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198 DYNAMICAL GEOLOGY. — [Boox III. — 
over areas thousands of square miles in extent, and when the 
molten rock, instead of issuing, as it does at a modern volcano, in 
narrow streams from one or more points, welled out from the rents, 
and flooded enormous tracts of country without forming any moun- 
tain or volcano in the usual sense of these terms. Of these “ fissure- 
eruptions,” apart from volcanic cones, no examples have occurred 
within the times of human history, unless some of the lava-floods of 
Iceland can be so regarded. ‘They can only be studied from the 
remains of former convulsions. ‘Their importance, however, has not 
yet been generally recognised in Europe, though acknowledged in 
America, where they have been largely developed. Much still 
remains to be done before their mechanism is as well understood 
as that of the lesser type to which all present volcanic action 
belongs. Hence in the succeeding narrative an account is first 
given of the ordinary and familiar volcano and its products; and 
in § 3 ii, some details are given of the general aspect and character 
of the more gigantic fissure eruptions. 
The openings by which heated materials from the interior now 
reach the surface include volcanoes (with their various accompany- 
ing orifices) and hot-springs. 
The prevailing conical form of a volcano is that which the ejected 
materials naturally assume round the vent of eruption. The summit 
of the cone is truncated (Fig. 32) and presents a cup-shaped or 
cauldron-like cavity termed the crater, at the bottom of which is 
the top of the main funnel or pipe of communication with the heated 
interior. A volcano, when of small size, may consist merely of one 
cone ; when of the largest dimensions, it forms a huge mountain, 
with many subsidiary cones and many lateral fissures or pipes, from 
which the heated volcanic products are given out. Mount Etna 
(I’ig. 32) rising from the sea to a height of 10,840 feet, and support- 
ing as it does some 200 minor cones, many of which are in themselves 
considerable hills, is a magnificent example of a colossal volcano. 
The materials erupted from volcanic vents may be classed as 
(1) gases and vapours, (2) water, (3) lavas, (4) fragmentary substances. 
A brief summary under each of these heads may be given here; the 
share taken by the several products in the phenomena of an active 
volcano is described in § 2. 
1. Gases and Vapours exist absorbed in the molten magma 
within the earth’s crust. They play an important part in voleanie 
activity, showing themselves in the earliest stages of a volcano’s 
history, and continuing to appear for centuries after all the other 
evidences of subterranean action have ceased to be manifested. By 
much the most abundant of them all is steam, which has been estimated 
to form =%,2°,ths of the whole cloud that hangs over an active voleano. 
In great eruptions it rises in prodigious quantities, and is rapidly 
condensed into a heavy rainfall. M. Fouqué calculated that during 
100 days one of the parasitic cones on Etna had ejected vapour enough 
to form if condensed, 2,109,000 cubic metres (462,000,000 gallons) 
