Lopolithic Intrusion of Basalt in the Intertrappeans at Amboli Hill, Jogeshwari, Bombay

At Amboli Hill, in Jogeshwari, is exposed a spectacular columnar jointed basalt outcrop which has been inferred to represent a sill-like body into the intertrappean beds. The structural deformation of the intertrappeans gives a distinct saucer-shaped bottom giving the impression of a lopolith. Petrographic and chemical studies of the basalt indicate that the rock is tholeiitic in composition and does not show any significant variation in mineral content and chemical composition across the exposed thickness of the body. The intertrappean beds are predominantly made of volcanic breccias with thin intercalations of sedimentary beds. Detailed study of the petrography and chemistry of the breccia fragments indicate that they are of spilitic character which leads to tbe conclusion tbat the rock is a hyaloc1astite formed by subaqueous eruption of the basaltic lava.