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. [Illustration: 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. In sinking a short shaft in a sunny country a large mirror, inclined at a suitable angle over the shaft, will give sufficient light. Lodes or veins following the general trend of the auriferous quartz are much more likely to be rich than are those that cross it. Gold is never distributed evenly in veins, though it may be in great beds of low grade material; but more often rich areas alternate with barren portions. Where quartz veins are small and the rich pockets separated by wide intervals of poor gangue the gravel of the district will usually be similar in character. As this condition obtains in the upper Yukon district as far as the gravels are concerned, it will probably be found to hold good for the quartz leads, when they shall have been discovered. The more nearly the gold formation approaches to the crystalline schists, the poorer will the quality of the gold be through the larger percentage of silver found in it. In slates the proportion may be 22 gold to 1 silver; in schists it has been known to be a ratio of 1 to 1.