THE NONMETALLIC MINERALS. 243 



is not infrequently separated from the coarser or heavier impurities by 

 a process of washing in running water, whereby the ochre, in a state of 

 suspension, is drawn off into vats, where it is allowed to settle, the water 

 decanted, and the sediment made up into bricks and dried, when it is 

 ready for grinding. 



The following description of the occurrence of umber and its prep- 

 aration at the Caldbeck Fells, in Cumberland, England, is taken from 

 the Journal of the Society of Chemical Industry for October, 1890, 

 p. 953: 



The vein of umber contains crystals of quartz, and lies in a granitic rock largely 

 decomposed. The method of working is as follows: The umber is brought down by 

 an overhead tramway and passed through a hopper into a wash barrel consisting of 

 a cylinder formed of parallel bars one-eighth of an inch apart, having a perforated 

 pipe conveying water, for its axis. By this means the umber is washed through, the 

 quartz being retained; the former then passes to an edge-runner, the casing of 

 which is of sufficient depth to allow of the submersion of the rollers. The rate of 

 revolution is about 14 to the minute, and the finer floating particles flow into the 

 drag mill. The bed of this mill is a single block of granite and over it the four burr- 

 stone blocks are dragged; the finer "floating" particles of umber pass to a second 

 mill of the same kind, then through a brass wire sieve (to remove particles of peat 

 and heather that have been floating throughout the process) to settling tanks, com- 

 posed of brickwork lined with cement. After settling for four hours four-fifths of the 

 water are drawn off, and the umber, now of the consistency of slurry, filter-pressed 

 and dried. It has the following composition: 



Ferricoxide 47.14 



Manganese dioxide 11.17 



Cupric oxide 3.23 



Alumina 7. 66 



Lime Trace. 



Magnesia _ Trace. 



Silica 24.70 



Combined water 6. 18 



100.08 



In this condition it may be put on the market, serving for colouring coarse brown 

 paper (that being the chief use to which umber is put), or it may be re-ground in a 

 conical burrstone mill and sold to paint and oil-cloth manufacturers and the makers 

 of the finer kinds of brown paper. The fine state of division to which it is reduced 

 may be judged from the facts that the workman in charge of the mill is compelled 

 to wear a respirator, and the stain is not easily removed from the hands. 



At the Lehigh Gap mines the ore, as it comes from the mines, is 

 free from refuse, great care having been taken to separate slate and 

 clay from it in the working places. It is hauled in wagons to kilns, 

 which are situated on a hillside for convenience in charging. The 

 platform upon which the ore is dumped is built from the top of the 

 kiln to the side of the hill. The ore is first spalled to fist size and 

 freed from slate, and is then carried in buggies to the charging hole 

 of the kiln. 



