HABIT OF IGNEOUS ROCKS Igneous rocks occur essentially in two different ways, either as lavas or intrusions.
Lavas or Effusive Types.—The lavas have been poured out at the surface and have consolidated after ejection, under conditions which are fairly well understood, seeing that they may be ex amined at active volcanoes in many parts of the world ; the intru sive rocks, on the other hand, have been injected from below into cracks and fissures in the strata and have cooled there be neath masses which conceal them from view till exposed by denudation at a subsequent period. The members of these two groups differ in many respects from one another, so that it is often possible to assign a rock to one or other of them on mere superficial inspection. The lavas (or effusive rocks), having cooled rapidly in contact with the air, are mostly finely crystalline or have at least a fine-grained ground-mass representing that part of the viscous semi-crystalline lava flow which was still liquid at the moment of eruption. At this time they were exposed only to atmospheric pressure, and the steam and other gases, which they contained in great quantity, were free to escape; many important modifications arise from this, the most striking being the frequent presence of numerous steam cavities (vesicular structure) often drawn out to elongated shapes subsequently filled up with minerals by infiltration (amygdaloidal structure). As crystallization was going on while the mass was still creeping forward over the sur face of the earth, the latest formed minerals (in the gound-mass) are commonly arranged in subparallel winding lines following the direction of movement, and the larger early minerals which had previously crystallized may show the same arrangement. Most lavas have fallen considerably below their original tempera tures before they are emitted. In their behaviour they present a close analogy to hot solutions of salts in water, which, when they approach the saturation temperature, first deposit a crop of large, well-formed crystals (labile stage) and subsequently precipitate clouds of smaller less perfect crystalline particles (metastable stage). In igneous rocks the first generation of crystals generally forms before the lava has emerged to the surface, that is to say, during the ascent from the subterranean depths to the crater of the volcano. It has frequently been verified by observation that freshly emitted lavas contain large crystals borne along in a molten, liquid mass. The large, well-formed, early crystals are generally admitted to be porphyritic ; the smaller crystals of the surrounding matrix or ground-mass belong to the post effusion stage. More rarely lavas are completely fused at the moment of ejection; they may then cool to form a non-porphyritic, finely crystalline rock, or if more rapidly chilled may in large part be non-crystalline rock, or glassy (vitreous rocks such as obsidian, tachylyte, pitchstone). A common and easily recog
nized feature of glassy rocks is the presence of rounded bodies (spherulites: Gr. oval.pa, a ball), consisting of fine divergent fibres radiating from a centre ; these divergent fibres consist of imperfect crystals of felspar, mixed with quartz or tridymite; simi lar bodies are often produced artificially in glasses which are al lowed to cool slowly. Rarely these spherulites are hollow or con sist of concentric shells with spaces between (lithophysae : Gr. Mos, a stone; cgo-a, bellows). Perlitic structure, which is also found to be common in glasses, consists in the presence of con centric rounded cracks owing to contraction on cooling (see PERLITE).
The phenocrysts (Gr. Octivey, to show ; Kpbara,XXoP, a crystal) or porphyritic minerals are not only larger than those of the ground-mass; as the matrix was still liquid when they formed they were free to take perfect crystalline shapes, not being inter fered with by the pressure of adjacent crystals. They seem to have grown rapidly, as they are often filled with enclosures of glassy or finely crystalline material like the material of the ground mass. Microscopic examination of the phenocrysts often reveals that they have had a complex history. Very frequently they show successive layers of different composition, indicated by variations in colour or other optical properties ; thus augite may be green at the centre and various shades of brown outside this; or may be pale green centrally and darker green with strong pleochroism (aegirine) at the periphery. In the felspars the centre is usually more basic and richer in lime than the surrounding faces, and successive zones may often be noted, each less basic than those which lie within it. Phenocrysts of quartz (and of other minerals), instead of sharp, perfect crystalline faces, may show rounded corroded surfaces, with the points of the crystals blunted and irregular tongue-like projections of the matrix into the substance of the crystal; it is clear that after the mineral had crystallized it was partly again dissolved or corroded at some period before the matrix solidified. Corroded phenocrysts of biotite and horn blende are very common in some lavas; they are surrounded by black rims of magnetite mixed with pale green augite. The horn blende or biotite substance has proved unstable at a certain stage of consolidation and has been replaced by a pseudomorph of augite and magnetite which may be partially or completely sub stituted for the original crystal but still retains its characteristic outlines.