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Parr I Szcr.i.§2] LAVA-STREAMS. 229 
"In passing from a fluid to a solid condition, and thus contracting, 
lava acquires different structures. Lines of divisional planes or 
joints traverse it, especially perpendicular to the upper and under 
surfaces of the sheet. These sometimes assume prismatic forms, 
a 


Fic. 44.—ELONGATION OF VESICLES IN DIRECTION OF FLow oF Lava. 
dividing the rock into columns, as is so frequently to be observed in 
basalt. They are described in Book IY. Part ii., together with other 
forms of joints. 
Vapours and sublimations of a lava-stream.—Besides 
- steam, many other vapours absorbed in the original subterranean 
molten magma escape from fissures of a lava-stream. ‘The points at 
which such vapours are copiously disengaged are termed fumaroles. 
Among the exhalations, chlorides abound, particularly chloride of 
sodium, which appears, not only in fissures, but even over the cooled 
crust of the lava, in small crystals, in tufts, or as a granular and 
even glassy incrustation, Chloride of iron is deposited as a yellow 
coating at fumaroles, where also bright emerald green films and 
scales of chloride of copper may be more rarely observed. Many 
chemical changes take place in the escape of these vapours. Thus 
specular-iron, either the result of the mutual decomposition of steam 
and iron chloride, or of the oxidation of magnetite, forms abundant 
scales, plates, and small crystals in the fumaroles and vesicles of some 
lavas. Sal-ammoniac also appears in large quantity on many lavas, 
_ not merely in the fissures, but also on the upper surface. This salt 
is not directly a volcanic product, but results from some decom- 
position, probably from that of the aqueous vapour, whereby a 
combination is formed with atmospheric nitrogen. 
Slow cooling of lava.—The hardened crust of a lava-stream 
is a bad conductor of heat. Consequently, the surface of the stream 
_ may have become cool enough to be walked upon, though the red- 
hot mass may be observed through the rents to lie only a few inches 
below. Many years therefore may elapse before the temperature of 
the whole mass has fallen to that of the surrounding soil. Eleven 
months after an eruption of Etna, Spallanzani could see that the 
lava was still red-hot at the bottom of the fissures, and a stick 
thrust into one of them instantly took fire. The Vesuvian lava of 
1785 was found by Breislak seven years afterwards to be still hot and 
steaming internally, though lichens had already taken root on its 
surface. The ropy lava erupted by Vesuvius in 1858 was observed 
by the author in 1870 to be still so hot, even near its termination, 
that steam issued abundantly from its rents, many of which were too 
