188 



PHYSIOGRAPHY 



found over great areas, far above sea-level. Most of the solid rock 

 beneath the Mississippi basin, for example, was laid down as sedi- 

 ment beneath the sea, as shown by the shells, etc., of the sea animals 

 which it contains. In the Appalachain Mountains, rocks formed in 

 the same way are found up to heights of several thousand feet; in 

 the Rocky Mountains up to 10,000 feet and more; in the Andes 

 up to 16,000 feet; and in the Himalayas to still greater heights. It 

 seems certain, therefore, that the changes of level have been great. 

 Future changes of level. Not only have changes of level be- 

 tween land and sea been taking place for untold ages, but they are 

 likely to continue. The wear of the land and the transfer of sedi- 

 ment to the sea raises the level of the water by partly filling the 

 basins which hold it. The rise of the water increases the area of 

 the sea and decreases the area of the land. In the past, there seem 

 to have been occasional sinkings of the ocean basins; and when an 

 ocean basin sinks, its capacity is increased and the sea-level is 

 drawn down, just as the surface of the water in a pan would be 

 drawn down if the bottom were lowered. This lowering of the sur- 

 face of the sea makes the continents appear to rise. Such changes 





Fig. 183. Gentle fold in limestone. Dumfriesshire, Scotland. (H. M. 



Geol. Surv.) 



are likely to occur in the future, so far as can now be seen, just as 

 they have in the past. It is probable that, in the course of the 

 earth's history, the lowering of the sea-level because of the sink- 



