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Metamorphism is a process within the Earth that changes one rock type into another rock type by submitting it to changes of temperature and pressure. These rock types may be igneous, sedimentary, or older types of metamorphic rock. The term "metamorphic grade" may refer to the type of rock produced in regard to crystal size and orientation. Textures of metamorphic rock fall into foliated and unfoliated categories. The foliated rocks, the one you asked about, are rocks where the axes of the crystals have rotated during the metamorphic process and are now perpendicular to the orientation of the shortening of the rock bands. Foliated textures generally cleave along clean even lines to reveal their crystal layers, while nonfoliated rocks do not. Slate is an example of a finely grained metamorphic rock, while phyllite is an example with medium-sized grains. Gneiss is an example of a coarse-grained metamorphic rock. Marble is an example of a nonfoliated metamorphic rock; it has no cleavage, so it allows itself for sculpture and architecture. So, in terms of high to low grades of metamorphism, slate, phyllite, gneiss, and marble, with marble representing the lowest degree of foliation.
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