320 CEMENTS, LIMES, AND PLASTERS. 



for use in Portland-cement manufacture occur in three widely separated 

 areas occupying respectively (a) parts of central Alabama and north- 

 eastern Mississippi, (6) southwestern Arkansas and central Texas, and 

 (c) parts of Iowa, Nebraska, North and South Dakota, Colorado, and 

 other States of the Great Plains region. Though the chalk is in all these 

 areas of approximately the same age and character, the formations 

 containing it have been given different names i.e., the Selma chalk, 

 in Alabama and Mississippi; the Whitecliffs chalk, in Arkansas; the 

 Austin chalk, in Texas; and the Niobrara chalk, in the Great Plains 

 region. 



In addition to the true chalks, soft limestones of Tertiary age occur 

 in all the Atlantic and Gulf coast States from Virginia to Mississippi 

 inclusive, as well as in California. These are the materials commonly 

 described as " marls" in the older geological reports, though they are 

 in no way related to the fresh-water marls now so largely used in Port- 

 land-cement manufacture, discussed in Chapter XXV. 



Physical Properties. When dry, the chalks and soft limestones, 

 are commonly considerably lighter than the hard limestones. As 

 noted on a previous page, the chalky limestones may run as low in 

 specific gravity as 1.85, corresponding to a weight of about 110 Ibs. per 

 cubic foot, while the hard, compact limestones in common use range in 

 specific gravity from 2.3 to 2.9, corresponding approximately to a range 

 in weight of from 140 to 185 Ibs. per cubic foot. 



The low weight above quoted is, however, exceptional, and the soft 

 limestones may be expected to range between 125 and 150 Ibs. per cubic 

 foot when dry. They are usually very porous, however, and but brief 

 exposure to water will increase their weight and moisture content re- 

 markably. This, indeed, is their single defect from the point of view 

 of the cement manufacturer, for during a rainy season or with a badly 

 drained quarry he may have to handle a material carrying 15 or 20 

 cent of moisture. 



Otherwise they are admirable cement materials, being soft and 

 easily quarried and ground. 



Composition of chalks and soft limestones used in cement-plants. 

 In composition the chalks and other soft limestones vary from a rather 

 pure lime carbonate low in both magnesia and clayey matter to an 

 impure clayey limestone of about the composition of the Lehigh district 

 cement rock. Magnesium carbonate is rarely present in quantities of 

 over 2 or 3 per cent, but alkalies, sulphur, and phosphoric acid may 

 occur in sufficient percentages to require careful considerations. 



