288 PEOF. J. E. BLAKE ON THE CAMBRIAiq- 



felsite. There are three varieties, close together and of small size, 

 probably insignificant intrusions. Then the mapping round Moel 

 Tryfaen is shown by the adit and cutting on that hill to be wrong. 

 There can be no fault between the conglomerate and the slate. 

 The great mass of felsite represented as occurring to the west is all 

 under drift-covered country, while the conglomerate wraps round 

 the felsite between Moel Tryfaen and Pen-y-groes. These correc- 

 tions would curtail the importance of the porphyry and make a great 

 fau]t unnecessary. 



Thus the Cambrian age of this mass of porphyry, as determined by 

 the nature of its under and its upper surface, is consonant with, and 

 confirmed by, the other tests to which the conclusion can be 

 submitted. 



We must now apply this determination to the question of the 

 age of the rocks near Bangor. Since the conglomerates of Llyn 

 Padarn and Moel Tryfaen are not the base of the Cambrian, there is 

 no reason why they should correspond with any particular band in 

 the series at Bangor, either towards the top or at the bottom. But 

 if these Padarn conglomerates are not the base, where is it? 

 Noiuliere in this particular area. We go downwards as we go 

 westwards till interrupted by a pair of trough faults, letting down 

 the Ordovician. When we get over this and come upon the older 

 beds again, what should they be but the continuations westward of 

 the Cambrian series ? There is scarcely a bed amongst them that we 

 cannot match somewhere or other in the higher parts of the series, 

 though on a smaller scale. Considering the lateral changes that 

 may take place in rocks, and the frequent dying-out of conglo- 

 merates, we cannot safelj^ say that all the Bangor beds underlie all 

 those from Dinas Dinorwic east : the correlation of bed with bed, or 

 the proof of their independence, if it were thought of importance, 

 would require a further long series of stratigraphical researches. 

 All we can say is, that we see to the south of Bangor the base of the 

 Cambrian down to a great felsite lava-flow : but whether this, like 

 that at Llyn Padarn, was contemporaneous and followed below by 

 earlier deposits still, or was erupted at a date that may be fairly 

 separated from the Cambrian period, there is not enough evidence 

 in this district to show. 



Conclusions. 



The results of the examination in the areas between Bangor and 

 Caernarvon, and from Llyn Padarn to Moel Tryfaen, of the rocks 

 which are found within them, are not very con fir mator}' of the views 

 of recent observers, but lead us back, with certain important modi- 

 fications, to the earlier teachings of the Survey on the district. They 

 may be summarized as follows : — 



1. In the Bangor to Caernarvon area : three distinct conglomerates 

 have been confounded together. The only one which, by its uncon- 

 formable overlap, sets an upper limit to the age of the rocks below 



