308 GEOLOGY. 



result must inevitably be, that the level of the ocean will 

 rise, unless there be some movements of the land to coun- 

 teract it. Elevations in the land or depressions in the 

 floor of the sea would do this. That elevations have 

 occurred within this age, and are occurring now, we 

 have the clearest evidence, as shown in Chapter X. It 

 may be that the heaving of volcanoes, adding to the 

 land, may occasion some corresponding depressions in 

 the floor of the sea in their neighborhood. At any rate, 

 in some way the general level of the ocean in relation to 

 the land is maintained about the same from century to 

 century. The preservation of this equilibrium in the 

 midst of constant changes is secured by the same all- 

 wise and all-powerful Providence that preserves the uni- 

 versal constitution of the atmosphere, as noticed in 85, 

 Part II. This appears very wonderful when we reflect 

 that the earth is essentially a molten ball covered with a 

 hard crust, which has vent-holes communicating with the 

 internal fires, and contains vast quantities of water in its 

 depressions. 



418. Present Activity of the Agencies Producing 

 Change. The causes of the present changes in the earth 

 were brought to view in 172, and were there stated to 

 be the same as have been in action in the different ages 

 of the earth's formation. All of them have been in op- 

 eration from the first except one, that is, life, which was 

 introduced upon the stage after the long periods of the 

 Azoic age were passed. An interesting question arises 

 in regard to the degree of activity of these agencies at 

 the present time. Lyell and some others claim that they 

 are as active now as they ever were ; but the evidences 

 in favor of the opposite opinion are very decisive. They 

 surely are not as strongly at work now as they were 

 when they raised up the lofty mountains, or when, in the 

 Azoic age, the crust of the earth was solidifying at the 

 first, and the germs of the continents were forming. 



419. Historic and Human Periods. What is called the 



