In the Musa Valley area rocks of the Papuan Ultramafic Belt outcrop discontinuously over an area of 25 miles by 45 miles. The rocks are part of a folded layered sequence ranging from magnesian dunite and peridotite, chemically and petrographically typical of the alpine ultramafic suite, to olivine gabbro and bytownite gabbro. Agglomerate like breccias consisting of fragments of ultramafic rock in a variable matrix occur as irregular vent-like bodies in the peridotite and dunite and as horizontal sheets in a Pleistocene-Recent sedimentary sequence. The breccias are interpreted as vent and extrusive breccias resulting from the penetration, brecciation and local entrainment (fluidization) of peridotitic country rock by volcanic gases. Olivine alkali basalt was probably the parental magma responsible for the gaseous activity. The breccias show a sequence of stability fields for the assemblages mesh-texture serpentine and bastite (i.e. chrysotile and lizardite mixtures); antigorite; olivine and enstatite; and these are correlated qualitatively with variation in intensity of the brecciation process. Chemically the brecciation process was responsible for the addition of H2O, SiO2, CO2, Al2O3 and O2 and the removal of Fe and Mg from the fragmented rock. Minerals which are of direct crystallization in the matrix fluid environment include chalcedony, fibrous serpentine (chrysotile), magnesite, quartz, andradite, and garnierite. There is no evidence of the presence of an ultramafic, serpentine magma in the genesis of the breccias.