The dispersionless KP and Toda hierarchies possess an underlying twistorial
structure. A twistorial approach is partly implemented by the method of
Riemann-Hilbert problem. This is however still short of clarifying geometric
ingredients of twistor theory, such as twistor lines and twistor surfaces. A
more geometric approach can be developed in a Hamilton-Jacobi formalism of
Gibbons and Kodama. AMS Subject Classifiation (1991): 35Q20, 58F07,70H99Comment: 20 pages, latex, no figure