Certain minerals are likely to be found associated. Cassiterite goes with boron and tourmaline, topaz, fluor spar and lithia-mica; all containing fluorine. It is also found with wolfram, chlorite and arsenical pyrites. Magnetite is often accompanied by rocks containing garnet, epidote and hornblende. Zinc blend and galena may occupy the same vein, which is likely to be of baryta or heavy spar. Much galena carries silver. Gold is associated with many metallic sulphides such as iron, magnetic, and copper pyrites, mispickel, galena, blend, stibnite and tetrahedrite. Gypsum accompanies salt. Surface indications may be described as: Form of ground, color, outcrop, decomposed and detached mineral, mineral deposits from springs, altered or peculiar vegetation and other similar guides. A hard quartz outcrop often stands up like a wall and is traceable for miles. The Rainbow silver bearing lode of Butte, Montana, stood 20 feet above the surface. Soft minerals, such as clay, are cut into and sunk below the surrounding level. Deposits of Kaolin or China clay are usually so found. Any special bright coloration of the rocks of a district merits investigation. Copper gives green, blue, and red stains; iron, red or brown; manganese, black; lead, green, yellow or white; cobalt, pink; cinnabar or quicksilver, vermilion. The nickel deposits of New Caledonia were made known to the world by the explorer Garnier in 1863, his curiosity having been aroused by the delicate green coating given the rocks by an ore containing water, quartz, nickel and magnesium. Hard beds of shale decompose on the surface into soft clay, and a still more noticeable change is the conversion of ores containing sulphur into oxides. This chemical change causes the gossan or "iron hat," for which token of underlying wealth the prospector should be eternally watchful. This alteration may extend downward four or five hundred feet from the surface, but in such cases the true weathering has ceased long before the limit of discoloration is reached, and the change of substance is due to the filtering of surface waters through the vein. Gossan varies greatly in its nature. Galena becomes anglesite, cerussite, pyromorphite and mimetite. Copper pyrite changes into native copper, melaconite, cuprite, malachite, chessylite, or perhaps into a phosphate, arsenate, or silicate of the metal. Carbonate of manganese gives the black oxides and silver sulphide ores are, after weathering, known as native silver, kerargyrite and embolite. The ore in the gossan is very generally more valuable than it will be below, and this is especially true of gold and silver ores. The gold having been set free from the close embrace in which the iron pyrite held it previous to the latter's oxidation, it is now readily caught by quicksilver. Silver under similar conditions becomes chloride, and likewise amalgamates without difficulty.