The Patio and Cazo Process. 9 



the stones would seem likely to produce a large loss of both 

 mercury and amalgam. It is surprising, however, that with 

 sueh a rude construction the loss of mercury is not very much 

 larger than in the better-constructed ones. This is owing to the 

 great e kill which the men have acquired, not only in working, 

 but in picking out the mercury and amalgam from the cracks, and 

 refilling with slimes. Such an arrastra will have to be run the 

 longest time possible, fifteen or twenty days, before a clean-up is 

 made. It will then generally be found expedient to remove the 

 tailings and work up all the material in the interstice?. A properly 

 constructed arrastra can, however, be cleaned up every few days 

 without disturbing the pavement. It is generally built of paving- 

 stones or slabs of quartzose porphyry. In the best works, the 

 edges of these stones are carefully dressed and they are jmt 

 together with cement, or when that cannot be had, with the very 

 fine tails which result from washing up the torta. These 

 stones are 0.75 m. in length. They are placed vertically. 

 When put in with care, the bottom will last for twelve months. 

 It will then be necessary to clean out all the cracks and repair 

 it, taking up the stones, carefully scraping them, and w? g 

 the dirt upon them and that beneath them, to rect any 

 mercury or amalgam that may have penetrated into the ground. 

 The sides are made generally of flat stones forming a rough 

 curbing 0.60 m. high, which projects enough to make the interior 

 about 0.60 m. deep. In the centre of the arrastra, raised above 

 the bottom, is a pivot hole for the central shaft, which carries four 

 arms, and is supported above and below. To each of these arms 

 one and sometimes two stones are attached, which act as mullers, 

 voladoras, to grind the ore. They are made of quartzose por- 

 phyry, which must have an open grain so as to present a good 

 grinding surface until it i? entirely worn out. A close-grained 

 stone would become smooth after a little wear, and would then 

 be no longer serviceable. They are usually, when there is only 

 one to each arm, a little smaller than the half diameter of 

 the arrastra and about 0.40 m. thick. Two holes are drilled 

 in each one ; into these, wooden plugs are driven to re- 

 ceive staples, by which they are fastened to the arms by means of 

 thongs, leather, or chains, in such a way that their front 

 edges will be about 0.05 m. above the bottom, while the rear 



