LIMESTONES. 309 



tion of a relatively small percentage of pure limestone. At a few Lehigh 

 district quarries, however, the "cement rock" is a little too high in 

 carbonate, rather than too low, so that it requires the addition of clay 

 and not of limestone. 



In the present volume the term "cement rock" will be used to cover 

 clayey limestones low in magnesia and carrying from 50 to 80 per 

 cent or so of lime carbonate, while limestones higher than 80 per cent 

 in carbonate will be called for convenience "pure limestones". 



Impurities of limestone. Whether a limestone consists entirely 

 of calcium carbonate or carries more or less of magnesium carbonate 

 in addition, it may also contain a greater or lesser amount of distinct 

 impurities. From the point of view of the Portland-cement manu- 

 facturer, the more important of these impurities are silica, alumina, 

 iron, alkalies, and sulphur, all of which have a marked effect on the 

 value of the limestone as a cement material. These impurities will 

 therefore be discussed in the order in which they are named above. 



The silica in a limestone may occur either in combination with 

 alumina as a clayey impurity or not combined with alumina. As 

 the effect on the value of the limestone would be very different in 

 the two cases, they will be taken up separately. 



Silica alone. Silica, when present in a limestone containing no 

 alumina, may occur in one of three forms, and the form in which it 

 occurs is of great importance in connection with cement -manufacture. 



(1) In perhaps its commonest form, silica is present in nodules, 

 masses, or beds of flint or chert. Silica occurring in this form will not 

 readily enter into combination with the lime of a cement mixture, and 

 a cherty or flinty limestone is therefore almost useless in cement-manu- 

 facture. 



(2) In a few cases, as in the hydraulic limestone of Teil, France, 

 a large amount of silica is present and very little alumina, notwith- 

 standing which the silica readily combines with the lime on burning. 

 It is probable that in such cases the silica is present in the limestone 

 in a very finely divided condition, or possibly as hydrated silica, pos- 

 sibly as the result of chemical precipitation or of organic action. In 

 the majority of cases, however, a highly siliceous limestone will not make 

 a cement on burning unless it contains alumina in addition to the silica. 



(3) In the crystalline limestones (marbles) and less commonly in 

 uncrystalline limestones, whatever silica is present may occur as a com- 

 plex silicate in the form of shreds of mica, hornblende, or other sili- 

 cate mineral. In this form silica is somewhat intractable in the kiln 

 and mica and other silicate minerals are therefore to be regarded as 



