AQUEOUS AGENCIES. 73 



ated springs, iron oxides from chalybeate springs, sulphur 

 from sulphur-springs, and silica from alkaline springs. 



FIG. 40. The Saracens' tent, Luray Cave. 



Deposits of Lime Carbonates. These deposits are in 

 carbonated springs in limestone regions, and especially 

 in volcanic countries. In order to understand why 

 deposit occurs, the student must remember 1. That 

 lime carbonate (limestone) is slightly soluble in water 

 containing C0 2 . 2. That up to a certain limit the solu- 

 bility is proportioned to the amount of C0 2 . 3. That the 

 amount of C0 2 taken up by water is proportioned to the 

 pressure. Now, besides the small quantity of C0 2 in air, 

 and therefore in all meteoric water, there are also subter- 

 ranean sources, especially in volcanic regions. Therefore, 

 if \vater circulating deep in the interior, and therefore 

 mi IT heavy pressure, come in contact with subterranean 

 sources of C0 2 , it will take up a corresponding amount, 



