TRAPP, in mineralogy, a species of siliceous earth. It is described by Dr Kirwan as nearly the same with basaltes: a dark grey or black stone, generally invested with a ferruginous crust, and crystallized in opaque, triangular, or polygonal columns, is called basaltes; that which is amorphous, or breaks in large, thick, square pieces, is called trapp. Their constituent principles, and relation to acids and fluxes, are exactly the same. The texture of this stone is either coarse, rough, and distinct, or fine and indistinguishable. It is often reddish; it is always opaque, and moulders by exposure to the air; some specimens give fire with steel very difficultly, though
it is always very compact; sometimes it is sprinkled over with a few minute shining particles: its specific gravity is 30.0.
When heated red-hot, and quenched in water, it becomes by degrees of a reddish brown colour: it melts per se in a strong heat into a compact slag. Borax also dissolves it in fusion, but mineral alkali not entirely.
According to Mr Bergman, 100 parts of the basaltes contain 12 of siliceous earth, 15 of argil, 8 of calcareous, 2 of magnesia, and 25 of iron; and with this Mr Meyer very nearly agrees.
For a more complete account of this species of stone, see M. Faujas de St Fond on the Nat. Hist. of Trapp.