238 ME. C. A. MATLEY ON THE [May I9OO, 



The close correspondence of the sequence here with that of the 

 Ordovician area south of the Green Series has heen previously 

 noticed. Prof. Hughes's description of the latter area 1 makes this 

 point sufficiently clear. 



(b) The Llanbadrig Series. 



The rocks of this group come in above the Green Series, and 

 below the Llandeilo Beds. Apart from its characteristic limestones 

 the series is a distinctly gritty one ; the shales contain abundant 

 quartz-grains, 2 are interbedded with bands of quartzose grit or 

 quartzite, and pass up into conglomerate. The prevailing colour 

 may be stated as a dull olive-brown, relieved by horizons of grey, 

 green, sage-green, or even black among the shales, of blue or pale 

 grey in the limestones, or of white in the massive quartzites. The 

 succession, which has not been fully made out, may be best studied 

 in the coast-sections. 



(i) Area west of the Hell's Mouth Fault. — Commencing 

 at the western end of the Complex, several masses of the Llanbadrig 

 limestones may be seen involved in the crush-zone of Wylfa Head, 

 but most of the quartzite-masses here seem to be altered fine grits 

 of the Green Series. 



On the eastern side of Porth y Wylfa, a narrow wedge of shattered 

 quartzite, with an included bed of impure limestone, is caught up in 

 the Green Series between thrust-planes, which hade as usual to 

 the north. 



At Penrhyn occurs the overthrust area figured in my previous 

 paper (op. cit pp. (560, 60^). The broken grits and slates which form 

 the ' crush-conglomerate ' here do not possess the distinctive green 

 coloration of the flaggy slates to the south, and on the eastern side 

 of Trwyn y Penrhyn the junction of the two groups is rather abrupt 

 and apparently faulted, though wedges of typical green slates appear 

 north of the break. The western boundary is more arbitrarily 

 drawn, and there may be a passage between these rocks and the 

 crush-conglomerates of the Green Series along the coast to the west. 



We pass now to the coast-section (PI. XIV, Section 1) at Pig y 

 Barcud and along the eastern shores of Cemaes Bay. The limestone 

 at Pig y Barcud is shaly, with black anthracitic partings, much 

 contorted, and intersected by numerous small thrusts. This and 

 two more limestone-masses to the west are isolated portions of the 

 Trwyn y Pare Limestone and occupy an overfolded syncline. 

 They are overridden by broken slaty and gritty beds (with some 

 bands of quartzite) of the same character and on the same horizon as 

 the crush- conglomerate at Penrhyn. We may call them Trwyn y 

 Pare Slates and Grits. In them occurs a crushed dyke, and 



1 Quart. Journ. Geol. Soc. vol. xxxviii (1882) p. 16. 



2 The quartz-grains of some of the gritty shales, grits, and quartzites are seen 

 under the microscope to be remarkably rounded. A grit from the broken-up 

 beds of the Green Series at Porth y Wylfa showed similar well-rounded grains. 



