CHEMICAL PRECIPITATES 209 



heads : a, Precipitates of the alkalies and alkaline earths ; b, sili- 

 ceous precipitates ; c, ferruginous precipitates. 



a. Precipitates of the Alkalies and Alkaline Earths 



Calcareous Tufa or Sinter, Travertine, Stalactite, Onyx Marbles, 



are all forms of carbonate of lime deposited from solution, either 

 around the vents of springs, or by percolating waters in limestone 

 caverns or in lakes. These deposits are made of crystallized cal- 

 cite (or aragonite), are very pure, and usually white, and more or 

 less translucent, though they may be stained by other substances 

 dissolved with the lime. In structure they are banded and show 

 rings of growth, which distinguishes them from the organic lime- 

 stones. The so-called " Mexican onyx " or "onyx marble" is a 

 beautifully banded travertine derived from ancient spring deposits. 



Oolite is a limestone composed of minute spherules of carbonate 

 of lime, cemented into a more or less compact mass, somewhat 

 resembling fish-roe, whence is derived the name, meaning " egg 

 rock." The spherules are made up of concentric layers of car- 

 bonate of lime, deposited from solution around some nucleus, it 

 may be a particle of sand or dust, or a calcareous fragment. The 

 beach rock of a coral reef (p. 169) is made in this fashion, and 

 calcareous sinter often has a similar structure. When the spheres 

 are larger, resembling peas in size and shape, the rock is called 

 pisolite. 



Gypsum (CaS0 4 , H 2 0) is a rock as well as a mineral (see 

 p. 23), and is deposited from solution in salt lakes and lagoons, 

 in which evaporation balances the influx of water (p. 151). When 

 pure, gypsum is white, but it is often coloured grey, brown, or red, 

 by iron stains, and it may even be black. It forms compact, 

 crystalline, or fibrous beds, looking like limestone, but much softer 

 and not effervescing with acid ; portions of the beds may consist 

 of transparent selenite crystals. Gypsum sometimes occurs in the 

 form of anhydrite (CaS0 4 ), but it is not known under what con- 

 ditions the anhydrous sulphate has been deposited from solution. 



Rock Salt (NaCl) is precipitated by evaporation from the dense 

 brine of salt lakes and lagoons, following the deposition of gypsum, 



