The writer has considered that magma has been re-heated in the course of ascending vent. Comparing the chemical composition of plutonic rocks with that of volcanic rocks, both are nearly equal in the amout of SiO2 and total iron and FeO/Fe2O3 is larger in plutonic rocks than volcanic rocks. H2, CH4, H2S and others detected in volcanic gas are considered as derived from magma, and consequently deep seated magma may be in reduced condition. When magma ascends into the oxidation zone of the earth crust, it contacts with air or water preserving high temperature. That causes exothermic reaction of FeO→Fe2O3.
Hornblende which contains volatile substances is remarkably affected by the re-heating, and this fact has been already noticed, green hornblen-de→brown hornblende→oxyhornblende→opacite. These phenomena take place under existence of oxygen. Green hornblende is found in glassy rock and pumice.
Brown hornblende and oxyhornblende are found in crystalline rock, and often opacitized along margin or on the whole. Phenocrystic pyroxene shows simillar phenomena to hornblende, though not so conspicuous. In this case, augite generally is more easily affected than hypersthene. Reaction rm of the pyroxene is found in the lava flow belonging to pigeonitic rock series, but not in the dyke of the same series. Comparing chemical composition of monoclinic pyroxenes in plutonic rocks with that in volcanic rocks, the ratio of FeO/Fe2O3 of the former is larger than that of the latter. It may be considered that corrosion of quartz, inversion into tridymite, opacitization of biotite and crystallization of minerals in groundmass are caused by re-heating.
