572 



GEOLOGY. 



lower average gradient than the surface observations. The compu- 

 tations from compression by Lunn, giving a central temperature of 

 36,000° F. (20,000° C), imply a still lower average rate, while the con- 

 vection hypothesis postulates no sensible increase at all below 200 or 

 300 miles. 



The amount of loss of heat. — The amount of loss of interior heat 



Average material of 



crust (Clarke's 



tables).! 



Norm minerals calculated 

 from Clarke's average. 



Mineral equiva- 

 lent (C. I. P. W. 



system). 



. 

 .2 



< 



Linear 

 expansion. 



Volume 

 expansion. 



SiOg 58.59 



AlA-.-»- 15.04 

 FeA-.-- 3.94 



FeO 3.48 



CaO 5.29 



MgO 4.49 



K2O 2.90 



Nap 3.20 



TiO^ 55 



Minor con- 

 stituents 

 omitted. 2.52 



Quartz 11.4 



Orthoclase. ... 17.2 



Albite 27.3 



Anorthite 17.8 



Diopside 6.8 



Hypersthene . . 10.2 

 Magnetite and 



Ilmenite 6 . 8 

 Minor constitu- 

 ents omitted. 2.5 



100.00 



Quartz 

 Anorthite 



Diopside 



Augite 

 (used for 

 hypersthene) 



Magnetite 



a 

 b 

 c 

 a 

 h 

 c 

 a 



h 

 c 



+ .00001206 

 + .00001906 

 -.000002035 

 -.000001495 

 + .000008125 

 + .000016963 

 -.000001707 

 + .000013856 



+ .00000272 

 + .00000791 

 + .000009540 



.00003618 



.00001553 

 .0000234 



.0000245 

 .00002862 



100.00 



which the earth suffers may be estimated by that which is observed t(» 

 be passing outward through the rock, or by computing the amount 

 which should be conveyed outwards with the estimated gradients and 

 with the conductivity of rock as determined by experiment. The 

 latter method is usually employed in general problems. Taking the 

 mean thermometric conductivity of rock as 0.0045, the gradient as 

 1° C. in 30 meters, the average specific heat of rock as . 5 small calories 

 per cubic centimeter, it is computed that in 100,000.000 years the 

 loss of heat would amount to 45° C. (81° F.) for the whole body of the 

 earth.^ Tait makes the more conservative estimate of 10° C. (18° F.) 

 in the same period.^ This is an exceedingly small result, and emphasizes 

 the low conductivity of rock. 



The amount of shrinkage from loss of heat. — To compute the amount 

 of shrinkage for a given amount of cooling, the average coefficient of 

 expansion of rock is required. This has been experimentally deter- 

 mined by several investigators. By combining the determinations of 

 others with his own, T. Mellard Reade found the linear coefficient to be 



1 Bull. 168 U. S. Geol. Surv., p. 14. 



2 Daniell's Physics, p. 407. 



3 Heat. Tait, p. 225. 



