346 PRINCIPLES OF STRATIGRAPHY 



stalactite then grows after the manner of an icicle by external addi- 

 tions, and may become of great size. By the confluence of a series 

 of stalactites, banded sheets of lime are formed, which hang from 

 the roof of the cavern, a curtain of stone screening nooks and cham- 

 bers. Stalagmites are formed on the floor of the caverns from the 

 lime still contained within the water after it drops from the stalac- 

 tite. They may be sheet-like, forming a solid floor for the cavern, 

 or they may grow up into mounds. In the latter case they are 

 often in the form of successive caps, one above the other, their 

 edges frequently fringed with stalactites, which give the surface of 

 the stalagmite mound a fluted appearance. 



Cavern deposits may sometimes form on such a scale that the 

 cavern is filled again by crystallized marble, which is often beautiful 

 enough to be sought for commercial purposes. In the Southern 

 Appalachians, and in Missouri and Arkansas, broken-down caverns 

 have made possible the quarrying of the stalagmitic deposits for 

 onyx marble. There are similar deposits in clefts and caverns in 

 the Eocenic limestones of the Nile valley in Egypt. These have 

 been extensively used for architectural and other purposes. In 

 Italy, too, at many localities such cavern deposits occur, and have 

 been worked since ancient times. 



Method of Deposition of Lime from Solution, 



According to Fresenius, carbonate of lime is soluble in pure 

 water, in the proportion of i part in 10,800 of cold, and i part in 

 8,875 parts of boiling water, while others have placed the proportion 

 at much less. "In carbonated w^aters the neutral carbonate of lime 

 unites with the carbonic acid to form the bicarbonate of lime, which 

 is readily dissolved in water to the- extent of 0.88 gram per liter 

 in water saturated with carbonic acid gas at the ordinary at- 

 mospheric pressure and a temperature of 10 degrees centigrade." 

 (Weed-68:(5?7.) With increased pressure the solubility increases, 

 with corresponding increase in the amount of CO, absorbed, until 

 the maximum is reached, which is about 3 grams per liter. When 

 the waters are free from carbonic acid, but contain alkaline and 

 earthy salts, the formation of unstable supersaturated solutions of 

 carbonate of lime is favored. From this the lime is gradually pre- 

 cipitated, the precipitation being more rapid when the salts in solu- 

 tion are the chlorides of the alkalies and alkaline earths than when 

 they are the sulphates of these same bases. Magnesium sulphate 

 and sodium sulphate form solutions with a certain amount of sta- 



