582 ERAGMENXAL ROCKS IN N.W. CAERNARVONSHIRE. [Nov. 1894. 



subsequently very much compressed and wrinkled. 1 The latter fact, 

 which seems quite clear in the three slices examined, appears con- 

 trary to Mr. Blake's view of the rocks. 



The ' Purple Slate ' is said by Mr. Blake to occur in mass along 

 the lower levels only (ibid. p. 450). 2 The rock is partly purple and 

 is slaty, but we fail to see any evidence which would correlate it 

 with the purple slate of the quarries, as seems intended by Mr. 

 Blake in his use of the term. Lithologically even, one of the most 

 characteristic structures in the ' post-Llanberis ' strata — the inter- 

 banding of fine grit and purple slate — is certainly indicated in one 

 part of the mass. It is followed in the railway-cutting by the beds 

 of the synclinal, which are taken as b post-Llanberis' (see our fig. 1, 

 7 b, 7 c, 7 d, 7 e, and 6, 7, 6, in map, fig. 4, p. 594), and the dip of 

 the ' Purple Slate ' (fig. 1, 7 a) is similar in direction to that in the 

 northern arm of the synclinal (about 70° S.E. by E.). The southern 

 arm, however, gives still more distinct evidence. Mr. Blake describes 

 and draws the conglomerate, g, as mounting " over the back of a 

 dyke of greenstone " (op. cit. p. 450, fig. 6). We find below this 

 conglomerate a purple slaty rock or banded and cleaved argillite, 

 indistinguishable in hand-specimens from that claimed as 'Cambrian ' 

 on the north (see our fig. 1, 7 b, 7 a). Bands similar in character 

 also occur in the ' post-Llanberis ' synclinal. Thus the supposed 

 ' Cambrian ' beds dip (at the north) with the ' unconformable ' strata 

 above, can be matched with bands intercalated among them, and 

 appear to be reproduced to the south, turning up in that arm of the 

 synclinal, exactly as we should expect if there were no unconformity. 



The other argument used here is the distribution of the con- 

 glomerate. It is said to occur at the spots marked a, c,g, m (op. cit. 

 p. 451, fig. 6), extending over different beds of the earlier series. 

 The southern mass m cannot give much help. Mr. Blake, we suppose, 

 faults it down, — we think it more probable that the rock is faulted 

 up. The conglomerate a, however, is supposed to pass over to that 

 marked c in the synclinal (see 3 and 7 b in our fig. 1, and 3 and 6 in 

 map, p. 594). Of these Mr. Blake says " there can hardly be a doubt 

 .... that [they] are parts of the same mass " (loc. cit.). But litholo- 

 gically they appear somewhat distinct, both in hand-specimens and 

 on microscopic examination. The conglomerate c has a less squeezed 

 look than the conglomerate a, is more purple, and contains clearly 

 fragments of rocks of more basic or intermediate composition, which 

 seem practically absent from the latter. Further, the thickness of c 

 is probably less than 20 feet, while that of a must be much greater. 

 We believe that it is a distinct layer at a higher horizon, and it is 



1 Undoubtedly a mass of felsite occurs just beyond ; but, as the slate is con- 

 spicuously wrinkled, its dip can hardly be parallel to tbe surface of the igneous 

 rock. 



- Again on p. 455, it is stated that 'nowhere are the felsite and Purple 

 Slate, which are seen at the level of the railway (11, 12), to be found above.' 

 The ' Purple Slate ' is discussed in the next few sentences. But a felsite also 

 occurs up the hill S. W. of Tan-y-pant Cottages. It is difficult on any theory 

 to explain this outcrop, but it undoubtedly is shown in several well-marked 



