Geophysical Laboratory work is especially applicable to quenched rocks, but does not lend itself to the solution of problems connected with the interpretation of structural relationships between the component minerals of deep-seated rocks. The detailed examination of a rock of simple mineral composition might be expected to yield a picture of the way in which phase-boundary adjustments are made in a rock crystallizing under such conditions. Some of the Bushveld basic rocks are of such a nature, and in the present paper an attempt is made to analyse and interpret the structural relationships between the two minerals bronzite and bytownite (An78-80), of which they are composed. The material studied was collected under the guidance of Dr. A. L. Hall during the International Geological Congress in 1929, supplemented by core specimens recently made available through the courtesy of an old student of the College.