Some important Terms applied to Rocks.

Sedimentary-composed of sediment which may be either a mechanically suspended detritus, such as mud, sand, shells, or gravel; or a chemical precipitate, as rock-salt and calcareous tufa. The various deposits which are accumulated on the floors of lakes, in river-courses, and on the bed of the sea, are examples of sedimentary rocks.

Fragmental, Clastic-composed of fragments derived from some previous rock. All ordinary detritus is of this nature.

Concretionary-composed of mineral matter which has been aggregated round some centre so as to form rounded or irregularlyshaped lumps. Some minerals, particularly pyrite (Fig. 64 c), marcasite, siderite, and calcite, are frequently found in concretionary forms, especially round some organic relic, such as a shell or plant (Figs. 61, 65). In alluvial clay, calcareous concretions which often take curious imitative shapes, are known as "fairy stones" (Fig. 64, a, b; see p. 177).

When nodules of limestone, ironstone, or cement-stone are marked internally by cracks which radiate towards, but do not

reach, the outside, and are filled up with calcite or other mineral, they are known as Septaria or septarian nodules (Fig. 65, and layer 13 in Fig. 80).

Oolitic-made up of spherical grains, each of which has been formed by the deposition of successive coatings of mineral matter round some grain of sand, fragment of shell, or other foreign particle (Fig. 66). A rock with this structure looks like fish-roe,

hence the name oolite or roe-stone; but when the granules are like peas, the rock becomes pisolitic (pea-stone, Fig. 67). This peculiar structure is produced in water (springs, lakes, or enclosed

parts of the sea), wherein dissolved mineral matter (usually carbonate of lime) is so abundant as to be deposited in thin pellicles round the grains of sediment that are kept in motion by the current (p. 88).

Stratified, Bedded-arranged in layers, strata, or beds lying generally parallel to each other, as in ordinary sedimentary deposits (Fig. 79, p. 172).

Aqueous laid down in water, comprising nearly the whole of the sedimentary and stratified rocks.

Unstratified, Massive having no arrangement in definite layers or strata. Lavas and the other eruptive rocks are examples (Chapter XIV).

Eruptive, Igneous-forced upwards in a molten or plastic condition into or through the earth's crust. All lavas are Eruptive or Igneous rocks, also called Volcanic because erupted to the surface by volcanoes. In the same division must be classed granite and allied masses, which have been thrust through rocks at some depth within the earth's crust and may not have been directly connected with any volcanic eruption; such rocks are sometimes called Plutonic or Hypogene.

Crystalline consisting wholly or chiefly of crystals or crystalline grains. Rocks of this nature may have arisen from (a) igneous fusion, as in the case of lavas, where the minerals have separated out of a molten glass, or what is called a Magma; (b) aqueous solution, as where crystalline calcite forms stalactite and stalagmite in a cavern; (c) sublimation, where the materials have crystallised out of hot vapours, as in the vents and clefts of volcanoes.

By the aid of the microscope many rocks which to the naked eye show no definite structure can be shown to be wholly or partially crystalline. Moreover, it can often be ascertained that the crystals or crystalline grains in a rock, as they were crystallising out of their solution, have enclosed various foreign bodies. Among the objects thus taken up are minute globules of gas, which are prodigiously abundant in certain minerals in some lavas; liquids, usually water, enclosed in cavities of the crystals, but not quite filling them, and leaving a minute freely-moving bubble (Fig. 68); glass, filling globular spaces, probably part of the original glassy magma of the rock; crystals and crystallites (rudimentary crystalline forms, Fig. 69) of other minerals. Thus a crystal, which to the eye may appear quite free from impurities, may be found to be full of various kinds of enclosures. Obviously the study of these en

FIG. 68. Cavities in quartz containing liquids (magnified).

closures cannot but throw light on the conditions under which the rocks enclosing them were produced.

There are various types of crystalline structure which can best be examined under the microscope, as Holocrystalline, composed entirely of crystalline elements without any interstitial glass -one of the most characteristic types of this structure is found in

FIG. 69. Various forms of crystallites (highly magnified).

granite, hence it is sometimes termed the granitic or granitoid structure; Semi-crystalline, consisting partly of crystals, but with a ground mass or base which may be partly glassy or variously devitrified; Felsitic or microfelsitic-composed of indefinite halfeffaced granules and filaments (p. 163).

Glassy, Vitreous-having a structure and aspect like that of artificial glass. Some lavas, obsidian for example, have solidified as natural glasses, and look not unlike masses of dark bottle-glass. In almost all cases, however, they contain dispersed crystals, crystallites, or other enclosures. These substances have generally multiplied to such an extent in most lavas as to leave only small interstitial portions of the original glass, while in many cases the glass has entirely disappeared. When a glass has thus been converted into a dull, opaque, stony, or lithoid mass, or into a completely crystalline substance, it is said to be devitrified. microscope enables us to prove many crystalline eruptive rocks to have been once molten glass which by a process of devitrification have been brought into their present more or less crystalline condition (p. 97).

The

Porphyritic-composed of a compact or crystalline base or matrix, through which are scattered conspicuous crystals much larger than those of the base, and generally of some felspar. Many eruptive rocks have this structure and are sometimes spoken of as "porphyries." The large crystals existed in the rock while still in a mobile state within the earth's crust, while the minuter crystals of the base were developed by a later process of crystallisation during the consolidation of the rock. In the successive zones of growth which porphyritic crystals often present, we may note by the enclosed minerals some of the successive stages of consolidation.

Spherulitic-composed of or containing small pea-like globular bodies (Spherulites) which show a minutely fibrous internal structure radiating from the centre (Fig. 71, A). This structure is

FIG. 71.--Spherulites and fluxion-structure. A, Spherulites, as seen under the microscope (with polarised light). B, Fluxion-structure of Obsidian, as seen under the microscope.

particularly observable in vitreous rocks, where it appears to be one of the stages of devitrification (p. 144).

Perlitic. Many vitreous rocks show a minute fissured structure as one of the accompaniments of devitrification. In the structure termed perlitic the original glass has had a series of reticulated and globular or spiral cracks developed in it, sometimes giving rise to globules composed of successive thin shells.

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