272 



PHYSICAL GEOLOGY 



quently overlap the older strata (Fig. 273). They were deposited 

 when the water in which they were laid down had a greater extent 

 than it had when the older strata were deposited. 



Constitution of the Earth's Interior 



The ancient Greeks and Romans in speculating about the under- 

 world came to the conclusion that its heat and other igneous phe- 

 nomena were due to the work of imprisoned giants. Our present 

 theories are less fanciful, but because of the inaccessibility of the 

 deeper portions of the earth's interior and our failure to reproduce 

 the conditions there, our knowledge of its constitution is far from 

 satisfactory. 



Zone of Variable Temperature. — The temperature at the surface 

 of the earth is variable because of the changes in daily and seasonal 

 temperature, but at a depth which in Java and Indra is about 12 feet 

 and in New York about 50 feet the temperature is constant through- 

 out the year. 



The Interior Heat of the Earth. — Below the level where seasonal 

 change in temperature occurs, the temperature increases with the 

 depth. This fact has been determined by well borings, by tunnels, 

 and by mines, but since the deeper borings are only a little more than 

 a mile in length, the statements in regard to the rate of increase at 

 great depths must necessarily be theoretical. The rate of increase 

 is usually estimated at i° F. for 60 to 75 feet, but it varies so widely at 

 different places that to strike an average is difficult. In the St. 

 Gothard tunnel (Italy to Switzerland) it is 1 ° F. for 82 feet ; at Calumet, 

 Michigan, the average for 4939 feet is i° F. for 103 feet ; in the British 

 Isles it varies from i°F. for every 34 feet to i° F. for every 130 feet. 

 A boring in West Virginia to a depth of 5386 feet showed an increase 

 of one degree for 80 or 90 feet for the upper half, and of one degree 

 for 60 feet in the lower half. Near Leipzig, Germany, a boring 5560 

 feet deep showed an average of one degree for 56 feet. In South 

 Dakota the artesian wells show a depth of from 17.5 to 45 feet for 

 each degree. The rapid increase shown in South Dakota, however, 

 is probably due to folding in the water-bearing stratum. The 

 presence of hot igneous bodies would also increase the tempera- 

 ture gradient. 



Since the temperature gradient follows the surface configuration, 

 the level of the Simplnn tunnel which connects Switzerland and 



