On Augite-diorites with Micropegmatite in Southern India

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Abstract
Prior to the great outburst of Deccan trap at the close of the Cretaceous period, there were two principal periods of volcanic activity in Peninsular India. One of these is indicated by the contemporaneous traps of the Dharwars—the oldest of our recognized Transition systems—and the other by the lava-flows of the Cuddapah system, whose precise position in the stratigraphical succession remains, on account of the complete absence of fossils, still undetermined. As a result of the great earth-movements which affected the strata of Peninsular India previous to the deposition of the Cuddapah system, the igneous rocks of Dharwar age have been greatly metamorphosed, and in this respect they stand in striking contrast to the Cuddapah traps, since the eruption of which the Peninsula of India has been remarkably free from geological disturbances. It is evidently in consequence of this remarkable freedom, both from extreme changes of level and the crushing effects of earth-movements, that rocks as old as the Cuddapahs have their original delicate structures, and primary constituents so susceptible as olivine and augite, preserved with striking freshness. The numerous dykes of basic igneous rocks which break through the ‘pyroxene-granulites’ and gneisses of the Madras Presidency, and which, for reasons that need not now be stated, are regarded as the dyke-representatives of the Cuddapah lava-flows, vary in composition from very basic olivine-augite norites approaching saxonites, through augite-norites, to augite-diorites with micro-pegmatite. Detailed descriptions of these and the peculiar hemi-crystalline and vitreous varieties, which form the selvages of the larger masses

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