25 



ous to be classed as 'cement rocks.'' I Because of their softness, 

 however, all the chalky limestones will be described together. 



Pure hard limestones. 



Soon after the American Portland cement industry had be- 

 come fairly well established in the Lehigh district, attempts 

 were made in Xe\v York State to manufacture Portland cement 

 from a mixture of pure limestone and clay. These attempts 

 were not commercially successful, and although their lack of 

 success was not due to any defects in the limestone used, a cer- 

 tain prejudice arose against tine use of the hard limestones. In 

 recent years, however, this has disappeared, and a very large 

 proportion of the America:, output is now made from mixtures 

 of limestone with clay or shale. (See page 21 for comparative 

 figures.) This reestablishment in favor of the hard limestones 

 is doubtless due, in great part, to recent improvements in grind- 

 ing machinery, for .the purer limestones are usually much harder 

 than argillaceous limestones like the Lehigh district "cement 

 rock," and it was very difficult to pulverize them finely and 

 cheaply with the crushing appliances in use when the Portland 

 cement industry was first started in America. 



A series of analyses of representative pure hard limestones, 

 together with analyses of the clays or shales with which they are 

 mixed, is given in the table. 



Analyses of pure hard limestones and clayey materials. 



Limestones. 



Silica (Si02) 1.72 0.86 0.56 0.40 



Alumina (A12O2) 1.63 0.'63 1.23) 0.44 



Iron oxide (Fe2O3) 6.59 1.03 0.29f 



Lime carbonate (CaCO3) 90.58 97.06 97.23 97.99 



Magnesium carbonate (MgCOS) 0.75 0.42 



Clays and Shales. 



Silica (SiO2) 63.56 55.80 56.30 60.00 



Alumina (A12O2) | T23.36 



Iron oxide (Fe2O3) | 27.32 30.20 29.86] 4.32 



Lime carbonate (CaCOS) 3.60 2.54 " 1.70 



Magnesium carbonate (MgCO3) 2.60 1.50 



