450 
CONTRIBUTIONS  TO  ECONOMIC  GEOLOGY,  1.903. 
[BULL.  225. 
alumina,  unci  iron  than  the  higher  beds  whose  analyses  are  given  on 
page  449.  The  lowest  beds  form  a  natural  transition  into  the  under- 
lying cement  rock. 
Trenton  limestone. — The  Lehigh  cement  rocks,  which  are  approxi- 
mately equivalent  in  age  to  the  lowest  Trenton  beds  of  New  York,  are 
made  up  of  a  series  of  more  or  less  argillaceous  limestones.  The  for- 
mation appears  to  vary  in  thickness  from  150  feet  in  New  Jersey  to 
250  feet  or  even  more  at  Nazareth  and  on  Lehigh  Kiver.  Its  upper 
beds,  near  the  contact  with  the  overlying  Hudson  shales,  are  very  shaly 
or  slaty  black  limestones,  carrying  approximately  50  to  60  per  cent  of 
lime  carbonate  and  40  to  50  per  cent  of  silica,  alumina,  iron,  etc. 
Lower  in  the  formation  the  percentage  of  lime  steadily  increases,  while 
that  of  clayey  material  decreases  correspondingly,  until  near  the  base  of 
the  formation  the  rock  may  carry  from  85  to  95  per  cent  of  lime  carbon- 
ate with  only  5  to  15  per  cent  of  impurities.  This  change  in  chemical 
composition  is  accompanied  by  a  change  in  the  appearance  and  phys- 
ical character  of  the  rock,  which  gradually  loses  its  slaty  fracture  and 
blackish  color  as  the  percentage  of  lime  increases,  until  near  the  base 
of  the  formation  it  is  often  a  fairly  massively  bedded  dark-gray  lime- 
stone. Even  so,  it  can  usually  be  readily  distinguished  from  the  mag- 
nesian  Kittatinny  limestone,  described  below,  for  the  cement  rock  is 
always  darker  than  the  magnesian  limestone  and  contains  none  of  the 
chert  beds  which  are  so  common  in  the  magnesian  rock. 
The  Lehigh  cement  rock  is  never  nearly  so  high  in  magnesia  as  is 
the  underlying  Kittatinny  limestone.  It  does,  however,  carry  con- 
siderable magnesia  (as  compared  with  other  Portland-cement  materials) 
throughout  its  entire  thickness,  and  few  analyses  will  show  less  than 
4  to  6  per  cent  of  magnesium  carbonate.  The  following  series  of 
analyses  is  fairly  representative  of  the  lower,  middle,  and  upper  beds 
of  the  formation.  The  specimens  from  the  upper  beds,  near  the  Hud- 
son shales,  show  considerably  less  lime  and  more  clayey  matter  than 
those  from  the  lower  parts  of  the  formation. 
Analyses  of  Trenton  limestone  [Lehigh  cement  rock.)® 
10, 
Silica(SiOo) 
Alumina  ( AL03) 
Iron  oxide  (Fe203) 
Lime  (CaO) 
Magnesia  (MgO) 
Carbon  dioxide  (C02). 
Per  ct. 
1.86 
.60 
.51 
53.64 
.81 
43.03 
Per  ct. 
5.03 
2.06 
1.23 
49.73 
1.02 
40.19 
Perct. 
8.38 
4.03 
1.32 
45.45 
1.34 
37.18 
Per  ct. 
11.90 
4.42 
1.70 
44.18 
1.18 
36.01 
Per  ct. 
11.71 
4.36 
1.62 
43.47 
1.82 
36.15 
Per  ct. 
11.11 
4.40 
1.91 
42.51 
2.89 
36.57 
Per  ct. 
17.04 
6.90 
2.13 
37.53 
2.17 
32.88 
Per  ct. 
22. 71 
5.84 
2.13 
36.50 
1.69 
30.52 
Per  ct. 
19.53 
6.03 
1.70 
35.71 
3.33 
32.73 
Perct. 
24.45 
5.68 
1.57 
35.00 
2.21 
29.89 
"Ann.  Rept.  New  Jersey  State  Geologist  for  1900,  p. 
