Veins, lodes, or ledges, may be found in stratified or unstratified rocks, and in the former they generally cut the beds at an angle. Veins are bounded by walls. The rock in which a vein is found is a country rock. Smooth walls are called "slickensides." The upper wall of an inclined vein is the hanging wall; the other the foot wall. A layer of clay between the veins and wall is a selvage. A mass of rock enclosed in the vein is a horse. The vein stone, or gangue, is all that part of a vein that is not mineral.
A FAULT.
The throw of a fault in a vein is measured by the amount of vertical displacement. When the miner comes to a fault, he should follow the greater angle in his attempt to recover the lode. For instance, on mining along A B to the line of fault X Y, the exploration will be continued downward, because the angle A B Y is greater than the angle A B X.
Mercury that has been "sickened," that is to say, has lost its brightness and power of amalgamating, may often be cured by washing with an extremely weak solution of sulphuric acid and adding a little zinc.
As regards the comparative merits of chlorination and cyanization, it may be said the one is the equal of the other. Under certain conditions, chlorine gives a higher percentage of gold; under others the same may be said of cyanide. A description of either process would be out of place, however, in a simple elementary work.
Handed down through the centuries, the primitive arrastra is still useful in certain contingencies. It is like a cider mill in its principle, and was probably suggested by recollections of that machine, or else of the Spanish wine-press. A circular, shallow pit, a dozen feet or more in diameter, is first paved with hard, uncut stones of granite, basalt, or other hard rock. This pavement is a foot thick, and beneath it is a bed of puddled clay 6 inches deep. A vertical shaft with an arm, or arms, revolves in the center of the arrastra. Grinding blocks weighing 400, or perhaps even 1,000 pounds, are fastened to the arms by chains or rawhide strips. The forward part of each stone is raised a couple of inches off the floor. Mule, horse, water or steam power may be used, the speed ranging from 4 to 18 turns a minute.
Nothing can be simpler, less expensive, or save a greater proportion of the value in the ore than the arrastra. Its limited capacity is its worst fault. An arrastra 10 feet in diameter will treat 500 or 600 pounds of ore at a charge, and handle one ton a day of 24 hours. Ores that were so poor they yielded nothing to the stamp mill have paid well with the arrastra.
This humble device may be used to advantage, probably in some of the poorer gold-bearing cemented gravels of the Northwest. The ore should be crushed to pigeon-egg size. Small quantities of mercury, about a tablespoonful to every five tons of gravel, has been found a satisfactory proportion in California.
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 1¼ 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.