GEOLOGY OF ANGLESEY. 



215 



round to the AberfFraw sands. On the shore near Bodowen, Llan- 

 gefni conglomerate is underlain by grit. On the east side of Porth 

 twyn mawr, it overlies slaty rock, and contains large unrounded 

 pieces of it. The same conglomerate, repeated by folding, occurs at 

 intervals as far as the AberfFraw sands. 



Qnartzite occurs in lenticular bands in pale-green slate .west of 

 Porth Cadwaladr. This association is also very common in Northern 

 Anglesey. 



Near, or at, the base of the slaty series of this district are several 

 masses of quartz rock of* obscure origin. The most prominent of 

 these is the craggy hill* called Craig fawr, above Llangefni. It 

 is a massive white rock, in which the quartzite structure is still 

 apparent. It is surrounded by Carboniferous strata, and it probably 

 formed an island in the Carboniferous sea. The same rock may be 

 traced at intervals along the strike to the south-west. One conspicuous 

 crag of it is seen at Bethel, north-east of Bodorgan station. I can 

 only suggest that we may have in these bosses the remains of an 

 ancient land, from which the Pentraeth gneiss derived its angular 

 fragments, and the Llangefni conglomerate its rounded pebbles and 

 its quartz grits. 



3. Cerrig -Ceinwen Slaty Group. — These rocks lie to the north- 

 west of the last group, with the same (north-west) dip ; and as they 

 present important lithological differences, they can hardly be repe- 

 titions. 



North east of Cerrig Ceinwen, a little south of the Holyhead 

 road, are green and purplish slaty beds. Some bands are calcareous ; 

 and the formation on the whole resembles the rocks forming the 

 ridge at Wugan bach. Large nodules of jasper are abundant. Their 

 origin is not absolutely clear ; but I am disposed to regard them as 

 included pebbles. The beds have a high dip to the north-west. 

 On about the same strike to the south-west are pale green ashy bands. 

 Close to Cerrig-Ceinwen church, to the north, are prominent ridges, 

 composed of a greenish rock, so tough that it was difficult to obtain a 

 specimen. It has a superficial resemblance to a greenstone, but is 

 undoubtedly an indurated sedimentary rock, presumably an ash. It 

 suggested the "greenstone" of Clegyr Eoia, St. David's. 



A fine exposure in a quarry west-south-west of Cerrig Ceinwen, 

 on perhaps a little lower horizon than the jasper conglomerate, 

 deserves attention. The rock is typical of the slaty series of 

 Anglesey. In the field it appears as a pale green slate or indurated 

 shale, sometimes faintly banded. Comparing it with some of the 

 Charnwood slates (Note 44, p. 234), the only difference I could detect 

 was that the Anglesey type displayed a slight lustre indicative of 

 incipient metamorphism. It sometimes passes into a rock in which 

 the alteration has been carried further, sometimes into a sort of 

 hornstone. The dip is still to the north-west. 



At Bod enlli we are apparently on the horizon of the jasper 

 conglomerate. The conglomerate itself is not exposed ; but we have 

 limestone, purple slates, and green breccias and shales, not unlike 

 those associated with the jasper. 



* Coloured "greenstone" on the map. 



q2 



