THE CAMBRIAN SERIES IN N.W. CAERNAUVONSHIKK. 



J535 



Not only do we fail to find any such evidence, but we are, more- 

 over, struck by the almost identical character of the two masses of 

 felsite. Mr. Blake does not even suggest the existence of any 

 difference between them ; yet these two rhyolitic masses, which are 

 practically indistinguishable from each other, are considered by him, 

 judging by his map, to be of entirely different periods. 



IV. The Bryn-Efail Sections. 



But the most important argument brought forward by Mr. Blake 

 rests upon his interpretation of the sections at Bryn Efail (a spot 

 just north of Llyn Padarn). They prove, in his opinion, that the 

 fault along the northern boundary of the felsite has no existence 

 here, and t!iat the Llyn-Padarn lava flowed over the sedimentary 

 rocks to the north. In short, to quote the words used, this spot 

 " provides a crucial test " *. I examined it accordingly with the 

 greatest possible care. 



(a) Strata north of the Valley. 



On the north-west side of the felsite, Mr. Blake describes a 

 low alluvial valley, and states that the rock on the opposite side 

 " is not a conglomerate," but slates with vertical cleavage, " dipping 

 towards the felsite at an angle of about :20°." Although no con- 

 glomerate occurs exactly in this position, we have not to go far 

 before we reach in the fields near Bryn Madog outcrops of it 

 associated with grits. It is thoroughly typical, with large well- 

 rounded pebbles of felstone and of quartzite. In one field a mass 

 of quartz-felsite intervenes, which might possibly be part of the old 

 floor of igneous rock with its overlying conglomerate rising up 

 again or brought up by a fault. Moreover, the argillites " quite 

 close to the valley " cannot be said to have a very persistent dip. 

 The general inclination for a short distance seems to be towards a 

 point to the north of east, not therefore directly " towards the 

 felsite '' : but the beds show much small faulting and some contor- 

 tions, and the dip varies in neighbouring outcrops, evidence which 

 on the whole is rather in favour of the faulted junction marked by 

 the Survey. Any inference drawn from m somewhat variable dip 



Fig. 1. — Section along Raihuay-cattinrj N. of Bryn Efail. 



'^<^'^'i^(^.l^'^'j^;0^^^^ 







>y^/: 



(This section shows the argillites nearest to the valley. The dip of the strata 

 at the southern end is somewliat variable, and is partly masked by joint- 

 planes. Length of section about 30 yards.) 



occurring along about ten yards, at the south end of the section shown 



i!i fig. 1, would not seem to have much force. But, further, it is 



doubtful whether the argillite exposed at Bryn Efail really represents 



* Quart. Journ. Geol. Soc. vol. xliv. (1888) p. 284. 



