Very few gold-bearing lodes contain nothing but free gold; on the contrary, they carry the bulk of their values in the form of sulphurets, having more or less gold incorporated, and even when the gold is native and free-milling at the surface, it is generally changed into sulphurets as depth is gained. So the miner has to consider methods of recovery more complicated and expensive than simple amalgamation with mercury, for upon gold included in pyrites mercury has no effect. Titanic iron, hematite, and tungstate of iron often hold gold, or soft clay ores carry it in their midst, and such combinations tax all the skill of the mining engineer merely to save a respectable percentage of the assay value. Sometimes chlorination and sometimes cyanization are the measures tried, but supposing the preliminary treatment to have been by stamps in the battery, concentrating is one of the main reliances of the mill man. The blanket table is undoubtedly the oldest type of concentrating machine, but it is very inferior to modern inventions. Percussion tables often do good work. In this system a sharp and frequently repeated blow is given the table, in such fashion as to make the heavy material separate from the light. "Shaking" and "rocking" tables are favored in some mills, and they give better results on fine gold than any of the previously mentioned devices. But the best machine so far invented is the Frue Vanner--an endless rubber band drawn over an inclined table, having both revolving and side motions. The lighter particles are carried off by water, and the heavier collected in a trough. [Illustration: FRUE VANNER.] 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. [Illustration: 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.