But in the vast majority of cases, the operations at a volcanic vent do not end with the first explosions. Clouds of ashes and stones are ejected, and streams of molten lava are poured forth. In some instances, the chimney may be finally choked with volcanic blocks, scoriæ, cinders, and ashes, in others with consolidated lava. Examples of both kinds of infilling are found, and also others where the two forms of volcanic material occur together in the same vent.

A volcanic chimney filled up in this way with volcanic materials, and exposed by the removal of the lava or ashes thrown out to the surface is known as a Neck (see Figs. 40, 41, 42, 111, and p. 208). As these materials are usually harder and more durable than the surrounding rocks, they project above the general surface of the ground. The stump of the volcano is left as a hill, the form and prominence of which will chiefly depend upon the nature of the material; hard tough lava will rise abruptly, as a crag or hill, above the surrounding country, while consolidated ashes, scoriæ, and other fragmentary stuff will give a smoother and less marked outline.

These features will be best understood from a series of diagrams.

FIG. 40.-Outline of a Volcanic Neck.

We may take, by way of illustration, a neck composed mainly of fragmentary ejections, but with a plug of lava reaching its summit. The usual outlines of such a neck are represented in Fig. 40. There is nothing in the general form of this hill to suggest a volcanic origin; yet, if we examine its structure and that of the ground around it, we may find them to be as represented in Fig. 41, where the surrounding rocks are supposed to consist of various sandstones, clays, limestones, and other sedimentary deposits (a), through which the volcanic vent (b, c) has been drilled. The

FIG. 41.-Ground-plan of the structure of the Neck shown in Fig. 40.

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neck is represented as elliptical in cross section, composed mainly of consolidated volcanic ashes and blocks (b), but with a mass of lava (c) in the centre. The structure of the hill is explained in the vertical section, Fig. 42 (see also Fig. 111). We there see that the vent has been blown through the surrounding strata (a, a), and has been filled up mainly with fragmentary materials (b, b); but that through its centre there has risen a column or plug of lava (c), which not improbably marks the last effort of the volcano to force solid ejections to the surface. The line s, s indicates the present surface of the ground, after the prolonged waste during which all the volcanic cone has been removed. But we

FIG. 42. Section through the same Neck as in Figs. 40 and 41.

can in imagination restore the original surface, which may have been somewhat as shown by the dotted lines, the position of the crater being indicated at e, and its crest on either side at d, d. No trace is here left of the original volcanic cone. The present form of the ground is due to denudation, which has left the more durable volcanic rocks projecting above the surrounding strata. The continued progress of superficial degradation will remove still more of the neck, but the downward continuation of the volcanic column must always remain, and will probably always project as a hill. A volcanic neck is thus one of the most enduring and unmistakable evidences of the site of a volcano (see p. 208).

Of

Besides vents or funnels, other openings are made by volcanic explosions in the crust, which serve as receptacles of lava and ashes, and remain as durable memorials of volcanic action. these the most important are Fissures, which are formed in large numbers in and around a volcanic cone, but which may also arise at a distance from any actual volcano. During the convulsions of an eruption, the cone and the surrounding country are sometimes split by lines of fissure, which tend to

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radiate from the centre of disturbance, somewhat as cracks do in a pane of glass through which a stone is thrown. Sometimes the two sides of a fissure close together again, leaving no superficial trace of the dislocation. More frequently steam and various volcanic vapours escape from the chasm, and may deposit along the walls sublimates of different minerals, such as common salt, chloride of iron, specular iron, sulphur, and sal-ammoniac. These deposited substances may even continue to grow there until they entirely fill up the space between. In such cases, the line of fissure is marked by a vertical or steeply inclined band of minerals interposed between the ends of the rocks that have been ruptured and separated. But in most instances, the opening is filled up by the rise of lava from below. At night, the vents opened on the outside of an active volcano may be traced from afar by the glow of the white-hot lava that rises in them to within a short distance from the surface. When the lava cools and solidifies in these fissures, it forms wall-like masses, known as Dykes (Fig. 43). Inside many volcanic craters, the walls are traversed with dykes which, though on the whole tending to keep a vertical direction, may curve about irregularly according to the form of the vents into which the lava rose. Like the necks above described, dykes form enduring records of volcanic action. The superficial cones and craters may disappear, but the subterranean lava-filled fissures will still remain as records of volcanic action.

In some volcanic regions, where enormous floods of lava have been poured forth, no great central cones have existed. Such regions extend as vast black plains of naked rock, mottled with shifting sand-hills, or as undulating tablelands carved by running water into valleys and ravines, between which the successive sheets of lava are exposed in terraced hills. Beyond the limits over which the lava-sheets are spread, dykes of the same kinds of lava rise in abundance to the surface. There can be no doubt that the dykes do not terminate at the edge of the lava-fields, but pass underneath them. Indeed, as they increase in number in that direction, they, are probably more abundant underneath the lava than outside of the lava-fields. Sometimes sections are exposed showing how, after rising in a fissure, the lava has spread out on either side as a sheet. In these vast lava-plateaux or deserts, the molten rock, instead of issuing from one main central Etna or Vesuvius, appears to have risen in thousands of fissures opened in the shattered crust, and to have welled forth from numerous vents on these fissures, spreading out sheet after sheet till, like a rising lake, it has not only overflowed the lower grounds, but even buried all the minor hills. Such appears to have been the history of vast tracts in Western North America. The area which has there been flooded with lava has been estimated to be larger than that of France and Great Britain together, and the depth of the total mass of lava erupted reaches in some places as much as 3700 feet. Some rivers have cut gorges in this plain of lava, laying bare its component rocks to a depth of 700 feet or more. Along

the walls of these ravines we see that the lava is arranged in parallel beds or sheets often not more than 10 or 20 feet thick, each of which, of course, represents a separate outpouring of molten rock.

Except where such deep sections have been cut through them by rivers, recent lava-floods can only be examined along their surface, and we are consequently left chiefly to inference regarding their probable connection with fissures and dykes underneath. But in various parts of the world, lava-plains of much older date have been so deeply eroded as to expose not only the successive sheets of lava but the floor over which they were poured, and the abundant dykes which no doubt served as the channels wherein the lava rose towards the surface, till it could escape at the lowest levels, or at weaker or wider parts of the fissures. In Western Europe important examples of this structure occur, from the north of Ireland through the Inner Hebrides and the Faroe Islands to Iceland. This volcanic belt presents a succession of lava-fields which even yet, in spite of enormous waste, are in some places more than 3000 feet thick. The sheets of lava are nearly flat, and rise in terraces one over another into green grassy hills, or into the dark fronts of lofty sea-washed precipices. Where this thick cake of lava has been stripped off during the degradation of the land, thousands of dykes are exposed, and many of these traverse at least the lower parts of the sheets of lava. They form, as it were, the subterranean roots of which these sheets were the subaerial branches; and even where the whole of the material that reached the surface, more than 3000 feet thick, has been worn away, the dykes still remain as evidence of the reality and vigour of the volcanic forces.

EARTHQUAKES.

The rise of hot springs and the explosions of volcanoes furnish impressive testimony to the internal heat of our planet; but they are by no means the only proofs that the pent-up energy of the interior of the globe reacts upon the outer surface. By means of delicate instruments, it can be shown that the ground beneath our feet is subject to continual tremors which are too feeble to be perceived by the unaided senses. From these minuter vibrations, movements of increasing intensity can be detected up to the calamitous earthquake, whereby a country is shaken to its foundations, and thousands of human lives, together with much valuable property, are destroyed. We do not yet know by what different causes these various disturbances are produced. Some of the fainter tremors may arise from such influences as changes of temperature and atmospheric pressure, and the rise and fall of the tides. But the more violent must be assigned to causes working within the earth itself. The collapse of the roofs of underground caverns, the sudden condensation of steam or explosion of volcanic vapours, the snap of rocks that can no longer resist the strain to which, by the cooling and consequent contraction of the inner hot nucleus, they have been subjected within the earth's crust-these and other influences may at different times come into play to