34 
CEMENT MATERIALS AND INDUSTRY 
Cm-Li, 243. 
Southern and Western States. They are all of approximately the same 
geologic ages — Cretaceous 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 
three widely separated areas, in (a) Alabama and Mississipppi, (b) 
Texas and Arkansas, and (c) Iowa, Nebraska, North and South Dakota. 
Composition. — In composition these chalks, or "rotten limestones, " 
vary from a rather pure calcium carbonate, low in both magnesia and 
clayey materials, to an impure clayey limestone requiring little addi- 
tional clay to make it lit for use in Portland-cement manufacture. Thej 
analyses in the table below show the range of composition of the 
chalky limestones. 
Analyses of chalky /inn stones. 
Silica (SiO,) 
Alumina ( A1 2 3 ) 
Iron oxide (Fe 2 3 ) , 
Lime carbonate (CaC0 3 ) 
Magnesium carbonate (MgCO 
Demopo- 
lis, Ala. 
12.13 
4.17 
3.28 
75. 07 
.92 
San Anto- 
nio, Tex. 
0. / / 
2.12 
90. 15 
.15 
Dallas, 
Tex. 
23. 55 
1.50 
70. 21 
.58 
White 
Cliffs, 
Ark. 
1.09 
88. 04 
.73 
Yankton, 
S. Dak. 
8.20 
7.07 
83.59 
Q.d. 
Milton, 
N. Dak. 
9. 15 
| 4. 80 » 
\ 2. 30i 
63. 75 ■ 
1.25 
FRESH-WATER MARLS. 
Marls, in the sense in which the term is used in the Portland-cement | 
industry, are incoherent limestones which have been deposited in the 
basins of existing or extinct lakes. So far as chemical composition is 
concerned, marls are practically pure limestones, being composed 
almost entirely of calcium carbonate. Physically, however, they dif- 
fer greatly from the compact rocks which are commonly described as 
limestones, as they are granular, incoherent deposits. This curious 
physical character is due to the conditions under which they have 
been deposited, and according^ varies somewhat. 
The above definition of marl is that commonly used in the cenien 
industry, but in geological and agricultural reports, particularly in 
those issued before the Portland-cement industry became prominen 
in this country, the term has been used for several veiy different sub 
stances. The following three uses of the term have been particularly 
common, and must be guarded against when such reports are bein|i 
examined in search of descriptions of deposits of cement materials: 
(1) In early days the terms "marls" and "marlytes" were applie< 
to calcareous shales and often to shales which were not particularb 
calcareous. This use of the term will be found in many of the earlie 
