IGNEOUS AGENCIES. 171 



Cause of Crust Movements. 



It is evident that the thing actually observed is only 

 changes in the relative level of sea and land. In the inte^ 

 rior of continents we have no means of determining such 

 movements, except by river beds, as just explained. The 

 cause of these slow changes is very obscure and can not be 

 discussed here.* Suffice it to say that the great inequali- 

 ties of the earth's crust, such as continents, ocean basins, 

 and mountain chains, are probably due to the slow cooling, 

 unequal shrinking, and consequent slight deformation of 

 the whole earth, progressive through all geological time. 



General Retrospect. 



We have discussed briefly the agencies now in operation 

 on the earth's surface, producing structure and form 

 under our eyes. We believe that similar agencies have 

 been at work through all time, and left their effects in the 

 structure and surface forms which we actually find. We 

 study the small and insignificant effects now produced in 

 order that we may throw light on those greater effects 

 which, accumulating through all geological times, are now 

 embodied in the earth's structure. We are now in a posi- 

 tion to examine the actual structure and forms of the 

 earth, and to interpret them by the light of the previous 

 discussions. 



Again : Of the agencies which we have been discussing 

 there are manifestly two groups. Atmospheric, aqueous, 

 and organic agencies constitute the one, and igneous 

 agencies the other. The one group tends to reduce the 

 inequalities of the surface, and, acting alone, would event- 

 ually bring all to sea-level, and are therefore called level- 

 ing agencies. The other originally caused, and has ever 

 tended to increase, the inequalities of the surface, and, 



* For fuller discussion, see the author's " Elements of Geology," 

 p. 131. 



