bears, hyænas, and called Bone-breccia.

other extinct and living species, as to be Layers of stone, full of the coprolites (fossil excrement) or of the rolled bones of various vertebrate animals, have, in recent years, been largely worked as sources of phosphate of lime for the manufacture of artificial manures.

II. ERUPTIVE ROCKS.

Under this division are grouped all the massive rocks which have been erupted from underneath into the crust or to the surface of the earth. They are composed chiefly of silicates of alumina, magnesia, lime, potash, and soda, with different proportions of free silica, magnetic or other oxide of iron, and phosphate of lime. The principal silicate is generally some felspar, the number of eruptive rocks without felspar being comparatively small. The felspar is, in different rocks, conjoined with mica, hornblende, augite, magnetite, or other minerals.

No perfectly satisfactory classification of the eruptive rocks has yet been devised; they have been grouped according to their presumed mode of origin, some being classed as plutonic or hypogene, from their supposed origin, deep within the earth's crust, others as volcanic, from having been ejected by volcanoes. They have likewise been arranged according to their chemical composition, and also with reference to their internal structure. In the following enumeration of some of the more abundant and important varieties, it may be enough to adopt an arrangement in three sections, according to the nature of the predominant silicate: viz. (1) Orthoclase rocks; (2) Plagioclase rocks; and (3) Olivine and Serpentine rocks. It has already been pointed out that the original condition of many lavas and other eruptive rocks has been that of molten glass, their present stony structure being due to the more or less complete devitrification and disappearance of the glass by the development of crystals and crystallites out of it during the process of cooling and consolidation (p. 144). Though there is no evidence that all crystalline eruptive rocks have once been in the state of molten glass, it may be useful to begin with the vitreous varieties, which we know to represent the earliest forms of many that are now quite crystalline.

(1) Orthoclase Rocks.

In this section the prevalent silicate is Orthoclase, either in its common dull, white, or pink form, or in the glassy condition

M

(sanidine). In many of the rocks, free quartz occurs either in irregular crystalline blebs or in definite crystals, which frequently take the form of double pyramids. Among other minerals, hornblende, white and black mica, and apatite are of common occurrence. The rocks of this division are the most acid of the eruptive series—that is, they contain the largest proportion of silica or silicic acid, sometimes more than 75 per cent. Some of them (granite) are only found as masses that have consolidated deep beneath the surface; others (trachyte, rhyolite, obsidian) are abundant as superficial volcanic products.

Obsidian- -a black, brown, or greenish (sometimes yellow, blue, or red) glass, breaking with a shell-like or conchoidal fracture and into sharp splinters, which are translucent at the edges. Examined in a thin section under the microscope, the rock is found to owe its usual blackness to the presence of minute opaque crystallites (Fig. 69) which are crowded through it, not infrequently drawn out into streaky lines and curving round any larger crystal that may be embedded in the mass (Fig. 71 B). These arrangements, called flow-structure (p. 146), have evidently been caused by the movement of the rock while still in a fused state, the crystallites and other objects being borne onward by the currents of molten glass. In some obsidians, little spherulites of a dull grey enamel-like substance have made their appearance as stages in the devitrification of the rock (Fig. 71); but the mass has consolidated before the stony condition could be completed. In other instances, the whole rock has passed into a stony enamel-like mass with perlitic structure (pearlstone, p. 145). Where a still molten obsidian has been frothed up by the expansion of steam or gas through it, so as to become a spongy cellular substance which will float on water, it is called pumice. Obsidian occurs in many volcanic regions, sometimes as streams of lava which have been poured forth at the surface, sometimes in dykes and veins, and often in fragments ejected with the other detritus that now forms tuffs.

Trachyte-a compact porphyritic rock, consisting mainly of orthoclase (sanidine), with some plagioclase and usually with some hornblende, or with augite, mica, magnetite, or other minerals; having a peculiar matrix which, under the microscope, is found to consist mainly of minute felspar-crystallites. Large crystals of orthoclase (sanidine) are frequent, and also scales of dark mica. This rock is found abundantly among some of the younger volcanic regions of the world, where it occurs in lava

streams and also in intrusive sheets and dykes.

Quartz-trachyte

(Liparite, Rhyolite) is a rock composed of a compact, often rough and somewhat porous base, through which are scattered crystals of felspar and blebs of quartz, often also with hornblende and mica.

Felsite an exceedingly close-grained rock, composed of an intimate mixture of quartz and orthoclase. The felspar often Occurs as large disseminated crystals, giving the porphyritic structure. Where the quartz appears as distinct blebs or crystals (sometimes double pyramids) the rock becomes Quartz-porphyry. The felsites and quartz-porphyries play an important part among the eruptive rocks of older geological time, occurring both in the form of lavas erupted to the surface and of intrusive masses that have consolidated below ground. Many of them can be proved to have been originally in the condition of molten glass which has been devitrified. Rocks which show the characteristic closeness of grain characteristic of the felsites are said to be felsitic or to have a felsitic ground mass (p. 144).

Syenite a thoroughly crystalline rock, consisting essentially of orthoclase and hornblende, and distinguished from granite chiefly by the absence or small amount of quartz. It occurs in bosses and veins which have been erupted into older rocks.

Granite a thoroughly crystalline (holo-crystalline) compound of felspar, quartz, and mica, the individual minerals being large enough to be distinctly recognised by the naked eye. Sometimes large crystals of felspar are porphyritically scattered through the rock. Granite occurs in large eruptive masses which have been intruded into many different kinds of rocks, also in smaller bosses and veins. Round the outside of a mass of granite there frequently diverge from it dykes and veins (p. 203) which, where of great width, may show the usual granitic structure; but which, when of small dimensions, are apt to appear as felsite or quartzporphyry. There can be no doubt that such fine-grained veins are actually portions of the same mass of rock as the granite, so that granite and felsite or quartz-porphyry are only different conditions of the same substance, the differences being probably due to variations in the circumstances under which the cooling and consolidation took place. In the crystalline-granular structure so distinctive of granite (granitic or granitoid, p. 144) the constituent minerals have not had room to assume perfect crystallised shapes, but occasionally they have been able to shoot out in perfect crystals where cavities occur. Fig. 77, for example,

shows a group of the ordinary crystals of this rock which have crystallised in a cavity of the granite of the Mourne Mountains, Ireland. It is in such cavities also that the rarer minerals of this rock, such as topaz and beryl, may be looked for.

FIG. 77.-Group of crystals of felspar, quartz, and mica, from a cavity in
the Mourne Mountain granite.

In this section the felspar is some variety of plagioclase, and the other most frequent silicate is either augite or hornblende. Though free quartz occurs in some of the rocks, they contain generally so much less silica than the orthoclase rocks that instead of being acid they are commonly basic compounds. A range

of texture can be observed in them similar to that characteristic of the orthoclase series, from a true glass up to a thoroughly crystalline granitoid rock. Some of them, more especially the coarsely crystalline varieties, are probably of deep-seated origin; others (and these include the great majority) are truly volcanic ejections which have risen in volcanic pipes and fissures, and have been poured forth at the surface as actual lava-streams.

Basalt-Rocks—a group of rocks consisting of plagioclase, augite, olivine, and magnetite or titaniferous iron, to which apatite and other minerals may be added. These rocks range in texture from a black glass up to a coarsely crystalline mass wherein the component minerals are distinctly visible to the naked eye. Different names are employed to distinguish these varieties.

Basalt-glass (Tachylyte, Hyalomelan) is a general epithet to denote the vitreous varieties. These are 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 or fissure 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 (see pp. 97, 143). Basalt—a black, compact, heavy, homogeneous rock, breaking with a conchoidal fracture, showing sometimes large porphyritic crystals of plagioclase, olivine, or augite, but too finegrained 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