6oo CHARLES R. VAN HISE 



is explained under physical and chemical principles in the terms of 

 energy. 



It is plain that the treatment of the energies of geology and the 

 treatment of the agencies of geology will overlap, sijice one cannot 

 be considered without also considering the other; but this is an 

 advantage rather than a disadvantage, for each of the two points of 

 view is very important in enabling the mind to grasp the composite 

 whole. Just as in the science of physics-chemistry it may some- 

 times be advantageous to consider the subject mainly from the point 

 of view of substance, and at another time mainly from the point of 

 view of energy, and the treatments from both points of view are neces- 

 sary to build up the science of physics-chemistry; so it is necessary 

 to consider the subject of geology from the points of view of energy 

 and of agent, if an approximation to adequate comprehension be 

 gained. 



As already intimated, another point of view from which geology 

 may be considered is the result. This was the chief point of view 

 of the early geological papers and text-books, which were content to 

 tell of phenomena. Phenomena may, and often are, observed and 

 described in advance of their physical-chemical interpretation. But 

 the naming or even the description of the phenomena of the earth 

 without reference to energy or agent is very unsatisfactory. And 

 usually the valuable descriptions of before unobserved phenomena are 

 made in connection with theories of their physical and chemical 

 significance. But it is still true that observation and description 

 present a third important point of view which interlocks with and 

 overlaps the treatment of geology from the points of view of energy 

 and agent. 



So complex is the earth that to enable the mind to comprehend 

 the intricately interlocking whole, the subject should be considered 

 from as many points of view as possible. If only the human mind 

 were sufficiently powerful, and means of expression adequate, the 

 ideal method of treatment would be simultaneous consideration and 

 exposition of all possible points of view. But since this method of 

 treatment is an impossibility, we must necessarily at any time con- 

 sider each portion of the subject in part and treat it in part. The 

 problem is then the selection of the various partial points of view 



