OCCURRENCE OF LEAD ORES. 69 



and most productive ore of lead, and is very frequently ex- 

 tremely rich in silver. It is found in rock formations of 

 various ages in lodes, pockets, flats, &c. 



The carboniferous or mountain limestones of England 

 yield most of the lead ore, while it is also worked in the 

 " killas " of Cornwall, a Devonian formation. 



It also occurs in Great Britain and other countries in the 

 Lower Silurian rocks, in granites, gneiss, &c. 



The carbonate of lead deposits of Leadville, Colorado, 

 best known for being richly argentiferous, occur between 

 blue limestone and porphyry (Fig. 41). 



Galena is generally associated with quartz, carbonate of 

 lime spar, fluor-spar, sometimes barytes, copper and iron 

 pyrites, &c. (For the assay of Galena, see Chap. IX.) 



If powdered galena be heated in an iron spoon, lead can 

 be obtained. The heat should be gradually raised at first 

 till the pieces cease to decrepitate. After this a red heat 

 will suffice. 



The following is a simple method of obtaining lead bul- 

 lion (though not the proper amount) from an ore, and may 

 be of use to the prospector. Erect a square furnace of 

 rough stones. Place rough logs of wood at the bottom, 

 above this split wood, then broken-up ore, and then wood. 

 The fire should be lighted at the entrance, and the lead 

 allowed to run out into a basin. 



MANGANESE. 



The principal ore is the black oxide (grey manganese or 

 pyrolusite). 



Found compact or granular ; the black powder in the 

 cavities will soil the fingers. Small brilliant crystals, like 

 cut steel, are sometimes met with ; also botryoidal masses 

 with a fibrous structure. 



Lustre submetallic. 

 Colour and streak black. 

 H. 2 to 2-5 ; S.G. 4-8 to 5. 



Composition 6 3 '3 per cent, manganese, the remainder 

 oxygen. 



Effervesces briskly with borax before the B.F. 



