Trap

Trap (Sw. trappa, 'a stair'). Under this term were formerly included various crystalline igneous rocks, most of which are basic in character. These frequently enter more or less conspicuously into the formation of certain hills and mountains—the igneous rocks being often arranged in horizontal or approximately horizontal beds, separated by intervening layers of some more yielding material, such as tuff, shale, sandstone, &c. Under the influence of denudation the relatively hard crystalline rocks come to stand out like huge steps on hill-faces and mountain-slopes, which thus acquire a step-like contour. The term is now disused. Most of the trap-rocks are varieties of Basalt (q.v.), while under the same head were formerly included certain diorites and porphyrites (see IGNEOUS ROCKS).