Iron is rarely found free, but it forms about five per cent. of the crust of the earth, and it is believed to form at least one-fifth of the unknown centre of the earth, the bulk of the globe, the weight of which we know, but concerning the substance of which we can say little that is positive. _Manganese_ is not a conspicuous element, but is found united with oxygen in purplish or black streaks on the sides of rocks. It is somewhat like iron, but much less common. _Calcium_ is the element that is the foundation of limestones. The skeletons and shells of animals are made of calcite, a common mineral formed by the uniting of carbon, oxygen, and calcium. Marbles are, perhaps, the most permanent form of the limestone rocks. "Hard" water has filtered through rocks containing calcite, and absorbed particles of this mineral. From water thus impregnated, all animal life on the earth obtains its bone-building and shell-building materials. _Carbon_ forms a large part of the tissues of plants and animals, and in the remains of these it is chiefly found in the earth's crust. When these burn or decay, the carbon remains as charcoal or escapes to the air in union with oxygen as the well known carbonic acid gas. This is one of the most important foods of plants. Joined with calcium it forms the mineral calcite, or carbonate of lime. _Hydrogen_ is one of the two gases that unite to form water. Oxygen is the other. Many kinds of rock contain a considerable amount of water. Surface water sinks into porous soils and rocks, and accumulates in pockets and veins which feed springs, and are the reserve water supply that keeps our rivers flowing, even through dry weather. More water is held by absorption in the earth's solid crust than in all the oceans and seas and great lakes. Hydrogen, combined with carbon, occurs in solid rocks where the remains of plants and animals have slowly decayed. From such processes the so-called hydrocarbons, rock oil and natural gas, have accumulated. When such decay goes on above ground, these valuable products escape into the air. Marsh gas, whose feeble flame above decaying vegetation is the will-o'-the-wisp of swamps, is an example. _Magnesium_, _potassium_, and _sodium_ are found in equal quantities in the earth's crust, but never free. In union with chlorine, each forms a soluble salt, and is thus found in water. Common salt, chloride of sodium, is the most abundant of these. Water dissolves salt out of the rocks, and carries it into the sea. Clouds that rise by the evaporation of ocean water leave the salt behind, hence the seas are becoming more and more salty, for the rivers carry salt to the oceans, which hold fast all they get.