440 Mr. Aikin on Witheritc found in Shropshire. 



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 dis- 

 tinguish it from heavy-spar; this latter from the looseness of its 



