Page:South African Geology - Schwarz - 1912.djvu/60

 of rocks composing the earth's crust: the sedimentary rocks, the metamorphic, and the igneous. The igneous rocks may be called the massive rocks, because they have hardened and crystallized in a continuous mass from a molten condition. The sedimentary rocks are called clastic or derivative, because they have been derived from other rocks which have been broken (clao in Greek means to break) by the processes of weathering. The metamorphic rocks are those which have undergone alteration by changes due to heat, pressure, and solvent water as they lie buried deep in the earth's crust. A metamorphic rock can therefore only appear on the surface by the removal of a great deal of rock that once covered it. Taking granite as the most typical igneous rock, we can show the relationship of the various types of rocks by the following diagrammatic scheme:

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