Deep mining may be divided into drifting and hydraulic mining. In the former the metal is won by means of tunnels and drifts or horizontal passageways along the length of the deposit. It is usually resorted to in districts where a flow of lava has covered the gold-bearing gravel, and made hydraulic mining impossible. It is followed in Alaska for another reason, viz., because the constantly frozen ground will not permit of the more remunerative method. The gravel is carried to the mouth of the tunnel and there dumped to be washed in the sluices. When "cemented" it must be broken up by stamps. Rich deep placers may be worked by drifting, but whenever practicable hydraulicing is to be preferred as giving better results. It yields from four to six times the amount of gold that drifting does. Thorough exploration should precede the expenditure of large sums in a hydraulic plant. Even should the explorations result in finding barren gravels the money will have been well spent in saving the cost of an unproductive plant. Black sand (magnetic iron) almost always accompanies gold, but this alone is no sign that gold is present, as black sand may usually be obtained by grinding and washing crystalline rocks. Ditches and flumes of wood or metal are used to bring the water for hydraulic mining from the region where it was impounded in a catch basin, often a distance of many miles. It is said $100,000,000 have been invested in ditches and flumes, mining and agricultural, in the western states, and new flumes are being planned every month. Some of them consist of wrought iron pipe carried over ravines by trestles 250 feet high. In planning a ditch the miner must see to it that his water supply is at a sufficient elevation to command the ground. The more pressure the water works under the better. The supply should be continuous, or at least be available during the whole working season. Ditches in regions of deep snow should have a southern exposure. All streams crossed by the ditch should be diverted into it, to counteract leakage and other loss. Waste gates must be provided every half mile. Ditches are better than flumes. Narrow, deep, and steep ditches are to be preferred in mountainous regions, and the reverse in valleys with soft soil. Some Californian ditches with a capacity of 80 cubic feet per second and grades of 16 to 20 feet per mile have been built. [Illustration: SECTION OF DITCH.] [Illustration: SECTION OF FLUME.]