Now it is found that those lava-streams which move slowly and present ropy surfaces give off but little steam during their flow, while those lava-streams which flow more rapidly and present a rough and cindery appearance give off vast quantities of steam. The extraordinary amount of vapour given off from the lava-streams which flowed from Vesuvius in 1872 is illustrated in the photograph copied in fig. 5 (facing page 24), in which the three lava-currents are each seen to be surmounted by enormous vapour-clouds rising to the height of several thousands of feet above them, and mingling with the column that issued from the central vent. By the escape of this enormous quantity of steam the surface of the lava was thrown into rugged cindery projections, and in some places little cones were formed upon it, which threw out small scoriæ and dust. The quantity of vapour was, in fact, so great, that little parasitical volcanoes were formed on the surface of the lava-stream. Some of these miniature volcanoes were of such small dimensions that they were carried away on boards to be employed as illustrations in the lecture-rooms of the University of Naples.

The arrangement of the materials forced out from fissures on the surfaces of lava-streams by the disengaged vapours and gases depends on the degree of fluidity of the lava, and the force of the escaping steam-jets. In very viscous lavas the materials may issue quietly, forming great concentric masses like coils of rope; such were described by Mr. Heaphy as occurring in New Zealand (see fig. 24).

In other cases the lava, if somewhat more liquid, may in issuing quietly without great outbursts of steam, accumulate in great bottle-shaped masses, which have been compared to 'petrified fountains.' Cases of this kind have been described by Professor Dana as occurring on the slopes of Hawaii (see fig. 25).

When the steam escapes with explosive violence from a spiracle ('bocca') on the surface of a lava-stream, minute cinder cones, like those described as being formed in 1872, are the result. Fig. 26 represents a group of miniature cones thrown up on the Vesuvian lava-stream of 1855: it is taken from a drawing by Schmidt.

Some of these appear like burst blisters or bubbles, while others are built up of scoriaceous masses which have been ejected from the aperture and have become united while in a semi-fluid condition. Other examples of these spiracles or bocche on the surfaces of lava-currents may be seen in the figs. 22 and 23, which are copied from photographs.

The facts we have described all point to the conclusion that the presence of large quantities of water imprisoned in a mass of lava contributes greatly to its mobility. And this conclusion is supported by so many other considerations that it is now very generally accepted by geologists. The condition of this imprisoned water in lavas is one which demands further investigation at the hands of physicists. It has been suggested, with some show of reason, that the water may exist in the midst of the red-hot lava as minute particles in the curious 'spheroidal condition' of Boutigny, and that these flash into steam as the lava flows along.

Lava, when extruded from a volcanic crater in a more or less completely fluid state, flows down the side of the cone, and then finds its way along any channel or valley that may lie in its course, obeying in its movements all the laws of fluid bodies. The lava-currents thus formed are sometimes of enormous dimensions, and may flood the whole country for many miles around the vent.

Lava-streams have been described, which have flowed for a distance of from fifty to a hundred miles from their source, and which have had a breadth varying from ten to twenty miles. Some lava-streams have a thickness of 500 feet, or even more. These measures will give some idea of the enormous quantities of material brought from the earth's interior by volcanic action and distributed over its surface. The mass of lava which flowed out during an eruption off Reykjanes in Iceland, in the year 1783, has been calculated to be equal in bulk to Mont Blanc.

There are many parts of the earth's surface, such as the Western Isles of Scotland and the North-east of Ireland, the Deccan of India, and large tracts in the Rocky Mountains, where successive lava-sheets have been piled upon one another to the height of several thousands of feet, and cover areas of many hundreds or even thousands of square miles.

The more fusible basic lavas are as a general rule more liquid in character than any others, and it is these very liquid lavas that are usually found forming plateaux built up of successive lava-streams. The less liquid lavas, like those of Hungary and Bohemia, are not usually found flowing to such distances from the vent, but form dome-shaped mountain-masses.

Lava-streams usually exhibit in their upper and under surfaces a scoriaceous texture due to the escape of steam from the upper surface, portions of the cindery masses so formed falling off from the end of the stream, and being rolled over by the stream so as to form its base. The thickness of this scoriaceous upper and lower part of a lava-stream varies according to the quantity of steam imprisoned in it; but all thick lava-streams have a compact central portion which is composed of hard, solid rock. Very good examples of the internal structure of lava-streams may sometimes be examined in the sea-cliffs of volcanic islands. In fig. 27 we have given a copy of a drawing made while sailing round the shores of Vulcano. The scoriaceous portions of lava-streams are sometimes employed, as at Volvic in the Auvergne, as a building material, or as at Neidermendig in the Eifel and in Hungary for mill-stones; the compact portions are employed for building and paving, and for road metal. The rock of some of the modern lava-streams of Vesuvius is largely quarried for paving the streets of Naples.

This solid portion of the lava-streams in slowly cooling down from its highly-heated condition undergoes contraction, and in consequence is rent asunder by a number of cracks. Sometimes these cracks assume a wonderfully regular arrangement, and the rock may be broken up into very symmetrical masses.

If we imagine a great sheet of heated material, like a lava-stream, slowly cooling down, it is evident that the contraction which must take place in it will tend to produce fissures breaking up the mass into prisms. A little consideration will convince us what the form of these prisms must be. There are only three regular figures into which a surface can be divided, namely, equilateral triangles, squares, and regular hexagons; the first being produced by the intersection of sets of six lines radiating at angles of 60° from certain centres; the second by the intersection of sets of four lines radiating from centres at angles of 90°; and the third from sets of three lines radiating from centres at an angle of 120°. It is evident that a less amount of contractile force will be required to produce the sets of three cracks rather than those of four or six cracks; or, in other words, the contractile force in a mass will be competent to produce the cracks which give rise to hexagons rather than those which form squares or triangles. This is no doubt the reason why the prisms formed by the cooling of lava, as well as those produced during the drying of starch or clay, are hexagonal in form.

The hexagonal prisms or columns formed by contraction during the consolidation of lavas vary greatly in size, according to the rate of cooling, the nature of the materials, and the conditions affecting the mass. Sometimes such columns may be found having a diameter of eight or ten feet and a length of five hundred feet, as in the Shiant Isles lying to the north of the Island of Skye; in other cases, as in certain volcanic glasses, minute columns, an inch or two in length and scarcely thicker than a needle, are formed; and examples of almost every intermediate grade between these two extremes may sometimes be found. The largest columns are those which are formed in very slowly cooling masses.

The columnar structure is exhibited by all kinds of lava, and indeed in other rock-masses which have been heated by contact with igneous masses and gradually cooled. The rocks which display the structure in greatest perfection, however, are the basalts.

Mr. Scrope first called attention to the fact that the upper and lower portions of lava-streams sometimes cool in very different ways, and hence produce columns of dissimilar character. The lower portion of the mass parts with its heat very slowly, by conduction to the underlying rocks, while the upper portions radiate heat more irregularly into the surrounding atmosphere. Hence we often find the lower portions of thick lava-streams to be formed of stout, vertical columns of great regularity; while the upper part is made up of smaller and less regular columns, as shown in fig. 28.

The remarkable grotto known as Fingal's Cave in the Island of Staffa has been formed in the midst of a lava-stream such as we have been describing; the thick vertical columns, which rise from beneath the level of the sea, are divided by joints and have been broken away by the action of the sea; in this way a great cavern has been produced, the sides of which are formed by vertical columns, while the roof is made up of smaller and interlacing ones. The whole structure bears some resemblance to a Gothic cathedral; the sea finding access to its floor of broken columns, and permitting the entrance of a boat during fine weather. Similar, though perhaps less striking, structures are found in many other parts of the globe wherever basaltic and other lava-streams exhibit the remarkable columnar structure as the result of their slow cooling. Portions of basaltic columns are often employed for posts by the road-sides, as in Central Germany and Bohemia, or for paving stones, as in Pompeii and at the Monte Albano near Rome.

Occasionally basaltic lava-streams exhibit other curious structures in addition to the columnar. Thus some basaltic columns are found divided into regular joints by equidistant, curved surfaces, the joints thus fitting into one another by a kind of ball-and-socket arrangement. Sometimes we find processes projecting from the angles of the curved joint-surfaces, which cause the blocks to fit together as with a tenon and mortise. This kind of structure is admirably displayed at the Giant's Causeway, Co. Antrim, in the North of Ireland. A portion of a basaltic column from this locality is represented in fig. 29.

While the ordinary columnar structures are very common in basalts, the ball-and-socket and tenon-and-mortise structures are exceedingly rare. The question of the mode of origin of these remarkable structures has given rise to much discussion, and the opinions of geologists and physicists are by no means unanimous upon the subject.

Sometimes we find masses of lava traversed by curved joints, and occasionally we find curious combinations of curved and plane joints, giving rise to appearances scarcely less remarkable than those presented by the columns of the Giant's Causeway. Some of the more striking examples of this kind have been described and explained by Professor Bonney.

In the Ponza Islands there occurs a remarkable example of a columnar pitchstone, which is also traversed by a member of curved concentric joints, causing the rock to break up into pieces like the coats of an onion. This remarkable rock-mass is represented in fig. 30.

A very similar structure is often seen in certain glassy lavas, when they are examined in thin sections under the microscope. Such glassy lavas exhibit the peculiar lustre of mother-of-pearly doubtless in consequence of the interference of light along the cracks. Lavas exhibiting this character are known to geologists as 'perlites.' The perlitic structure has been produced artificially by Mr. Grenville Cole in Canada Balsam, and by MM. Fonqué and Michel Lévy, in chemically deposited silica. See fig. 31.

A thick lava-stream must take an enormous period to cool down—probably many hundreds or even thousands of years. It is possible to walk over lava-streams in which at a few inches below the surface the rock is still red-hot, so that a piece of stick is lighted if thrust into a crack. Lava is a very bad conductor of heat, and loose scoriæ and dust are still worse conductors. During the eruption of Vesuvius in 1872, masses of snow which were covered with a thick layer of scoriæ, and afterwards by a stream of lava, were found three years afterwards consolidated into ice, but not melted. The city of Catania is constantly supplied with ice from masses of snow which have been buried under the ejections of Etna.

During the cooling down of lavas, the escape of steam and various gases gives rise to the deposition of many beautiful crystalline substances in the cavities and on the surfaces of the lava. Deposits of sulphur, specular-iron, tridymite, and many other substances are often thus produced, and the colour and appearance of the rock-masses are sometimes completely disguised by these surface incrustations, or by the decomposition of the materials of the lava by the action of the add gases, and vapours upon it.

Very frequently the surface of a lava-stream becomes solid, while the deeper portions retain their fluid condition; under such circumstances the central portions may flow away, leaving a great hollow chamber or cavern. In consequence of this action, we not unfrequently find the upper surface of a lava-current exhibiting a depression, due to the falling in of the solidified upper portions when the liquid lava has flowed away and left it unsupported, as in fig. 32.