The characters of igneous rocks in _dykes_, that is, of those types that have consolidated in fissures, resemble in many respects the characters of lava-flows. Chilling being usually equal on both surfaces, glassy or compact types of rock occur on both sides, and the dyke is, as previously observed, more crystalline in the centre. Columnar structures arise from both surfaces, the dyke also shrinking parallel to its margins. In the outer layers so formed, the columns are small, and they increase in diameter nearer the centre. In small dykes and veins, the columns may run continuously from side to side; in larger ones, they meet along a central surface, which forms, on weathering, a plane of weakness in the rock. Dykes may thus become worn away, decay spreading from the central region, and leaving the more resisting and more glassy portions clinging to the bounding walls. Where, however, the surrounding rocks are more easily worn away than the igneous invader, as very often happens, the dykes stand out on the surface as great ribs and walls. The rocks cooled in the deep-seated cauldrons, under what are styled _plutonic_ conditions, have parted with their gases so slowly that they do not show scoriaceous structure. They may become very coarsely crystalline, like many of the Scandinavian granites; minerals, moreover, may be produced which are unstable or difficult to form nearer the surface. Crystals developed in plutonic surroundings become carried forward when the partially consolidated mass is pressed up to a volcanic orifice, and may undergo resorption on the way. Many, however, escape, and impart a _porphyritic structure_ to lavas. The deep-seated rock, from causes that promote the growth of one mineral and the retention of another in solution, may also become "porphyritic" _in situ_, smaller crystals, or even a eutectic intergrowth, finally filling in the ground. The viscidity of igneous rocks may cause any of the types to show a _fluidal structure_. Constituents already formed become dragged along in parallel series as the mass moves forward. Sometimes a group of spherulites, or a knot of "felsitic" matter caused by the dense growth of embryo-crystals, is stretched out into a sheet, and on fractured surfaces a _banded structure_ characterises the mass. These banded rocks record, in their crumpled and obviously fluidal layers, the formerly molten condition of the mass. Even completely crystalline rocks may show parallel arrangement of their minerals, owing to flow during the last stages of consolidation, or to pressure from the walls of the cauldron, influencing the positions taken up by crystals that possess a rod-like or platy form. [Illustration: Fig. 16. Granite invading Mica-Schist. Clifton, near Cape Town. Adjacent sections were studied by Charles Darwin (see p. 156).]