GEOLOGY OF ANGLESEY. 



221 



" 338. Hornblende-schist, passing into a more compact rock (339), 

 not schistose, or very slightly so ; and this, again, into hornblende 

 and felspar rock, very similar in appearance to that at Malvern (340). 



" This is an interesting case, as the above varieties were taken 

 from different parts of the same large block. As in Malvern rocks, 

 epidote occurs near the joints. No. 340 might be called horhblendic 

 gneiss ; the felspar is well crystallized in many parts of the mass. 

 In this place there is also hornblende and mica rock ; and in some 

 parts hornblende and felspar occur in separate layers. The granite 

 has a gneissose structure ; and to my mind the evidence is clear that 

 the whole form a series of metamorphic rocks." 



I am glad to quote confirmatory evidence by so competent a 

 lithologist, and trust that it will be unnecessary to multiply refuta- 

 tions of the old " granite-vein " hypothesis. 



4. Structure of the Zone. — For structural purposes it is necessary 

 to include in this band the gneissic area between Ty Croes and 

 Aberffraw. The whole forms a complex and shattered anticline. A 

 central fault margins the granitoidite on the east from Caernarvon 

 Bay to a mile north-east of Bodwrog ; then it curves a little to the 

 east, and, passing near Llangwyllog, leaves room for the underlying 

 gneisses. The strata dip in opposite directions on each side of the 

 fault. This dislocation thus appears to pass along the summit of 

 the anticline; but it does so obliquely, permitting the south-east 

 side of the arch to open out towards the south-west, and the north- 

 west side to expand towards the north-east ; so that we have an 

 almost complete section between Llangwyllog and Llanerchymedd 

 as we have between Porth Nobla and Aberffraw, but with the oppo- 

 site dip. 



The large area occupied by the granitoidite may seem to require 

 explanation. As this rock rarely exhibits bedding, its relations 

 are frequently obscure ; but, fortunately, in the district between 

 Gwalchmai and Llechyn farwy the dark Holyhead schist, which 

 immediately underlies it, is brought up by repeated undulations. 

 Near Gwalchmai, for example, the granitoid rock dips north-west ; 

 but to the west it is thrown off in opposite directions in the Craig- 

 yr-allor anticline. The granitoidite is, then, a band of no great 

 thickness, repeated by contortions. There is also evidence of repe- 

 tition by faults. 



The generalized section (fig. 1) represents the relations of the 

 principal rock-groups in Anglesey. 



IV. rToETHEEN A£EA. 



This district is bounded on the south by the curved fault which, 

 starting from near Carmel's Point on the west, passes round by way 

 of Llanffiewin and Paris Mountain to the east coast at Porth-y- 

 corwg. The rocks are very much contorted and broken ; but there 

 appears to be, on the whole, an ascending series from the fault to the 

 north coast. The dip is generally to the north ; but on the west it 

 is to the north-east. 



1. Volcanic group of Paris Mountain. — The chief types of this 



