In a permanent arrastra a layer of neatly-dressed and pointed stones is laid in hydraulic cement. A fair-sized arrastra will require 50 pounds of quartz to charge it, and the material must be broken into pigeon-egg size. After the machine has been started, and a little water added from time to time, little else need to be done for four or five hours, and this is perhaps one of the reasons for which it has always been so favored in indolent Mexico. At this stage, the quartz and ore will be very finely pulverized, and water should be added until the pulp is as thin as cream. Quicksilver must now be added in the proportion of 1-1/4 ounce for every supposed ounce of gold in the ore being treated. Two hours' further grinding is given, and water then admitted until the paste is quite thin, the speed of the arrastra being reduced at the same time so as to allow the amalgam and quicksilver to sink to the bottom. A half an hour of this treatment suffices and the thin mud is run off, leaving the gold and amalgam on the floor of the arrastra. A second charge of broken quartz is put in and the operation repeated, the clean-up not taking place oftener than every ten days, and sometimes only at intervals of a month or so. The rougher the bottom the longer the interval between clean-ups, as all the stone work must be taken up each time and all the sand and mud between them must be washed carefully. The arrastra is extremely valuable to the poor man who, having discovered a gold-bearing vein, wishes to transfer some of the metal into his own pocket, at the least possible outlay. Its cheapness places it within reach of all, while a stamp will cost a good deal. Then again the amalgamation being more perfect in the arrastra than in any other mill, it is particularly suited for the poor, lean ores. It is, however, only adapted to those that are free-milling, others not being suited to this form of apparatus, nor, indeed, to any save very costly plants. Some arrastras have been built to treat old tailings, and have paid well when water power could be used. Free-milling gold and high-grade silver and gold ores are those usually treated.