12 
NATURE AND CAUSES OF VOLCANIC ACTION 
BOOK I 
these rocks lieen plicated into endless foldings, the axes of which traverse 
the British Islands with a north-easterly trend ; they have likewise been 
dislocated l)y many gigantic ruptures, which tend on the whole to follow 
the same direction. Tlie line of the Great Glen, the southern front of the 
Highlands, and the northern boundary of the Southern Uplands of Scotland, 
are conspicuous examples of the position and effect of some of the greater 
fractures in the structure of this country. 
The ridging up of any part of the terrestrial crust will afford some relief 
from pressure to the parts of the interior immediately underneath. If, as 
is probable, the material of the earth’s interior is at the melting point 
proper for the pressure at each depth, then any diminution of the pressure 
may allow the intensely heated substance to pass into the liquid state. It 
would be along the lines of terrestrial uplift that this relief would be given. 
It is there that active volcanoes are found. The molten material is forced 
upward under these upraised ridges by the subsidence of the surrounding 
regions. And where by rupture of the crust this material can make its way 
to the surface, we may conceive that it will be ejected as lava or as stones 
and ashes. 
"STewed in a broad way, such appears to be the mechanism involved in 
the formation and distribution of volcanoes over the surface of the earth. 
But obviously this explanation only carries us so far in the elucidation of 
^■olcanic action. If the molten magma flowed out merely in virtue of the 
influence of terrestrial contraction, it might do so for the most part tran- 
quilly, though it would probably be affected l)y occasional sudden snaps, as 
the crust yielded to accumulations of pressure. Human experience has 
no record of the actual elevation of a mountain-chain. We may believe 
that if such an event were to happen suddenly or rapidly, it would be 
attended with gigantic catastrophes over the surface of the globe. We 
can hardly conceive what would be the scale of a volcanic eruption 
attending upon so colossal a disturbance of the terrestrial crust. But 
the eruptions which have taken place within the memory of man have 
been the accompaniments of no such disturbance. Although they have 
been many in number and sometimes powerful in effect, they have 
seldom been attended with any marked displacement of the surrounding 
parts of the terrestrial crust. Contraction is, of course, continuously and 
regularly in progress, and W’e may suppose that the consequent subsidence, 
though it results in intermittent wrinkling and ^iplifting of the terrestrial 
ridges, may also be more or less persistent in the regions lying outside these 
ridges. There will thus be a constant pressure of the molten magma into 
the roots of volcanoes, and a persistent tendency for the magma to issue at 
the surface at every available rent or orifice. The energy and duration of 
outflow', if they depended wholly upon the effects of contraction, would thus 
vary with the rate of subsidence of the sinking areas, probably assundng 
generally a feeble development, Imt sometimes Imrsting into fountains of 
molten I'oek hundreds of feet in height, like those observed from time to 
time in Hawaii. 
