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). [Illustration: Fig. 24.--Concentric Folds on mass of cooled Lava.] 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). [Illustration: Fig. 25.--Mass of cooled Lava formed over a spiracle on the slopes of Hawaii.] [Sidenote: MINIATURE CONES ON LAVA-STREAMS.] 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. [Illustration: Fig. 26.--Group of small Cones thrown up on the Vesuvian Lava-current of 1856.] 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.