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.
Seams containing native sulphur often show no trace of that element on the surface, having weathered into a soft, white, gray or yellowish-white granular, or pulverulent, variety of gypsum.
Veins of asbestos often decompose into a white powder found in the crevices of the rocks; fibrous asbestos existing in the interior.
Petroleum shows in an iridescent film upon still pools, and the odor is a sure guide to its nature.
A "dipping-needle" is valuable to the prospector on the lookout for iron ore; by its use he may discover masses of magnetic ore and trace their extent. As he carries the compass over the ground the needle dips toward any iron mass he approaches; directly over the ore it becomes vertical.
MINER'S DIPPING NEEDLE.
In a wilderness country strength of body and endurance are important qualifications. The prospector must, moreover, have such general knowledge of geology and mineralogy as to be able to recognize all valuable minerals and confirm his conjecture by simple tests. Pick, shovel and pan must be handled skillfully, while the rifle, shotgun and paddle must also be understood. For in the unsettled parts of the country the traveler must even yet rely to some extent upon the fish and game he may be able to secure, and every old prospector becomes a trained hunter and camper. Knowing how to bake bread is sometimes more valuable than much mathematics; ability to build a rough boat is often the one hope of salvation.