

718 



DYNAMICAL GEOLOGY. 



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

 parts of Europe ; and so over much of the globe. 



Such facts strongly favor the view that igneous eruptions must for 

 the most part have come either from liquid rock beneath the crust, or 

 from great fire-seas, within it, which originated from the earth's liquid 

 interior. This point is further discussed on page 822. 



B. 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 Earth's cooling. — The 

 amount thus received at the surface may have been sufficient to have 

 modified somewhat the temperature of the oceans, and the earth's cli- 

 mates, during early geological time. At present it is inappreciable ; 

 and yet, according to Thomson, 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, *0085 millimeter thick, to water at 32° F., 

 or bring 777 cubic miles of ice to the same state. 



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

 vapors. 



(c.) By an Accumulation over large Regions of a. great thickness of 

 Sedimentary Deposits. — It follows from the conditions of a globe hav- 

 ing 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 parallel 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 thou- 

 sand 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 up- 

 ward 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 de- 

 pression of the isogeothermal planes. 



With reference to a covering of ice over a large region of the globe in a Glacial era, 

 Mr. O. Fisher has deduced by calculation, allowing for the difference in the conductiv- 

 ity of ordinary rock and ice (-00581 and -00218), and for the lowering of the melting 

 point 0-0137° F. for each additional atmosphere of pressure, and taking one sixtieth of a 

 degree Fahrenheit as the average increase for a foot of descent in the earth's strata, 

 that, if the mean temperature of the ice at surface were 0° F., the ice would have for its 

 limit of thickness at which its lower surface would begin to melt, 714 feet. Further, the 

 heat used in the melting diminishes thus much the amount that would otherwise be con- 

 ducted through into space; but all will be used up in the melting after the ice has at- 

 tained a thickness of sixteen miles. The above if is one suggested by Croll with ref- 

 erence to a polar ice-cap ; it could not be a fact in any Glacial era for zones outside of 



