The PLACER DEPOSITS of California and other regions are gold- bearing deposits of gravel and sand in river beds. The heavy gold is apt to be found mostly near or upon the solid rock, and its grains, like those of the sand, are always rounded. How the gold came in the placers we may leave the pupil to suggest. Copper is found in a number of ores, and also in the native metal. Below the zone of surface changes the ore of a copper vein is often a double sulphide of iron and copper called CHALCOPYRITE, a mineral softer than pyrite--it can easily be scratched with a knife--and deeper yellow in color. For several score of feet below the ground the vein may consist of rusty quartz from which the metallic ores have been dissolved; but at the base of the zone of solution we may find exceedingly rich deposits of copper ores,-- copper sulphides, red and black copper oxides, and green and blue copper carbonates, which have clearly been brought down in solution from the leached upper portion of the vein. ORIGIN OF MINERAL VEINS. Both vein stones and ores have been deposited slowly from solution in water, much as crystals of salt are deposited on the sides of a jar of saturated brine. In our study of underground water we learned that it is everywhere circulating through the permeable rocks of the crust, descending to profound depths under the action of gravity and again driven to the surface by hydrostatic pressure. Now fissures, wherever they occur, form the trunk channels of the underground circulation. Water descends from the surface along these rifts; it moves laterally from either side to the fissure plane, just as ground water seeps through the surrounding rocks from every direction to a well; and it ascends through these natural water ways as in an artesian well, whenever they intersect an aquifer in which water is under hydrostatic pressure. The waters which deposit vein stones and ores are commonly hot, and in many cases they have derived their heat from intrusions of igneous rock still uncooled within the crust. The solvent power of the water is thus greatly increased, and it takes up into solution various substances from the igneous and sedimentary rocks which it traverses. For various reasons these substances stances are deposited in the vein as ores and vein stones. On rising through the fissure the water cools and loses pressure, and its capacity to hold minerals in solution is therefore lessened. Besides, as different currents meet in the fissure, some ascending, some descending, and some coming in from the sides, the chemical reaction of these various weak solutions upon one another and upon the walls of the vein precipitates the minerals of vein stuffs and ores. As an illustration of the method of vein deposits we may cite the case of a wooden box pipe used in the Comstock mines, Nevada, to carry the hot water of the mine from one level to another, which in ten years was lined with calcium carbonate more than half an inch thick.