278 



PEOr. J. F. BLAKE OX THE CAMBEIAIT 



drawn on the Survey Map running up from Pen-y-swintau to 

 Friddod (see Map, fig. 1), as its distribution would suggest intrusion. 

 This tongue is not correctly laid down in the map either of Prof. 

 Bonney or of Prof. Hughes, but is drawn on the Survey Map with 

 considerable accuracy. 



I think, however, the probability is that it is not intrusive. In 

 the first place the structure of the rock, as obtained at Pen-y- 

 swintan or at the extreme end at Priddod, is not that of a dyke, 

 which such a tongue would practically be, but the matrix shows 

 flow-structure, and it is crowded with crj'stalline fragments, the 

 rock itself not being much fractured, thus suggesting a faulted 

 original flow. Again, as will be seen by the stratigraphy of the 

 higher beds, the country here must be much broken up by faults, 

 one of which, on its north-western side, might easily bring it to its 

 present position ; and lastly, it is followed in several places on the 

 south-east side by a small conglomerate, some fragments in which, 

 in one specimen at least, are similar in structure to the porphyry. 



The beds, however, which lie between the porphyry and the Arenig 

 grit are perhaps the most important, at all events in their relation 

 to the rocks of Anglesey. They are divided into two areas by the 

 fault which runs along the Bangor valley ; and if we can be certain 

 of them in one area, the stratigraphy of the other becomes more of a 

 geological puzzle than a matter of general interest. 



Pig. 5. — Plan north of Gorsbach. 



^^^orsiMmiiim^^^^^^^^^ 



1. Felsite. 



2. Quartz-felspar grit. 



3. Grit. 



4. Pelsite conglomerate. 7. Breccia. 



5. 8. Green Bangor Beds. 9. Arenig grit. 



6. Halleflinta. 



Taking the eastern area, we find the first incoming of these beds 

 just north of Gorsbach, near Llandeiniolen, where the geology is 

 rather complicated. The ground-plan is given in fig. 5*. On the 

 western side of the Arenig grit, which can be traced all along here, 

 we find, next to the porphyry, a coarse quartz-felspar grit, which 

 forms a knob at Fachell, and is scarcely to be distinguished from 

 the porphyry, except under the microscope; next comes a band of 

 grit, and then the dark muddy conglomerate, full of large pebbles of 

 red felsite. Higher still is a thick mass of compact green-banded 

 rock, such as occurs so abundantly at Bangor, and then finer material 

 * The Map, fig. 1, is not quite accurate here. 



