60 C. J?. VAN HISE 



average rock, that it expands 13 per cent. 1 in changing from 

 the crystalline to a liquid condition. The reverse passage from 

 the liquid to the crystalline condition would involve a contrac- 

 tion of more than 12 per cent. 



Even if the earth is now solid and crystalline to the center, 

 as believed by some geologists, it by no means follows that this 

 was the case through the major part of geological history. If the 

 changes above mentioned have largely occurred during geolog- 

 ical time, this has been a very important cause for contraction. 

 However, there is no way by which the amount can be quanti- 

 tatively estimated without involving so many uncertain hypoth- 

 eses that it is not considered advisable to make the attempt. 



Another subordinate cause for contraction is a change from 

 less complex to more complex molecules. In so far as this 

 change is involved in that of change from a liquid to an amor- 

 phous state or from either of these states to the crystalline condi- 

 tion, it has already been counted; but as a result of chemical 

 interactions all substances, even crystalline compounds, tend to 

 rearrange themselves under given conditions, especially where 

 the temperature and pressure are great, so that they will have 

 the most compact molecules. In so far as this has occurred, it 

 is a cause for contraction, although its importance cannot be 

 assumed to be great. 



These changes in the physical state of matter and the conse- 

 quent earth contraction are independent of the numerical results 

 due to change of pressure and loss of heat given on a previous 

 page ; for all the estimates in reference to secular cooling and 

 changing pressure are upon the hypothesis that the matter con- 

 tinues in the same state. The loss of heat and the increase of 

 pressure are undoubtedly among the causes which promote 

 change of physical condition, but in so far as change of state 

 has occurred the resultant contraction must be added to the 

 quantities assigned to the amounts due to secular cooling and 

 increased pressure. 



r The contraction of molten rock, by C. Barus : Am. Jour. Sci., Vol. XLII, 

 1891, pp. 498-499- 



