TRAP or TRAPPEAN ROCKS, an important section of the igneous rocks (q.v.), asso ciated with primary and secondary strata, so called from the Swedish trappa, a stair, because these rocks, having resisted, from their greater hardness, the abrading influ ences which have destroyed the softer sedimentary strata, stand out like huge steps on the faces of the hills and mountains in some places where they occur. Unlike granite, the trap rocks are free from silica crystallizing as a separate constituent of the rock ; from the modern volcanic rocks the structural difference is very slight, consisting only of the manner in which the silicate of magnesia and lime, common to both, is crystal lized—in the older rocks appearing as hornblende, while in the newer it exists as augite.
Trap rocks are composed of feldspar and hornblende, and the different varieties founded on the chemical composition of the mass depend on the relative proportion of these two minerals. When the feldspar predominates, the rock belongs to the feldspathic trap or feldstone series; and when the hornblende is abundant, it is a hornblendic trap or greenstone. • This latter series contains the best-known trappean rocks. The dark horn blende preponderates, and gives to the rock a dull green color, from which it derives its well-known name of greenstone, a translation of the German grunstein. It has, how ever, been shown by Delesse that many trappean rocks owe their color to a dark variety of feldspar which exists in them, and such rocks belowg rather to the feldspathic than to the hornblendic series. Some greenstones are very light green, others are so dark as to appear black, and all intermediate shades of color occur. These rocks vary also very greatly in texture; in some, the crystals are sufficiently large to be detected with the naked eye; while others are so fine-grained and compact that it is difficult to resolve the separate crystals even with the help of a lens. Experiments have shown that the size of the crystal in an igneous rock increases in proportion to the length of time during which the mass remains fluid, and so permits the continued crystalline segregation of its various in gredients. The vitreous trap and obsidian would accordingly represent a speedily cooled flow of liquid rock. In fine-grained basalt, the crystalline force has been slightly devel oped; while greenstones of different textures exhibit its more continued operations in proportion to the coarseness of their texture. The principal varieties of hornblendic trap are greenstone, whinstone, or trap proper. When th, crystals are extremely minute, and there is a tendency in the rock to become columnar, it is a basalt. If the feldspar is a soda-feldspar, either albite or oligoclase, it is diorite. Euphotide, diallage rock, or gab bro, is a compound of Labrador feldspar and diallage, a variety of hornblende; it is a coarse, or sometimes fine-grained rock, with a granitic or porphyritic aspect. Hypers
thene rock, or hyperite. is made up of Labrador feldspar and hypersthene, another variety of hornblende; it is also a granitic-looking rock, very tough, and of a grayish or greenish black color: it is very abundant in the isle of Skye. Different varieties of hornblendic trap are based upon the structure of the rock, as well as upon its chemical composition. Trappean obsidian is not a common rock, but it is occasionally found. Porphyritie trap is more abundant; a very black variety has received the name of metaphyre. Amyg daloid is a trap with round or almond-shaped cavities. filled with agate, calcite, or other minerals, scattered through it. Trap tuff consists of fragments of scoriae, volcanic dust, and pieces of other rocks, forming a_coarse irregular mass, sometimes bound together 15y a calcareous cement.
The characteristic rock of the feldspathic series is feldstone, compact feldspar, or petro silex. It is a light-colored, greenish, or bluish, very compact, homogeneous, and trans lucent rock, with a flinty-looking appearance. It forms a large proportion of the con temporary intruded trap rocks in the silurian measures of Wales. Clinkstone, or phono lite, is a variety found in layers or slabs which give a metallic ring when struck with the hammer. Aphanite, or cornean, scarcely differs from true feldstone, except that it is a more compact and tougher rock. Pitehstone, or retinite, is a vitreous feldstone, less glassy than obsidian, and of a green color and resinous luster: a dyke 30 ft. wide occurs on the eastern shores of the island of Arran, cutting through the sandstone rocks. When distinct crystals of one or more minerals are scattered through an earthy or compact base of feldstone, a feldspathic porphyry is formed. The ancient red porphyry of Egypt, known as rosso antico, belongs to this set of rocks; it consists of a red feldspathic base, iu which are disseminated rose-colored crystals of oligoclase, with some plates of blackish horn Mende and grains of oxidized iron ore.
As true igneous rocks come up from below, there is always a connection of some kind between the ejected mass and the inferior source of supply, except when the ejected materials have been subsequently arranged by atmospheric or aqueous agency. Pipes and dykes form such connections, and they are generally associated with tabular masses which have either spread themselves over the surface, or inserted themselves between the beds of the sedimentary strata. When the materials have been mechanically arranged, the igneous rocks are contemporaneous with the deposits in which they occur; but in all where strata are cut through by dykes or pipes, or arc covered by .flows of liquid rock, the igneous rocks are newer than the sedimentary strata with which they are asso ciated.