26 



The first limestone analysis given in the above table repre- 

 sents a curious type, used in several plants in the Middle West. 

 As will be noted, it is a relatively impure limestone, but its 

 principal impurity is iron oxide. It contains 8.22 per cent, 

 of iron oxide and alumina, as compared with 1.72 per cent, of 

 silica : and therefore demands great care in the selection of a 

 suitable high-silica clay to mix with it. 



Soft Limestones'. Chalk. 



Origin and general character. Chalk, properly speaking, is 

 a pure carbonate of lime composed of the remains of the shells 

 of minute organisms, among which those of foraminifere are 

 especially prominent. The chalks and soft limestones discussed 

 in this chapter agree, not only in having usually originated in 

 this way, but also in being rather soft and therefore readily 

 and cheaply crushed and pulverized. As Portland cement ma- 

 terials they are, therefore, almost ideal. One defect, however, 

 which to a small extent counterbalances their obvious advan- 

 tages is the fact that most oif these soft, chalky limestones ab- 

 sorb water quite readily. A chalky limestone which in a dry 

 season will not carry over 2 per cent, of moisture as quarried, 

 may, in consequence of prolonged wet weather show as high 

 as 15 or 20 per cent, of water. This difficulty can, of course, be 

 avoided if care be taken in quarrying to avoid unnecessary ex- 

 posure to water and, if necessary, to provide facilities for stor- 

 ing a supply of the raw materials during wet seasons. 



Geographic and geologic distribution in the United States. 

 The chalks and chalky limestones are confined almost entirely 

 to certain southern and western States. They are all of ap- 

 proximately the same geologic ages, Cretaiceous or Tertiary, 

 and are mostly confined to one division of the Cretaceous. 

 The principal chalk or soft limestone deposits available for use 

 in Portland cement manufacture occur in threei widely separated 

 areas, occupying respectively (a) parts of Alabama and! Mis- 

 sissippi ; (b) parts of Texas and Arkansas; and, (c) parts of 

 Iowa, Nebraska, North and South Dakota. 



