2IO THE SEDIMENTARY ROCKS 



which explains the very common association of the two rocks 

 in successive beds. The salt may be present only as an ingredi- 

 ent of shale (saline shale), or may form thin layers, indicating 

 brief periods of deposition, followed by freshening of the water. 

 Again, it may occur in enormously thick masses, the result of 

 long-continued precipitation. One such mass, near Berlin, ex- 

 ceeds 5000 feet in thickness. Rock salt is often very pure, and 

 then it is transparent and colourless ; but it is frequently stained 

 by iron, or mingled with dust blown into the lake or lagoon which 

 deposited the salt, or mixed with clay and other mechanical sedi- 

 ments. 



b. Siliceous Precipitates 



These are much less common and extensive than the calcareous, 

 and are formed under exceptional conditions. 



Geyserite, or Siliceous Sinter, is deposited in dense and hard 

 masses around the mouths of geysers, partly by the evaporation of 

 the water which holds the silica in solution, and partly by the 

 action of Algae (see p. 130). Large terraces of this rock have 

 been built up by the geysers of the Yellowstone Park. Geyserite 

 also occurs as an uncompacted white powder. 



Cherts (Flint or Hornstone) are exceedingly dense and fine- 

 grained masses, which the microscope shows to be made up of 

 very minute grains of chalcedony mixed with more or less amor- 

 phous silica and crystals of quartz. The mode of origin of these 

 masses is not at all well understood, but is believed to be by 

 precipitation from sea-water. In the Lake Superior region occur 

 cherty rocks which are mixtures of chalcedony and carbonate of 

 iron, from which the iron ores are derived by a process of weath- 

 ering. 



c. Ferruginous Precipitates 



These rocks may, with almost equal propriety, be classed with 

 those of organic origin, because, as we have already learned (p. 135), 

 the concentration into beds of the iron, which is so widely diffused 

 through nearly all rocks and soils, is chiefly due to the action of 

 decomposing vegetable matter. But as the action is by means of 



