168 Trap dikes CHAPTER VIII. VOLCANIC ROCKS - continued. sometimes project — sometimes leave fissures vacant by decomposition - Branches and veins of trap — Dikes more crystalline in the centre-Foreign fragments of rock imbedded Strata altered at or near the contactObliteration of organic remains - Conversion of chalk into marble—and of coal into coke - Inequality in the modifying influence of dikes Trap interposed between strata — Columnar and globular structure- - Relation of trappean rocks to the products of active volcanos Submarine lava and ejected matter corresponds generally to ancient trap. HAVING in the last chapter spoken of the composition and mineral characters of volcanic rocks, I shall next describe the manner and position in which they occur in the earth's crust, and their external forms. Now the leading varieties, such as basalt, greenstone, trachyte, porphyry, and the rest are found sometimes in dikes penetrating stratified and unstratified formations, sometimes in shapeless masses protruding through or overlying them, or in horizontal sheets intercalated between strata. - Volcanic dikes. Fissures have already been spoken of as occurring in all kinds of rocks, some a few feet, others many yards in width, and often filled up with earth or angular pieces of stone, or with sand and pebbles. Instead of such materials, suppose a quantity of melted stone to be driven or injected into an open rent, and there consolidated, we have then a tabular mass resembling a wall, and called a trap dike. It is not uncommon to find such dikes passing through strata of soft materials, such as tuff or shale, which, being more perishable than the trap, are often washed away by the sea, rivers, or rain, in which case the dike stands prominently out in the face of precipices, or on the level surface of a country. (See the annexed figure.) * Dike in inland valley, near the Brazen Head, Madeira. In the islands of Arran, Sky, and other parts of Scotland, where sandstone, conglomerate, and other hard rocks are traversed by dikes of trap, the converse of the above phenomenon is seen. I have been favoured with this drawing by Captain B. Hall. The dike having decomposed more rapidly than the containing rock, has once more left open the original fissure, often for a distance of many yards inland from the sea-coast, Haven Fig. 90. her Fissures left vacant by decomposed trap. as represented in the annexed view. (Fig. 90.) In these instances the greenstone of the dike is usually more tough and hard than the sandstone; but chemical action, and chiefly the oxidation of the iron, has given rise to the more rapid decay. There is yet another case, by no means uncommon in Arran and other parts of Scotland, where the strata in contact with the dike, and for a certain distance from it, have been hardened, so as to resist the action of the weather more than the dike itself, or the surrounding rocks. When this happens, two parallel walls of indurated strata are seen protruding above the general level of the country, and following the course of the dike. As fissures sometimes send off branches, or divide into two or more fissures of equal size, so also we find trap dikes bifurcating and ramifying, and sometimes they are so tortuous as to be called veins, though this is more common in granite than in trap. The accompanying sketch (Fig. 91.) by Dr. through both the limestone, c, and the trap, b, In Fig. 92. a ground plan is given of a ramifying dike of greenstone, which I observed cutting through sandstone on the beach near Kildonan Castle, in Arran. The larger branch varies from five to seven feet in width, which will afford a scale of measurement for the whole. Fig. 92. Ground plan of greenstone dike traversing sandstone. Arran. In the Hebrides and other countries the same masses of trap which occupy the surface of the country far and wide, concealing the subjacent Fig. 93. Trap dividing and covering sandstone near Suishnish in Sky. (MacCulloch.) stratified rocks, are seen also in the sea-cliffs, prolonged downwards in veins or dikes, which probably unite with other masses of igneous rock at a greater depth. The largest of the dikes represented in the annexed diagram, and which are seen in part of the coast of Sky, is no less than 100 feet in width. Every variety of trap rock is sometimes found in these dikes, as basalt, greenstone, felsparporphyry, and more rarely trachyte. The amygdaloidal traps also occur, and even tuff and breccia, for the materials of these last may be washed down into open fissures at the bottom of the sea, or during eruptions on the land may be showered into them from the air. Some dikes of trap may be followed for leagues uninterruptedly in nearly a straight direction, as in the north of England, showing that the fissures which they fill must have been of extraordinary length. ·Dikes more crystalline in the centre. In many cases trap at the edges or sides of a dike is less crystalline or more earthy than in the centre, in consequence of the melted matter having cooled more rapidly by coming in contact with the cold sides of the fissure; whereas, in the centre, the matter of the dike being kept long in a fluid or soft state, the crystals are slowly formed. In the ancient part of Vesuvius a thin band of half-vitreous lava is found at the edge of some dikes. At |