The islands of Bombay, Salsette, and Trombay expose basalts and associated rocks belonging to the Upper Deccan group, probably of Tertiary age. The basalts dip 10°–20°W. throughout the area. Interbedded with the basalts are sediments mainly of tuffaceous origin and in part deposited in fresh water, and relatively small extrusives and intrusives of highly mafic rocks (monchiquite, ankaramite, oceanite) and of felsic rocks (rhyodacites, rhyolites). Previous work in the area is summarized.

Petrographic descriptions of the prevailing rock types and nine new chemical analyses are given. Sixty-four analyses of Deccan rocks taken from the literature are reviewed. Averages are calculated for the Deccan basalts and for the Upper and Lower Deccan groups. The analyses are plotted on four types of chemical diagrams. Like many basaltic rock suites, the Deccan province shows an initial stage of iron enrichment, followed by dominant alkali enrichment. The average Deccan basalt has somewhat more FeO and less Al2O3 and MgO than most tholeiitic basalts.

The absence or scarcity of pigeonite in Deccan basalts is attributed to the chemical characteristics of the magma and rapid cooling with incomplete crystallization of the basalts. The felsic rocks are believed to represent differentiates of the basaltic magma, produced by crystal fractionation and diffusion of volatiles. Magmatic differentiation was directed initially toward a rhyodacitic residuum and only in the final stages toward rhyolitic melts with more K2O than Na2O.

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