ESTIMATES AND CAUSES OF CRUSTAL SHORTENING 6 1 



Loss of water and gas.- — Finally, as suggested by Fisher, 1 nucleal 

 contraction may have resulted from loss of originally occluded 

 water. Chamberlin suggests that water and gas may have been 

 emitted which have been lost to the earth. 2 Both of these losses 

 would result in contraction of the nucleus. Probably the quan- 

 titative value of such contraction and consequent crustal shorten- 

 ing is small. 



General. — Doubtless as the study of the earth continues, 

 causes other than those assigned will be discovered for crustal 

 shortening. 



However, it is believed that the cumulative effects of the 

 various causes assigned for nucleal contraction, and for crustal 

 corrugation, are possibly sufficient to account for the phenomena 

 of mountain-making. 



We have seen that there are four important causes for crustal 

 corrugation. These are secular cooling, vulcanism, change of 

 oblateness, and change of pressures. Possibly there should be 

 included among the important causes also that of change in 

 physical condition and cementation. 



It is impossible to make any accurate quantitative compari- 

 son of the several causes. However, it is to be noted that the 

 change in surficial area due to oblateness of 210,000 square miles 

 is about equal to that which Darwin estimated would result from 

 secular cooling in 10,000,000 years, 228,000 square miles. It is to 

 be further noted that the contraction due to increased pressures 

 at a minimum estimation, 1,700,000 square miles, is 7^ times 

 as great as the amount which Darwin estimated would occur in 

 10,000,000 years as the result of secular cooling, and is there- 

 fore equivalent to the effects of secular cooling for 75,000,000 

 years, or for a longer period than Darwin allows for the history, 

 of the earth since the separation of the earth-moon couple. At 

 present we are, and probably we shall long continue to be, unable 

 to give any accurate quantitative value to the crustal shortening 



1 Physics of the earth's crust, by Osmond Fisher : London, 1861, pp. 87, 180, 218. 



2 A group of hypotheses bearing on climatic changes, by T. C. Chamberlin : 

 Journ. Geol., Vol. V, 1897, pp. 656-668. 



