Trap
a term rather loosely and vaguely applied by the earlier geologists to some or all of the multifarious igneous rocks that belong to the palæozoic and secondary epochs, as distinct from granite on the one hand and the recent volcanic rocks on the other. Trap rocks often assume a terraced appearance, whence their name from trappa, the Swedish for stair. Their composition may be described as consisting chiefly of feldspar and hornblende. Trap rocks of crystalline structure are distinguished as greenstones, basalts, clink stones, compact feldspar, and feldspar porphyries, while the softer and more earthy varieties are known as claystones, claystone porphyries amygdaloids, trap tuffs, and mackes. BASALT (q. v.) is the most compact, the hardest, and the heaviest of the trap rocks. The hill scenery of trappean districts is often picturesque.