28 ME. C. A. MATLET ON THE [Feb. IQOI, 



III. Fossils. 



The few fossils that have been discovered have all been collected 

 from the Black-Slate zones. The zone of interbedded grits and 

 -slates a,bove the Garn Conglomerate has yielded none near the Garn 

 itself ; but similar rocks are exposed a few miles away to the south- 

 east, in a quarry near Llanbabo Church referred to by Dr. Callaway,'^ 

 and I there collected Lingula sp. (common), and the following 

 graptolites : — Diplograptus teretiusculus (common), Leptograptus 

 cf. validus, and possibly, though doubtfully, Glimacograptus sp. 

 These have been identified by Miss E. M. E,. Wood, who remarks that 



' the age of the beds as determined by the graptolites is Upper Llandeilo, 

 about the age of the Coenograptus-heds. Prof. Lapworth has also examined 

 these specimens.' 



At Bwlch, south of Mynydd-y-Garn, in the Black Slates without 

 grits, I obtained a single, graptolite — Leptograptus, probably the 

 same species as above. 



Distant J mile from Bwlch is the oolitic rock already described, 

 which contains an occasional horny brachiopod : — Lingula or Sipho- 

 notreta (?), Acrotreta sp. (brachial valve only). 



IV. Earth-Movements in the District around Mynydd-t-Gaen. 



The region around Mynydd-y-Garn has been affected since Llan- 

 deilo times by earth-movements acting from two directions. This 

 is easily inferred by studying the strike of the rocks and the trend 

 of the faults ; for both faults and strikes fall readily into two groups, 

 according as they run more or less (1) eastward or (2) south- 

 eastward. About 5 miles south-east of the Garn a third movement 

 has produced a third system of faults and bedding-strikes, which 

 run north-eastward. This last-mentioned movement, which has 

 .affected the largest part of Anglesey, has not been studied by me, 

 ;as it has produced but slight effects in the area under notice. 



The easterly strike is the result of a powerful movement acting 

 from the north. All the country west of the Garn and away to 

 Oarmel Head has felt its effects markedly. At Carmel Head green 

 phyllites with broken beds of quartzite and limestone have been 

 driven over Llandeilo Slates^; while, farther south, Llandeilo Slates 

 and older rocks alternate with and are dovetailed into each other in 

 augen-like outcrops, the Llandeilo Beds tailing out to the west, the 

 older rocks to the east. The rocks are faulted, overthrust, cleaved, 

 .crushed, and shattered, and, with the exception of the Llandeilo 

 Slates, are largely in the condition of crush-conglomerates. 

 The boundaries between the rock-groups are sometimes fault- 

 dykes. 



The south-easterly strikes have been produced by movement 

 acting from the north-east, and its effects are best seen at Llanfair- 

 j'nghornwy and in the country lying north-west and south-east of 



1 Quart. Journ. Geol. Soc. vol. xl (1884) p. 580. 



2 im. vol. Iv (1899) p. 646. 



