116 Proceedings of the Eoyal Physical Society, 
suddenly produced by pressure acting on it from beneath, 
or in any other direction ; 
4^A, Occasionally, but rarely, to the recoil from mighty 
explosive effects at volcanic foci, as when a mass of rock 
weighing 200 tons was shot from the crater of Cotopaxi to 
the distance of nine miles, or when nearly one-half of the 
crater of Vesuvius was blown away. Earthquake pheno- 
mena are therefore considered to furnish very strong argu- 
ments in favour of a central heat. 
III. Rocks of Igneous Formation. — The argument derived 
from the structure of the igneous class of rocks assumes that 
the felstone and greenstone series came up from below the 
stratified rocks at a period long after these were deposited ; 
and as they bear marks of having ascended in a state of 
fusion, or nearly so, they afford direct evidence, as it were, 
that there is a very high temperature beneath the crust of 
the earth, and that melted rock has been in existence at no 
great depth below the earth's surface, long after it had 
become the habitation of organised beings. 
lY. Hot Springs, — The argument derived from the exist- 
ence of thermal springs is likewise considered to possess 
considerable weight. Hot springs are found in many parts 
of the world, at a temperature varying from a few degrees 
above the mean temperature of the climate up to the boil- 
ing point. Those springs possessing the highest tempera- 
ture generally occur in the vicinity of volcanoes, such as 
the well-known Geysers in Iceland, and may therefore be 
attributed to the operation of the same causes which pro- 
duce volcanic activity. There are other springs, however, 
scattered over the world at a distance from volcanoes, the 
high temperature of which it is difficult to account for by 
any known chemical cause. Humboldt relates an interest- 
ing example of common river-water sinking to a great 
depth, and again reappearing at the surface in the form of 
hot springs. In September 1759, JoruUo, in the plain of 
Mexico, was suddenly elevated by volcanic action to a 
height of 1682 feet above the surrounding plain. Two 
