Abstract Harzburgitic dunite mylonite, with lesser blastomylonite, olivine schist, talcose ultramafic schist, and pegmatite, form a vertical septum 400-3000 ft wide, at least 14 miles long, in steeply folded and mylonitised gneiss on the Fiordland coast near Milford Sound. The mylonites are foliaceous and porphyroclastic, the schists xenoblastic granulitic to schistose. Contacts with adjoining gneiss are faults; that at Poison Bay is the site of a hydrogen-methane gas seep probably generated by serpentinisation reactions. Associated Lower Paleozoic rocks are medium to moderately high grade (kyanite zone) gneisses of the amphibolite facies of metamorphism, now extensively mylonitised. Mapped as Thurso, Saint Anne, and Milford Formations, they formerly consisted of a basic igneous assemblage (gabbro, pyroxenite, etc.) probably associated with the ultramafites, as well as greywacke, tuffaceous sandstone, mudstone, and limestone. Minor basic dikes invaded the gneisses at a late stage of the Upper Paleozoic re...