Vesicular, Cellular-containing spheroidal or irregularly shaped cavities. In many eruptive rocks (as in modern lavas) the expansion of interstitial steam, while the mass was still in a molten condition, has produced this cellular structure (Fig. 35), the vesicles have usually remarkably smooth walls; they may form a comparatively small part of the whole mass, or they may so increase as to make pieces of the rock capable of floating on water. Where the vesicular structure is conjoined with more solid parts, as in the irregular slags of an iron furnace, it may be called slaggy. Where, as in the scoriæ of a volcano, the cellular and solid parts are in about equal proportions, and the vesicles vary greatly in numbers and size within short distances, the structure may be termed scoriaceous. The lighter and more frothlike varieties that can float on water are said to be pumiceous, or to have the characters of pumice (p. 162). Exposed to the influence of percolating water, vesicular rocks have had their vesicles filled up by the deposition of various minerals from solution, especially quartz, calcite, and zeolites. These substances first begin to encrust the walls of the cells, and as layer succeeds layer they gradually fill the cells up (Fig. 52); as the cells have not infrequently been elongated in one direction by the motion of the rock before consolidation was completed (Fig. 37), the mineral deposits in them, taking their exact moulds, appear as oval or almond-shaped bodies. Hence rocks which have been treated in this way are called Amygdaloids, and the kernels filling up the cells are known as Amygdules (Fig. 35). An amygdaloidal rock, therefore, was originally a molten lava, rendered cellular by the expansion of its absorbed steam and gases, its vesicles having been subsequently filled up by the deposit in them of mineral matter, often derived out of the surrounding rock by the decomposing and rearranging action of percolating water. Flow-structure, Fluxion-structure an arrangement of the crystallites, crystals, or particles of a rock in streaky lines, the minuter forms being grouped round the larger, indicative of the internal movement of the mass previous to its consolidation. The lines are those in which the particles flowed past each other, the larger crystals giving rise to obstructions and eddies in the movement of the smaller objects past them. This structure is characteristic of many once molten rocks; it is well seen in obsidian (Fig. 71, B). But it is also found in rocks which, by enormous stresses within the earth's crust, have been crushed and made to undergo an interstitial movement like that of the flow of

liquids. The most solid gneisses and granites have in this way been so sheared and squeezed that their component minerals have been crushed into a fine compact mass, through which the streaking lines of flow are sometimes displayed with singular clearness.

Mylonitic-a name sometimes applied to rocks which by terrestrial movement have had their original structure entirely obliterated, and which now present only a dull, crushed felsitic mass, sometimes partially or completely recrystallised.

Schistose, Foliated-consisting of minerals that have crystallised in approximately parallel, wavy, and irregular laminæ, layers,

or folia (Fig. 72). Such rocks are called generally schists. They have, in large measure, been formed by the alteration or metamorphism of other rocks of various kinds by the vast terrestrial movements referred to in the foregoing paragraphs (see Chapter XIII).

CLASSIFICATION OF ROCKS

Various schemes of classification of rocks are in use among geologists, some based on mode of origin, others on mineral composition or structure. For the purpose of the learner, perhaps the most instructive and useful arrangement is one which as far as possible combines the advantages of both these systems. Accordingly, in the following account of the more important rocks which enter into the structure of the earth's crust, a threefold

subdivision will be adopted into: (i) sedimentary rocks; (ii) eruptive rocks; (iii) schistose rocks.

I. SEDIMENTARY ROCKS.

This division includes the largest number, and to the geologist the most important of the rocks accessible to our notice. It comprises the various deposits that arise from the decay of the surface of the land and are laid down on the land or over the bed of the sea, together with all those directly or indirectly due to the growth of plants and animals. It thus embraces those which constitute the main mass of the earth's crust so far as known to us, and which contain the evidence whence the geological history of the earth is chiefly worked out. It is, therefore, worthy of the earliest and closest attention of the student.

Sedimentary rocks, being due to the deposition of some kind of sediment or detritus, are obviously not original or primitive rocks. They have all been derived from some source, the nature of which, if not its actual site, can usually be determined. In no case, therefore, can sedimentary rocks carry us back to the beginning of things; they are themselves derivative and presuppose the existence of some older rock or material from which they could be derived.

One of their most obvious characters is that, as a rule, they are stratified. They have been deposited, usually in water, sometimes in air, layer above layer, and bed above bed, each of these strata marking a particular interval in the progress of deposition (Chapter XII). As regards their mode of origin, they may be subdivided into three great sections: (1) fragmental or clastic, composed of fragments of pre-existing rocks; (2) chemically precipitated, as in the deposits from mineral springs; and (3) formed of the remains of organisms, as in peat and coral-rock.

(1) Fragmental or Clastic Rocks.

These are masses of mechanically-formed sediment, derived from the destruction of older rocks; they vary in coherence from loose sand or mud up to the most compact sandstone or conglomerate; they are accumulating abundantly at the present time in the beds of rivers and lakes, and on the floor of the sea, and they have been formed in a similar way all over the globe from the earliest periods of known geological history. Some of the more frequent kinds are the following:

Cliff-Debris-coarse angular rubbish, including large blocks of stone, disengaged by the weather from cliffs and other bare faces of rock. This kind of detritus is formed abundantly in rugged and mountainous regions, especially where the action of frost is severe; it slides down the slopes and accumulates at their foot, unless washed away by torrents. In glacier-valleys it descends to the ice, where, gathering into moraines (Chapter VI), it is transported to lower levels. The perched blocks of such valleys are some of the larger fragments of this cliff-debris left stranded by the ice, and from around which the smaller detritus has been washed away (Fig. 23).

Soil, Subsoil, described in Chapter II, represent the result of the subaerial decomposition of the surface of the land.

Breccia a rock composed of angular fragments. Such a rock shows that its materials have not travelled far; otherwise, they

would have lost their edges, and would have been more or less rounded. Ordinary cliff-debris may consolidate into a breccia, more especially where it falls into water and is allowed to gather on the bottom. The angular fragments shot out of a volcano often accumulate into volcanic breccia (Fig. 73). A rock with abundant angular fragments is said to be brecciated.

Gravel-loose rounded water-worn detritus, in which the pebbles range in average size between that of a small pea and that of a walnut; where they are larger they form Shingle. They may

consist of fragments of any kind of rock, though having resulted from more or less violent water-action, as a rule, pieces of only the more durable stones are found in them. Quartz and other siliceous materials, from their great hardness, are better able to withstand the grinding to which the detritus on an exposed seashore, or in the bed of a rapid stream, is subjected. Hence quartzose and siliceous pebbles are the most frequent constituents of gravel and shingle.

Conglomerate-a name given to gravel and shingle when they have been consolidated into stone, the pebbles being bound together by some kind of paste or cementing material, which may

be fine hardened sand, clay, or some calcareous, siliceous, or ferruginous cement (Fig. 74). As above remarked with regard to gravel, the component materials of conglomerate may have been derived from any kind of rock, but siliceous pebbles are of most common occurrence. Different names are given to conglomerates, according to the nature of the pebbles, as quartz-conglomerate, flint-conglomerate, limestone-conglomerate.

Sand a name given to fine kinds of detritus, the grains of which may vary from the size of a small pea down to minute particles that can only be detected with a lens. In general, for the reason already assigned in the case of gravel, the component grains of sand are of quartz or of some other durable material. Examined with a good magnifying glass, they are seen to be usually rounded, water-worn, but sometimes angular, unworn particles of indefinite shapes which, except in their smaller size,