THE PROBLEMS OF GEOLOGY 595 



ment takes place and work is done, some portion of the energy 

 becomes kinetic. Work and kinetic energy are inseparable. As 

 multifarious kinds of work are always going on in the world, potential 

 and kinetic energy are always existent. For the most part we can 

 trace the kinetic energy back to one or more of the various classes 

 of energy above mentioned, but some part of it may be derived from 

 other unnamed sources. 



Any of the forms of energy may exert force, hence we have the 

 terms "force of gravitation," force of heat," "force of elasticity," 

 "force of cohesion," "chemical force," "electrical force," "mag- 

 netic force," -and "radiant force." 



Any or all of these forces may be exerted both under static and 

 dynamic conditions. When the conditions are static, the energy is 

 potential. When the conditions are dynamic and work is done, 

 some portion of the energy is kinetic. To illustrate: For many 

 years a cliff may stand; but finally a portion of it falls and geological 

 work is done. The force of gravitation is exerting the same pressure 

 upon the material concerned during all the years of quiesence and 

 during the brief period of movement, and for that matter continues 

 to be exerted after movement ceases. During the static conditions the 

 energy of gravitation is potential. During movement some part of it, 

 by pressure of the force of gravitation, passes into kinetic energy. 

 And this energy, through the agency of the falhng part, the agent, 

 does further geological work upon the material at the foot of the cliff. 



All of the forms of energy and force are important in geology, 

 but the geological work of some of them has been more clearly 

 discriminated than that of others. For instance, the geological 

 results produced by electricity and magnetism have not been worked 

 out, although I have no doubt that electrical and magnetic energy 

 have produced important permanent effects upon the earth which 

 ultimately will be discriminated. 



Geological use of the words " force, '^ " energy, ^^ and ''work.'" — 

 To the present time the geologist has much more frequently used 

 the word "force" than "energy." This is because the geologist is 

 usually more concerned with the exertion of force by an agent than 

 he is with the source or amount of energy which the agent contains. 

 Physical investigations seem to show that substance contains enor- 



