Igneous rocks, owing to their range of mineral composition and of structure, combined with their general hardness, lend themselves to various economic purposes. While the granites, resisting atmospheric attack admirably in a polished state, provide our handsomest building-stones, dolerites and fine-grained diorites, which owe their toughness largely to the interlocked relations of their constituent minerals, serve as our most satisfactory road-metals.
THE SCENERY OF IGNEOUS ROCKS
Volcanic landscapes, where activity is very recent or still in progress, present a number of characteristic surface-forms. The cones that have accumulated round the vents surpass all other hills in regularity of outline, and the crater in the summit is often relatively large. Lava-cones may be steep-sided bosses when formed of protrusions of viscid rocks rich in silica, like the remarkable domes in the north of Bohemia, or they may present very gentle slopes where fluid basic lavas have been extruded.
Tuff-cones are liable to be breached on one side, owing to the outflow of lava which the crater-wall could not sustain, and they then assume the form of a mountain in which glacial influences have hollowed out a cirque.
Rain washes down the loose materials from great volcanic cones, and emphasises the concave curve of the mountain sides, the form that is so beautiful in Fuji-yama in Japan, and which Hokusai, with pardonable and affectionate exaggeration, reproduced in a hundred illustrations. Ultimately, however, grooves appear on the flanks of the cone, in which permanent streams gather, and the slopes are dissected and worn away. During this process, the tuffs yield steep and fantastic forms, and wall-like dykes weather out among them. The dykes are usually the last features to decay.
Where the vent has been plugged with lava at the close of its activity, the neck of rock often remains standing above the surrounding country. The site of cone after cone can be picked out in this way in the Cainozoic volcanic areas of central Germany. The jutting crag of trachyte or of basalt has often been seized on as the site of a feudal castle, under which the dependent agriculturists still gather at nightfall in their red-roofed town. The group of sheer-sided necks in the Hegau in southern Württemberg, the Hohentwiel, Hohenkrähen, and the rest, form a very striking landscape amid undulating Cainozoic lands.
The lava-beds that cover wide areas are naturally of basic composition. Basalts thus form enormous plains with rugged surfaces, on which at last a red-brown soil collects. When exposed to denudation from the edge of the region inwards, they develop a marked terrace-structure, through which the rivers cut steep and grim ravines. Grass may grow on the ledges and the tables; but the scarps, controlled by the well-marked vertical jointing of the lavas, remain sharp and prominent, and the rock falls away from these walls in whole columns at a time. This structure is characteristically seen in northern Mull and the adjacent smaller isles, and is still more impressive from the centre to the north of Skye, where the rain swept terraces covered by grass and bog and scanty oatfields, and the black steps of rock between them, present a scene of strange monotony and desolation.
In regions less exposed to stormy weather, the lava-plateaus may provide good soils. For instance, after the great seaward scarp of the basalts has been crossed in the counties of Antrim and of Londonderry, the lava-fields, dropped by faults towards Lough Neagh, are seen to be occupied by prosperous farms. In arid countries, however, the savage surface of the flows merely becomes modified by red dust and scoriaceous gravel, worn by wind and changes of temperature from the upstanding portions of the land.
Where a stratified country has been freely invaded by sheets of lava along its planes of bedding, the stratification is emphasised in any part exposed to weathering. The resisting igneous rock stands out in scarps along the hills, and marks out any folds that have been formed since the epoch of its intrusion.
When the beds remain fairly level, and are also uplifted, flat-topped hills are formed by the intrusive sheets, like those that may be carved out of a country flooded over by lava-streams. The crystalline rock, very probably a dolerite, protects what lies below it. The kopjes north of the Great Karroo in the centre of the Cape of Good Hope are thus level on the crest and bounded by a steep wall or krans of rock.