<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> <P> Metasomatism is the drastic change in the bulk chemical composition of a rock that often occurs during the processes of metamorphism . It is due to the introduction of chemicals from other surrounding rocks . Water may transport these chemicals rapidly over great distances . Because of the role played by water, metamorphic rocks generally contain many elements absent from the original rock, and lack some that originally were present . Still, the introduction of new chemicals is not necessary for recrystallization to occur . </P> <P> Contact metamorphism is the name given to the changes that take place when magma is injected into the surrounding solid rock (country rock). The changes that occur are greatest wherever the magma comes into contact with the rock because the temperatures are highest at this boundary and decrease with distance from it . Around the igneous rock that forms from the cooling magma is a metamorphosed zone called a contact metamorphism aureole . Aureoles may show all degrees of metamorphism from the contact area to unmetamorphosed (unchanged) country rock some distance away . The formation of important ore minerals may occur by the process of metasomatism at or near the contact zone . </P>

Metamorphic rocks which are formed from shale and lime stone