ON THE ARCII.EVN ROCKS OF GREAT BRITAIN. .341 



vertical extent of this formation there is still a difference of opinion. 

 I regard it as consisting of the series of green argillites and breccias, 

 well exhibited in Bangor mountain, and of a lower series of argillites, 

 grits, and peculiar breccias. The latter, however, are regarded by Professor 

 Hughes as repetitions of the Bangor series and of the Cambrian con- 

 glomerate by faults ; but to myself the zones appear to be too well marked 

 and traceable over too large areas for this to be possible. On the existence 

 of a volcanic series between the rhyolite and the Cambrian conglomerate, 

 all of the ' newer school ' are agreed. 



Schists, of a type resembling some of those described below, are said 

 to occur in the Lleyn peninsula on the west side, but further information 

 is needed. From my personal knowledge, T can only say that the rock 

 mapped at Porthdinlleyn as serpentine has no claim to the name. 1 



As regards the island of Anglesey, it is admitted on all hands that a 

 portion of the shore at the southern end of the Menai Straits, like the 

 opposite mainland, is fringed by Carboniferous limestone, and that there 

 is a tract of similar rock at the north-east angle, not far from Beaumaris. 

 Carboniferous limestone, with overlying Coal-measures, and possibly 

 Permian in one part, extends across the island from slightly north of the 

 above-named tract to the western shore near Maeldreth Marsh. To the 

 north of this is a thin strip of ' Lower Silurian,' and yet further north a 

 considerable tract which extends diagonally across the island, and sends 

 off a prolongation at right angles to the northern shore. 2 Fringing the 

 part just named on the south, and extending to the western shore, is a 

 strip of ' granite,' and the rest of the island is coloured on the Geological 

 Survey map as metamorphosed Cambrian, &c. As to the unaltered 

 Palaeozoic rocks there is substantial agreement, except that the age of the 

 lower portion is uncertain, Professor Hughes believing that he has 

 identified Tremadoc beds in Anglesey, which is doubted by Dr. Callaway. 

 The Survey view may be stated in the words of the Memoir : 3 ' The 

 Cambrian strata of Anglesey being wholly metamorphic, and the Silurian 

 rocks being metamorphosed in part, there is reason to believe that their 

 metamorphism was contemporaneous and of Lower Silurian date, being 

 connected with the presence of granite, probably of the same age with 

 the imperfectly granitic rock and quartz porphyry on the opposite side of 

 the Straits ' (as has been shown above, both these rocks underlie the 

 Cambrian). There is no doubt much in the very complicated and diffi- 

 cult geology of Anglesey which must still be regarded as unsettled, but I 

 think that there is a general concurrence of all who have studied the 

 subject, both in the field and with the microscope, as to the following 

 points : — 



(a) That this region of ' metamorphic Cambrian and Lower Silurian ' is 

 separable into two, one strictly speaking metamorphic — schists, micaceous 

 and chloritic, fine-grained gneisses, quartzites, &c. : the other hypometa- 

 morphic, as it has been termed by Dr. Callaway, consisting of slaty or 

 schistose rocks, sometimes apparently of volcanic origin. Further, the 

 so-called granite consists in part of coarse gneisses and micaceous or 

 hornblendic schists, and it is doubtful (as at Twt Hill and the Ercal) 

 whether even the most granitoid beds are a true granite. Including this 

 coarsely crystalline group with the schists, the metamorphic rocks of 



1 Q. J. G. S., vol. xxxvii. p. 40. 



- There is also a little Lower Silurian near Beaumaris. 



3 



Mem. Geol. Surrey, vol. iii. p. 1 



i i . 



