eruption has ceased and the volcano relapses into quiescence ; or (0) 
after volcanic action has otherwise become extinct. Of the former — 
phase instances are on record at Vesuvius where an eruption has 
been followed by the emission of this gas so copiously from the 
ground as to suffocate hundreds of hares, pheasants, and partridges. 
Of the second phase good examples are supplied by the ancient vol- 
canic regions of the Eifel and Auvergne, where the gas still rises in 
prodigious quantities. Bischof estimated that the volume of carbonic 
acid evolved in the Brohl Thal amounts to 5,000,000 cubic feet, or 
300 tons of gas in one day. Nitrogen, derived perhaps from the 
decomposition of atmospheric air dissolved in the water which — 
penetrates into the volcanic foci, has been frequently detected 
among the gaseous emanations. At Santorin it was found to form 
from 4 to 88 per cent. of the gas obtained from different fumaroles.’ 
With these gases and vapours are associated many substances 
which, sublimed by the volcanic heat or resulting from reactions 
among the escaping vapours, appear as deposits along crevices and 
surfaces wherein they reach the air and are cooled. Besides sul- 
phur, there are several chlorides (particularly that of sodium, 
and less abundantly those of iron, copper, and lead); also free 
sulphuric acid, sal-ammoniac, specular iron, oxide of 
copper, boracic acid, alum, sulphate of lime, and other 
substances. Sodium chloride sometimes appears so abundantly that — 
wide spaces of a volcanic cone, as well as of the newly-erupted 
lava, are crusted with salt, which can even be profitably removed 
by the inhabitants of the district. Considerable quantities of these 
chlorides may thus be buried between successive sheets of lava, 
and in long subsequent times may give rise to mineral springs, as 
has been suggested with reference to the saline waters which 
issue from volcanic rocks of Old Red Sandstone and Carboniferous 
age in Scotland.? The iron-chloride forms a bright yellow and 
reddish crust on the crater walls, as well as on loose stones on the slopes 
of the cone. Specular iron from the decomposition of iron-chloride 
forms abundantly as thin lamelle in the fissures of Vesuvian lavas. In 
the spring of 1873 the author observed delicate brown filaments of 
tenorite (copper-oxide, CuO) forming in clefts of the crater of 
Vesuvius. ‘They were upheld by the upstreaming current of vapour 
until blown off by the wind. J ouqué has described tubular vents 
in the lavas of Santorin wherein crystals of anorthite, sphene and 
pyroxene have recently been formed by sublimation. 
2, Water.—In connection with the aqueous vapour of volcanoes, 
reference may be made here to the abundant discharges of water which 
accompany some volcanic explosions. ‘Three sources of this water may 
be assigned :—(1) from the melting of snow by a rapid accession of 
temperature previous to or during an eruption; this takes place 
from time to time on Ktna, in Iceland, and among the snowy ranges 
of the Andes, where the cone of Cotopaxi is said to have been entirely 
! Fouqué, loc. cit. * Geikie, Proc. Roy. Soc. Edin, ix. p. 367, 

B: 
202 DYNAMICAL GEOLOGY. [Boo IIT. 
