308 CEMENTS, LIMES, AND PLASTERS. 



the sea-bottom and thus aid in the formation of calcareous deposits. 

 Microscopic organisms acting in this way are the cause of the forma- 

 tion of chalk, as noted later (p. 318). Vegetable life, acting in a more 

 indirect way, appears to be an important agency in the deposition of 

 marl (p. 338).. Ordinary limestones may have originated in any of 

 the ways noted above. After their formation, if subjected to sufficient 

 heat and pressure, normal limestones may be converted into crystalline 

 limestones or marbles. 



All the varieties of limestone above named may vary in composition 

 and degree of purity within wide limits. 



Composition of limestones. The term limestone is used, in its most 

 general sense, to include all rocks composed largely or entirely of lime 

 carbonate, or of lime carbonate plus magnesium carbonate.* A limestone 

 of ideal purity will of course consist of 100 per cent of these carbonates; 

 but few limestones attain even approximate purity and many are very 

 impure. As the percentage of impurities increases, the limestone 

 becomes more and more clayey or sandy or shaly, until at last the name 

 limestone is no longer applicable. The exact lower limit of the group 

 it would be difficult to fix, because the change is gradual, but probably 

 all would agree that a rock containing less than 50 per cent of carbonates 

 can hardly be called a limestone, but should rather be termed a cal- 

 careous clay or sandstone or shale, as the case may be. In the present 

 volume, therefore, the lower limit in composition of limestones will be 

 accepted as that above noted i.e., 50 per cent of carbonates. 



As the average composition of a good Portland -cement mixture is 

 about three fourths lime carbonate and one fourth clayey matter, it 

 is obvious that such a composition could be secured either by mixing 

 a pure limestone and a pure clay in the proportions of about three parts 

 limestone and one part clay, or by starting with a clayey limestone 

 carrying, say, 60 to 85 per cent lime carbonate and adding enough clay 

 or pure limestone to bring this percentage up or down to the required 

 75 per cent. The " cement rock" of the Lehigh district is an example 

 of a highly argillaceous limestone, usually too low in lime carbonate to 

 be a good Portland-cement material of itself and requiring the addi- 



*When discussing Portland -cement materials, the term "limestone" may be 

 still further restricted so as to entirely exclude the highly magnesian limestones. 

 At present all the Portland cement made is kept as low in magnesia as possible, 

 because of the fear that this ingredient may do some harm to the cement. As a 

 cement carrying over 4 per cent of magnesia (MgO) would be hard to market, a 

 limestone carrying over 6 to 8 per cent of magnesium carbonate (MgCO 3 ) can 

 hardly be classed as a possible Portland-cement material at present. 



