<P> The layering within metamorphic rocks is called foliation (derived from the Latin word folia, meaning "leaves"), and it occurs when a rock is being shortened along one axis during recrystallization . This causes the platy or elongated crystals of minerals, such as mica and chlorite, to become rotated such that their long axes are perpendicular to the orientation of shortening . This results in a banded, or foliated rock, with the bands showing the colors of the minerals that formed them . </P> <P> Textures are separated into foliated and non-foliated categories . Foliated rock is a product of differential stress that deforms the rock in one plane, sometimes creating a plane of cleavage . For example, slate is a foliated metamorphic rock, originating from shale . Non-foliated rock does not have planar patterns of strain . </P> <P> Rocks that were subjected to uniform pressure from all sides, or those that lack minerals with distinctive growth habits, will not be foliated . Where a rock has been subject to differential stress, the type of foliation that develops depends on the metamorphic grade . For instance, starting with a mudstone, the following sequence develops with increasing temperature: slate is a very fine - grained, foliated metamorphic rock, characteristic of very low grade metamorphism, while phyllite is fine - grained and found in areas of low grade metamorphism, schist is medium to coarse - grained and found in areas of medium grade metamorphism, and gneiss coarse to very coarse - grained, found in areas of high - grade metamorphism . Marble is generally not foliated, which allows its use as a material for sculpture and architecture . </P> <P> Another important mechanism of metamorphism is that of chemical reactions that occur between minerals without them melting . In the process atoms are exchanged between the minerals, and thus new minerals are formed . Many complex high - temperature reactions may take place, and each mineral assemblage produced provides us with a clue as to the temperatures and pressures at the time of metamorphism . </P>

Where is the earth's greatest bulk composition located