IGNEOUS AGENCIES. 165 



ally so slow as to escape popular observation. But, 

 though so inconspicuous, they are the most important of 

 all forms of igneous agency, since it is by movements 

 such as these that continents and sea-bottoms, mountains 

 and great valleys, have been formed. Volcanoes and 

 earthquakes occur suddenly, fill the mind with terror, 

 and pass away, leaving behind little effect on the config- 

 uration of the earth ; but gradual movements of the 

 crust, acting over large areas, and without ceasing, 

 through inconceivable ages, have produced all the great 

 inequalities of the earth's surface. Thus is it always 

 the causes producing the most far-reaching effects are 

 ever those which, acting slowly, but everywhere and at all 

 times, are scarcely recognized except by the thoughtful 

 mind. 



But although the effects of this form of igneous agency 

 are so important, yet they are so obscure, and so little 

 has been accomplished by them in the present geological 

 epoch, that little is known of them, and our account must 

 therefore be brief. It is their accumulated effects through 

 all geological times, as shown in the structure and config- 

 uration of the earth, that alone are conspicuous. These 

 we shall treat of in Part II. In the meantime, however, 

 a few examples of their action now will prepare us for the 

 discussion of these effects. 



Elevation. 1. South America. We have already 

 mentioned (page 157) that in 1822 and again in 1835, 

 after severe earthquakes, the southwest coast of South 

 America was elevated several feet along a line of many 

 hundreds jof miles. It is not probable that very much is 

 accomplished in this paroxysmal way, but the fact is 

 important as showing the connection of earthquakes with 

 bodily elevation of large tracts. Suppose, then, any 

 force beneath tending to elevate the southern end of the 

 South American Continent, but resisted by the stiffness 

 of the crust : if the crust yielded gradually as the force 



