622 MININa INDUSTEY. 



the separation of the galena in a very thorough manner from not only the 

 earthy gangue, but from the lighter metallic minerals, such as the zincblende 

 and gray copper. The last two are obtained together, owing to the simi- 

 larity of their specific gravities, and they are also mingled with heavy spar 

 and some quartz. 



The general arrangement of this crushing and concentrating machinery 

 is as follows: The ore is brought upon the receiving floor, where the 

 larger pieces are broken sufficiently to admit the fragments to the crusher. 

 The clean pieces of galena and zincblende are also selected by hand, as 

 far as possible, before the material is sent to the dressing machinery. After 

 passing ''through the crushers the ore falls upon a screen, furnished with 

 a No. 6 sieve, that is, having six meshes to the lineal inch. Whatever passes 

 over this screen without falling through must be still further reduced in size 

 before going to the washing machines, and passes, therefore, from the screen 

 to a set of Cornish rollers, placed below. The material that falls through 

 the screen enters an elevator and is raised to the sizing sieve, that stands 

 above the washers. The elevator also brings the material delivered from the 

 rollers, still further reduced by them in size, to the same point. The sizing 

 sieve or screen consists of a frame about 6 feet long by 18 inches wide, 

 slightly inclined from one end to the other. The upper end of the frame is 

 fixed on a pivot, while to the lower end is attached a long arm and connect- 

 ing rod, by means of which a revolving cam raises the lower end of the frame 

 about two inches and lets it drop again upon a fixed support below. The 

 movement is rapid enough to impart a constant jigging motion to the screen 

 and thus to assist the material upon it to slide down over its surface. The 

 upper half of the screen is furnished with a No. 9 sieve, while the lower half 

 has a No. 6 sieve. The material that passes through the first goes to the 

 finer washing machines ; that which falls through the second goes to the 

 coarser machines, while all that passes entirely over is returned to the rollers 

 for finer crushing and a repetition of the process. The material then goes 

 to the ore-washers. Of these there are four double machines, or eight sieves. 

 Two of the double machines, containing four sieves, stand on a raised floor, 

 sufficiently elevated above the other two that the material delivered from the 

 outlet pipes, <?, of the first may flow to the sieves of the second. One of the 



