76 DYNAMICAL GEOLOGY. 



reddish deposit ; in fact, such deposit is the usual sign of 

 the existence of iron in the waters. 



The explanation of the deposit is as follows : Iron has 

 a powerful affinity for oxygen. As soon, therefore, as the 

 water reaches the surf ace the iron exchanges C0 2 for oxy- 

 gen of the air and forms a peroxide, which, being insol- 

 uble, is deposited. In regard to how the iron came in 

 the solution, we shall speak again under Organic Agency. 



Deposits of Sulphur. A yellowish deposit is usually 

 seen about sulphur-springs. These springs contain hydric 

 sulphide (H 2 S) in solution. The oxidation of this by 

 the contact of air forms water and deposits sulphur (H 2 S 

 + = H 2 + S). 



Deposits of Silica. Silica (quartz, sand, flint, etc.) is 

 usually regarded as extremely insoluble, but it is soluble 

 to a limited extent in alkaline carbonate waters, and the 

 solubility increases with the heat. Now, alkaline carbon- 

 ate waters are common in volcanic regions, and are often 

 hot. Such hot alkaline springs take up silica in their 

 subterranean course, and, coming to the surface, deposit 

 abundantly, partly by cooling, mostly by drying. Perhaps 

 the best example of such deposits is found at Steamboat 

 Springs, Nevada. Here, over an area of half a mile long 

 and a quarter of a mile wide, the whole surface is covered 

 with a deposit of silica twenty feet thick, and over the 

 whole area clouds of steam are seen issuing from many 

 vents. The deposit takes a great variety of forms some- 

 times a tufaceous material called sinter ; sometimes more 

 solid and regularly banded ; sometimes milky-white chal- 

 cedony ; and sometimes white quartz like loaf sugar. De- 

 posits of silica are found in all geysers. We shall, there- 

 fore, again speak of this under that head. 



Chemical Deposits in Lakes. 



Saline Lakes ; Salt Lakes. Salt lakes are found 

 only in dry climates. They are formed in two ways 



