1. Fractionation in igneous processes.- 2. Compositional variation in magmas.- 3. Phase diagrams — introduction.- 4. Ternary systems — I.- 5. Ternary systems with solid solutions page.- 6. The interpretation of two-element variation diagrams.- 7. Petrographic aspects of volcanic rocks.- 8. Quaternary systems.- 9. Experimental work on natural basaltic and allied rocks.- 10. Water-bearing basic rock systems.- 11. Compositionally zoned magma bodies and their bearing on crystal settling.- 12. Petrographic aspects of plutonic rocks.- 13. The interpretation of data for plutonic rocks.- 14. Trace elements in igneous processes.- 15. The use of isotopes in petrology.- Appendices.- 1. Nomenclature of igneous rocks.- 2. Average major element compositions and CIPW norms of common igneous rock types.- 3. Norm calculations.- Calculation of the CIPW norm.- Molecular norms.- 4. Calculation of plotting parameters for O’Hara (1968) polybaric phase diagram.- 5. Some representative mineral analyses.- Answers to exercises.- References.