46 C. R. VAN HISE 



for crustal corrugations. None of the contraction is available 

 to explain surficial deformation until after a solid outer shell has 

 formed, either by solidification from the center or from the sur- 

 face. 



Also, making no hypothesis as to the early condition of the 

 earth, the lessening of the solid surface available for corrugation 

 does not include the full amount obtained by calculations based 

 upon radial contraction. So far as the exterior shell was hotter 

 than at present, its cooling would cause circumferential contrac- 

 tion, and consequent lessening diameter, without crustal corruga- 

 tion, just as in the case of a steel jacket which in a heated con- 

 dition is put upon the core of a gun, and which upon cooling 

 shrinks. 



Furthermore, as pointed out by Davison, the outer spherical 

 shell might continue to contract circumferentially faster than the 

 average contraction of the interior, because nearer the surface, 

 and more rapidly losing heat. 1 This would cause tension in the 

 outer part of the earth, just as in the case of the jacket on the gun, 

 which, after it has shrunk to the core, continues to contract and 

 so firmly clasps the core as to be under great tension; or just as 

 a large steel ingot, at a high uniform temperature, by rapid cool- 

 ing may so much more rapidly contract on its outer part than in 

 its core as to form surface tensional cracks, because of the ten- 

 sile stretching during the early stages of cooling. 



How important this circumferential contraction is in the case 

 of the earth is unknown. For we do not know the average 

 temperature of the outer shell of the earth during early geo- 

 logical time, nor do we know very exactly its present tempera- 

 ture. We can only say that certainly some quantity must be 

 subtracted from the total surficial decrease, resulting from loss 

 of heat, in order to obtain the amount which is available to 

 account for crustal shortening. Estimates which disregard this 

 correction would be true only so far back in geological time as 

 we can assume the temperature of the outer shell to be prac- 

 tically the same as at present. 



1 Loc. cit., pp. 231-242. 



