ORIGIN OF LIMESTONES. 793 



The material deposited by animals may be amorphous or crystalline 

 aragonite or calcite. The material deposited by corals and crinoids is 

 crystalline, and is in the form of aragonite; the material deposited by the 

 mollusks is in the form of calcite, and to a less extent of aragonite. The 

 continuous precipitation of calcium carbonate of the sea by organisms, and 

 the building up of organic deposits through geological periods, combined 

 with mechanical rearrangement and with recrystallization (see pp. 795-797), 

 result in the great limestone formations. 



CHEMICAL PRECIPITATES. 



Chemical precipitates of calcium carbonate include the deposits of 

 springs and streams; inland seas with no outlets, like the Dead Sea, Great 

 Salt Lake, etc.; and possibly chemical precipitates in the ocean or in seas 

 connected with the ocean. 



springs and streams. — Tufa deposits are frequently formed by streams just 

 after issuing- from underground, and they are formed from water in caves 

 as it issues from the confined passages into the open caverns. In both 

 these cases the causes of the supersaturation and precipitation are the 

 same — release of pressure, with escape of carbon dioxide, and evaporation. 

 Spring and stream deposits of tufa are usually small and unimportant. 



inland seas with no outlets. — In inland seas with no outlets the calcium car- 

 bonate contributed by the streams steadily accumulates. On the average, 

 evaporation balances the additions of water. Concentration of the calcium 

 carbonate is the result, until supersaturation follows, and finally precipita- 

 tion. These deposits have generally been called tufa, and they are finely 

 illustrated by the deposits of the ancient lakes Lahontan and Bonneville. 

 Such deposits are forming' at the present time in Salt Lake, Pyramid Lake, 

 Walker Lake, and Winnemucca Lake." The tufa forming in lakes and 

 inclosed seas is likely to be rather impure in consequence of the simul- 

 taneous precipitation of other compounds, especially sodium chloride and 

 gypsum. 



Possible chemical precipitates in the ocean or in seas connected with the ocean. Willis holds it 



to be extremely probable that extensive limestone formations have been 

 chemically precipitated in the sea. 6 At the present time the ocean and the 



« Russell, I. C, The geological history of Lake Lahontan: Mon. U. S. Geol. Survey, vol. 11, 1885, 

 pp. 188-223. Gilbert, G. K., Lake Bonneville: Mon. U. S. Geol. Survey, vol. 1, 1890, pp. 167-169. 

 & Willis, Bailey, Condition of sedimentary deposition: Jour. Geol., vol. 1, 1893, pp. 519-520. 



