Yol. 56.] THE GEOLOGY OF NORTHERN ANGLESEY. 233 



13. On the Geology of Northern Anglesey : Part II. By Charles 

 A. Matley, Esq., B.Sc, F.G.S. (Read January 10th, 1900.) 



[Plates XIII & XIV— Map & Sections.] 



Contents. 



Page 



I. Stratigraphy of the Northern Complex 233 



(a) The Llandeilo Strata. 



(b) The Llanbadrig Series. 



(i) Area west of the Hell's Mouth Fault, 

 (ii) Area east of the Hell's Mouth Fault, 

 (iii) Further Remarks on the Quartzites. 



(c) Structure of the Northern Complex. 



II. The Intrusive and Associated Rocks of Northern Anglesey ... 247 



(a) Pre-movement Intrusions, 



(b) Post-movement Intrusions. 



In a recent communication l I discussed the stratigraphical 

 relationships of the rocks of Northern Anglesey, and showed that 

 the region had been profoundly affected by earth-movements. 

 Having now completed the mapping of that part of the district 

 which I called the ' Northern Complex/ that is to say the area 

 between Cemaes and Bull Bay, I venture to bring the results before 

 the Society, with map and sections. It has been ascertained that 

 some of the dykes of Northern Anglesey are composite in character, 

 and a short account of these and of the other intrusive rocks is 

 also given. 



I. Stratigraphy op the Northern Complex. 



In my previous paper the Northern Complex was stated to con- 

 tain rocks of undoubted Ordovician age, rocks of the Green Series, 

 and a third group characterized by limestones and quartzites. The 

 last-mentioned group will be here referred to as the Llanbadrig 

 Series, from its development in the neighbourhood of Llanbadrig 

 Church. The succession, in descending order, is as follows : — 



C. The Llandeilo Rocks — conglomerates, grits, sandy and argillaceous 



shales and slates, and ironstone. 

 B. The Llanbadrig Series — quartzites, limestones, slates, gritty slates 



with bands of grit, and pebbly slates. 

 A. The Green Series — represented in the ' Northern Complex ' mainly 



by greenish and bluish slates. 



The rocks strike in an east-south-easterly direction, and the dip, 

 almost always northerly, is usually high. A well-marked transverse 

 fault runs south-westward from Hell's Mouth, dividing the Complex 

 into western and eastern portions. 



1 Quart. Journ. Geol. Soc. vol. lv (1899) pp. 635-75. 



