f.ckfl.]  AMERICAN    CEMENT    INDUSTRY. 
Geologic  grouping  of  Portland-cement  materials. 
501 
System. 
States  and  number  of  plants  using. 
Total 
plants 
using. 
Pre-Cambrian  _  ... 
None 
0 
Cambrian  ._______•. 
None 
0 
Ordovician 
New  York  (1),  New  Jersey  (3),  Pennsylvania 
(14),  Georgia  (1). 
19 
Silurian 
New  York  (4) ,  Virginia  (1) 
5 
Devonian 
New  York  (1),  Michigan  (2) 
3 
Carboniferous: 
Mississippian 
West  Virginia  (1),  Indiana  (1),  Missouri  (3).. 
5 
Pennsylvanian 
Pennsylvania  (1),  Ohio  (4),  Illinois  (3).  Kan- 
sas (2). 
10 
Triassic 
None 
0 
Cretaceous . 
Alabama  (1),  Arkansas  (1),  Texas  (1),  South 
Dakota  (1),  Colorado  (1). 
5 
Tertiary ...... 
Quaternary 
None                                . 
0 
New  York  (4),  Ohio  (4),  Indiana  (2).  Michi- 
gan (10). 
20 
Horizon  doubtful    .    . 
California  (3)                      .      . 
3 
Slag,  alkali -waste 
Pennsylvania  (1),  Michigan  (1),  Illinois  (1)  _. 
3 
Total  
73 
NATURAL  CEMENTS. 
The  material  used  in  the  manufacture  of  natural  cement  is  inva- 
riably a  clayey  limestone,  carrying  from  13  to  40  per  cent  of  clayey 
material,  of  which  10  to  22  per  cent  or  so  is  silica,  while  alumina  and 
iron  oxide  together  may  vary  from  4  to  16  per  cent.  These  clayey 
materials  give  the  resulting  cement  its  hydraulic  properties.  Stress 
is  often  carelessly  or  ignorantly  laid  on  the  fact  that  many  of  the 
best-known  natural  cements  carry  large  percentages  of  magnesia, 
but  magnesia  (in  natural  cements  at  least)  ma}7  be  regarded  as  being 
almost  exactly  interchangeable  with  lime,  so  far  as  the  hydraulic 
properties  of  the  product  are  concerned.  The  presence  of  magne- 
sium carbonate  in  a  natural-cement  rock  is,  then,  merely  incidental, 
while  that  of  silica,  alumina,  and  iron  oxide  is  essential.  The  3D  per 
cent  or  so  of  magnesium  carbonate  which  occurs  in  the  cement  rock 
of  the  Rosendale  district,  New  York,  could  be  replaced  by  an  equal 
amount  of  lime  carbonate  and  the  burnt  stone  would  still  give  a 
hydraulic  product.  If,  however,  the  clayey  portion  (silica,  alumnia, 
and  iron  oxide)  of  the  Rosendale  rock  could  be  removed,  leaving  only 
the  magnesium  and  lime  carbonates,  the  burnt  rock  would  lose  all  of 
its  hydraulic  properties  and  would  yield  simply  a  magnesian  lime. 
