Vol. 56.] GEOLOGY OF NORTHERN ANGLESEY. 247 



that this fault, at any rate, has been produced during the great 

 period of movement, and has permitted the folding and faulting to 

 take p]ace independently on either side of the plane of fracture. 



The structure of the Green Series and of the Ordovician rocks to 

 the south of them is still very imperfectly known, but when fully 

 worked out the beds will no doubt show a structure similar to that 

 obtaining along the northern coast. From a careful perusal of 

 Prof. Hughes's and Dr. Callaway's descriptions 1 of the Ordovician 

 rocks, and from my own observations on the rocks of the Green 

 Series, it appears to me that the generalized structure of the country 

 from the south of Llanerchymedd to the northern coast is somewhat 

 as represented in fig. 4 (p. 24S). 



A glance at a geological map is sufficient to show that Anglesey 

 as a whole has undergone other movements than those enumerated 

 in the foregoing paragraphs. In Central and Eastern Anglesey the 

 rocks strike south-westward, and the principal faults run in the same 

 direction (parallel, it may be noticed, to the Hell's Mouth Fault). 

 Was the movement that produced this south-westerly strike con- 

 temporaneous with, or later than, the northerly thrust? There is 

 something to be said in favour of both views. The post-Bala and 

 pre-Silurian movement from the south-east which affected the rocks 

 of Caernarvonshire would bring about such a strike ; on the other 

 hand, the Carboniferous and Permian strata have the same strike, 

 whence it may be inferred that the movement was later than the pre- 

 Carboniferous northerly thrust, or at any rate was prolonged into 

 Permian times. In either case the northerly thrust was not con- 

 fined in its effect to the north of the island, as Prof. Bonney and 

 Miss Eaisin have observed thrusting from the north in the gabbros 

 and serpentines of the Holyhead district. 2 Whether the two move- 

 ments were simultaneous or successive, it is clear that the south- 

 easterly one predominates in Southern and Eastern Anglesey, while 

 the northerly movement is predominant in the north of the island. 

 In the intervening area, between Holyhead Bay and Dulas Bay, may 

 be expected a torsion of the beds caused by the interference of the two 

 directions of movement, which, while adding to the stratigraphicai 

 difficulties, should prove a fascinating study for the geologist" who 

 endeavours to unravel the complex arrangement of the strata. 



II. The Intrusive and Associated Rocks of 

 Northern Anglesey. 



The following account is limited to such igneous rocks as are 

 found within the Northern District, that is to say within the area 

 north of the Llanfflewin overthrust. A petrographical description 

 of some of these rocks has been already given by Prof. Blake, 3 while 

 their mode of occurrence in the field has been briefly mentioned by 

 Ptamsay, Callaway, Blake, Geikie, and other writers. These rocks 



1 Quart. Journ. Geol. Soc. vol. xxxviii (1882) p. 16 & vol. xl (1884) p. 567. 



2 Ibid. vol. lv (1899) p. 301. 3 Eep. Brit. Assoc. 1888 (Bath) p. 367. 



