Six mineral assemblages have been defined within a small portion of a layered two-pyroxene amphibolite. The relationships between bulk composition, mineral composition, and mineral assemblage were determined from X-ray fluorescence, electron microprobe, and optical data. It is shown that the layer assemblages and the mineral compositions are, in general, directly related to the bulk compositions of the layers. Graphical representation of the phase assemblages is impossible, due to the number of determining components; however, an attempt is made to tabulate the phase relation-ships in 9-space. Linear regression analysis has been used to show that the six mineral assemblages owe their differences purely to different bulk compositions, and that no isochemical reactions can be written between them. The computed equations relating the mineral assemblages indicate that water was immobile during metamorphism.