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)..

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. 118). Stratified, Bedded-arranged in layers, strata, or beds lying generally parallel to each other, as in ordinary sedimentary deposits (Fig. 79, p. 227).

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

All

plastic condition into or through the earth's crust. lavas are Eruptive or Igneous rocks. 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.

Crystalline-consisting wholly or chiefly of crystals or crystalline grains which have taken their forms by crystallising where they are now found. 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 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 FIG. 68. Cavities in quartz leaving a minute freely-moving containing liquids (magnified). bubble (Fig. 68); glass, filling globular spaces and probably portions of the original glassy magma, out of which the crystals formed; 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 enclosures can

FIG. 69.-Crystallites (highly magnified).

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

Glassy, Vitreous-having a structure and aspect like that of artificial glass. Some lavas, obsidian for example, are natural glasses, and look not unlike masses of dark bottleglass. In almost all cases, however, they contain dispersed crystals, crystallites, or other enclosures, and these substances have sometimes multiplied to such an extent as to take the place of the original glass. When a glass is thus converted into a dull, opaque, stony, or lithoid substance, it is said to be devitrified. The microscope enables us to detect traces of an original glassy condition in many crystalline eruptive rocks which, consequently, are thus ascertained to have been once molten glass that has been devitrified by the development of crystals and crystallites.

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 or as being porphyritic. The large crystals have probably been formed in the rock while still in a mobile state within the earth's crust, while the minuter crystals of the base have been developed during the consolidation of the rock at the surface; in the successive zones of growth which porphyritic crystals often present,

we may note by the enclosed minerals the stages of consolidation of the rock.

Spherulitic-composed of or containing small pea-like

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.

globular bodies (spherulites) which show a minutely fibrous internal structure radiating from the centre (Fig. 71, A). This