CHAPTER XXIX.
VOLCANIC ROCKS—continued.
Trap dikes — 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 contact — Obliteration 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 — Structure and physical features of Palma and some other extinct volcanos.
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.[378-A])
Fig. 439.
Dike in inland valley, near the Brazen Head, Madeira.