The intrusion of the Cassia batholith in Idaho was accompanied by clearly denned contact phenomena expressed by the impregnation of "igneous" material into quartzite, and in such a manner as to make it resemble granitic rock. While the magma crystallized as granodiorite, it sent forth emanations into the invaded rock and by progressive replacements produced therein a broad zone of porphyritic granite. As a result of the progressive granitization of the quartzites in the contact zone, the batholith is now about three times as large as it would have been had "replacement" not been a factor in its intrusion. It is altogether likely that the changes noted in the contact zone of the Cassia batholith may also be expressed in the crystalline schists in extensive metamorphic regions. The replacement origin of the microcline phenocrysts in the hybrid porphyritic granite also directs attention to the probability that the microcline phenocrysts in normal granitic rocks also may have been formed late by replacement.