GEOLOGY OF ANGLESEY. 



211 



Introduction. 



I have to submit evidence in support of the conclusion that there 

 are represented in Anglesey two Precambrian or Archaean groups, 

 each distinguished by well-marked lithological characters. They 

 may be named respectively the Slaty and Gneissic formations. The 

 " Pebidian " schists, as recognized by Dr. Hicks, I include in the 

 gneiss group ; but, in other parts of the island, I have worked out 

 a great assemblage of fraginental felspathic rocks, which I believo 

 tobe of true Pebidian age. I shall also attempt to prove that the 

 quartz rocks of Anglesey are included in the gneissic series. The 

 microscopic notes, kindly furnished by Prof. Bonney, are based upon 

 the specimens only, and have all the value of independent evidence. 

 The growing importance of these ancient rocks would seem to re- 

 quire that the provisional term "Precambrian" should give place 

 to a word which can be permanently used : and I have ventured to 

 adopt the term " Archaean/' so widely employed by Continental and 

 American geologists. To the words " Azoic " and " Eozoic " there 

 are obvious objections. 



A. Description op Areas and Sections. 



I. Menai Anticline. 



Eor convenience I accept this name for the rocks lying between 

 Menai Straits and the great fault which throws down the newer 

 Palaeozoic rocks of Malldraeth Marsh, though the term is not strictly 

 applicable. It is true that the strata on the south-east side of the 

 area dip to the south-east, and those on the north-west side to the 

 north-west ; but between these two extremities the undulations are 

 frequent, and some of the dips are probably overthrows. 



a. Gneissic Series. 



Two Varieties of Gneiss. — The structure of the area is rendered 

 much clearer by the recognition of two prevailing gneissic types — 

 a dark micaceous or hornblendic variety, associated with chloritic 

 schists*, and a greyish or light-red rock clearly foliated into an or- 

 dinary ternary gneiss. In the Craig-yr-allor anticline f the two 

 varieties are also recognizable, the grey gneiss passing up through 

 the dark type into the granitoidite. In the Menai district the dark 

 schist holds the same relation to the grey variety, and the passage 

 between the two may be seen at many points. 



1. Gneiss south of Pentraetli. — ISText to the fine section at Gaer- 

 wen %, the rocks south and east of Pentraeth require notice. The 

 band coloured " greenstone," running from Tai hirion north-north- 

 east to PMs-gwyn lodge, appears to be a highly- altered hornblendic 

 gneiss, with a north-west dip. In the field to the south-east of the 

 lodge is a small quarry of typical grey gneiss ; and in the plantation 



* Throughout the paper the terms " schist " and " schistose " are strictly con- 

 fined to foliated rock, 

 t Geol. Mag., March 1880, p. 119. + Geol. Mag., March 1880, p. 121. 



