particularly to be observed along the edges of dykes and other intrusive masses, where they represent the outer surface of the basalt that was suddenly chilled and consolidated by coming in contact with the cold walls of the vent into which it was injected, and where they no doubt show what was the original state of the whole basalt before devitrification converted the rock into its present crystalline structure. Basalt a black, compact, heavy, homogeneous rock, breaking with a conchoidal fracture, showing sometimes large porphyritic crystals of plagioclase, olivine, or augite, but too fine-grained for the component minerals of the base to be determined except with the microscope. The coarser varieties, where the minerals can be recognised with the naked eye, are known as dolerite. The basalt-rocks are pre-eminently volcanic lavas, occurring both as intrusive masses that consolidated underground, and as sheets that were poured out in successive streams at the surface. The black, compact kinds (true basalt) are particularly prone to assume columnar forms (Fig. 78), whence columnar rocks are sometimes spoken of as basaltic. In some varieties of basalt the mineral Leucite takes the part of the plagioclase; and in others this is done by another mineral, Nepheline.

DIABASE-a name given to some ancient basalt-rocks in which, owing to alteration of their augite or olivine, a greenish chloritic discoloration has often taken place. The lavas of early geological time are to a large extent diabase.

ANDESITE is closely allied to basalt; but contains no olivine. It sometimes includes free quartz and hornblende may be substituted in it for augite. Hornblende-andesite and augite-andesite are lavas which have been extensively erupted in later geological time.

DIORITE-a crystalline aggregate of plagioclase and hornblende, usually with magnetite and apatite, sometimes with augite and mica. The hornblende is black or dark green and often more or less decomposed, giving rise to a greenish chloritic discoloration of the felspar. From its prevalent green colour, the rock was formerly known as "greenstone." It occurs in intrusive masses, and seems generally if not always to have consolidated below ground instead of being poured out at the surface.

GABBRO, DIALLAGE-ROCK-a thoroughly crystalline granitoid aggregate of plagioclase and the variety of augite known as diallage, which appears in distinct brown or greenish crystals, with a peculiar metalloidal or pearly lustre; it is found in bosses associated with granite, gneiss, etc., and also sometimes with volcanic rocks.

(3.) Olivine and Serpentine Rocks.

In this group may be included a comparatively small number of rocks which consist principally of olivine, and which by gradual alteration pass into serpentine (Fig. 58). Olivine-rocks (Peridotites) are liable to remarkably rapid changes of texture and composition. In some places they are mainly made up of olivine, augite, or hornblende, magnetite, and brown mica, but some of these minerals may disappear and some felspar may take their place. They are intrusive masses which appear to have been generally injected into the crust in connection with volcanic eruptions, rather than to have been poured out at the surface in true lava-streams.

SERPENTINE-a compact, dull, or faintly glimmering rock, with a general dark dirty green colour, variously mottled, greasy to the touch, easily scratched and giving a white powder which does not effervesce with acids. It is a massive form of the mineral serpentine described on p. 178; frequently containing disseminated crystals of the minerals bronzite, enstatite, and chromic iron, and veins of a delicately fibrous silky variety of serpentine known as chrysotile. Many serpentines were originally olivine-rocks which, by hydration and alteration of their magnesian silicates, have assumed their present characters. Serpentine occurs in bosses, dykes, and veins, which were evidently of eruptive origin and were at first probably olivine-rocks; it is also found in thick beds associated with limestones and crystalline schists, where it may be a metamorphosed sedimentary rock.

(iii.) THE SCHISTS

AND THEIR ACCOMPANIMENTS.

This section includes a remarkable series of rocks of which the leading character is the possession of a schistose or foliated character (Fig. 72). They are, in their more typical varieties, distinctly crystalline; but some of them shade off into ordinary fragmental rocks, such as shale and sandstone. Several of them agree in chemical and mineral composition with some of the eruptive rocks already enumerated, but differ from these in the peculiar foliated arrangement of their minerals, though gradations can also be traced between them.

In the schists, therefore, we see an assemblage of rocks which, though possessing distinct characters of their own, may yet be observed to shade off into fragmental rocks on the one side, and into eruptive rocks on the other. In chapter xiii. some further account of the schists will be given, and it will there be shown on what grounds they have been regarded as metamorphic or altered rocks. For the present, in taking notice of their composition and structure, it will be enough to state that in many cases they can be shown to be more or less altered and crystalline, but originally sedimentary rocks; in other instances, they are crystalline eruptive masses, which have been subjected to such enormous pressure and shearing, that a foliated structure and recrystallisation of minerals have been superinduced in them.

CLAY-SLATE-a hard fissile clay-rock, through which minute scales of mica and crystals or crystallites of other minerals have been developed; generally bluish-grey to purple or green, and splitting into thin parallel leaves. As this rock often contains remains of marine animals and plants, and is interstratified with bands of sandstone, grit, conglomerate, and limestone, it was undoubtedly, at first, in the condition of soft mud on the sea-bottom. Sometimes the organic remains in it are so curiously elongated or distorted in one general direction as to show that the rock has been drawn out by intense pressure and shearing (Figs. 98, 103, 104). The planes along which clay-slate splits are generally independent of the original surfaces of deposit, sometimes cross these at a right angle, and have been superinduced in the rock by mechanical movements, as explained in chapter xiii. Different varieties of clay-slate have received special names. Roofing-slate is the fine compact durable kind, employed for roofing purposes and also for the manufacture of cisterns, chimney-pieces, writing-slates; Alum-slate-dark, carbonaceous, and pyritous, the iron disulphide oxidising into sulphuric acid, and giving rise to an efflorescence of alum ; Whet-slate, honestone-exceedingly hard, fine-grained, and suitable for making hones; sometimes owing its hardness to