52 A TREATISE ON METAMORPHISM. 



to be presumed that under such low temperatures changes in rocks would 

 be so slow as to be negligible. 



How deep below the surface of the earth the heat of the sun produces 

 an effect can not be accurately determined. It is highly probable that it 

 has an important effect to a depth of thousands of meters, probably beyond 

 the limits of the zone of observation. If the sun were not furnishing heat 

 to the earth, and the increment of increase in temperature were the same 

 as at present (1° C. for 30 meters), and the temperature at the surface were 

 200° G. lower it would be necessary to penetrate to a depth of 6,000 meters 

 to reach a temperature as high as that at the surface under the present 

 conditions. Below 6,000 meters the temperature would increase approxi- 

 mately as it does now from the surface downward. However, it is not to 

 be supposed that the effects of metamorphism would be the same as those 

 in the outer 6,000 meters at the present time, for the conditions of pressure 

 would be very different. (See Chapter I, p. 43, and Chapter IV, pp. 159- 

 160.) The assumption that the increment of temperature would remain 

 the same were not the sun giving heat to the earth is only approximately 

 true; but when it is remembered that 6,000 meters is an exceedingly small 

 fraction of the earth's radius, it seems probable that the increment of 

 increase of heat with depth in the outer part of the crust of the earth 

 would not be greatly different, even if the sun had long ceased to be a 

 source of heat; but if it were not for the heat of the sun, the temperature 

 of that part of the lithosphere directly under observation would be so low 

 that all chemical changes would be very slow, if indeed they were not 

 inappreciable. 



The absolute temperature at the surface is also dependent upon 

 latitude. The average temperature at the warmest tropical regions is about 

 300° C. absolute, or, stated in the ordinary scale, 27° C; the average 

 temperature of the coldest polar region where observations have been made 

 (latitude 81° 44') is 252.9° absolute, or, in the ordinary scale, —20.1° C.° 

 At intermediate latitudes there are all gradations between these extremes. 

 At any place the temperature may be presumed to increase with depth from 

 these surface temperatures at the rate of 1° C. per 30 meters. 



It would be fruitless to attempt a discussion of the changes of the 

 temperature of the outer part of the earth due to the solar cycle. So far 



" Hann, Julius, Handbueh der Klirnatologie, J. Engelhorn, Stuttgart, 1SS3, p. 733. 



