In the eastern part of the Maizuru zone, southwest Japan, ultramafic masses are classified into two types; type I refers to those arranged along the northern side of the Maizuru zone and type II refers to those occurring in the Yakuno complex inside the Maizuru zone. Petrographical differences between the two can be recognized. In type I ultramafic masses, dunite is most dominant accompanying harzburgite, clinopyroxenite, hornblendite and albitite. In type II ultramafic masses, peridotitic rocks are dominant over dunite and characteristic spinels are universally found. Type II ultramafic masses are subdivided into three subtypes; that is H-, W- and G-subtypes. H-subtype is composed of dunite, harzburgite, orthopyroxenite and clinopyroxenite, among which harzburgite is most dominant. W-subtype is composed of dunite, wehrlite, olivine clinopyroxenite and clinopyroxenite, and olivine clinopyroxenite is most dominant in them. G-subtype is composed of hornblende clinopyroxenite and clinopyroxene hornblendite. The modal composition of clinopyroxene and hornblende in G-subtype is variable. The occurrence of each subtype is characteristic. H-subtype occurs at the boundary of the Maizuru zone and the Tanba zone, and W-subtype occurs in the gabbroic rocks of the Yakuno complex but is not transitional to them. G-subtype occurs as a layer in the gabbroic rocks of the Yakuno complex from a centimeter to 5 m thick and is transitional to the latter. It is suggested that type I ultramafic masses have something to do with the Sangun metamorphism, and type II ultramafic masses have a relation to the gabbroic rocks of the Yakuno complex, of which, H-subtype has a relation to the formation of the zonal arrangement of the Maizuru zone
