Yd. 56.] GEOLOGY OF NOKTHERN ANGLESEY. 241 



The upper beds are, in ascending order : — 

 Slates with grit- and quartzi te-bands (seen near Llanbadrig 



Church). 

 Pebbly slates (west of Ogof Gynfor ; west of Porth Llanlliana ; and 



southern slopes of Llanlliana Head). 

 Quartzi te (Ogof Gynfor to Is-allt ; west of Porth Llanlliana; and 



Llanlliana Head). 



The calcareous zone west and south of Porth Llanlliana may be 

 referred with some probability to the horizon of* the Trwyn y Pare 

 Limestone, but the quartzites and shales of Porth Padrig are of 

 uncertain horizon. They perhaps come in between the upper and 

 lower divisions of the series. 



(ii) Area east of the Hell's Mouth Fault. — The slaty beds 

 of the Green Series are found much farther north on the eastern than 

 on the western side of the Hell's Mouth cross-fault. Limestone is 

 seen to the north of the Amlwch Hoad, with an outcrop in shape 

 resembling the quarter of an ellipse. Its presence among the Green 

 Series is due to earth-movements, and the green slates near it are 

 contorted or crushed to fragments. With this exception the rocks 

 as far as Hell's Mouth are greenish and bluish slaty beds of the 

 Green Series, which seem to pass up quite uninterruptedly into 

 pebbly slates, these being followed by a quartzite-ridge that runs to 

 Craig Wen. 



Prom evidence already obtained in the western part of the 

 Complex, it is permissible to infer that the pebbly slates and the 

 quartzite form the upper part of the Llanbadrig Series, wherefore 

 there must be a fault or an unconformity here between them and 

 the Green Series. I have not detected either a fault or an uncon- 

 formity on the coast, where indeed the exposures suggest a passage 

 from the Green Slates into the pebbly group, but the stratigraphy 

 requires a break here, and in this area of compressed beds some 

 faulted junctions so closely resemble bedding-planes as to be easily 

 overlooked. On the map (PI. XIII) therefore I provisionally insert 

 a fault here. 



As this Llanbadrig zone is followed to Porth Wen Bay a second 

 quartzite appears on a lower horizon, south of the pebbly beds. 

 In the south-western part of Porth Wen Bay the rocks are 

 quartzite and quartzose grit interbedded with shaly beds, and 

 resemble the rocks of Porth Padrig. 



South of Craig Wen the lower quartzite is succeeded by banded 

 dark siliceous shales, which exhibit tiny faults, and pass locally into 

 quartzite. In places they assume a conglomeratic aspect, which is, 

 however, due to crush, some of the ' pebbles ' still showing the same 

 banding as the matrix, but they pass up into beds with numerous true 

 pebbles of quartzose grit, upon which the upper quartzite reposes. 



In Porth AVen Bay a cross-fault throws back the southern 

 boundary of the Llanbadrig Series about 100 yards southward, and 

 the disposition of the beds along the eastern side of the Bay is 

 that figured in PI. XIV (Section <3). This shows the rocks to form 

 an overfolded syncline of the three series (Green, Llanbadrig, and 



