CHAPTER II 
The Nature and Causes of Volcanic Action — Modern Volcanoes. 
A VOLCANO is a conical or clome-sliaped hill or nioniitain, consisting of 
tnaterials whicli have been ernpte.cl from an orifice leading down from the 
surface into the heated interior of the earth. Among modern and recent 
volcanoes three types may he recognized. In the first and most familiar 
of these, the lavas and ashes ejectetl from the central vent have gathered 
around it by successive eruptions, until they have huilt up a central cone 
like those of Etna and Vesuvius. As this cone grows in height and 
diameter, lateral or parasitic cones are formed on its Hanks, and may become 
themselves the chief actively erujrting vents. This type of volcano, which 
has been so long w'oll known from its Mediterranean examples, was until 
recently believed by geologists to be the normal, or indeed the only, phase of 
volcanic energy on the face of the earth. 
A modification of this type is to be found in a few regions where frag- 
mentary discharges are small in amount and where the eruptions are almost 
wholly confined to the emission of tolerably licpiid lava. A vast dome 
with gently sloping declivities may in this way he formed, as in the 
Sandwich Islands and in certain parts of Iceland. 
The second type of volcano is at the present day extensively developed 
only in Iceland, hut in Tertiary time it appears to have had a wide range 
over the globe, for stupendous memorials of it are preserved in North- 
Western Europe, in Western America, and in India. It is distinguished 
by the formation of numerous parallel fissures from wliich the lava gushes 
forth, either with or without the formation of small cinder-cones along the 
lines of the chasms. 
The third type is distinguished by the formation of groups of cinder- 
cones or lava-domes, which from their admirable development in Central 
France have received the name of Pays. From these vents considerable 
streams of lava have sometimes been discharged. 
Without entering here 'into a detailed imjuiry regarding the nature and 
causes of Volcanic Action, we may with advantage consider briefly the two 
main factors on which this action appears to depend. 
1. Much uncertainty still exists as to the condition and composition of 
the earth’s interior. The wide distribution of volcanoes over the globe, 
together with the general similarity of materials brought by them up to the 
