PRE-CAMBRIAN ROCKS OF BANGOR. 143 



I will now endeavour to reconcile this interpretation with the 

 observations of other authors on various similar districts, pointing 

 out some modifications of their views which the results of my work 

 lead me to suggest. 



In the Llyn-Padarn section we have a more intense cleavage than 

 in the Caernarvon and Bangor area, and a sharper folding of the 

 beds near the base of the Cambrian ; but as far as I could follow 

 the section, it seemed to me that, where the conglomerate did not 

 turn up again so as to pass over the porphyritic series, it always 

 ended off at a fault or a greenstone dyke, which so often in that 

 district runs along a fault. The conglomerate is too persistent in 

 its occurrence along the flanks of the porphyritic ridge not to make 

 the explanation that metamorphism happens to have stopped just 

 there rather improbable. The observed metamorphism affects the 

 rocks in vertical, not horizontal extension — i. e. in the now tilted 

 beds is seen ascending through the beds, not extending along the 

 same bed. 



I feel sure, therefore, that the Cambrian conglomerate is the 

 base of a distinctly overlying series, whatever we may be able to 

 say as to its conformity and further relations to the volcanic 

 series below. 



With regard to Mr. Maw's view, that there is a visible uncon- 

 formity in the greenish schists seen in the railway-cutting on the 

 S. side of Llyn Padarn, his most carefully drawn and accurate sec- 

 tion enabled me easily to find the exact spot referred to. I can- 

 not, however, agree with his interpretation of the phenomena. The 

 irregularity seemed to me to be due to lateral pressure acting on 

 beds of unequal texture and character. The coarser and more sandy 

 beds were crumpled and contorted, while the finer beds were com- 

 pressed and cleaved, and, being thrust into the folds of the harder 

 beds, gave the appearance of a denuded jagged edge of slaty rock, 

 on which sand had been deposited, filling the clefts and notches. It 

 would be very improbable that denudation, such as furnished the 

 material of the Lower Cambrian beds, could have left such a surface ; 

 and a close examination of the junction shows that it is a larger 

 case of what is common enough, on a smaller scale, in the very 

 same district, where small beds of different texture are crumpled up 

 together, one taking cleavage the other not. 



A somewhat similar phenomenon may be seen close to the Di- 

 norwic workshops at the S.E. corner of Llyn Padarn, where there is 

 a large surface exposed of what looks at first sight like ripple-marks, 

 but which seems to be really due to the edges of cleaved slate thrust 

 into a thin parting bed of yellowish pasty rock, not more than three 

 quarters of an inch in thickness. 



There seems to be very good reason, from similarity of lithological 

 character and correspondence in the sequence of beds, for correlating 

 respectively the Bangor and Caernarvon series with the Pebidian and 

 Dimetian of Mr. Hicks. But while the St.-David's section seems to 

 me to strengthen the probability of an unconformity between the 

 Cambrian conglomerates and the Bangor beds, as the distinction be- 



