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This is the place for discussions of basic geology and how it has affected the land of the west. Diving right in, there are three basic classifications of rocks which are Igneous, Sedimentary, and Metamorphic.
Igneous rocks: These are the rocks that are formed when magma cools and forms solid rock. By definition, heat is required to melt the rock to form the magma. Igneous rocks are the most common type of rocks that are found on the earth and make up some 80% of the continents. One of the larger areas in the west of Igneous rocks is found in the Pacific Northwest on the Columbia Plateau. Both the Sierra Nevada Mountains of California and the Cascade Mountains of the Pacific Northwest are mostly made up of Igneous rocks.
There are two basic types of Igneous rocks, Pegmatites and Porphyritic rocks. The two types are determined by where the magma came from and where it cooled. Pegmatites contain large crystals and here is where many gemstones are found. Porphyritic rocks have smaller crystals such as Quartz, Feldspars, Hornblende, and Olivine.
Sedimentary rocks: Sedimentary rocks are those rocks that have been formed by the erosion and redeposition of other rocks. Some 70% of the continents are covered by Sedimentary rocks (do not be confused by the makeup of the continents which is mostly Igneous rock). The erosion and redeposition of the rocks form beds sometimes several miles thick over the underlying rocks. This action is by rivers, snow (glaciers), and by the wind (think of that the next time you are caught in a wind storm in the desert). There are many classifications of Sedimentary rocks including Conglomerates, Sandstone, Shale, Rock salts, Limestone, Chalk, and Bituminous coal.
Metamorphic rock: This is rock that has been essentially remade by heat, pressure, and some chemical reaction until it has recrystallized into another form. Some of the more common types of Metamorphic rocks are Quartzite, Marble, Slate, Schist. There are several terms associated with metamorphism. Of particular importance to mining is Contact Metamorphism. See Metamorphism.
Superposition: This rule states that the of the Sedimentary rocks layers making up the earth's surface, the oldest ones will be found at the bottom. Except in rare cases where the layers were overturned by tectonic activity (earthquakes and folding), this rule holds true.
Original Horizontality: This is another rule that comes from common sense. It states that Sedimentary rocks were originally formed as horizontal layers due to the forces of gravity during the deposition. Makes sense, but again, tilting and folding can change the original layers.
Cross-Cutting Relationships: Again, common sense rules, it states that a fault or an intrusive body (such as a quartz seam or volcanic dike) is younger than the host rock. In other words, the host rock had to be there in the first place for the intrusion to have something to intrude!
Faunal Succession: The use of fossils to date a particular group of rocks requires the assumption that fossils become more complex over time and that evolution of species does not repeat itself (except in the Jurassic Park) throughout the ages.
| Geologic Time Scale Chart | Faults and Faulting | Tertiary Gold Deposits |
| Placer Mining Page 1 | Metamorphism | |
| Trilobite Fossils, CA | Buffalo Canyon Fossils, NV |
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