766 G. MAW ON AN UNCONFORMABLE BREAK OP' 



Professor Hughes, in his recent paper, pointed out the uncon- 

 formity of the Cambrian series with the great porphyry-like mass 

 north-west of Moel Tryfane, though he disputed the existence of a 

 break in that part of the series included in the Llanberris railway- 

 catting. 



Now I believe that both may pertain to the same break in the 

 sequence ; that the beds included in the synclinal at the north- 

 west end of the cutting are the unaltered portion of the great 

 metamorphic porphyry-like mass at the lower end of the lake into 

 which they graduate ; and that towards Moel Tryfane these are 

 overlapped by the Cambrians, which rest unaltered against the 

 metamorphic mass. The synclinal of what I believe to be rocks 

 older than the Cambrians is repeated on the north-east side of the 

 lower end of Llyn Padarn, graduating, as on the south-west side, 

 into the metamorphic grey rock. 



The continuity of sequence between the stratified synclinal and 

 porphyry-like rock is evident from the occurrence here and there 

 in the latter of thin beds of slaty rock following the dip of the beds 

 at the north-west end of my section, though stratification has been 

 obliterated in the intercalated porphyritic mass. 



Knowing that it is often difficult in the older cleaved rocks to 

 distinguish cleavage from stratification, I have, within the last few 

 days, revisited Llanberris to test the observations I made in 1867, 

 and I can only confirm the conclusions I submitted to the British 

 Association at the Dundee meeting ; and I cannot agree with Pro- 

 fessor Hughes that the appearance of unconformity in the cutting 

 can be explained by any peculiarity of rock-structure. The singular 

 line of separation clearly defines the underlying slates dipping in 

 one direction and the unconformable overlying grits in the other. 

 The relative direction of the stratification of the two series is per- 

 fectly clear and unmistakable, and cannot be confounded with 

 cleavage, which also occurs, but in a different direction to the stra- 

 tification. 



If, as I believe, the conclusions I have arrived at will be verified 

 by other observers, I may claim for the Llanberris railway-section 

 some importance in the history of the classification of the older 

 rocks of Wales, as, ten years ago, no unconformable break was 

 known to occur at the base of the Cambrians in North Wales, 

 though the previous observations of Dr. Hicks in the St.-David's 

 district, and those of Professor Hughes subsequently made in the 

 near neighbourhood of the Llanberris Eailway-section, have esta- 

 blished the fact of the existence of an unconformable series of older 

 stratified rocks. 



Discussion. 



Dr. Hicks said that Mr. Salter and himself had published 

 their conclusions as to the existence of pre-Cambrian rocks at 

 St. David's as early as 1864. He added that, in company with 

 Prof. Hughes, he had visited the North- Wales district, and although 



