Chemical Classification of the Intermediate and Acid Rocks (Deccan Trap) of Salsette Island, Bombay

The intermediate and acid rocks of Salsette Island, Bombay, that were collectively identified as rhyolites, rhyodacites and trachytes by previous workers, have been remapped to locate the outcrops of acid and intermediate rocks separately. Eighty-one samples, collected systematically, have been analysed for Na₂O and K₂O contents and thirty-five of these for major elements. On the basis of geochemistry and mineralogy these rocks have been classified into five groups, viz., (I) potassic rhyolites, (2) sodipotassic rhyolites, (3) potassi-sodic rhyolites, (4) quartz-rich trachytes, and (5) quartzbearing trachytes.