5G0 GEOLOGY, 



sions involving internal temperatures/ The famous studies of Lord 

 Kelvin are based on the assumption of a uniform initial temperature of 

 7000° Fahr.^ Other temperatures have been assumed in similar studies 

 by others, but the results do not differ materially. On this hypothesis 

 there would be no deep-seated change of temperature until a tempera- 

 ture-gradient, extending to the deeper horizons, had been developed by 

 surface cooling. In the earliest eras, the loss of heat would be felt solely 

 in the outer zone. By surface cooling, a temperature gradient would 

 be slowly developed, and gradually changed from age to age, as shown 

 by the curved lines in Fig. 450, each of which shows the temperature 

 at the successive stages stated in the legend. The computations for 

 these curves were based on the methods and assumptions of Lord Kelvin. 

 The two lower curves represent greater periods than those usually 

 assigned by geologists to the whole history of the earth. It will be seen 

 that the modification of the original temperature line extends only 

 about 160 miles below the surface for the 100,000,000-year period, only 

 about 240 miles for the 237,000,000-year period, and only about 320 miles 

 for the excessive period of 600,000,000 years. The superficial nature 

 of the whole thermal problem under this hypothesis is thus made clear 

 and impressive. 



Center I 



6000° C. 

 ^^3900° C 



0= C. Surface 



.0 .1 .2 .3 .4 .5 .6 .7 .8 .9 1.0 



Fig. 450. — Diagram showing the original distribution of heat assumed by the con- 

 vection hypothesis and the modifications of this distribution near the surface in 

 successive long periods The base-line of the figure represents divisions of the earth- 

 radius with center at the left and surface at the right. The vertical lines represent 

 temperatures ranging from 0° C. to 5000° C. The assumed initial temperature 

 3900° C. (7000° ¥.) is represented by the horizontal line TC, full at the left and dotted 

 at the right to indicate the original extension of the initial temperature to the sur- 

 face. The upper curve at the right shows how nmch the temperature will have been 

 modified at the end of 100,000,000 years, computed according to the method of Lord 

 Kelvin. The middle curve shows the change at the end of 237,000,000 years, and 

 the lower curve the change at the end of 600,000,000 years. Similar curves may be 

 found in an article by Clarence King, Am. Jour, of Sci., XLV, 1893, p. 16. 



After the outer shell had cooled so as to be in approximate equilibrium 

 with the environment of the earth, it suffered practically no contraction. 



1 See Woodward's address, Mathematical Theories of the Earth, Proc. Am. Assc. 

 for Adv. Sci., 1889, pp. 59-63. 



2 Nat. Phil. Thompson and Tait, Pt. II, p. 477. See also Popular Lectures 

 and Addresses, 1894, II, p. 313. 



