GEOLOGY OF ANGLESEY. 



217 



seen on Porth gwyfen with the normal south-east dip. On the 

 shore, near the church, an undulation brings up grey gneiss. 

 Then the south-east dip is resumed ; but at Trwyn du, just before 

 reaching the Aberffraw estuary, the beds turn up to the south-east, 

 and a syncline is formed. The succession described is shown in 

 fig. 4. 



2. Section on the Railway from Ty Croes to Bodorgan. — The suc- 

 cession is substantially the same as the last ; but it furnishes one 

 or two additional facts. At Pelin bont is the halleflinta first 

 noticed by Dr. Hicks, who rightly conjectured, as the last section 

 proves, that it passed to the south-east beneath the schist at Ty 

 Croes*, though he was in error in placing it above the granitoidite, 

 which lies at the very summit of the gneiss series. West of Ty 

 Croes quartzose schist is exposed in the road. 



Coming to the railway-cuttings, we find to the east of the station a 

 good section of grey quartzose gneiss (Note 34, p. 233) dipping south- 

 east. This rock is one of the most prominent types seen west of Porth 

 Trecastell. Grey and white gneiss are seen for more than a mile 

 with the same dip. In the cutting from near Bodgedwydd to near 

 Graig bach, the grey gneiss grows gradually darker and darker, and 

 passes without a break into the dark green Menai type, which is 

 continued to the end of the exposure. The average dip is 40 ° to 

 south-south-east. Leaving the line, we find in a road-section north- 

 east of Tre' Iddon that the dip of the dark schist has changed to the 

 north-west. East of this point we come almost at once to the slaty 

 beds and conglomerates of the newer series. 



The railway-section is thus seen to agree precisely with the suc- 

 cession on the coast. In some points it is less complete ; but it dis- 

 plays very clearly the passage of the grey into the dark schist, so 

 well seen in the Menai anticline. 



3. Distribution of the gneissic Subdivisions to the North-east. Halle- 

 flinta. — This band gradually narrows towards the north-east, being 

 cut out by the fault that brings the lower part of the gneiss series 

 against the granitoidite ; so that at Gwalchniai it is hardly visible, 

 and at the section f north of the Holyhead road the grey gneiss is 

 in immediate contact with the granitoidite. Some of the gneissic 

 rocks of Gwalchmai display a transition towards the halleflinta 

 types. 



Quartz-schist. This zone is not well exposed near Ty Croes. To 

 the north-east it is seen atMelin Ddrydwy and Glan'raf on ; but it may 

 be best studied at Gwalchmai near the church, on the road to the south- 

 west of the church, and on the Holyhead road. In the last locality 

 the granitoidite is brought against it by the fault. The dip is to 

 the south-east, except on the. Holyhead road, where it is reversed. 

 There is probably a broken anticline at this point. The prevailing 



* I was at first disposed (Geol. Mag. March 1880, p. 123) to associate this 

 halleflinta with the granitoidite, since similar rock, together with quartz- 

 felsites, is included in that group ; but the coast-section has cleared up several 

 difficulties. 



t Geol. Mag. March 1880, p. 124. 



