242 proceedings of the geological society. [feb. 1, 



February 1, 1854. 



Charles Robert des Ruffieres, Esq., Alexander George Gray, jun., 

 Esq., George Milner Stephen, Esq., Barrister, and Edward Harman 

 Sheppard, Esq., were elected Fellows. 



The following communications were read : — 



] . On the Geology of the Gold-bearing District of 

 Merionethshire, North Wales. By Prof. A. C. Ramsay, 

 F.R.S., F.G.S. 



That part of Merionethshire to which this notice refers lies between 

 Dolgelli and the Moelwyn and Manod range, north of Ffestiniog*. 



A small part of this area has for some years been distinguished by 

 discoveries of gold, of more or less importance. Its geological rela- 

 tions are as follows : — On the N. and W. of the lower part of the 

 River Mawddach lie the lower part of the Lingula-flags and the Cam- 

 brian rocks. The latter consist of the coarse, thick-bedded, greenish- 

 grey grits of Barmouth and Harlech. Their upper boundary is marked 

 by a sinuous line which strikes in a north-easterly direction from Bar- 

 mouth to Rhaiadr-Mawddach, and from thence trends northerly a little 

 E. of Trawsfynydd to the turnpike road about a mile S. of Ffestiniog. 

 The line then strikes S.W. to Morfa Harlech, on the coast of Car- 

 digan Bay. These grits are overlaid by that part of the Lower Silu- 

 rian rocks known as the Lingula-flags, which here consist mostly of 

 blue slaty beds, generally more or less arenaceous, and partly inter- 

 stratified with courses of sandstone. A well-marked portion of the 

 series, composed of rusty ferruginous slate, occupies part of the cliffs 

 of Moel Cynwch that overhang Dol-fawr on the Mawddach. 



Both Cambrian and Silurian rocks have been penetrated by nume- 

 rous greenstone-dykes. Many of them are of a light grey colour and 

 highly calcareous. Others assume the colour and texture of ordinary 

 greenstone. Some of them are magnetic. Among the Cambrian 

 sandstones they run in all directions, sometimes with, but more 

 generally across, the strike. In the Silurian region they more usually 

 run more or less parallel with the lines of bedding. 



In the hard and solid Cambrian sandstones, the fractures into 

 which they were injected were capricious and irregular ; while in the 

 Silurian shales they have more frequently been intruded between the 

 beds. Some of them fill cracks which pass into lines of lode. A case 

 of this kind occurs on the N. and W. of Tyn-y-groes. A strong 

 lode, bearing quartz and lead, commences between Dol-y-melynen 

 and the fifth milestone on the Dolgelli and Ffestiniog road. For 

 about three-quarters of a mile it divides the Cambrian and Silurian 

 rocks. Passing into the former on Cefn-coch, the fracture, as it 

 crosses Craig-y-Cae and Y-garn, becomes filled with greenstone. 

 Several other metalliferous lodes occur in this neighbourhood. They 

 all traverse the Lingula-flags or their associated igneous rocks. There 

 * Geological Survey Maps, 75 N.E., S.E., and 59 N.E. 



