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. [Illustration: Fig. 27.--Natural section of a Lava-stream in the Island of Vulcano, showing the compact central portion and the scoriaceous upper and under surfaces.] [Sidenote: COLUMNAR STRUCTURE OF LAVAS.] 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.