Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/450

 frequently met with of all sizes, from that of a walnut to that of a small room: many of the lesser cavities are filled with petroleum, the others are lined with crystals of heavy-spar, calcareous spar, and quartz, having their bottoms or floors covered with aggregated masses of the above crystals, with crystals and potatoe-shaped pieces of galena, with carbonate of lead, and with a black powder which is principally pulverulent galena. In some parts there is a saalbande or sticking of grey clay an inch or two in thickness, and here the vein is the least productive; in other parts the rock is hardened by an infiltration of quartz, and these are uniformly found to be the most productive. There is no regularity in the arrangement of the contents of the vein, but generally the galena occupies the sides, and the sparry veinstones the centre. Riders, or ribs of rock inclosed within the vein occur, but not frequently. The ore is for the most part foliated galena and striated galena, called by the miners pot ore and steel ore: iron pyrites is the next in quantity, and lastly blende, this latter being for the most part so intimately mixed with the galena as to be scarcely visible. The veinstones are calcareous spar, often approaching to schiefer spar, and foliated heavy-spar (called here water spar, because from the looseness of its aggregation most of the water drains through it into the mine.) Quartz is more rarely met with; and in the lower part of the mine, where the vein is very thick and sparry, the witherite is found in irregular masses, weighing from 40 lbs. to 2 or 3 cwt., imbedded in heavy spar. The name given to this substance by the miners is yellow spar, not that this is its real colour by day-light, but its transparency is so considerable that if a lighted candle be placed behind a mass of it the whole will glow with a yellowish light, a circumstance by which the miners distinguish it from heavy-spar; this latter from the looseness of its