62 C. R. VAN HISE 



resulting from vulcanism and cementation and from change in 

 physical condition, but it appears possible, perhaps probable 

 (see pp. 47-52), that in vulcanism we have an explanation of as 

 large, or even a larger, fraction of the phenomena of crustal 

 corrugations than is furnished by any other single cause. 



The various causes for crustal shortening may be divided 

 into two classes : ( 1 ) those which involve a change in the volume 

 of the earth; and, (2) those which involve transfers of material. 

 ( 1 ) The loss of heat due to secular cooling, the increased pres- 

 sures due to lessening rotation, and the changes of physical con- 

 dition involve a contractional change of volume. Changing 

 oblateness due to changing rotation, vulcanism, cementation and 

 nucleal loss of water and gas, involve no appreciable change 

 of volume. (2) Changing oblateness is only possible by deep- 

 seated transfers of material which cause a change in the form 

 of the earth resulting in surficial contraction. Vulcanism results 

 in crustal expansion and nucleal contraction, and therefore in 

 crustal corrugation. The surficial expansion due to cementation 

 compensates for a part of the crustal corrugation. Loss of water 

 and gas produces slight nucleal contraction, and consequently 

 some crustal corrugation. 



Furthermore, it is to be remembered that the entire effect of 

 all these changes is available to account for crustal corrugation, 

 with the exception of contraction due to loss of heat, which, as 

 explained (pp. 44-45), is only partially available to account for 

 crustal deformation. Upon the other hand, the transfers of 

 material by vulcanism from the nucleus to within the shell has an 

 added effect in producing crustal corrugation much greater than 

 that due to nucleal contraction. 



The crustal shortening due to changing oblateness, and 

 increased pressures resulting from lessening rotation must have 

 been large in the remote past. According to Darwin, 1 56,810,- 

 000 years ago the rotation period of the earth was 6 hours 45 

 minutes, and 46,300,000 years ago the period was 15 hours 30 



'On the precession of a viscous spheroid and on the remote history of the earth, 

 by G. H. Darwin : Phil. Trans. Roy. Soc, Vol. CLXX, Pt. 2, 1897, p. 494. 



