Ixii PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



general south-easterly direction, but on approaching the higher for- 

 mations of Denbighshire, though interrupted in many places by great 

 faults, they tend generally to dip beneath those formations. Further 

 eastward from the Berwyns the structure of the district becomes 

 extremely irregular and dislocated. 



Returning to the country west of the Caernarvon antichnal, we find 

 N.W. and S.E. sections presenting minor undulations, but on the 

 whole an ascending series from the shores of the Menai to the anti- 

 clinal, the strike of the beds maintaining an approximate parallelism 

 with the direction of the anticlinal. As we proceed southward, how- 

 ever, the direction of the strike changes, till about Tremadoc it be- 

 comes nearly perpendicular to its normal direction. The beds here 

 dip in a north-easterly direction into the Snowdonian synclinal trough 

 forming the south-western extremity of that western portion of the 

 trough directly under the crest of the Caernarvon range*. The 

 eastern part of this trough is continued southward from the valley of 

 Festiniog along the western flank of the Merioneth anticlinal to the 

 Barmouth estuary. 



We shall best form some distinct conception of the general struc- 

 ture of this district, independently of the great dislocations which 

 traverse it, by representing to ourselves the surface of any one of its 

 continuous strata. In the section just referred to, given by Professor 

 Sedgwick, which runs N. by E. from near Tremadoc, the lowest 

 beds are found at its southern extremity, dipping with the super- 

 incumbent beds in a northerly direction. These are the Lingulae 

 beds with the superincumbent black slates which Professor Sedg- 

 wick, in the paper just referred to, identifies for the first time with the 

 beds of similar mineral character on the shores of the Menai Straits. 

 This identification has been established since on better evidence by 

 the gentleman of the Ordnance Survey. We may therefore conclude 

 that the position of the Lingulse beds in the series of formations is 

 not remote from the surface on the shores of the Menai. Fol- 

 lowing then the surface of these beds in imagination from thence 

 eastward, we shall have it near the level of the sea along the Menai, 

 and forming ridges and troughs till it rises up to the Caernarvon 

 anticlinal to which the ridges and troughs are parallel. To the east 

 of that line the surface will sink rapidly to form the Snowdonian 

 trough, at the south-western end of which it again rises to above the 

 sea-level. If a section could be continued to the S.W. from 

 Tremadoc under tbe sea into Cardigan Bay, we should probably 

 find beds lower in the series than these Lingulse beds, so that the 

 surface of these latter must be there continued as an imaginary 

 surface rising, perhaps, to a considerable height above the sea-level, 

 and forming possibly an anticlinal line along the prolonged axis of 

 the Snowdonian trough. On the E. of this line they would, at all 

 events, dip towards the S.E. under the coast to rise again, as they 

 are observed to do along the western flank of the Merioneth range, 

 thus forming the synclinal trough above mentioned as the southern 

 continuation of the wider trough between the Caernarvon and the 



, * Quart. Journ. Geol. Soc. vol. iii. p. 139. Section, by Prof. Sedgwick. 



