228 



C. CALLAWAY 0;\ T THE ARCHiEATT 



The Jialleflinta forms a band extending from Porth Nobla to 

 near Gwalchmai. It is probably represented north of Llangwyllog. 



The quartz-schist extends on the strike from Mynydd baen to 

 Gwalchmai. It may occur in its place north of Llangwyllog ; but 

 it emerges in the craggy mass of Bodafon Mountain and in the 

 small inlier to the east. In Western Anglesey it forms the eleva- 

 tions of Holyhead Mountain and Mynydd Rhoscolyn. 



The grey gneiss is brought up through the dark schist on the 

 western side of the Menai anticline at Gaerwen and east of Pentraeth. 

 In central Anglesey it runs from near Porth Trecastell to north- 

 east of Bodwrog, reappearing soon after on the opposite side of the 

 anticline north of Llangwyllog, and further to the west in the Craig- 

 yr-allor anticline. To the north-east it is seen in the area south- 

 east of Paris Mountain. 



The dark schist occupies the greater part of the Menai anticline. 

 It emerges west of the Llangefni " syncline," but is gradually faulted 

 out towards Gwalchmai. On the opposite side of the central anti- 

 cline it forms a band as far to the north-east as Plas Llannhangel, 

 and is brought up through the granitoidite in the oval dome of Craig 

 yr allor. It also occurs south-east of Paris Mountain. In Western 

 Anglesey it constitutes the schistose area south of the Porth-y-defaid 

 fault and north of the quartzose masses. 



The granitoidite is the " granite " band of the Survey. The details 

 are inaccurately laid down in their map at many points ; but little 

 theoretical importance attaches to these errors. There are slight 

 traces of this rock on the west side of the Menai anticline. 



2. Stratigraphical Distribution. — One of the most difficult pro- 

 blems in Anglesey was to make out the succession of the subdivisions 

 of this group. In no single section, owing to the faulting, is the 

 series entire ; but the subdivisions are recognizable by their litho- 

 logical characters ; and by comparing localities the fragments 

 are pieced together into a complete succession. The facts will be 

 best expressed in the following table, the groups being taken in de- 

 scending order : — 





1. Holy- 

 head. 



2. Craig 

 yr allor. 



3. Coed- 

 ana. 



4. Dulas 

 Bay. 



5. Ty 

 Croes. 



6. Menai. 



Granitoidite ... 





* 



* 



* 





Traces. 



Dark sjhist ... 











* 



* 



Grey gneiss . . . 





*- 



* 







* 



Quartz-schist . . . 















Halleflinta ... 















The star indicates the existence of the subdivision in the locality 

 at the head of the column. By comparing 5 with 2, 3, and 4, it 

 will be seen that two groups are common to the four areas ; and 

 they serve to link together the lowest and highest subdivisions. 



