20 MR, C. A. MATLEY ON THE [Feb. I9OI,- 



2. The Geolo&t of Myntdd-t-garn (Anglesey). By Charles A.. 

 Matley, Esq., B.Sc, F.G.S. (Read November 2l8t, 1900.) 



[Map on p. 24.] 



Contents. 



Page- 



L Introductory 20 



II. Stratigraphy, and Description of the Eocks 20 



III. Fossils 28 



IV. Earth-Movements in the District around Mynydd-y-Garn 28 



V. Summary 29 



I. Introductory. 



Aboye the Tillage of Llanfair-y'ngbornwy, in the north-west of 

 Anglesey, stands a hill known as Mynydd-y-Garn or The Garn. 

 Although rising to a height of less than 600 feet above Ordnance 

 datum, it is one of the highest hills in the county, and is so well 

 elevated above the surrounding country that from its crest the 

 view embraces the greater part of the island, the Caernarvonshire- 

 mountains, and large expanses of sea. 



Ramsay in his memoir on the ' Geology of !N'orth "Wales ' makes 

 but slight mention of the Garn,^ merely noting that it lies in a 

 much-faulted district. Prof. Blake gave a brief notice of the 

 geology of the hill in his ' Monian System of Rocks ' ; ^ and the 

 present writer gave, incidentally, a further short account in a recent 

 paper.^ The results of a more detailed examination of the 

 geological structure made during the past summer seem to warrant 

 a more extended description than any hitherto made. 



II. Stratigraphy, and Description of the Rocks. 



The mass of the hill consists of an inlier of sericitic and chloritic 

 schistose phyllites, usually green, surmounted by a massive con- 

 glomerate, and surrounded by black slates and shales. The slates, 

 apparently of Upper Llandeilo age, form a strip about a mile in 

 width, and are bounded both on the north-east and south-west by 

 fractures which bring them against older rocks. 



A geologist making a traverse from Craig-y-gwynt over the hill 

 in a north-north-westerly direction (see fig. 1, p. 22) and then 

 north-north-eastward to the northern coast, will meet with the 

 under-mentioned rocks in the following order : — 



1. Black Slates (with an anticline of grit at Craig-y-gwynt). 



2. Green breccias, with partings of black shale. 

 •3. Gnarled green phyllites. 



4. Green, bluish, and brown phyllites and fine grits, less fissile than the 

 preceding and but little gnarled. 



' Mem. Geol. Surv. 2nd ed. (1881) p. 246. 



2 Quart. Journ. Geol. Soc. toI. xliv (1888) p. 473. 



3 IbicL vol. Iv (1899) pp. 649-50, also p. 676. 



