DEFINITION AND RELATIONS OF NATURAL CEMENTS. 197 



for this purpose. The five essential ingredients of that cement, as shown 

 by the analysis, are: 



Silica (SiO a ) ..................................... 19.89 



Alumina (A1 2 O 3 ) ................................. 11 . 61 



Iron oxide (Fe 2 O 3 ) ............................... 1 .35 



Lime (CaO) ..................................... 29.51 



Magnesia (MgO) ................................. 20.38 



These values are substituted in the following formula: 



(2. 8 X percentage silica) + 

 (1 . 1 X percentage alumina) + 



(. 7 X percentage iron oxide) 



Cementation Index =7^5 .. . , ,., - - r-r 



(Percentage lime) +(1.4 X percentage magnesia) 



= (2.8X19.89) +(1.1 X11.61) + (.7X1.35) 



(29.51) + (1.4X20.38) 

 ^55.692 + 12.771 +0.945 



29.51+28.532 

 = 69.408 

 58.042 

 = 1.19. 



As will be later seen, this value is fairly characteristic for many natural 

 cements. 



Basal assumptions. It has previously been stated (pp. 170, 171) 

 that the applicability of the Cementation Index depends upon the fact 

 that it is the exact equivalent in percentages of a formula which involves 

 the following assumptions: 



(1) That the hydraulic activity of any material depends on the 

 formation of certain compounds of lime and magnesia with silica, alumina, 

 and iron oxide. 



(2) That in a hydraulic cement, lime combines with silica in such 

 proportions as to form the tricalcic silicate ( = SCaO.SiC^) ; while it 

 combines with alumina in such proportions as to form the dicalcic 

 aluminate (2CaO.Si0 2 ). 



(3) That in a lightly burned natural cement at least magnesia may 

 be regarded as molecularly interchangeable with lime, though of course 

 the differences in their combining weights must be allowed for when 

 the calculation is based on percentages. 



(4) That iron oxide may, in similar fashion, be regarded as molec- 

 ularly interchangeable with alumina. 



Of these assumptions, the third and fourth may be questioned by 

 other investigators, but it will be seen later that the hydraulicity of 



