536 GEOLOGY 



from time to time and place to place, but the above statement is 

 broadly true. There are other branches of geology, the principles of 

 which have been mainly developed from results accomplished rather 

 than from observation of the present actions of energies and agents. 

 In such branches the probable energies, agents, and processes which 

 produced the observed results were developed from a consideration of 

 the methods by which chemical and physical energy through the 

 agents available could have produced the results observed. For in- 

 stance, the development of the solar system occurred but once. Dur- 

 ing that development the earth was formed, including the atmosphere, 

 hydrosphere, and lithosphere. The process of differentiation was not 

 observed by man, cannot be repeated by him. The only method of 

 reaching a probable conclusion as to the manner of accomplishment 

 of the complex result is to consider in what possible ways physical 

 and chemical energy may have acted upon the enormous masses of 

 universe stuff out of which the earth was constructed, and to check 

 this reasoning by the attainable knowledge of what is now occurring 

 upon other heavenly bodies. 



The qualitative and quantitative stages of explanation. The task of 

 explaining geology in terms of processes involving energy and agent 

 has two stages the qualitative stage and the quantitative stage. 

 For most problems we have as yet been unable to go beyond the 

 qualitative stage. In the qualitative stage of a problem it is shown 

 that a cause is real. In this stage the question is not asked as to 

 how far the explanation applies; i. e., its quantitative importance. 

 Most geologists are content when they reach the qualitative stage. 

 A certain cause is determined to be real in the explanation of certain 

 phenomena. It is then usually assumed that this cause is the only 

 cause. For instance, it has been generally accepted that the loss of 

 heat by the earth results in decreased volume, and that such con- 

 densation is a cause for crustal deformation. Many geologists have 

 stopped at this point satisfied. They have not asked the question: 

 To what extent can loss of heat by the earth explain crustal deforma- 

 tion, and are there any other causes which can be assigned? Some 

 years ago I listed a number of causes, each of which partly explains 

 deformation. In addition to secular cooling, they are as follows; 

 volcanism, cementation, change of oblateness of the earth, change 

 of pressure within the earth, change of physical condition of the 

 material of the earth, and loss of water and gas from the interior. 1 

 Evidently, in order that we may have even an approximately correct 

 idea of the chief causes for crustal deformation, the question must 

 be answered as to the quantitative importance of each of the causes. 



1 C. R. Van Hise, Estimates and Causes of Crustal Shortening, Journal of Geo- 

 logy, vol. vi (1898), pp. 10-64. 



