GEOGNOSY AND GEOLOGY. 211 
The crater of Kirauea (pl. 47, fig. 4), on the island of Hawaii, with a depth 
of 1000 feet, and a circumference of eight miles, has fifty of such small cones 
of eruptions ; a night scene in this crater is shown in p/. 49, fig. 1. 
Similar phenomena are exhibited in the crater of a volcano on the island 
of Hawaii (pl. 47, fig. 5). Some volcanic cones are inclosed by a wall of 
gentle slope outside, but dipping abruptly towards the cone. Such is 
the Somma which surrounds Vesuvius, and is probably the wall of the 
ancient crater as it existed in the time of the Romans. | Pl. 45, fig. 4, 
is a supposed view of Vesuvius in the time of Pliny: fig. 5, as seen at the 
present day. 
The lava streams which accompany a volcanic eruption do not generally 
pour over the edge of the crater, but escape through fissures which may be 
formed in the sides. Among the most important volcanic products may be 
mentioned, lavas, pumice, various ejected matters, volcanic conglomerates. 
sublimates, and rocks altered by heat and vapors. 
By the term Java is meant all volcanic matters exhibiting a liquid molten 
character. Lavas have a very different appearance under different cir- 
cumstances; which difference, however, is rather accidental than essential. 
Eyen the same species of rock may exist under very different forms; thus 
pumice-stone is nothing else than trachyte in a frothy condition, and 
obsidian is the same, of a glassy and compact texture. Lavas may exist 
under the various forms of fillings, of strata, and as streams. ‘The fillings 
generally occur in fissures through which an eruption has taken place,and 
present a striking resemblance to some of the veins we have already 
considered. Lava strata are pseudo-morphous, deriving their stratiform 
character by penetrating between true strata. This, however, is not 
always the case ; it may happen that an earlier stream of lava, with the 
usual incumbent scoria and ashes, is covered by one of subsequent origin, 
and this, in like manner, by a third, &c., so that an alternation of stratiform 
masses of lavas and scoria may exist. The peculiarities of the masses are 
seen most conspicuously in the lava streams. These streams flow down the 
sides of the cone as far as the amount of the lava and the peculiarities of the 
soil may allow. 
The greatest lava stream of Mount Vesuvius had a length of 47,500 feet. 
That which took place in 1805, was 16,735 feet long, with a breadth of 8,542, 
and depth of 30-40 feet. 
Lava currents must naturally obey the laws which regulate other liquid 
masses. Should they meet with some obstruction in their course, such as a 
mountain or large rock, they divide into arms; flowing in a trough or 
channel they fill it up; pouring over precipitous descents, they form fiery 
cascades. They frequently run into the sea, and there sometimes form 
conspicuous features. Thus the lava stream which poured forth from 
Vesuvius, in 1794, ran into the sea, and formed a peninsula, eight hundred 
feet broad and seventeen feet high above the level of the sea. The surface 
of molten lavas soon cools, and forms a stiff crust beneath which the liquid 
mass still flows on. If this interior current be interrupted, it frequently 
breaks through the incumbent crust, and piles up on the surface, this 
ICONOGRAPHIC ENCYCLOPEDIA.—VOL. L 4l 641 
