CHAPTER XXIII. 

 LIMESTONES. 



THE Portland-cement materials which are discussed in this and the 

 following chapters (XXIV, XXV) under the names of pure hard lime- 

 stone, chalk, argillaceous limestone, or "cement rock", and marl, agree 

 in that they are all forms of limestones, though they differ sufficiently 

 in their physical, chemical, and economic characters to be discussed 

 separately and under different names. In order to avoid unnecessary 

 repetition, no general discussion of limestone will be presented here, 

 but reference should be made to Chapter VI, where the origin, varie- 

 ties, composition, and properties of limestones are described in detail. 

 In the present chapter these general facts will be briefly summarized, 

 and certain features common to all the types of limestone used in Port- 

 land-cement manufacture will be noted, after which these different 

 types will be separately discussed. 



Limestones in General. 



Varieties and origin. Limestones are rocks composed largely or 

 entirely of lime carbonate, or of lime carbonate with magnesium carbo- 

 nate. Though one or both of these carbonates will necessarily be the 

 principal ingredients in the rock, various impurities may occur. In 

 addition to the chemical differences which are thus caused between 

 different samples or kinds of limestone, they may also differ in their 

 physical characters, or in their methods of origin, or in both of these 

 points. 



Limestones are primarily formed by the deposition of lime carbonate 

 from sea- or lake-water which carries this salt in solution. This deposi- 

 tion may be direct, caused by chemical processes, or it may be effected 

 through the agency of living organisms. Travertine and tufa are chem- 

 ically deposited limestones formed by surface waters. Molluscs are able 

 to abstract lime carbonate from sea-water and utilize it in the forma- 

 tion of their shells. On the death of the animals, these shells sink to 



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