Ixiv PROCEEDINGS OF THE GEOLOGICAL SOCIETY, 



not A'ery far from the sea-level, and such appears also to be the cas6 

 east of Bangor slate quarries. Consequently the geological level of 

 this part of the district is probably at least some 15,000 or 16,000 

 feet higher than that of Denbighshire. The depth of the synclinal 

 trough formed by the same beds between the Caernarvon and Merio- 

 neth anticlinals must probably be much smaller than the above. 

 On the south-eastern side of the IMerioneth anticlinal, the surface of 

 our imaginary bed would form a deep valley, bounded on the S.E. 

 by the escarpment of the enormous fault just mentioned, on the 

 south-eastern side of which it would form a still deeper parallel 

 valley. Hence in N. Wales there are three great parallel geological 

 troughs, the Snowdonian one extending from Tremadoc to the north 

 of Suowdon, and the other two deeper ones terminating on the N.E. 

 in the deep depression of Denbighshire. Towards the S.W. the 

 more eastern of these troughs is continued into the Silurian basin of 

 the southei'n part of the Principality, while the intermediate one 

 must terminate about Cader Idris. 



I have now to speak of the faults of this district. Wherever the 

 continuity and regularity of the longitudinal troughs are preserved, 

 we find a corresponding regularity in the longitudinal faults running 

 parallel to them. This regularity is particularly observable on both 

 sides of the Menai Straits. Further to the south-east, the most 

 important fault of this kind is the enormous one already mentioned as 

 running along the Dolgelly and Bala valley. From the former of 

 those places, to near the northern extremity of Bala Lake, it steadily 

 preserves its north-easterly course ; but, at the latter point, it is 

 gi'adually deflected towards the east, so as to preserve an approxi- 

 mate coincidence to the strike of the beds, which, in the valley of 

 the Dee, instead of being N.E. and S.W. becomes nearly £. and W., 

 the beds dipping under the Upper Silurians of Denbighshire. This 

 fault is finally hidden under the New Red Sandstone at its north- 

 eastern extremity, after dislocating the Millstone Grit and the Coal 

 as well as the lower formations, showing the date of this portion, at 

 least, of the fault to be posterior to the Coal*. Another leading 

 longitudinal fault proceeds from the estuary of Traeth Maur, on 

 the west of the Merioneth anticlinal. It runs N.E. till it approaches 

 the Denbighshire depression, when it is deflected towards the north 

 so as to follow the south-western and western boundary of the 

 Denbighshire depression, which it embraces on that side as the 

 former-mentioned fault embraces it on the south and south-east. 

 No considerable fault has been observed to pass from the S.W. 

 directly into that basin, which is surrounded by faults running 

 along its curvilinear boundary on the south and west where the beds 

 dip towards the centre of the basin, as well as on the east where 

 they dip towards the east. These faults, since they coincide ap- 

 proximately with the strike of the beds, are properly longitudinal 

 faults. It is at these places, where the strike of the beds changes 

 suddenly from its normal north-easterly direction, that we always find, 

 as we might expect, the most numerous dislocations. Very striking 

 * I infer this from the Geological Map of the Ordnance Survey. 



