SHARPE ON NORTH WALES. 29S 



Rhaidr-cwm Section (Section 3). — From Rhaiadr-cvvm to Cynicht, 

 a distance of about seven miles, the beds dip at considerable angles 

 and with great regularity to the N.W. or N.N.W. On the west of Cy- 

 nicht, for a short space, they are nearly vertical ; and thence to the 

 east flank of Snowdon inclusive, a distance of about four miles, they 

 dip south-east. The centre of Snowdon is much disturbed by green- 

 stone dikes and faults, but its western flank is formed of schists and 

 slate dipping north-west : further westward the section is cut off by 

 great masses of igneous rock. 



There is consequently a synclinal axis on the west of Cynicht ; 

 and an anticlinal axis in the centre of Snowdon, though on the 

 western side of the anticlinal the beds are very irregularly disposed. 

 In this section therefore the oldest beds will be found at Rhaiadr 

 Cwm on the one side, and in the centre of Snowdon on the other ; 

 and the newest beds will be found at the synclinal axis west of Cynicht, 

 and again on the north-west flank of Snowdon at some distance 

 from its centre. 



The two sections thus examined in detail give similar results, and 

 establish two principal features in the geology of Carnarvonshire ; 

 viz. a great anticlinal axis of elevation along the centre of the Snow- 

 don chain, and a great synclinal axis running parallel to that chain 

 at a distance of about five miles east of the anticlinal axis. 



Course of the great Synclinal Axis. — The great synclinal axis of 

 the trough which lies between the Bryn-y-ddinas dike on the one 

 side and the Snowdon chain on the other, may be traced from Tre- 

 friw on the Conway, in a S.S.W. direction, to the forty-first mile- 

 stone on the Holyhead road ; and thence, in a south-west direction, 

 to the Traeth mawr, two miles below Aberglaslyn bridge. The 

 course of the axis is generally marked by a band of disturbed beds 

 of unequal width, through which greenstones have forced their way 

 to the surface, either in one great mass or in several minor dikes or 

 ridges. To these belong a dike w^hich runs across the east side of 

 Moel Siabod, Craig-y-llyn-llagi and Yr Arddu, one of the most rug- 

 ged of the Welsh hills. 



Age of the Beds near the Synclinal Axis. — The author has found 

 but few fossils in the beds near the axis of the trough. To the 

 east of the axis in the northern portion of the trough, and to the 

 west of the axis in the southern portion of the trough, he found 

 the fossils hereunder specified at the localities respectively annexed 

 to their names. All the known species in these lists are Lower 

 Silurian species ; and the absence of any that are peculiar to the 

 Llandeilo flags renders it probable that the beds near the axis 

 belong to the Caradoc, or upper portion of the Lower Silurian 

 system, — a conclusion which harmonizes with the position of the 

 beds at the top of the series of rocks of the east of Carnarvonshire. 



