TRAP, in mineralogy, is a Swedish term signifying stair. It was first applied to designate a certain class of mountains, • composed of nearly horizontal strata, with perpendicular breaks, which were supposed to give a rude resemblance to a flight of stairs. Hence many species of rock, differing very much frotn each other, were called by the same name,' which caused much confusion. According to Werner, there are three distinct classes or formations of rocks to which the term trap may be applied : of these the first class belongs to the primitive mountains, the second to the transition mountains, and the third to the secondary moun tains. Primitive traps are composed es sentially of hornblende, mingled with fel spar, and sometimes with pyrites and, mi ca. Of rocks belonging to this formation there are four distinct species, viz. the common hornblende, the schistose horn blende, primitive griinstein, and schistose grtinstein. Transition traps are composed principally of granular grtinstein, but the mixture of the ingredients is more inti mate, the grain is finer, and the mass ap pears more homogeneous. There are two principal varieties, viz. 1. The amygda bold, which is a rock of schistose horn blende in a state of semi-decomposition, resembling fine ferruginous clay. It con tains a number of globular cavities, from the size of a pea to that of a small apple: of these cavities some contain nothing but air, and are coated on the inside with a kind of varnish ; others contain balls of calcareous spar, quartz, chalcedony, &c.
The toadstone of Derbyshire is considered by Werner as belonging to this variety. 2. (=lobular trap, composed or schistose griinstein, in a state of semi-decomposi tion, arranged in spheroids of various magnitudes, and composed of thick con centric lamellar distinct concretions. Se condary or doetz traps are divided into those which are peculiar and characteris tic of it, and those which are accidental. The former are basalt, porphyry, Etc. Among the latter may be classed rubble and sandstone, clay, coal, and bituminous wood. The proper base to the secondary trap tbrmation, or in other words the sub stance, which appears to have immediate ly preceded it in the order of formation, is secondary limestone ; it is, how ever, not unfrequently found resting on sandstone, on argillite, on gneiss, and even on granite. The general order in the strata of this formation, is the following, viz. coarse sand, fine sand, sandy clay, unctuous clay, wakke, basalt, amygdaloid, porphyry, and giiinstein. It hardly ever happens that all these strata are met with in the same mass of moun tain. No metallic veins are found in this class of mountains, but the remains of - vegetable and marine organized bodies are of frequent occurrence. See noel:.