116 Proceedings of the Royal Physical Society. 



suddenly produced by pressure acting on it from beneath, 

 or in any other direction ; 



4:th, 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. 



IV. 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, Jorullo, in the plain of 

 Mexico, was suddenly elevated by volcanic action to a 

 height of 1682 feet above the surrounding plain. Two 



