Maw — Cambrian Rocks of Llanberis. 121 



III. — On a New Section of the Cambrian Rocks in a cutting 

 OP the Llanberis and Carnarvon Railway, and the Banded 

 Slates of Llanberis. 



By George Maw, F.G.S., etc. 

 (PLATES VI. AND VII.) 



THE importance of the question of conformity in the classification 

 of the earliest stratified rocks induces me to give a short 

 description of a new section of a part of the Cambrian rocks of 

 Llanberis, exhibiting an apparent case of unconformity towards their 

 base. 



The section has only been exposed within the last few months, 

 and at the time the district was mapped by Professor Ramsay, this 

 complicated part of the series was hidden under a roughly weathered 

 and glaciated surface, from which few of the details of structure 

 could be determined. The railway in course of formation, along 

 the southern side of Llyn Padam, has exposed for nearly a quarter 

 of a mile the lowest and most intricate part of the Cambrian rocks, 

 a representation of which is given in Plate VI. A new tunnel in the 

 neighbouring Glyn quarries has also opened up another section of 

 the beds given in the engraving (see Woodcut, Fig. 1). 



The upper part of the Cambrian series, not included in these 

 sections, consists of three or four alternations of blue and purple 

 slates interstratified with conglomerates and beds of a greenish rock. 

 From the Lingula flags downwards they appear to be perfectly 

 conformable ; but the conformity of the lowest workable slates in 

 the Grlyn quarries, with the underlying beds, appears to me to be 

 less certain. 



These lower grits and conglomerates, which are visible on both 

 banks of Llyn Padarn, graduate into the great mass of porphyry 

 crossing its western end. The gradation and metamorphism 

 of the beds is well seen beyond the north-western end of the 

 cutting; but whilst the change of the conglomerates to crystalline 

 porphyry, from east to west, is evident, the overlying workable 

 slates of the Glyn quarries resting directly upon them are entirely 

 unaltered at the point of junction. In the south-east end of the 

 section (Plate VI.) two masses (a a) of the blue slates are faulted 

 down between the underlying altered grits and conglomerates h and c, 

 and the same thing is observable in the tunnel. Fig. 1. Irregular 

 bosses of these underlying rocks, termed "Hards" by the quarry- 

 men, rise up in several places among the purple slates, but whether 

 this was solely due to faulting, or to the deposition of the slate on a 

 pre-existing irregular surface of the metamorphic rock, is not easily 

 determinable : the contortions in the latter do not, however, appear 

 to be repeated in the overlying slate, and the irregular line of 

 junction as seen in parts of the tunnel cannot, I think, be altogether 

 accounted for by the faults that have thrown the porphyry and 

 slates together. 



Between 150 and 470 feet from the south-east end of the cutting, 



VOL. V. — NO. XLV. 9 



