TRAP, in geology, is a term applied to older volcanic rocks which have flowed over or between other rocks, assumino. the form of stairs (Trappa, SW., a stair), and diainguished as eruptive trap, overlying trap, interposed trap, and trap-dykes. Trap, in its mineralogical structure, is of augite, hornblende, or basalt. The greatest outburst of trap in the world is that which was first described by Colonel Sykes as the trap formation of the Dekhan. It extends from Nemuch, in lat. 21° 27' N., to the banks of the Kistna, over an area of 250,000 square rniles.
In Central India, volcanic trap-rocks are observed to spread east and west from Nemuclitin tho form of basalt, basaltic greenstone, greenstone, and greenstone antygdaloid, mid southwards by Ujjain and Saugor acrosa the Vindhya, assuming the structure of columnar basalt in the steep descent to the Nerbadda. Crossing this river, the trap is seen to spread over all the Aurangabad province down through Kandesh and the Konkan to Bom bay, and southwards to Malwan, in lat. GO° N., its southern limits being observed south of Punderpur, through Bijapur to the right bank of the Kistna, in the valleys near Ilomnabad, where it is found beneath, but never penetrating the laterite hills south and east of Beder, and Maharajapetta, 30 miles west of Hyderabad. The eastern edge
of this vast tract of trap-rocks, after crossing the. Nerbadda to the south, skirts the town of Nagpur in Berar, passes Nandeir, and to the westward of the city of Hyderaba.d to its southern limit, just mentioned. South of this, as well as to the east ward, the trap only appears as great dykes, from fifty to a hundred yards broad, which run east and west parallel with each other. These dykes can at places be traced for 150 miles, bursting • through the granite and other rocks, tearing the highest of the hills asunder, and filling the chasms and crevices with its dark and compact structure. In these dykes the elements of the trap-rock assume a variety of appearances,—greenstone, porphyritic greenstone, basaltic greenstone, horn blende rock, and basalt. They are particularly numerous in Hyderabad, the Balaghat, Ceded Districts, Carnatic, and Mysore, alrnost to the southern cape of the Peninsula, and, with very rare exceptions, run due east and west.—Colonel Sykes ; Carter's Geology ; Baronzetrical Sections.