276 PROF. J. r. BLAKE 02«- THE CAMBEIAlf 



So it is coloured by the Survey, and every consideration tends to 

 prove them right. It is at the base of the Arenig that palaeonto- 

 logists have suggested a break in the strata. It is the base of the 

 Arenig that contains the overlapping conglomerates in Anglesey 

 and Ireland*, and this is the lowest series that is most widely spread 

 in Europe t. Hence it is nothing extraordinary that Arenig rocks 

 should here have a conglomerate at their base, and should be asso- 

 ciated with the porphyry (all Cambrian rocks being absent), and 

 so it is that we find here the key to the interpretation of the 

 district. My explanation, therefore, of the rocks near Caernarvon 

 is as follows : — The granite is intrusive into older rocks than the 

 Arenig, now hidden from view, though perhaps their fragments may 

 be recognized ia the conglomerate. This conglomerate once rested 

 on the surface of these rocks, perhaps not far from the curved and 

 bay-like line which now forms the junction with the granite ; but 

 in the upheavals of the district the union has been broken, and 

 slippings have taken place, producing faults. 



Passing now to the north-east of Caernarvon, the undulations of 

 this base-line of the Arenig are found to be very considerable. They 

 are traced on the Map, fig. 1, p. 272. The conglomerate almost cuts 

 out the porphyry behind Ty-coch, where it actually dips towards it. 

 It then passes rapidly eastward, to be found near the river-side south 

 of Pengelli. It then apparenth^ curves back till it runs nearly north 

 to Careg Goch, and all the fields to the east are covered with the 

 Ordovician shale. Then we come to the great grit-beds south-south- 

 east of Tan-y-maes, and the conglomerate, which must be almost 

 welded to the porphyrj^" at the cottages, and certainly overlies it. 

 East of this we find it thrown out to the farm at Wern, where the 

 junction with the shale is seen. It must then be thrown east again, 

 to curve round the farm at Cefn and undulate along the road to 

 Gorsbach. Here we come upon a different set of underlying beds, 

 and the conglomerate gets finer and graduaU)^ degenerates into a 

 grit — we find this grit in the valley to the east of Gorsbach, to the 

 south-west of Ty-mawr, and further on in a quarry near the cross 

 roads on the western side. I have not traced it for the next 

 1^ mile to the north, but find it again at Caer-hun, in quarries to 

 the east of the road, again on the west of Pen-yr-allt, and on the 

 east of Cae Seri, then cutting the road by the "i " in "Bryniau," 

 and after crossing this it rises into a good escarpment of massive 

 grit in a wood running parallel to the road which branches ofi" to the 

 right. After the crossing of the next lane, and so on to the Holy- 

 head road, very little sign of grit can be found — it may be concealed 

 or have died out, or there may be a slight fault here. In tracing 

 this line it will be perceived that the portion on the Bangor sheet 

 is very accurately laid down on the map, but the part on the Caer- 

 narvon sheet has not previously been surveyed with precision. It 

 will be seen also that the conglomerate and grit base of the series 

 is much thicker south of Gorsbach, where it is in contact with the 



* See Eamsay, Mem. Geol. Surv. vol. iii. 



t See Hicks, Quart. Journ. Geol. Soc. vol. xxxi. 1875. 



