CHEMICAL DEPOSITS IX LAKES. 79 



have evidently been formed in a somewhat similar way. In the region 

 of the Yellowstone Park, deposits of travertine from waters of hot 

 springs running down a steep incline, in a succession of cascades, 

 assume the most beautiful forms, as shown in the accompanying figure, 

 taken from Hayden. 



Deposits of Iron. — Iron carbonate, like lime carbonate, is to some 

 extent soluble in water containing carbonic acid. Subterranean waters, 

 therefore, which always contain atmospheric carbonic acid, when they 

 meet this carbonate, will take up a small quantity in solution. Such 

 waters are called chalybeate. On coming to the surface the iron gives 

 up its carbonic acid, is peroxidized, becomes insoluble, and is deposited. 

 As the presence of organic matter is usually necessary to bring the iron 

 into a soluble condition, the full discussion of this very interesting sub- 

 ject is reserved until we take up organic agencies. 



Deposits of Silica. — Silica is soluble in alkaline waters, especially if 

 the waters be hot. Such waters, reaching the surface and cooling, de- 

 posit the silica in great abundance, often at first in a gelatinous con- 

 dition, but drying to a white porous material called siliceous sinter. 

 Examples of such deposits are found in all geysers, as in those of Ice- 

 land, and in the Steamboat Springs in Xevada, and especially in the 

 wonderful geysers of Yellowstone Park. Such deposits are confined 

 to volcanic regions, the volcanic rocks furnishing both the alkali and 

 the heat. We will discuss these again under Igneous Agencies. 



Deposits of Sulphur and Gypsum. — Springs containing sulphide of 

 hydrogen (H 2 S), usually called suljjhur-springs, sometimes deposit 

 sulphur by oxidation of the hydrogen (H 2 S+0=H 2 0+S), and some- 

 times gypsum. This latter deposit is caused by the more complete oxi- 

 dation of the sulphide of hydrogen, forming sulphuric acid (H 2 S-(-40 

 =H 2 S0 4 ), and the reaction of this acid on lime carbonate held in solu- 

 tion in the same water. 



Chemical Deposits in Lakes. 



Salt Lakes and Alkaline Lakes. — Salt lakes may be formed either 

 — 1. By the isolation of a portion of sea-water in the elevation of sea- 

 bottom into land ; or, 2. By indefinite concentration of river-water in 

 a lake without an outlet. Thus, the Dead Sea, Lake Elton, and the 

 brine-pools of the Russian steppes, are probably concentrated remains 

 of isolated portions of the sea,* for their waters are highly-concen- 

 trated mother-liquors of sea-water, having a composition very similar 

 to the mother-liquors of the salt-maker. The Caspian Sea, on the other 

 hand, although elevated lake-margins show that much of its waters 

 has dried away, is still much fresher than sea- water. This fact, to- 



* Bischof, Chemical and Physical Geology, vol. i. p. 396. 



