| | Hanes Chwarela - The Slate Industry Of Ffestiniog (Site not responding. Last check: 2007-11-03) |
 | | The slate-bearing strata of Festiniog [Ffestiniog] belong to the Ordovician or Lower Silurian systems, and are of an uniform soft blue-grey colour, although the North Vein or topmost bed shows a tendency to be slightly darker. |
 | | The chemical analysis of a standard Festiniog [Ffestiniog] slate shows silica 55 per cent., alumina 25 per cent., iron oxide 11 per cent., the remainder being composed of various elements of which the chief is magnesia. |
 | | The several slate deposits worked at Festiniog [Ffestiniog] are of considerable thickness, often from 150 ft. up to as much as 300 ft., measured on the horizontal, inter-stratified with beds of chert and other rocks of superior hardness to the workable slate. |
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