and the Nature of the Liquidity of Lavas. 189 
not in a state of complete fusion; that a large proportion at 
least, if not all, of the crystalline or granular particles of which, 
when cooled and consolidated, they appear composed, are already 
formed and solid, their mobility being aided by the intimate 
dissemination through the mass of a minute but appreciable 
quantity of some fluid,—in all probability water,—which is pre- 
vented from expanding wholly into vapour by the pressure to 
which it is subjected while within the volcanic vent, or in the 
interior of the current, until that pressure is sufficiently reduced 
to allow of its expansion in bubbles, or its escape through pores 
or cracks, by which it passes into the open air from the surface 
of the intumescent lava. 
I was strengthened in this opinion by several concurrent con- 
siderations :— 
1, If all lavas are (as they are usually supposed to be) in a 
state of complete fusion when they issue from a volcano, how is 
it that they do not all present the same glassy texture which is 
seen in some, the obsidians, the pitchstones, and pumiceous 
lavas especially, and in the ropy, cavernous, filamentous basalts 
of Kilauea, Iceland and Bourbon, and which these very erystal- 
line and stony lavas themselves put on when melted under the 
blowpipe or in a furnace? The usual answer is, that the gra- 
nular and crystalline texture is acquired subsequently to emission 
by slow cooling ; and the experiments of Gregory Watt and Sir 
James Hall are cited in support of this assertion. In the present 
day, probably the process by which Messrs. Chance and Co. of 
Birmingham, devitrify a mass of fused basalt (from the Rowley 
rag, near Dudley) by causing it to cool slowly in an “annealing 
furnace,” would be considered as a strong confirmatory fact. 
But there is no fact more certain than this, that the superficial 
portions at least of a lava-current flowing in the open air do not 
cool slowly. On the contrary, they are rapidly, I might say 
instantaneously, upon their exposure, consolidated and cooled 
down to a temperature which permits them to be handled and 
even walked upon without damage. How is it that this scori- 
form crust, or the solid cakes and slabs which so instantly form 
upon every exposed surface of lava, nay, even the scorise which 
are tossed up in a liquid state by the eruptive jets, and harden 
while yet in the air before they fall, exhibit on fracture no glassy 
texture, but much the same earthy or stony grain, and occasion- 
ally crystals of considerable size in the solid matter separating 
their cellular cavities, as is found in the interior of the current 
which is known to have cooled very slowly? How is it that 
some lava-currents are stony throughout, others vitreous through- 
out, as, for example, some of the large pumice-streams of Lipari, 
Iceland, and the Andes ? 
