ROCKS IN N.W. CAERNARVONSHIRE. 289 



is of Arenig, i. e. of Ordovician age, and can only be called Cam- 

 brian by the misleading use of the term, which embraces all rocks 

 up to the Bala series. 



2. The rocks of the southern and central portion are essentially 

 of igneous origin. 



3. These may clearly be distinguished into two groups, of which 

 the southern is probably intrusive, and the northern is certainly 

 eruptive. 



4. There is an interval of time between their production ; but 

 how great an interval, there is no evidence to show. That the 

 southern mass is of later date than the Arenig, and intrusive into 

 it, is a tenable hypothesis ; but that it is of earlier date and over- 

 lain by the northern portion, is far more probable. 



5. The Bangor beds are derived from the denudation of the 

 volcanic series and of rocks which may have been associated 

 with it. 



6. These beds contain a series of conformable conglomerates, of 

 which the great conglomerates near Bangor are members. 



7. They are the continuation downwards of the Cambrian rocks 

 seen to the east, and have not undergone very serious alteration 

 compared with the latter. 



8. The porphyries of Llyn Padarn and Moel Tryfaen are contem- 

 poraneous lava-flows in the midst of the Cambrian series, the over- 

 lying conglomerates being derived from them and from the Cam- 

 brian sedimentary rocks to the west. 



And hence, finally, there is no certain proof of there being any 

 Pre-Cambrian rocks in the whole district, though there is great 

 probability that the rock near Caernarvon belongs to a distinct epoch 

 anterior to the Cambrian. 



I may add that nothing here observed invalidates former con- 

 clusions as to the essential independence of the group which lies 

 below the true basal Cambrian conglomerate in the St. David's dis- 

 trict. That group does not bear sufficient resemblance to the series 

 in north-west Caernarvonshire to justify their correlation, and the 

 unconformity there observed is of a totally different order, and com- 

 parable rather to the overlap of the Ordovician beds in this district. 



Discussion. 



Prof. T. M^'K. Hughes said the question was too large and in- 

 tricate to discuss thoroughly at so late an hour. His reading of 

 the stratigraphy of the Twt-Hill quarry and of the sections near 

 Llandeiniolen and Bangor differed altogether from that of Prof. 

 Blake, whose correlation of the various conglomerates appeared very 

 doubtful. 



Dr. Hicks felt the same difficulty. In his opinion Prof. Blake 

 had misunderstood the beds and had not sufficiently considered the 

 eff'ects produced by the faults. The pebbles of felsite in the Cambrian 

 conglomerates showed a cleavage produced before they were broken 



