STRUCTURE OE SOME WELSH ROCKS. 145 



a diabase ; it has been a basalt, and much resembles a flow in its 

 structure. 



No. III. Lowest beds, Llysmeirion, Caernarvon (p. 142). — A highly- 

 altered rock, consisting largely of quartz (dull in tint with polarized 

 light, and containing many minute cavities and enclosures), with 

 felspar, not very characteristic as to species, partly decomposed, and 

 traversed by lines of minute secondary products, golden-coloured 

 with crossed Nicols ; also a little iron peroxide, and some vermicular 

 Of rosette-shaped aggregates of a slightly dichroic golden-green 

 niineral, probably allied to chlorite. The quartz, in parts of the 

 sjide (which varies much in texture), is distinctly in broken grains, 

 slubangular in form ; and the rock is certainly of clastic origin. 

 I No. IV. Felsiiic Series N.W. of Llanddeiniolen, Caernarvon 

 (ip. 142). — The ground-mass is a devitrified glass, showing in parts a 

 fluidal structure very well, containing distinct crystals of felspar, 

 sometimes of rather a broken aspect, and grains or imperfect crystals 

 of quartz. There is also a very little iron peroxide, some minute secon- 

 dary microliths, golden, as above, especially in the larger felspar 

 crystals, where both orthoclase and plagioclase can be recognized. 

 Three or four crystals, partly occupied by iron peroxide, show also 

 a fibrous pseudomorph, rich- coloured,; with crossed Nicols, and are 

 probably pseudomorphs after hornblende or mica ; one or two crystals 

 resemble decomposed ilmenite. The rock is probably from an old 

 flow of rhyolitic lava. 



No. Y. Coarse Breccia ; Lower Moor, St. Davids. — A highly altered 

 rock, consisting mainly of quartz and felspar (both kinds), having 

 a general resemblance to No. III. It is difficult to pronounce on 

 the exact origin ; but much of the quartz and felspar appear to be 

 fragmental and more or less angular, while the texture of the rock 

 seems to vary much. There is a fair amount of viridite, and but 

 little iron peroxide. Some of the fragments show a graphic struc- 

 ture. The larger fragments do not come into the slide. I do not 

 find any devitrified lapilli, and think this rock probably not one of 

 volcanic origin, but formed by the denudation of a granite or gra- 

 nitoid gneiss, the fragments of which were soon deposited — like the 

 " arkose " of Auvergne. 



No. VI. Brithdir, Bangor. — A devitrified ground-mass, exhibiting 

 flow-structure well, with many minute grains of iron peroxide, nu- 

 merous grains and imperfect crystals of quartz, and crystals of fel- 

 spar of both kinds, often with a broken aspect, not unlike some of 

 those found in the Wrekin District. Devitrified rhyolite, probably 

 from a flow. 



No. VII. Agglomerate. Pre- Cambrian ; West Shaft to Tunnel, 

 Bryniau, Bangor. — Composed of fragments, very variable in size 

 and nature, in a ground-mass resembling a fine felspathic dust, much 

 altered ; many secondary products, difficult to describe or name ex- 

 actly. Larger fragments, quartz, altered felspar, slaty rock, and 

 some lapilli, one or two showing plagioclase, which, however, are 

 not very characteristic. The slaty fragments seem highly altered, 

 and contain microliths, apparently felspathic and of secondary origin ; 



Q. J. G. S. No. 133. l 



