552 GEOLOGY, 



1. The centripetal agencies. 



Gravity. — The most obvious of the concentrating forces is gravity, 

 and in most questions relating to great segmental movements, it has 

 been thought sufficient to consider gravity alone, but it is by no means 

 certain that this does not lead to serious error. In studying the causes 

 and effects of earth movements, it is necessary to consider both gravita- 

 tional energy and gravitational force. Gravitational energy is greatest 

 when the mass is most widely dispersed, and least when most concen- 

 trated. Gravitational force is greatest when the mass is most concen- 

 trated, and least when most dispersed. The gravitational energy of 

 the earth matter was at its maximum when it was most widely diffused 

 in the supposed nebulous condition. It will perhaps reach its minimum 

 at some future period when the shrinkage shall reach its limit. In passing 

 from an expanded condition to a more concentrated condition, potential 

 energy, or energy of position, is transformed into other forms of energy, 

 chiefly heat. The heat thus developed is an important factor in the 

 earth's dynamics. The total amount of gravitational energy involved 

 in the earth's evolution is unknown, for neither the maximum disper- 

 sion of the earliest state, nor the ultimate condensation, is known. It is 

 not difficult, however, to compute the amoimt of transformation of 

 gravitational energy into heat, or other forms of energy, during a given 

 degree of condensation. If a mass equal to that of the earth were origi- 

 nally infinitely scattered, the gravitational energy given up by it in con- 

 densing into a homogeneous sphere of the earth's present size would, if 

 all transformed into heat, suffice to raise the temperature of an equal 

 mass of water 8900° C. (Hoskins), or an equal mass of rock (specific heat 

 of .2), 44,500° C. If the mass were more condensed toward the center, 

 as is the actual case, the heat would be considerably greater. If the 

 condensation toward the center followed the Laplacian law (p. 564), the 

 heat would be sufficient to raise the earth mass 48,900° C, assuming its 

 specific heat to be .2, which is about the average specific heat of rock 

 at the surface (Lunn). A further shrinkage of one mile would trans- 

 form an additional amount of gravitational energy into heat about equal 

 in amount to Tait's estimate of the loss of heat from the surface of the 

 earth in 100,000,000 years (see p. 572). If the radial shrinkage has been 

 32 miles, or even 16 miles, the amount of heat generated is very much 

 greater than the estimated loss from the surface. 



