ARCH^AN. 43 



The Caernarvon Beds are regarded as equivalent in part of the 

 Dimetian, and are subdivided into — 



Upper = Crug Beds, pink and grey, compact and fine-grained 



fclspathic rocks. 

 Lower = Twt-Hill Beds, consisting of a coarse crystalline 



aggregate of quartz and felspar. 



The Cae-Seri and Tairflfynnon breccias belong to the Cambrian 

 Beds. The conglomerates at the base of the Cambrian in Caer- 

 narvonshire and Anglesey contain pebbles derived from the 

 Archaean rocks. 



Anglesey. — Micaceous and chloritic schists may be seen near 

 Beaumaris, and these, together with gneissic rocks, grits and 

 quartz-rock, constitute a great portion of Anglesey. They are 

 traversed by igneous dykes, and associated with masses of granite 

 and granitoid rocks. The rocks were long ago described by the 

 Rev. J. S. Henslow.^ The exact age of some of the strata on 

 parts of the north coast of the island is by no means certain. 

 Dr. Hicks regards all the altered "Cambrian" of Anglesey, and 

 its so-called intrusive granite, as Pre-Cambrian.^ (See p. 55.) 



In Anglesey Dr. C. Callaway would identify two Archaean or 

 Pre-Cambrian groups, distinguished as the Slaty and Gneissic for- 

 mations. In the Slaty series are included the Llangristiolus grits 

 and slates, Llangefni conglomerates and shales, Cerrig-Ceinwen 

 slaty group, Chloritic schists of Mynydd Mechell, Llanfechell grits, 

 Rhosbeirio shales, Amlwch slates and Cemmaes limestones. 



The Gneissic or granitoid series comprises halleflinta, quartz- 

 schist, grey gneiss, schist, and granitoidite. These beds are faulted 

 wherever seen in contact with the slaty series : but their more 

 intense metamorphism points to their greater antiquity.^ 



The correlation of the Slaty series with the Pebidian, and of the 

 Gneissic series with the Dimetian, has been suggested, while certain 

 halleflintas and quartz-felsites are identified as Arvonian. Sir A. 

 C. Ramsay maintains that the altered rocks of Anglesey are the 

 metamorphosed representatives of the Cambrian.* 



Holyhead Island and the North and South Stack islets are com- 

 posed of gneiss, grits, and schists, showing very violent contortions. 

 The rocks may be, in part, Pre-Cambrian.^ The quartz-schists in 

 Holyhead mountain are regarded as Arvonian by Dr. Hicks. The 

 Lleyn district (the promontory that separates Caernarvon Bay from 

 Cardigan Bay) contains rocks believed to be of Pre-Cambrian age. 

 These are the metamorphosed rocks or schists on the south side of 



^ Trans. Cambr. Phil. Soc. i. 359. 



^ G. Mag. 1878, p. 461 ; Proc. Geol. Assoc, vii. 70. See also Pop. Science 

 Review, 1881, p. 2S9 ; T. G. Bonney, Q. J. xxxix. 476; and Address to Geol. 

 Soc. 1886, p. 87. 



^ Q. J. xxxvii. 210; xl. 567 ; G. Mag. 1886, p. 117 ; 1881, p. 348. 



^ Q- J- xxxvii. 237. 



* W. S. Synionds, Records of the Rocks, p. 29 ; see also Murchison's Siluria, 

 ed. 5. P- 35- 



