GEOLOGY OF ANGLESEY. 



227 



The chloritic rocks of this area are rather more quartzosc than in 

 most Anglesey localities ; but they resemble some of the varieties in 

 Mynydd Llwydiarth, and there are indications in both districts 

 that a source of quartz-derivation was not far distant. 



3. JRlioscolyn District. — -The south-eastern half of Holyhead Island 

 is similar in structure to the Hotyhead area ; but the rocks are 

 affected by an upheaving force to the south, so that the prevailing 

 dip is to the north. The green schists, which occupy the chief part, 

 need no further description. The southern fault, which separates 

 the quartz and chloritic groups south of Holyhead, passes under the 

 sea, and holds the same relation near I^hoscolyn. It is first seen 

 near Bwa du, and again appears on the coast to the south-east at Borth 

 wen, passing in its course near Rhoscolyn church, where the quartz- 

 schist (JNo. 32, p. 233) is exposed, dipping northerly. A few yards to 

 the north the chloritic group dips in the same direction, at about the 

 same angle. At Bwa du the junction is well seen, both series dip- 

 ping northerly, the green schist at a lower angle than the other. 

 The quartz group forms an elevation. Mynydd Bhoscolyn, as is usual 

 in Anglesey. 



At Borth Saint, south of Bwa du, the contortion is intense. There 

 is evidence, from the folding of quartz veins, that the rock has been 

 squeezed to one fourth of its original bulk. At one spot there is 

 a very curious specimen of ripple-mark, distorted so as to be almost 

 unrecognizable. Quartzite dips to the north, and is underlain by 

 soft schist. The under surface of the former overhangs, and is 

 covered with rounded projections, which are evidently the squeezed 

 casts of ripple-marks. The schist was the mud which received the 

 impressions of the ripples; and the quartzite represents the sand 

 which covered in and preserved the marks. I have elsewhere * 

 figured and described the above phenomenon. It is important to 

 record ripple-marks in so ancient a formation. 



South of the last locality is a considerable thickness of chloritic 

 quartz-schist (Note 33, p. 233). 



B. Summary oe Results. 



I. Distribution" op the Rocks. 

 [See Map (PI. VIII.) and Pig. 1.] 



a. Gneissic Series . 



1. Geographical Distribution. — This series occupies the greater 

 part of the Menai anticline. It reappears west of the Llangefni 

 slaty area, and forms the whole of the central zone, except the 

 faulted mass south-west of Bodafon Mountain. It then reemerges 

 west of the Palaeozoic, and covers AVestern Anglesey south of the 

 Porth-y-defaid fault. About three fifths of the area coloured 

 "altered Cambrian and Silurian" consists of gneissic and schistose 

 rocks. 



* 'Science for AH,' part xxxi. pp. 203 and 2QS. 



