ASSIMILATION in petrography (from the Latin word "assimilatio" (assimilation, merging) * EN: assimilation, magmatic digestion, magmatic dissolution; DE: Assimilierung, Assimilation; FR: assimilation; ES: asimilacion, digestion magmatica; RU: ассимиляция) is the process of capturing, remelting, and subsequent complete digestion of the substance of the country (lateral) rocks by the intruding magma, without the preservation of the relics of the rocks, which have been devoured.

Assimilation is one of the forms of manifestation of the reactions of interaction of the magma with country rocks, during the process of introduction of the superheated magma into the country rocks, the latter ones are cracking themselves under the influence of heating and uneven expansion. The rocks of the lateral walls and roof, which have been mechanically crushed and trapped by the magma, are sometimes preserved in the form of the xenoliths, which are sharply differing in composition from the magma, which has captured them. During the heating to the temperatures, which are near to the temperature of the magma, as the result of the reactions with the magmatic melt, the composition of the xenoliths becomes similar to the composition of the magma. With significant quantity of the xenoliths, the composition of the magmatic melt, which is digesting them, also changes.