Parr I. Sect.i.§1.] VOLCANIC PRODUCTS. 203 
divested of its snow in a single night by the heating of the mountain ; 
(2) from the condensation of the vast clouds of steam which are 
discharged during an eruption; this undoubtedly is the chief source 
_ of the destructive torrents so frequently observed to form part of the 
phenomena of a great volcanic explosion; and (3) from the disruption 
of reservoirs of water filling subterranean cavities, or of lakes 
occupying crater-basins; this has several times been observed among 
the South American volcanoes, where immense quantities of dead 
fish, which inhabited the water, have been swept down with the 
escaping torrents. The volcano of Agua, in Guatemala, has never 
been known to discharge anything but water. In the beginning of 
the year 1817 an eruption took place at the large crater of Idjen, 
one of the volcanoes of Java, whereby a hot steaming lake of acid 
water was discharged with frightful destruction down the slopes of 
the mountain. After the explosion the basin filled again with water, 
but its temperature was no longer high. S 
In many cases the water rapidly collects volcanic dust as it 
rushes down, and soon becomes a pasty mud; or it issues at first in 
this condition from the volcanic reservoirs after violent detonations. 
Hence arise what are termed mud-lavas, or aqueous lavas, which in 
many respects behave like true lavas. This volcanic mud eventually 
consolidates into one of the numerous forms of tuff, a rock which, as 
has been already stated (p. 161), varies greatly in the amount of its 
coherence, in its composition, and in its internal arrangement. 
Obviously, unless where subsequently altered, it can possess none of 
the crystalline structure of true lava. As a rule it betrays its 
aqueous origin by more or less distinct evidence of stratification, by 
the multifarious pebbles, stones, blocks of rock, tree-trunks, branches, 
shells, bones, skeletons, &c., which it has swept along in its course and 
preserved within its mass. Sections of this compacted tuff may be 
seen at Herculaneum. The érass of the Brohl Thal and other valleys in 
the Eifel district, referred to on p. 164, is another example of an 
ancient volcanic mud. 
3. Lava.—The term lava is applied generally to all the molten 
rocks of volcanoes." The use of the word in this broad sense is of 
great convenience in geological descriptions, by directing attention 
to the leading character of the rocks as molten products of volcanic 
action, and obviating the confusion and errors which are apt to arise 
from an ill-defined or incorrect lithological terminology. Precise 
definitions of the rocks, such as those above given in Book IL. can 
be added when required. A few remarks regarding some of the 
_ general lithological characters of lavas may be of service here; the 
behaviour of the rocks in their emission from volcanic orifices will 
be described in § 2. : 
While still flowing or not yet cooled, lavas differ from each other 
in the extent to which they are impregnated with gases and 
1 Alles ist Lava was im Vulkane fliesst und durch seine Fliissigkeit neue 
Lagerstatter eiunimmt” is Leopold yon Buch’s comprehensive definition. 
