500 
CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904.         [bull.  200. 
cement  mixture  could  be  prepared  by  combining,  in  an  almost  infinite 
number  of  ways  and  proportions,  many  possible  raw  materials. 
Obviously,  too,  we  might  expect  to  find  gradations  in  the  artificial- 
uess  of  the  mixture,  varying  from  the  one  extreme  where  a  natural 
rock  of  absolutely  correct  composition  was  used,  to  the  other  extreme 
where  two  or  more  materials,  in  nearly  equal  amounts,  are  required. 
The  almost  infinite  number  of  raw  materials  which  are  theoretic- 
ally available  are,  however,  reduced  to  a  very  few  under  existing 
commercial  conditions.  The  necessity  for  making  the  mixture  as 
cheaply  as  possible  rules  out  of  consideration  a  large  number  of 
materials  which  would  be  considered  available  if  chemical  composi- 
tion was  the  only  thing  to  be  taken  into  account.  Some  materials, 
otherwise  suitable,  are  too  scarce  and  some  are  too  difficult  to  pulverize. 
In  consequence,  a  comparatively  few  combinations  of  raw  materials 
are  actually  used  in  practice. 
The  various  combinations  of  raw  materials  which  are  at  present 
u<v(\  in  the  United  States  in  the  manufacture  of  Portland  cement 
may  be  grouped  under  six  heads :  (1)  Argillaceous  limestone  (cement 
rock)  and  pure  limestone;  (2)  pure  hard  limestone  and  clay  or 
shale;  (3)  soft  chalky  limestone  and  clay;  (4)  marl  and  clay;  (5) 
alkali  waste  and  clay;  (6)  slag  and  limestone. 
Production  of  Portland  cement  from  various  materials. 
Year. 
"  Cement  rock"  and 
pure  limestone. 
Hard  limestone  and 
clay.« 
Marl  and  clay.b 
Chalk  and  clay. 
Barrels. 
Per 
cent. 
Barrels. 
Per 
cent. 
Barrels. 
Per 
cent. 
Barrels. 
Per 
cent. 
1898 . . 
1899.... 
1900 .... 
1901  .... 
1902 .  _ 
1903 . ; 
2, 682, 304 
4,010,132 
5, 919, 629 
8,  503, 500 
10,923.922 
12, 483, 694 
74.9 
70.9 
70.3 
66.  S 
63.6 
55.7 
315,608 
458, 000 
874,715 
1,710,773 
3,673,790 
6,338.520 
8.8 
8.1 
10.4 
13.5 
21.3 
28.3 
545, 372 
1,095,934 
1,444,797 
2,001,200 
2,214,519 
3, 052, 946 
15.2 
19.4 
17.1 
15.8 
12.9 
13.6 
39, 000 
88, 200 
184, 400 
495, 752 
372,412 
457, 813 
1.11 
2  ' 
3. 
o 
o 
"  Including  also  the  product  from  two  plants  using  slag  and  limestone. 
b  Including  also  the  product  from  one  plant  using  alkali  waste  and  clay 
Geologic  age  of  cement  materials. — Though  Portland-cement  mat* 
rials  vary  greatly  in  geologic  age,  the  rocks  of  certain  periods  seel 
to  have  contained  an  especially  large  amount  of  limestone  low  i 
magnesia,  while  in  those  of  other  periods  limestones  are  either  ver 
scarce  or  are  commonly  high  in  magnesia. 
