Many granites pass by insensible gradations into schist, quartzite, or other metamorphic rocks. There have been numerous suggestions that the granite at such places resulted from metasomatic replacement of older rocks, but it is hard to find a really conclusive large example that has been described in detail. Several well-known papers are here given critical comment.
The problem is difficult for many reasons, and reconnaissance no longer suffices as a basis for the suggestion of large-scale metasomatic origin. It is hoped that one or more of the promising districts may be given very thorough study and careful discussion to provide a type to which others may be compared. The features of a good report to establish such a type are described in some detail. Until such a type district is found, considerable caution should be exercised by those who suggest metasomatic origin of igneous-looking rocks.