'-Hap. n 
VOLCANIC ERUPTIONS 
13 
2. The actual phenomena of volcanic eruptions, however, show that a 
source of explosive energy is almost always associated with them, and that 
while the transference of the subterranean molten magma towards the 
''olcanic vents may be referred to the results of terrestrial contraction, the 
violent discharge of materials from those vents must be assigned to some 
idnd of energy stored up in the substance of the earth’s interior. 
The deep-seated magma from which lavas ascend contains various 
Vapours and gases which, under the enormous pressure within and beneath 
the terrestrial crust, are absorbed or dissolved in it. So great is the tension 
of these gaseous constituents, that when from any cause the pressure on the 
loagma is suddenly relieved, they are liberated with explosive violence. 
A volcanic paroxysm is thus immediately the efiect of the rapid escape 
these imprisoned gases and vapours. With such energy does the 
Explosion sometimes take place, that the ascending column of molten lava is 
blown into the finest impalpable dust, which may load the air around a 
Volcano for many days before it falls to the ground, or may be borne in the 
'^'Pper regions of the atmosphere round the globe. 
The proportion of dissolved gases varies in different lavas, while the 
lavas themselves differ in the degree of their liquidity. Some flow out 
tranquilly like molten iron, others issue in a pasty condition and rapidly 
Congeal into scoriae and clinkers. Thus within the magma itself the amount 
explosive energy is far from being always the same. 
It is to the co-operation of these two causes— terrestrial contraction and 
Its effects on the one hand, and the tension of absorbed gases and vapours 
On the other — that the phenomena of v'olcanoes appear to be mainly due. 
There is no reasoir to believe that modern volcanoes differ in any essential 
respect fronr those of past ages in the earth’s history. It might, indeed, 
have been anticipated that the general energy of the planet would manifest 
itself in far more stupendous volcanic eruptions in early times than those of 
the modern period. But there is certainly no geological evidence in favonr 
of such a difference. One of the objects of the present work is to trace the 
continuity of volcanic phenonrena back to the very earliest epochs, and to 
show that. So far as the geological records go, the interior of the planet has 
reacted on its exterior in the same w’ay and with the same results. 
We may now proceed to inquire how far volcanoes leave behind them 
evidence of their existence. I shall devote the next two or three chapters 
lo a consideration of the proofs of volcanic action furnished by the very 
iiature of the materials brought up from the interior of the earth, by the 
arrangement of' these materials at the surface, by the existence of the actual 
I^jnnels or ducts from which they were discharged above ground, and by the 
*^^isposition of the masses of rock which, at various depths below the surface, 
Jiave been injected into and have solidified within the terrestrial crust. 
