8 The Patio and Cazo Process. 



stone, 1.65 m. in diameter and 0.38 m. wide. It is bound to- 

 gether with an iron tire 0.10 m. thick. It rotates on a hori- 

 zontal shaft attached to a vertical one. The other end of the 

 shaft projects so that a mule can be harnessed to it. The wheel 

 runs in a circular space made of iron, which is 0.50 m. wide, 

 on the inside of which there is a screen of five or six meshes to 

 the inch. When there are two wheels, the axis is generally 

 about three meters high, and turns on pivots — one fixed in a 

 step raised above the bottom of the grinding-space, and the 

 other held by a frame above. The arm on which the wheels re- 

 volve is fixed to this axis, and the power is communicated by an 

 arm fixed above on the axis. This arm may be single for one 

 mule, or may project on both sides: in which case yokes are at- 

 tached so that a mule can be harnessed at each end. The num- 

 ber of these mills depends on the amount of work to be done. 

 When the amount is small, mules are always used; but when it 

 is large, water pow r er or steam is the motor. 



In some of the w r orks both the Chilian mill and the stamps 

 h«ve been abandoned for a series of crushers, Quehraderos, or for 

 i a single one. In this way, by a machine readily managed and 

 repa^ed, a much larger amount of material can be prepared for 

 the arras tra than by either of the other machines. 



2. Grinding and Amalgamating the Ore 

 in the arrastra. 



The crushed ore goes from the stamps or Chilian mill to the ar- 

 rastra, which is a very important part of the process, as the yield 

 of the ore depends very largely on the work which is done in it. 

 Its action is very slow, but no machine yet invented can 

 compete with it in the efficiency of its work. The arras- 

 tra is generally circular and somewhat below the level of the 

 ground. It is from 3 to 4 meters in diameter. The bottom 

 is sometimes made of the hardest boulders that can be found in 

 the country, bedded in clay with their smooth sides turned up 

 and ground to something like even surfaces before the operation 

 begins. This is' a bad construction, as the open places between 



