HEAT. 697 



V. HEAT. 



The effects of heat here considered are those affecting the rock-ma- 

 terial of the globe, exclusive of the comprehensive changes resulting 

 from the earth's gradual refrigeration. They include (1) expansion and 

 contraction ; (2) fusion, solidification, and attending igneous phe- 

 nomena ; (3) metamorphism and vein-making, besides chemical deposi- 

 tions and changes. After some observations on (1) the Sources of 

 Heat, these subjects are considered under the following heads : (2) Ex- 

 pansion and Contraction ; (3) Igneous Action and Results ; (4) Meta- 

 morphism ; (5) Mineral Veins. 



1. SOUECES OF HEAT. 



The Earth has three prominent sources of heat: (1) The Sun; 

 (2) Chemical and mechanical action ; (3) The igneous condition of the 

 Earth's interior. 



1. The Sun. — The heat of the earth's surface derived from the sun 

 has depended on (1) the condition of the sun, and (2) the density of 

 the earth's atmosphere. The atmosjohere absorbs and retains heat, 

 and is thus like a blanket about the sphere. Moreover,, the heat it 

 takes varies with its density. Hence, the ancient globe had through 

 its atmosphere more warmth than the modern, the atmosphere having 

 then been denser than now, through the presence of more carbonic acid 

 and more moisture ; and, for the same reason, elevated regions have 

 less warmth than lower ones. Some facts with regard to the local 

 distribution of heat over the earth are stated on pages 41 to 44. 



As the Sun, like other heated spheres, has been losing heat through 

 all time, the earth receives less now than in Archaean time. But it is 

 not probable that the diminution since the commencement of the 

 Paleozoic has produced any appreciable change on the climate of the 

 globe. 



The amount of heat received from the sun varies not only with the 

 seasons, but also with the variations in the eccentricity of the earth's 

 orbit. 



The eccentricity passed one of its maxima, according to Stockwell's calculations, 1 

 about 100,000 years since ; another, higher, 200,000 years; another, not so high, 300,- 

 000 years; a rather low minimum, 410,000 years; a low maximum, 475,000 years; a 

 very low minimum, 520,000 years; a maximum, 570,000 years; two maxima, the second 

 750,000 years; a very low minimum, 800,000 years; an extreme maximum, 850,000 

 years; another very low minimum, 900,000 years; a high maximum, 950,000 years, and 

 so on. In future time, there will be a very low minimum, 24,000 years on ; a low max- 

 imum, 150,000 years; another low maximum, 250,000 years; a very low minimum, 

 300,000 years; a low maximum, 400,000 years: a very high maximum, 515,000 years; 



1 Am. Jour. Sci., II. xlvi. 1868. 



