and the Nature of the Liquidity of Lavas. 199 
The phenomena of active volcanoes, and the protrusion of in- 
tumescent crystalline matter on so many points of the earth’s 
surface, and at all periods of its history, may be admitted to 
prove the continued existence beneath a very large area of that 
surface—if not the whole—of a mass of intensely heated crystal- 
line matter, having disseminated throughout its substance (in 
the manner already dwelt upon) some fluid or, fluids, such as 
water, affording an imperfect liquidity to the mass, and, by its 
intense elastic force, communicating to it a powerful tendency to 
expansion. Now suppose any considerable diminution to occur 
locally in the amount of pressure confining this expansible mass 
beneath the crust of the globe,—such as might be brought about 
by any extraordinary concurrence of the ordinary barometric, 
tidal, oceanic, or excavating causes (not to suggest others),— or, 
on the other hand, any considerable increase of its expansive 
tendency, owing to a local increase of temperature, from some 
unknown, but easily imagined, cause,—we should anticipate, as 
the necessary result, the violent fracture and elevation of the 
overlying crust of rocks, and the extrusion through some prin- 
cipal fissure, or line of fracture, of a ridge of the subterranean 
intumescent crystalline matter. 
It seems very probable that under such circumstances the cen- 
tral axis of the protruded ridge may retain its irregularly crystal- 
line grain and structure, but that the portions of crystalline 
matter that from either side would rush or be thrust up by 
pressure from behind (consisting partly of the weight of the 
overlying rocks on the semi-liquid matter below them) towards 
the opening should be subjected to so much internal friction of 
their crystalline particles, and so much pressure at right angles, 
or nearly so, to the direction of the movement, as must stretch 
and draw them out into parallel planes,—just as happened evi- 
dently to the striped and ribboned trachytes in the protruded 
dykes of Ponza and Palmarola. This friction and pressure 
would be extreme, of course, along the lateral parts of the pro- 
truded mass, that is, the selvages of the great dyke; which, if 
the original mass were granite, would thus appear composed of 
an axis of granite, passing on either side into gneiss (or squeezed 
granite) and further on into mica-schist. 
But every irregularity, whether on the large or the small scale, 
obstructing more or less the even motion of the layers, must 
create a waving or contortion in them, especially in the planes 
of slippery mica-plates, such as is exemplified even in hand- 
specimens of the Ponza trachytes, and also on the largest scale 
in the same locality. And the extreme irregularities of motion, 
occasioned on the upper layers of the intumescent mass by the 
pressure and resistance of the overlying beds, may be expected 
