192 Mr. G. P. Scrope on the Formation of Craters, 
chytes, the broken and dislocated appearance of the leucites, 
felspars, and other crystals in many basalts ; the frequent arrange- 
ment of their longest axes in the direction of the bed of the rock, 
that is, of the movement of the lava when liquefied; the finer 
grain often exhibited towards the tail or extremity of a current 
than at its source, the brecciated lavas which appear to have 
enveloped fragments in great number of the same material with- 
out any fusion even of their finest angles. So also might be 
explained the more or less spongy, porous, and loosely crystal- 
line texture of many trachytes, and their disposition im thick 
beds or dome-shaped bosses, attesting their protrusion in a very 
imperfect state of liquidity, more resembling the intumescence 
of some kinds of dough in an oven than the fusion of metal in 
a furnace. 
And here let me remark, that Dr. Daubeny, and some other 
writers on volcanic phenomena, have spoken of the vesicles or 
air-bladders in lavas, as being proofs of their having been in a 
state of complete fusion. But have the loaves baked in our 
ovens been in fusion? The comparison of a cellular scoria with 
a loaf or a French roll will show that vesicles of precisely similar 
appearance to those of lavas are producible in substances of a 
pasty consistence, which owe their liquidity to an aqueous vehicle, 
the heat applied being only sufficient to develope the contained 
gases. Other kinds of baked cakes are porous rather than cel- 
lular, and aptly represent the texture of the earthy and porous 
trachytic lavas. 
Plutonic Rocks.—This theory as to the nature of the liquidity 
of many lavas appeared to me so reasonable, that I proceeded to 
examine its applicability to the still more generally crystalline 
plutonic rocks, from the alteration of which by heat lavas are 
usually supposed to derive. I asked myself, what would pro- 
bably be the effect on a mass of granite, for example, contaming 
water intimately combined with its molecular particles, and con- 
fined beneath overlying rocks and seas, under circumstances of 
intense compression, and at the same time high and increasing 
temperature? Surely a tendency to intumescence, which, where- 
ever, and in proportion to the extent to which it takes place, 
must elevate and fracture the overlying rocks, and likewise dis- 
integrate more or less the crystalline particles of the swelling 
mass, through the irregularities of their internal movements and 
mutual friction, Many of the crevices broken through the 
neighbouring rocks would be injected by the intumescent mat- 
ter. Some may be sufficiently enlarged to allow of its forcing 
its way into the open air as a lava, perhaps accompanied by 
eructations of the gases and vapours developed in the lower parts 
of the mass; or, should the liquefaction not be sufficient to admit 
