This is a widely─extended rock; it sometimes forms whole mountains, and even chains of mountains; but these usually have a gentle acclivity. The summit of the celebrated mountain called Skiddaw in Cumberland is of clay slate. 258. _PRIMITIVE LIMESTONE is a simple mountain rock of crystalline or granular structure; and generally of white, yellowish, greenish, or reddish colour._ To this species of rock belong many of the rich and beautiful kinds of marble already described (143, &c.). Carrara, or statuary marble (146), is a familiar instance of it. Whole mountains in Stiria, Carinthia, Carniola, and the Pyrenees, and three mountains in Switzerland, 10,000 feet in height, are of primitive limestone. The mountain of Filabres in Spain, is said to consist of one block of white granular marble, 2,000 feet high, and three miles in circuit; without intermixture of other earths or stones, and almost without a fissure. Various mineral ores, in beds and veins, as lead, zinc (241), and iron, are occasionally found in this kind of rock. 259. _PRIMITIVE TRAP is a mountain rock composed of a black mineral called hornblende, mixed, in some varieties, with felspar (110), and, in others, with mica (123)._ The word _trap_ is of German origin, signifying a _stair_; and rocks of this formation are called trap rocks, because their strata, when exposed, usually jut out, one beneath the other, somewhat like a stair. Under this term is comprehended a series of rocks, distinguished chiefly by the hornblende, which they all contain. Rocks belonging to this formation are numerous. They occur in Scotland; and abundantly in Derbyshire and some other parts of England. In many countries they constitute considerable hills. They abound in ores. 260. _SERPENTINE is a primitive rock, usually consisting of quartz (76), magnesia (198), alumine (197), with a portion of oxide (21) of iron._ This rock and its various uses, have been already described (132). It generally occurs in shapeless masses and beds; and seldom in distinct strata. It is found in Cornwall, the island of Anglesea, and several parts of Scotland; but it rarely forms mountains. Ores of lead, silver, and copper, are sometimes found in serpentine. 261. _PORPHYRY is a primitive rock, consisting of quartz (76) or felspar (110), or both, embedded in a solid and compact cement or ground._ _The ground or basis of porphyry varies in the different kinds. In some it is claystone, in others pitchstone, hornstone, or compact felspar._ When not covered by other formations, porphyry sometimes forms single rocks; but it never constitutes elevated mountains. It occurs in beds of considerable magnitude, but never appears in distinct and well─defined strata.