Some very important criticisms of the wider generalizations of modern petrology were made by Henry S. Washington in a paper read before the International Geological Congress in 1913 on “Volcanic Cycles in Sardinia”.

The Sardinian volcanoes belong to three different volcanic cycles. A careful study of the sequence of the various lava-flows brings out the following points. The sequence as a whole is discontinuous but recurrent. The close of one magmatic cycle is simultaneous with the ending of one volcanic phase, and the recurrence of a new one coincides with a change in the intensity or character of the vulcanicity. In the several successive cycles there is a progressive decrease in differentiation; thus the earliest cycle comprises rhyolites, trachytes, andesites, and basalts; while in the latest felspar-basalts are the only lavas which have been extruded. Each cycle closes with the extrusion of felspar-basalt. The sequence here observed is not in accordance with the ‘law’ of divergence from an average magma. Harker would, however, regard such exceptions as due to the suppression of one of two divergent lines.