170 MR. F. EUTLEY ON FELSITIC LAVAS [^^y 1 899, 



9. On a Small Section of Felsitic Lavas and Ttjefs near Conway 

 (Caeenarvonshike). By Feank Eittlet, Esq., P.G.S. (Bead 

 January 18th, 1899.) 



[Map on p. 172, and section on p. 174,] 



The felsites of Conway Mountain were described by Prof. Bonney 

 more than 16 years ago.^ Among specimens which I collected itt 

 North Wales in 1877, but from which sections have only recently 

 been prepared, there are a few which may be of interest, since they 

 w^ere procured at short intervals from a point at the mouth of the 

 Eiver Conway, and may be regarded as representing the southern 

 portion of the felsite near the fault which, on the Geological Survey 

 map, is shown to separate the felsites of Conway Mountain from 

 those of Diganwy, throwing the latter a considerable distance to 

 the north. 



The spot at which the specimens were collected is marked Bod- 

 londeb on the Ordnance Survey map. A path runs round this point,^ 

 bounded on the west by an exposure of rock which has been dressed 

 back so as to form a low natural wall, while a lower wall has been 

 built between the eastern side of the path and the estuary (see 

 map, p. 172). The specimens, or small chips, were consecutively 

 numbered in the order in which they were collected, from south to 

 north. On the south the felsite is succeeded by Bala Beds, which 

 about a mile to the west are marked on the Geological Survey map 

 as dipping 60° southward, but at Bodlondeb the felsites appeared to 

 dip at a higher angle. 



1^0. i is a pale pinkish-grey rock, somewhat resembling the 

 yellowish felsite which occurs to the west below Castell Caer Seion. 

 The fluxion-structure is, however, more delicate and better defined. 

 The bands are not continuous, but extend only for short distances, 

 thinning away towards their extremities. They are cryptocrystalline, 

 and are more translucent than the rest of the rock, which has, as a 

 rule, a microcrystalline character, and is rendered turbid by diminu- 

 tive grains and flecks of opaque white matter, the latter apparently 

 due to the kaolinization of the felspathic constituents. The structure 

 may be defined as an impersistently banded, wavy fluxion. 



No. 2, taken some 10 or 15 feet farther north than the pre- 

 ceding specimen, is a pale bluish-grey rock, consisting of felsitic 

 matter, sufficiently coarse in texture to permit the investigation of 

 individual grains in convergent polarized light. Some of the grains 

 show a positive uniaxial interference-figure : these are quartz. 

 Other grains, also colourless, show the emergence of a bisectrix or 

 else a dark brush which sweeps obliquely across the field during 

 rotation : these are grains of felspar, and they seem to be more 

 numerous than those of quartz. In transmitted light, the section 

 shows an irregular, hazily-defined fluxion -structure ; while, between 



^ * On some Nodular Felsites in the Bala Group of North Wales,' Quart. 

 Journ. Geol. Soc. vol. xxxviii (1882) p. 289. 



