How the Earth was Formed
Mining of Metals
Quartz and the Silicates
Mineral GuideIt is well known that the intensely heated interior of the earth is subjected to enormous pressure caused by the weight of the overlying rocks. With cooling comes the slow shrinking and contraction of the crust or outer portions. Under such conditions of heat and pressure, many substances are molten or fluid, but become hard if they reach the surface or otherwise become cool.
An example of these is lava, which is poured in a more or less fluid condition from the craters of active volcanoes. While in the fluid condition there is a strong tendency for the ingredients of rock to gather together into masses of varying size, and these, upon cooling, form crystals. If the cooling be slow, the crystals have ample time for formation, and will, therefore, be large. By "slow" is meant a very long period of time-perhaps a thousand years.
Examples of such formations are the diamond and the garnet, the pictures of both of which show both the central crystal and the surrounding material, called matrix. The colors of these gems are due to various ingredients such as iron, manganese, cobalt, etc. The white diamond is practically pure carbon.
It often happens, however, that when rocks solidify, cavities or pockets are formed, perhaps from gas bubbles, and into these there later penetrates water which is on its way upward to the surface from the great depths below. When under the influence of the intense heat and the enormous pressure of the interior, water will directly dissolve certain substances which ordinarily it would not, or it may dissolve certain minor substances, thus forming strong acids or alkalies, which further dissolve the most refractory materials.
Through cracks, crevices, or sometimes open vents, this water, with its load of dissolved materials, slowly percolates, finally rising toward the surface. As the pressure and heat diminish the materials which cannot be carried in solution are deposited along the sides of the passageway or around the walls, and in cavities into which the water has penetrated. The crevices become filled and other channels may be opened at other places. In any event, the result is the formation of a mineral vein or a nodule, the characteristic structure of both of which is well illustrated in a number of the plates. The agate was probably made during a long period of time and the successive layers, being composed of unlike substances, formed bands of dissimilar colors. Such, in a very general way, is the story of the formation of a vein of gold-bearing quartz, of lead, of silver, and other materials, and that of the innumerable agates and carnelians.