"Tie-line data are presented for twelve analysed igneous pyroxene pairs, from which it is demonstrated that a precise point of intersection, at or close to Wo 75 En 25 in the ternary diagram, does not exist. Eight tie-lines for pyroxene pairs from ultrabasic nodules in basalts are also considered. The tie-lines for metamorphic pyroxenes are compared with the igneous, and are shown to intersect over a similarly wide range, the average (Ca 82 Mg 18 ) being the same for both assemblages. It is shown that no conclusions can be drawn as to the genesis of the pyroxene assemblage, from the assumption of a characteristic tie-line intersection point. Optical determinations are considered, and shown to be of little practical value in this particular problem." In the discussion by Brown, additional data, including four chemical analyses, are given for augite and pigeonite from the Skaergaard intrusion.