a peculiar metalloidal or pearly lustre; it is found in bosses associated with granite, gneiss, etc., and also sometimes with volcanic rocks in centres of eruption.

(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. 134; 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. Some of them shade off into ordinary fragmental rocks, such as shale and sandstone; others agree in chemical and mineral composition with some of the eruptive rocks already enumerated, into which they may often be traced by imperceptible gradations.

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 them will be given, with special reference to their probable origin, and to the grounds on which 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 transformations of what were originally sedimentary rocks; and that in other instances they represent original 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. The essential feature which unites masses of such different origin is the possession of that common schistose structure which they have derived from having all been alike subjected to the same kind of intense terrestrial movements,

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 by mechanical movements (Cleavage), 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, chimneypieces, writingslates; Alum-slate-dark, carbonaceous, and pyritous, the irondisulphide 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 the presence of microscopic crystals of garnet; Chiastolite-slate-containing disseminated crystals of chiastolite, and found especially around eruptive bosses of granite. By increase of its mica-flakes a clay-slate passes into a Phyllite, which has a

more silvery sheen, and represents a farther stage of metamorphism. Phyllite, by increase of the mica, becomes Mica-slate, so that a transition may be traced from sedimentary fossiliferous rocks through clay-slate and phyllite into thoroughly crystalline schist. Clay-slate occurs extensively among the older geological formations in all parts of the world.

Amphibolites-rocks composed mainly of hornblende, but with quartz, orthoclase, and other minerals in minor proportions; sometimes they are massive and granular (Hornblende-rock), and in this condition doubtless represent eruptive rocks. Gradations can be followed from such rocks (originally diorite, diabase, etc.) into perfect schist (Hornblende-schist), so that the development of the schistose structure can be traced from rocks that were at first as structureless as any amorphous eruptive mass can be. Amphibolites occur among the crystalline schists in most parts of the world as occasional bands or bosses, which probably mark zones of basic igneous rock, either intruded into the accompanying masses, or contemporaneously erupted with them.

Chlorite-schist- -a scaly, schistose aggregate of greenish chlorite with quartz, and often with felspar, mica, and octahedra of magnetite (Fig. 54); it occurs in beds associated with gneiss and other schists. Some chloritic schists may represent old lavas or other erupted rocks which have been crushed down and become schistose; others, especially where they contain pebbles of quartz, etc., and are banded with quartzites and schistose conglomerates, not improbably mark where fine volcanic ashes fell over a sea-bottom, and were then mingled and interstratified with the ordinary sediment that happened to be accumulating at the time.

Mica-schist (Mica-slate)—a schistose aggregate of quartz and mica, the two minerals being arranged in irregular but nearly parallel wavy folia. The rock splits along the laminæ of mica, so that its flat surfaces have a bright silvery sheen, and the quartz is not well seen except on the cross fracture, where only the thin edges of the mica-plates present themselves. Mica-schist is often remarkably crumpled or puckered a structure bearing witness to the intense compression it has undergone (Fig. 114). It abounds in most regions where schists are extensively developed (Chapter XVI). Some mica-schists contain fossil shells and corals (Bergen), and must thus represent what were originally sedimentary deposits; others may be highly deformed eruptive rocks.

Gneissa schistose aggregate of orthoclase, quartz, and mica,

varying in texture from a fine-grained rock up to a coarse crystalline mass which, in hand specimens, may not be distinguishable from granite. There is no difference indeed as regards composition between gneiss and granite; gneiss may be called a foliated granite. There is good reason to believe that some, if not all, true gneisses have been made out of granite or allied rocks by the process of shearing above referred to. Gneiss occurs abundantly among the oldest known rocks of the earth's crust, and may be found in most large regions of crystalline schists (Chapter XVI).

A few rocks which are found associated with the schists, or with evidence of metamorphism, may be noticed here-marble, quartzite, and schistose conglomerate.

Marble-a crystalline granular aggregate of calcite, white when pure, and having the texture of loaf-sugar, but passing into various colours according to the nature of the impurities. It occurs in beds among the schists, and is no doubt a limestone, formed either by chemical precipitation or by organic agency, which has been metamorphosed by heat and pressure into its present thoroughly crystalline character. Some of the fossiliferous limestones through which the Christiania granite rises have been changed into crystalline marble, but their original corals and shells have not been wholly effaced (see Chapter XIV).

Quartzite a hard, compact, granular rock, composed of adherent quartz-grains, and breaking with a characteristic lustrous fracture. It occurs in beds and thick masses, not infrequently associated with slates, mica-schists, and limestones; it sometimes contains organic remains; and is evidently an indurated siliceous sand.

Schistose Grit and Conglomerate. Interstratified with clayslates and mica-schists there are sometimes found beds of grit and conglomerate, the grains and pebbles of which consist of quartz or other durable material, imbedded in slate or schist. The original fragmental character of such rocks admits of no doubt; they were obviously at one time sheets of fine and coarse gravel mixed with sandy mud; and their presence among schistose rocks furnishes additional corroborative evidence of the original sedimentary character of some of these rocks. The clay or mud which formed the matrix has been metamorphosed into a more or less thoroughly crystalline micaceous substance, while in many cases the pebbles have been flattened and pulled out of shape. Hence these rocks afford important evidence as to the nature of the processes whereby the schists have been produced.