84 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



One of their most conspicuous members is quartzite, which, besides 

 occurring sporadically all over the island, forms the prominent mass 

 of Holyhead Mountain. There are likewise flaggy chloritic schists, 

 green and purple phyllites or slates, and bands of grit, while parts 

 of the so-called " grey gneiss " consist of pebbly sandstones that 

 have acquired a crystalline structure. That some order of sequence 

 among these various strata may yet be worked out is conceivable, 

 but the task will be one of no ordinary difficulty, for the plications 

 and fractures are numerous, and much of the surface of the ground 

 is obscured by the spread of Palaeozoic formations and superficial 

 deposits. 



These rocks are so obviously an altered sedimentary scries that it 

 is not surprising that they should have been regarded as metamor- 

 phosed Cambrian strata. All that can be positively afl[irmed regard- 

 ing their age is that they are not only older than the lowest fossili- 

 ferous rocks of Anglese}- — that is, than Arenig or even Tremadoc 

 strata — but that they had alread}' acquired their present metamorphic 

 character before these strata were laid down unconformably upon 

 them. There is no actual proof that they include no altered Cam- 

 brian rocks. But when we consider their distinctly crystalline 

 structure, and the absence of any such structure from any portion of 

 the Cambrian areas of the mainland ; when, moreover, we reflect that 

 the metamorphism which has affected them is of the regional type, 

 and can hardly have been restricted to merely the limited area of 

 Anglesey, we are led to realize that, in spite of the absence of posi- 

 tive proof of their true geological horizon, they must be of much 

 higher antiquity than the Cambrian rocks of the neighbourhood. 

 "No one familiar with the Dalradian rocks of Scotland and Ireland 

 can fail to be struck with the close resemblance which these younger 

 Anglesey schists bear to them, down even into the minutest details. 

 Petrographically they are precisely the counterparts of the quartzites 

 and schists of Perthshire and Donegal, and a further connexion 

 may be established of a paloeontological kind. I can now announce 

 that the upper part of the Holyhead quartzite was last autumn 

 found by Mr. B. N. Peach and myself to be crowded with annelid- 

 pipes, and that I subsequently found the same to be the case with 

 the flaggy quartzites near the South Stack. 



Por my present purpose the feature of greatest interest about 

 these younger schists of Anglesey is the association of igneous rocks 

 with them. They include bands of dark basic material, the less 

 crushed portions of which resemble the diabases of later forma- 



