542 eeport — 1884. 



Anglesey occupy three areas — one, a strip roughly parallel with the 

 Menai Straits, extending from the western to the eastern shore, consist- 

 ing mainly of micaceous or chloritic schists, with some rather decidedly 

 crystalline hornblendic schists ; another (in which the granitoid rocks 

 are included), extending from the western shore at Porth Nobla, near 

 Ty Croes and Llanfaelog, about two-thirds way across the island, with 

 sundry outcropping patches to the north-east which show that an Archaean 

 floor underlies the Palaeozoic rock right up to the eastern shore; and, 

 lastly, a rudely triangular patch, consisting of the island of Holyhead, and 

 a still larger tract on the adjacent mainland. 



(b) That at the base of the indubitably Palaeozoic series, there is in 

 many places a conglomerate which contains fragments of the granitoid 

 rock, the schists, and the schistose or slaty beds of the ' metamorphic 

 Cambrian and Lower Silurian region' of the Survey. Hence, that whether 

 this conglomerate is Cambrian, as is the opinion of Professor Hughes and 

 Dr. Hicks, or very low down in the Ordovician, as seems to be the view of 

 Dr. Callaway, 1 there is a very important break between it and the ' meta- 

 morphic series,' in which also the upper member contains pebbles of the 

 lower. 



(c) The greater antiquity of the granitoid series and its non-intrusive 

 character is placed beyond doubt by the following section : 2 Near Llan- 

 faelog the granitoid series and the so-called Lower Silurian can be seen 

 in close proximity 3 and traced for a considerable distance. Not only are 

 grits in the latter formed of debris closely resembling the constituents of 

 the former, but also there are occasionally bands of well-rounded pebbles, 

 sometimes more than four inches in diameter, which are absolutely indis- 

 tinguishable macroscopically and microscopically from the varieties of the 

 granitoid rocks of the Llanfaelog ridge. 



When we come to the classification of the truly metamorphic series, 

 there is at present considerable discordance of opinion, and it will doubt- 

 less take much careful work, both in the field and with the micro- 

 scope, before the subject can be regarded as settled. Dr. Hicks gives 

 the following succession in ascending order : — (1) Granitoid series. 

 (2) Compact quartzose rocks. (3) Quartz-schist, and chloritic or mica- 

 ceous schist (the most abundant rock in the metamorphic series of 

 Anglesey). The lowest series he correlated with the Dimetian, the ' halle- 

 flinta ' with the Arvonian, and the ' chloritic series ' with the Pebidian. In 

 my opinion, over much importance has been assigned to the ' hiilleflinta ' 

 zone, which is made up partly of some compact quartzomicaceous or 

 quartzochloritic rocks not very distinctly foliated (for which it is 

 difficult to find a satisfactory name), partly of some felstones, almost 

 certainly intrusive, and so giving no help in classification. Further, the 

 great 'chloritic' series cannot be correlated with the Pebidian (i.e., the 

 series underlying the Cambrian conglomerate at St. David's and near 

 Bangor), without setting aside all the conclusions to which we are led 

 by the use of the microscope in studying tha British rocks. Dr. Calla- 

 way gives the following succession in ascending order : — (1) Hiilleflinta. 

 (2) Quartz-schist. (3) Grey gneiss. (4) Dark schist. (5) Granitoidite. 

 The dark schist (4) includes the dull lead-coloured or greenish, chloritic 



« Q. J. G. S., vol. xl. p. 567. 



* Hicks, Q. J. G. S., vol. xl. p. 1S7. 



3 About 30 yards at the nearest point — Hides, supra-, p. 1P2. 



