ln many parts of the world it has be en shown that granite and allied rocks have formed by replaccment of pre-existing rocks. Indeed, hardly any geologist today will doubt the importance of metasomatic processes in petrogenesis. However, the physical naturc of the active agents and the mechanism of the process are still much debated.
It has been proposed that the transfer of the chemical material was effected for example as mineral molecules or as oxides by a magma, a pore liquid, a gas, or by a hydrothermal solution. More recently various autihors have maintained that free atoms, loosened from the crystalline lattice, are reponsible for the transfer.