THE PROBLEMS OF GEOLOGY 597 



was the word used for this place by Hutton in the opening pages of 

 his epoch-making paper on the "Theory of the Earth."' 



It is also to be noted that the word "work," as above defined, is 

 also technicalized, having reference only to the exertion of force in 

 producing change of state of motion. With this meaning it has no 

 relation to the material results. To illustrate: By the expenditure 

 of energy, the crust of the earth may be fractured, or material be 

 transported from one place to another. In the general sense used 

 by geologists, these results are often spoken of as "work." It is 

 certainly a very grave question whether geologists can afford to 

 restrict the word "work" to its physical definition, and thus be 

 obliged to discontinue its use in an indefinite sense, both for the 

 expenditure of the energy, and the effect of such expenditure, or 

 for either alone. While this is so, it may be said tliere are very con- 

 siderable advantages in having a technical word for the physical 

 meaning of work. This w^ould assist the geologist to think clearly 

 and discriminate between the expenditure of energy and the material 

 effects of such expenditure. 



Whatever meaning the geologist assigns to the words "force" 

 and "work," he should have a clear understanding of the conceptions 

 which the physicists have of their meaning, and should attempt to 

 express these conceptions in some way. Also he should make it 

 clear, in case he decides not to use the words "force" and "work" 

 in the physical sense, that the old general usage is retained for them. 

 In this paper I shall use "force" in its technical sense, but retain the 

 common usage for the word "work." 



The agents of geology. — We are now ready to classify the agents of 

 geology. They may be grouped into ether, gases, liquids, and solids. 

 Possibly organisms are so peculiar a combination of gases, liquids, 

 and solids that they should constitute a fifth group, and in this case 

 the agents may be classified into ether, gases, liquids, solids, and 

 organisms. From another point of view the agents may be classified 

 into their chemical elements, some seventy or more in number, but of 

 which only about twenty are so abundant as to be important. 



The small number of categories of energies and agents given 



I Charles Hutton, "Theory of the Earth," Philosophical Transactions of the 

 Royal Society of Edinburgh, 1785, pp. 212-14. 



