as on the other side of the synclinal. But an unconformity or its 



W.S-W 



E,S.E. 



Fig. 2. — South-east end of the synclinal in the Llanberis railway cutting. (1) Purple 

 slate, (2) green (St. Ann's ?) grit, (3) greenstone, (4) conglomerate, (5) grit. 



absence cannot be proved in this sort of section ; it is too obscure. 

 Its only use is to show how far an unconformity, otherwise proved, 

 extends over the underlying rocks. Still, what we see here is more 

 favourable to an unconformity than the reverse. I pointed out that 

 the conglomerate " mounts up over tbe back of the greenstone boss " 

 and passes to the other side, as now shown in Fig. 2. This looks 

 like a transgression over the outcrop of slate.^ 



I also described the conglomerate as leaving the felsite along 

 the north-west boundary and disclosing between them a difierent 

 succession. I briefly described the immediate successor of the 

 felsite here as a hard purple slate, but Miss Kaisin goes into details 

 and records between the felsite and conglomerate a breccia, a pebbly 

 grit, a banded grit and argillite (a greenstone), and a purple grit, — 

 just what we might expect to follow the felsite in spots now further 

 west, representing hollows on its surface at the time of their deposit. 



^ The following quotation from Professor Bonney's paper (C) will show that he 

 at that time gave quite a different account of this section from mine and Miss 

 Raisin's, and considered the conglomerates now distinguished as inseparable except 

 as Yarieties. " The cutting .... passes into fine green grits or 'bastard 

 slate,' beyond which we find a thick mass of iuterbedded conglomerate and similar 

 grit, then another band of grit, followed by a band of small rolled fragments of 

 felsite about as large as hemp-seed." It is plain that either what he here calls fine 

 green grit or bastard slate is the same as that now called purple argillite, or he has 

 missed the first conglomerate in the cutting, and takes the next band to be the 

 *' Cambrian" conglomerate, calling it ''the finer variety,'' and saying that " with 

 considerable variety of detail the general character of these is similar." 



