eckel.]    CEMENT-ROCK  DEPOSITS  OF  LEHIGH  DISTRICT,   PA.-N.   J.    453 
been  opened  on  beds  of  cement  rock  running-  considerably  higher  in 
lime  carbonate,  and  occasionally  running  so  high  (80  per  cent,  etc.)  as 
to  require  the  addition  of  shale  or  clay  rather  than  of  pure  limestone. 
Character  and  composition  of  the  pure  limestones. — The  pure  lime- 
stones added  to  the  cement  rock  are  commonly  gray,  and  break  into 
rather  cubical  fragments.  The  fracture  surfaces  show  a  finely  granu- 
lar structure,  quite  distinct  in  appearance  from  the  slaty  cement  rock. 
In  composition  the  limestones  commonly  used  will  carry  from  90  to 
96  per  cent  of  lime  carbonate,  with  rather  less  magnesium  carbonate 
than  is  found  in  the  cement  rock.  All  of  the  cement  plants  own  and 
operate  their  own  cement-rock  quarries,  but  most  of  them  are  com- 
pelled to  buy  the  pure  limestone.  When  this  is  the  case  only  very 
pure  grades  of  limestone  are  purchased,  but  when  a  cement  plant 
owns  its  limestone  quarry  material  running  as  low  as '85  per  cent  of 
lime  carbonate  is  often  used. 
Quarry  practice. — In  most  of  the  cement-rock  quarries  of  the  Lehigh 
district  the  rock  dips  from  15°  to  25°,  usually  to  the  northwest.  At 
a  few  quarries,  particularly  in  New  Jersey,  the  dip  is  much  steeper. 
The  quarries  are  opened,  preferably,  on  a  side  hill,  and  the  overlying 
stripping,  which  consists  of  soil  and  weathered  rock,  is  removed  by 
scrapers  or  shoveling.  The  quarry  of  the  Lawrence  Cement  Company 
has  been  extended  in  its  lower  levels  so  as  to  give  a  tunnel  through 
which  the  material  is  hoisted  to  the  mill.  Several  other  quarries  have 
been  carried  straight  down,  until  now  they  are  narrow  and  deep  pits, 
from  which  the  material  is  hoisted  vertically.  The  Bonneville  Port- 
land Cement  Company  quarry  is  an  extreme  example  of  this  type. 
In  quarries  opened  on  a  side  hill,  so  as  to  have  a  long  and  rather 
low  working-  face  and  a  floor  at  the  natural  ground  level,  the  rock  is 
commonly  blasted  down  in  benches,  sledged  to  convenient  size  for 
handling  and  crushing,  and  carried  by  horse  carts  to  a  point  in  the 
quarry,  some  distance  from  the  face,  where  the  material  can  be  dumped 
into  cars,  which  are  hauled  by  cable  to  the  mill.  Occasionally  the 
material  is  loaded  at  the  face  into  small  cars  running  on  temporary 
tracks.  The  loaded  cars  are  then  drawn  by  horses  or  pushed  by 
men  to  a  turntable,  where  they  are  connected  to  the  cable  and  hauled 
to  the  mill.  While  these  methods  seem  clumsy  at  first  sight,  the}^  are 
capable  of  little  improvement.  The  amount  of  rock  used  every  day  in 
a  large  mill  necessitates  very  heavy  blasting,  and  this  prevents  per- 
manent tracks  and  cableways  from  being  laid  near  to  the  working  face. 
At  several  quarries  the  loading  into  the  cars  or  carts  is  accomplished 
by  means  of  steam  shovels.  The  cement  rock  seems  to  be  well 
adapted  for  handling  by  steam  shovels,  but  even  then  much  sledging 
is  necessary,  and  the  blasting  operations  are  interfered  with. 
Mill  practice. — What  may  be  considered  as  typical  American  prac- 
tice in  the  manufacture  of  Portland  cement  from  dry  materials  owes 
