190 Mr. G. P. Serope on the Formation of Craters, 
I have recently visited the manufactory of Messrs. Chance and 
Co., at Oldbury, near Birmingham, for the purpose of examining 
the mode in which the basalt used there (and which is the same 
upon which Mr. Gregory Watt experimented) conducts itself in 
their furnaces, and I found that when the liquid and fused con- 
tents of a furnace at a white heat are poured out upona brick or 
other floor into the open air, so as to represent a stream of lava 
flowing out of a volcanic vent, the mass consolidates throughout, 
whatever its bulk, into a homogeneous and purely vitreous black 
obsidian, in fact an absolute glass, with a conchoidal fracture 
and sharp cutting edges. It is only when made to consolidate 
very slowly in an oven kept at a high temperature for some days, 
that it assumes the deadened and semi-crystalline texture of the 
manufactured article. 
If this process be interrupted, it is found to have commenced 
by the formation, at numerous points within the vitreous mass, 
of globular coneretions about the size of a small pea, of a lighter 
colour than the base, and having a pearly lustre and radiated 
structure. The multiplication and confusion of these crystallites 
or sphzrulites ultimately destroy the glassy character of the sub- 
stance altogether, and give to it a pearly semi-crystalline texture, 
without, however, restoring the far more crystalline aspect of the 
basaltic rock. A similar change may be often observed to have 
taken place in nature among the vitreous lavas, which pass into 
pearlstone and pitchstone by the formation of the same kind of 
spherulitic concretions, and of course there is no question as to 
the complete state of fusion in which such lavas have been pro- 
duced. But there is no trace of such a process in any of the 
ordinary earthy, and stony or crystalline and porphyritic lavas. 
I am not aware of a single current from either Etna or Vesuvius 
having ever exhibited, even on its most rapidly cooled surfaces, 
any passage into true obsidian, or sphzerulitic pearlstone, or any 
portion of such vitrifactions. A pellicle or glaze of a semi- 
vitreous appearance coats the surface in some parts, or lines the 
cellular cavities ; but it seems evident that the bulk of the matter 
could not have been at the time of its emission in that thoroughly 
fused condition which it assumes when melted in a furnace or 
under the blowpipe. 
2. It struck me that temperature does not alone determine 
the fusion or liquefaction of substances ; and that compression 
may prevent the liquefaction of a solid at a high temperature, 
just as it prevents the vaporization of a liquid, in the common 
experiment of boiling water at a lower temperature in a rarefied 
atmosphere. If so, the intense pressure to which heated lava 
must be subjected before it rises from the bowels of the earth to 
discharge itself on the surface, intensified by the reaction of its 
