ECKEL.] 
LIMESTONES. 
33 
argillaceous limestones like the Lehigh district "cement rock," and it 
was very difficult to pulverize them finely and cheaply with the crush- 
ing appliances in use when the Portland cement industry was first 
started in America. 
A series of analyses of representative pure hard limestones, together 
with analyses of the clays or shales with which they are mixed, is 
given in the following table: 
Analyses of pure hard limestones and clayey materials. 
Silica (Si0 2 ) 
Alumina (A1 2 2 ) 
Iron oxide (Fe 2 ; .)... 
Lime carbonate 
(CaC0 3 ) 
Magnesium carbonate 
(Mgccy 
Limestones. 
1.72 
1.63 
6.59 
90. 58 
0.86 
.63 
1.03 
97.06 
0.56 
1.23 
.29 
97.23 
.75 
0.40 
\ ■« 
97.99 
.42 
Clays and shales. 
63. 56 
27.32 
3.60 
2.60 
55. 80 
30.20 
2.54 
56.30 
29.86 
60.00 
j 23. 26 
I 4.32 
1.70 
1.50 
The first limestone analysis given in the above table represents a 
curious type, used in several plants in the Middle West. It is a rela- 
tively impure limestone, its principal iinpurity r being iron oxide. It 
contains 8.22 per cent of iron oxide and alumina, as compared with 
1.72 percent of silica, and therefore great care is required in selecting 
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, those of foraminifera being especially prominent. The 
chalks and soft limestones discussed 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 very 
of jreadily. A chalk}^ limestone which in a dry season will not carry over 
12 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 diffi- 
culty 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. 
Geographic and geologic distrihution in the United States. — The 
chalks and chalky limestones are confined almost entirely to certain 
Bull. 243—05 3 
