258 DYNAJkllCAL GEOLOGY. 



endurance of the miners. The heat in this case was of local origin, as the 

 region is one of former igneous eruptions. 



2. The wide distribution of volcanoes over the globe affords evidence of 

 internal heat. Moreover, the ejection of melted rock through fissures has 

 taken place over all the continents ; in Nova Scotia, Canada, New England, 

 New Jersey, and the states south, the region of Lake Superior, the Kocky 

 Mountains, and western America; in Ireland, Scotland, and various parts 

 of Europe ; and so over much of the globe. Such facts favor the idea of an 

 internal source of heat. The heat of the earth's interior has reached toward 

 or to the surface for geological work in three ways. 



(a) By conduction outward attending the earths cooling. — The amount 

 thus received at the surface may have been sufficient to modify somewhat 

 the temperature of the oceans, and the earth's climates, during early geo- 

 logical time. At present it is inappreciable ; and yet, according to Kelvin, 

 the amount of heat now lost by the earth, as a consequence of cooling, is such 

 as would melt annually a complete covering of ice 0-0085 millimeter thick, to 

 water at 32° F., or bring 777 cubic miles of ice to the same state. 



(&) By fractures of the crust, and the escape of melted rock or hot vapors. 



(c) By an acctiniulation of sedimentary deposits over large regions. — It fol- 

 lows from the conditions of a globe having an internal source of heat, that 

 equal temperatures will exist, as a general thing, at equal depths ; in other 

 words, that isothermal planes, or more precisely, isogeothermal, will be par- 

 allel to the surface ; and that they will even bend upward to correspond with 

 the general curve of the broader mountain regions, and downward beneath 

 the oceanic depressions. Consequently, the isogeothermal planes will rise a 

 thousand feet for every thousand feet in depth of deposits spread out over 

 a wide area; and, as urged by Babbage, solidification, crystallization, and 

 other chemical changes may thus be occasioned in the inferior beds, provided 

 the accumulation reaches a depth adequate to raise upward the requisite 

 amount of heat. 



Again, the removal of rock-material from wide areas, as is done in the 

 slow processes of erosion, will tend to produce an equivalent depression of 

 the isogeothermal planes. 



Chemical and Physical Changes and Mechanical Action as 



Sources of Heat. 



Heat is evolved by chemical changes in which there is condensation, as in 

 liquids becoming solids, or gases becoming liquids, and in oxidation, etc. It 

 is often an effect of the natural decomposition of minerals, or vegetable or 

 animal matter. The oxidation of sulphides, and especially of the most com- 

 mon of them, pyrite and marcasite, is a source of heat in many mines, and 

 for many warm springs. In the formation of a pound of water from vapor, 

 heat enough is given out, says Tyndall, to melt five pounds of cast iron. 



The heat of lightning has also its effects among geological phenomena. 



