INTERIOR HEAT OF THE EARTH. g3 



deposits which are increased by the frequent alternate influx and evapo- 

 ration of sea- water. Conglomerate rocks are thus forming at the pres- 

 ent time in the Canaries and many other places. 



CHAPTER III. 



IGNEOUS AGENCIES. 



The agencies thus far considered tend to reduce the inequalities of 

 the earth by cutting down the continents and filling up the seas. Their 

 final effect, if unopposed, would be to bring the whole surface to one 

 level, and thus to make the empire of the sea universal. This is pre- 

 vented by igneous agencies, which tend, by elevation of land and de- 

 pression of sea-bottoms, to increase the inequalities of the earth-surface, 

 and thus to increase the area and the height of the land. All the dif- 

 ferent forms of igneous agency are connected with the interior heat of 

 the earth. This must, therefore, be first considered. 



Section 1. — Interior Heat of the Earth. 

 Stratum of Invariable Temperature.— The mean surface temperature 

 of the earth varies from 80° at the equator to nearly 0° at the poles. 

 The rate of decrease in passing from the equator to the poles is not the 

 same in all longitudes ; the isotherms, or lines joining places of equal 

 mean temperatures, are therefore not parallel to the lines of latitude, 

 but quite irregular. The mean temperature of the whole earth-surface 

 is about 58°. There is also in every locality a daily and an annual 

 variation of temperature. As we pass below the surface both the daily 

 and annual variations become less, until they cease altogether. The 

 stratum of no daily variation is but a foot or two beneath the surface; 

 but the stratum of no annual variation, or stratum of invariable tem- 

 perature in temperate climates, is about sixty to seventy feet deep. The 

 temperature of the invariable stratum is nearly the mean temperature 

 of the place. The depth of the invariable stratum depends upon the 

 amount of annual variation ; it is, therefore, least at the equator, and 

 increases toward the poles. At the equator it is only one or two feet 

 beneath the surface ; * in middle latitudes about sixty feet, and in high 

 latitudes probably more than a hundred feet, f It is, therefore, a sphe- 

 roid more oblate than the earth itself. The temperature of the earth 

 everywhere within this spheroid is unaffected by external changes. 



* Humboldt, Cosmos, Sabine's edition, vol. i, p. 165. 



f In polar regions, or the region of perpetual ground-ice, the stratum of invariable 

 temperature probably again rises nearer the surface, on account of the property of ice of 

 retaining its temperature by melting. 



