Petrographers in recent years have tended to place increasing emphasis upon the importance of replacement processes in igneous rocks. The movement has received impetus from the study of pegmatites, especially by Brögger and Fersman in Europe and by Hess, Landes and Schaller in the United States. These authors have presented convincing evidence for a replacement sequence in the development of these bodies. Other investigators have described instances in which large masses of plutonic rocks appear to have undergone important modifications in chemical and mineral-ogical composition since their consolidation.

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