Placer Gold Placer deposits of gold are formed by the concentration of the gold from the debris of gold veins and other deposits of gold in rock. As the loose material is washed down hillsides into gullies and valleys, the gold, being six or seven times as heavy as quartz and other rock- minerals, quickly … Read more
The complete study of faulting belongs to mining rather than to prospecting. But there are surface results of importance to the prospector. As explained, faulting is due to the breaking of the rock under strain; there has been displacement at the same time, so that the two sides of the break do not fit. If the … Read more
Mechanical Deposition of Sedimentary Mineral Deposits In some cases, the ore mineral, on account of its resistance to weathering and its hardness, survived the destruction of many of the rock minerals; and, during sedimentation, on account of its weight, it was sorted out from the fragments of quartz and other resistant minerals. Thus, the ore-mineral was deposited, … Read more
Vein Mineral Deposits includes a considerable variety of ore deposits formed under various conditions by the concentration of the ore from the molten rock in process of cooling, and its subsequent deposition in fissures and joints, crevices, and pores; or the accompanying solvents make room for the ore by dissolving the rock and carrying it … Read more
An ore is defined as a mineral or rock from which some valuable constituent, usually a metal, can be profitably extracted. Thus hematite and magnetite are ores of iron; galena is an ore of lead; zinc blende is an ore of zinc. With the ore-mineral is more or less material of no value, called gangue; … Read more
Rocks that have been formed from other rocks, sedimentary and igneous, by internal changes caused by pressure, heat, and the chemical action of the solutions and hot gases, are called metamorphic rocks and here is a list: Crystalline Limestone Crystalline Limestone is a rock that results from the crystallization of the calcite in sedimentary limestone. … Read more
Porphyry. — This name refers to the structure, which is formed of larger crystals set in a fine-grained or glassy ground-mass. The porphyritic structure may be plainly visible, the larger crystals being easily seen (phenocrysts), or it may require the microscope to bring it out. Porphyritic structure may appear in any igneous rock, but the … Read more
We can list volcanic rocks that are formed at or near the surface by comparatively quick cooling of lava, cannot be classified without the aid of the microscope and chemical analysis, except roughly as light-colored and dark lavas, conveniently called rhyolite, and basalt or trap. Rhyolite Rhyolite is made of light-colored acid lava flows, often called … Read more
It is strongly recommended that these descriptions and examples of plutonic rocks be read with the specimens in hand, obtainable, as noted before, from the Geological Survey. Granite Granite is a coarse or medium- grained rock, composed of quartz, an acid feldspar or two of them (orthoclase, microcline, albite), and generally mica (biotite, or muscovite, or … Read more
Volcanoes A volcano is the most striking evidence of a hot interior. There are about 250 active volcanoes at present, and about as many more that have been active in historic times. The cone shape of a volcano is due to its gradual building up by the ejection of material from the center hole, the … Read more