Vol. 50.] ANNIVERSARY ADDRESS OF THE PRESIDENT. 99 



Whilst apparently regarding the great igneous mass between Caer- 

 narvon and Bangor as pre-Cambrian, he held that the quartz-felsites 

 of Llyn Padarn and Moel Tryfaen are contemporaneous lava-flows 

 in the midst of the Cambrian series, the overlying conglomerates 

 being derived from them and from the sedimentary Cambrian rocks 

 to the westward. These views were supported by detailed sections. 

 It is worth while noticing that Prof. Blake still had faith in the 

 ' true basal Cambrian conglomerate ' in the St. David's district, since 

 the underlying beds in that area do not bear sufficient resemblance 

 to the series in North-west Caernarvonshire to admit of their 

 correlation. 



This paper was presently followed by one from Miss Raisin on the 

 lower limit of the Cambrian series in North-west Caernarvonshire. 

 After discussing the position of the Bangor Beds, she allows that the 

 age of the northern beds must depend to a great extent upon the 

 classification adopted for the Llyn Padarn rocks, remarking that it 

 has recently been proposed to regard the quartz-felsite of this dis- 

 trict as a lava-flow of Mid-Cambrian age, and the beds to the north 

 as lower strata included in the same great series. It was very 

 clearly shown on this occasion that a part, at least, of the evidence 

 for the former view was based on a misapprehension, and the Author 

 pointed out other difficulties in accepting Prof. Blake's interpreta- 

 tion, viz. the enormous thickness of beds which must be cut out by 

 the supposed Arenig unconformity at Caernarvon ; the difficulty of 

 assigning two felsite masses lithologically similar to two distinct 

 periods, and the occurrence of conglomerates similar to those which 

 are elsewhere admitted to be basal Cambrian. 



Quite lately Prof. Blake has returned to the charge, having 

 favoured the Society with two papers containing an immense 

 amount of detailed stratigraphy in support of his general conten- 

 tion as to the absence of pre-Cambrian rocks in North-west Caer- 

 narvonshire. He now ventures upon a definite succession in his 

 Cambrians, commencing with the Pale or Green Slates, which have 

 attracted attention as lying at the top of the series in the great 

 quarries of Penrhyn and Llanberis, and terminating with the 

 Brithdir quartz-felsite grit, which may or may not be a true base in 

 accordance with the age assigned to the Dinorwic felsite. A triple 

 subdivision is assigned to the entire series, the highest including the 

 Purple Slates with some associated grits, the middle subdivision 

 consisting of Pale-banded Slates and Halleflintas, whilst the lowest 

 subdivision is made up of Laminated Grits and Conglomerates. 



