THE NONMETALLIC MINERALS. 



267 



It is stated that the presence of more than very small quantities of 

 sand, iron oxides, or vegetable matter in the clay is detrimental. A 

 good cement mud before burning may contain from 68 to 78 per cent 

 of calcium carbonate, 21 to 15 per cent of silica, and from 10 to 7 per 

 cent of alumina. 



The following analyses from the same source as the above serve to 

 show (I) the composition of the clay; (II) the mixed clay and chalk or 

 "slurry," as it is called, and (III) the cement powder prepared from 

 the same: 



Several brands of Portland cement are manufactured in America, 

 usually from a mixture of materials, the proportions of which have 

 been worked out by experiment. At the Coplay Cement Works, in 

 Lehigh County, Pennsylvania, a blue-gray crystalline limestone and 

 dark gray more siliceous variety are ground and mixed into the desired 

 proportions, molded into a brick, and burnt to the condition of a 

 slag. The material is then ground to a powder and forms the cement. 

 Through the courtesy of the manager, the Museum collections contain 

 samples of the crude and manufactured materials, as follows: Lime- 

 stone (Specimen No. 53541, U.S.N.M.); cement rock (No. 53542, 

 U.S.N.M.). Composition formed by admixing the two rocks (No. 

 53543, U.S.N.M.); and the clinker (No. 53544, U.S.N.M.) obtained by 



