318 CEMENTS, LIMES, AND PLASTERS. 



silica, alumina, iron oxide, or magnesium carbonate, it will be best to 

 drill at the intermediate 25-foot point. 



With the data thus obtained operations can be conducted with 

 some confidence. Sections should be plotted to correspond to each 

 row of drill-holes, and at the proper vertical points the drill record should 

 be shown graphically, using different colors or conventions to distin- 

 guish rocks of different composition. The direction and amount of 

 dip or slope of the rock-beds can be determined from observation of 

 natural outcrops; and this will guide the engineer in drawing lines on 

 the sections to connect the different borings. 



The sections or profiles will serve as a basis for determining the 

 amount and location of the different grades of rock. In this connec- 

 tion it will be safe to assume a weight of 160 Ibs. per cubic foot for lime- 

 stone in the quarry, and to recollect that one rotary kiln will use about 

 190,000 cubic feet of limestone per year. This would correspond to a 

 thickness of about 4^ feet, over one acre, per year per kiln. 



Chalk and Other Soft Limestones. 



Chalk, properly speaking, is a pure carbonate of lime composed of 

 the remains of the shells of minute organisms, among which those of 

 Foraminifera 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 materials they 

 are therefore almost ideal. One defect, however, which to a small 

 extent counterbalances their obvious advantages is the fact that most 

 of these soft, chalky limestones absorb water quite readily. A chalky 

 limestone which in a dry season will not carry over 2 per cent of moist- 

 ure 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 exposure 

 to water and, if necessary, to provide facilities for storing a supply of 

 the raw materials during wet seasons. 



Origin of chalk. The term chalk is properly applied to a fine-grained 

 and usually very pure limestone, formed largely or entirely of the cal- 

 careous shells of microscopic organisms. These shells are chiefly of 

 the minute Foraminifera, though equally small and smaller calcareous 

 particles of various shapes also occur. Calvin describes * a section of 

 chalk from Iowa as follows: 



* Reports Iowa Geological Survey, vol. 3, p. 224. 1895. 



