^^^' 57-] ^^^ GEOLOGY OP MYNYDD-r-GARN. 25 



There is sufficient evidence to show that the rock-sheet of which 

 the Garn Conglomerate forms part was once of fairly wide extent ; 

 and its frequent absence from the edges of the Black-Slate areas is 

 the result of faulting and not due to lack of deposition. Portions 

 of it are met with on the eastern slope of Pen Bryn-yr-Eglwys/ 

 2 miles to the north-west ; at Porth Padrig near Mynachdy in the 

 -same neighbourhood, where it contains fossiliferous limestone- 

 nodules ^ ; and at Clymwr,^ 4 miles to the south-east, whence it 

 has been traced for some miles southward. The conglomerates and 

 grits of JS^orthern Anglesey are, in all likelihood, parts of this same 

 rock-sheet.* 



The Garn Conglomerate is inferred to be of Llandeilo age, 

 partly because it lies immediately below Black Slates containing 

 Upper Llandeilo fossils, and also because it can be correlated with 

 neighbouring conglomerates of Llandeilo age. 



Several microscopic sections have been examined, and they bear 

 out the descriptions just given. Crushing and shearing are 

 observable in most of them. A small well-rounded pebble obtained 

 from the Conglomerate near Cae-engan was sliced [N.A. 141] and 

 was found to consist of rock-fragments of schistose grits, etc. 

 embedded in a slaty matrix. The schistose area from which this 

 pebble obtained its fragments must have been of considerable 

 .antiquity, because it is clearly separated in age by two strong 

 unconformities from the Llandeilo conglomerate.' 



Black Slates and Shales. 



On the north-eastern slopes of the Garn these contain at their 

 l)ase bands of grit and breccia, which connect them with the 

 underlying Conglomerate. The beds above this transition-zone are 

 Wack laminated shales with courses of hard black unlaminated 

 mudstone. About 600 feet in all of these rocks is here seen ; they 

 dip steadily north-eastward at angles of 35° to 45°, and are suddenly 

 terminated on the lower slopes of the hill by an overthrust of the 

 Llanfair-y'nghornwy Beds, near which they are usually quartz- 

 veined and very pyritous. 



South of the Garn inlier they are black or dark -blue slates of a 

 rather uniform and monotonous character, and their strike follows 

 the general curvature of the boundary- thrust. The dips, often wavy, 

 .are high, averaging about 70°, and usually are northerly, though 

 southerly dips occur. These dips may sometimes be merely 



1 Quart. Journ. Geol. Soc. vol. xHv (1888) p. 474. 



2 Bid. vol. Iv (1899) p. 647. ^ Ibid. vol. xl (1884) p. 571. 

 4 Ibid. vol. Ivi (1900) pp. 234 et seqq. 



^ Compare also the occurrence of schistose and gneissose fragments in the 

 Green Series grit of Llanfeehell (Bonney, Quart. Journ. Geol. Soc. vol. xxxvii, 

 1881, pp. 234-35, and Watts, ibid. vol. Iv, 1899, p. 676). The evidence derived 

 from this pebble supports Mr. Greenly 's interpretation of the coast- section at 

 'Oareg-Onen in the eastern corner of Anglesey, that below the Ordovician rocks 

 ■of the island there are two older groups of rocks unconformable one to the 

 other and to the Ordovician (Quart. Journ. Geol. Soc. vol. Hi, 1896, p. 620). 



