272 "THE GEOLOGIST. 



I^To fossils have yet been found in the Cambrian rocks of Merioneth- 

 shire. Bands of purple slate occur in them ; but in this district they 

 have never been worked to profit. Workable slates of an inferior 

 quality here and there occur in the Lingula-flags, together with copper- 

 lodes, which are frequently worked, and as frequently abandoned. At 

 Cwm Eisen, gold has also for a long time been extracted at intervals from 

 a lode of quartz, though never with a steady profit ; and, a few years ago, 

 much sensation was excited in the mining-world by what were, at 

 the time, considered promising discoveries of that metal at Dol-y- 

 ffrwynog, the Clogau, Prince of Wales, and the Cambrian mines, 

 and in various other localities. This fact is of scientific interest, 

 because in the Ural, South Australia, Canada, and other parts of the world, 

 gold occurs in rocks of the same general age, and apparently under the 

 same circumstances. At Bol-y-ffrwynog, the quartz gold-bearing vein 

 traverses a talcose schist, which is a metamorphosed part of the 

 Lingula-flags, intimately associated with neighbouring ramifying 

 intrusive masses of greenstones ; and here, for a time, gold was 

 found in such quantity, that in the lode itself it was visible, 

 speckling the surface of the quartz. The greenstone that is intruded 

 into this part of the Lingula-flags, is probably connected under- 

 ground with the rock of Rhobell-fawr, which itself forms by far the 

 largest mass of greenstone in Wales. From Dolgelli it spreads, 

 in nearly an unbroken sheet of eight or nine miles in length, to 

 the upper part of the river Mawddach ; and at Rhobell-fawr it is 

 more than two miles wide, rising in great broken, and bare 

 undulations to the very top, near which it is overlaid by a strip of highly 

 porcelained slate. 



On the south-east side of Cader Idris, and east of the Arans, are 

 beds of black slate, forming probably beds of passage from the Llandeilo 

 flags to the Bala beds. In places they contain OrtTiis Jctonice, 0. hiforataj 

 Lept(Bnaq^uinquecostata, with species of Asaphus, Ampyx, Cyhele, and other 

 fossils. These, in turn, are overlaid by undoubted Caradoc or Bala beds, 

 in the midst of which lies the well-known Bala limestone, which, 

 beginning at Dinas Mowddwy, in interrupted faulted lines, stretches 

 north by Bala across the broken hills south of Cerrig-y-Druidion, and 

 from thence westward to Penmachno. Throughout this course, on two 

 or three horizons, this highly fossiliferous limestone is underlaid by 

 certain thin bands of felspathic volcanic ash. The limestone itself is 

 sometimes slightly ashy. 



