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C. CALLAWAY ON" THE AKCHiEAN 



Over a mile to the west-south-west, at Cerrig engan fawr, there 

 is a considerable exposure of a very tough, green, ashy-looking rock, 

 similar to the variety north of the church. 



All the above rocks dip to north-west ; but a little further to the 

 west, at Ty'n-y-buarth, the beds dip to south-east. The strata at 

 this locality are highly contorted on a small scale, and are very 

 similar to the gnarled rocks east and south-east of Amlwch, the 

 alteration, as in that district, having proceeded further than at 

 Cerrig Ceinwen. In all other respects these altered rocks resemble 

 the Cerrig-Ceinwen slates. 



b. Gneissic Series. 



The band of gneissic rocks is hardly a mile in width at Bodwrog, 

 where it is separated from the Pebidian by a faulted strip of Palae- 

 ozoic shale ; but it gradually expands to a breadth of over two miles 

 in the coast section on the south-west. The fault separating it 

 from the slaty series runs from near Bodwina, north of the 

 Holyhead road, in a south-south-west direction, losing itself under 

 the sands of AberfFraw. On the west of the sands are nothing but 

 schists ; on the east are nothing but rocks of the slaty series. It 

 will be necessary to describe two sections across this zone. 



1. Coast Section from Porth Nobla to Aberffraiv Sands. — On the 

 coast, west of Porth Nobla, is a considerable exposure of greenish 

 felspathic rock similar to the halleflinta seen on the strike to the 

 north-east, near Ty Croes. It appears to form an anticline. On 

 Porth Nobla it clearly dips south-east, and passes gradually up into 

 grey gneiss. To the west it presents but slight indications of 

 stratification: but as we proceed eastward planes of separation grad- 

 ually appear, which, at first obscure, become more and more distinct, 

 till the rock is undistinguishable from a true gneiss. I have long" 

 since noticed an unbroken passage between halleflinta and gneiss 

 in the Wrekin ; and Dr. Hicks has recorded the presence of an 

 incipient foliation in the halleflinta ("Arvonian") of St. David's. 

 The grey gneiss soon passes up into quartzose gneiss, quartz-schist, 

 and quartzite. These siliceous types, which are of much theoretical 

 interest, occur in great force, and form the main part of the 

 headlands of Mynydd baen and Pen y cnwc, which project between 

 Porth Nobla and Porth Trecastell. On the west side of Porth 

 Trecastell is a band of calcareous quartz-schist, or quartzose foliated 

 limestone. The silicate appears to be talc or chlorite. There is 

 a break in the section at the centre of the bay ; but as the cal- 

 careous schist reappears on the east side, there can be but a slight 

 break, if any, in the succession. Overlying the last-named rock is 

 a considerable exposure of decomposed grey gneiss interstratified with 

 bands of quartz rock, the whole being much contorted and broken. 

 A greenstone dyke appears here. After a few undulations, the de- 

 composed rock disappears, and ordinary white and grey gneiss, some- 

 times rather quartzose, dips steadily to the south-east for some 

 distance. Then comes in the well-known green schist of the 

 Henai type, which is continued to the Aberffraw sands. It is well 



