Although ortho-amphibolites can often be distinguished chemically from para-amphi-bolites by their higher contents of Cr, Ni, and Ti, and lower Niggli k ratios, no criterion based on abundance levels is generally applicable because many basic igneous rocks have low Cr, Ni, and Ti contents, and alkali metasomatism in metamorphic terrains often disturbs the k ratio. Of much more value is the distinction between igneous and sedimentary trends of variation. This requires enough analyses to establish a reliable trend. Some critical plots are suggested, and the Karroo dolerites from South Africa and the Connemara striped amphi-bolites from western Ireland are compared with amphibolites from Langøy (N. Norway), Bakersville-Roan area (N. Carolina, U.S.A.), and NW. Queensland (Australia). The plots confirm the origin of the ortho-amphibolites from these regions but some of the supposed para-amphibolites may be ortho-amphibolites and more analyses are needed to define reliable trends of variation.