Summary: In order to aid the graphical presentation of analyses of igneous rocks, a pseudo-quaternary system R2O3-XO-YO-ZO2 has recently been used (O'Hara, 1968, fig. 4). To test its ability to present the analytical data of tholeiitic basalts, the analyses of selected lavas have been projected into the system and then sub-projected into two planes. Lines, which represent the loci of liquid compositions in certain isobaric, pseudo-univariant equilibria, have been drawn on the basis of the melting relations of the basalts. After reading the pseudo-ternary phase diagrams resulting from the sub-projections, some genetic aspects of the lavas of Kilauea and Mauna Loa are discussed from a phase equilibrium standpoint.