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Removal of these constituents from the magma naturally results in marked differences in the composition of the rest magmas. Assimilation country rocks of country rocks may result in changes in the composition of the magma, but it is thought that limitations in the supply of heat necessary for such Assimilation country rocks restrict the importance of this process in producing on a major scale igneous rocks of differing compositions. Fluxes may again help this process along, and of course if the country rocks are already at a high temperature, more material can be assimilated.

The width of the contact aureole depends on the temperature and the rate of cooling of the magma, the nature and composition of the country rocks, and the fluxes originally present in the surrounding rocks or given off by the magma. In pneu-matolysis (the transformation of rocks through the action of gases or vapors liberated by the cooling of magma), the effects of boron, chlorine, and fluorine are especially noticeable, and the composition of the country rocks may be changed through their action. Since these elements are acidic, they will have strong reactions with basic contact rocks such as limestones, with which they yield skarn rocks consisting of minerals like pyroxene and garnet.

In metasomatism, by which rocks are changed through the ionic replacement of original substances by new ones from outside, much water is available inside the rocks, and again there may be significant changes in the composition of the country rocks. Thus shales and tuffs surrounding granitic batholiths may recrystallize and change in composition to resemble the Granite itself. This process may proceed in two stages, in which the rocks are first basified. This is accomplished by the outward migration and accumulation of magnesium, iron, and calcium— Mg, Fe, and Ca. Next the rocks are granitized by the accumulation of sodium, potassium, and aluminum—Na, K, and Al.