534 
CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904. 
I  bull.  200. 
stones  being  highly  magnesian,  others  being  almost  pure  calcium 
carbonate,  while  a  third  group  contains  a  considerable  amount  of 
clayey  material.  According  to  the  character  of  the  rock,  and  also  in 
agreement  with  the  fossil  contents,  four  divisions  may  be  recognized 
in  the  Shenandoah  limestone,  in  ascending  order,  as  follows:  (1) 
Beds  of  magnesian  limestone  of  Cambrian  age  from  1,000  to  2,000 
feet  in  thickness;  (2)  300  or  more  feet  of  cherty  limestone  bearing  fos- 
sils of  Beekmantown  (Calciferous)  age;  (3)  60  to  100  feet  of  a 
coarsely  crystalline,  light-colored,  highly  fossiliferous  limestone;  (4) 
200  to  350  feet  of  dark-colored  argillaceous  limestone,  the  Trenton 
cement  rock.  Members  1  and  2  are  apparently  uniformly  developed 
throughout  the  valley,  but  3  and  4,  although  having  a  wide  distri- 
bution, are  sometimes  absent. 
Cambrian  limestone. — On  account  of  the  lack  of  continuous  expo- 
sures and  the  difficulty  in  distinguishing  the  various  beds,  the  thick- 
ness of  this  division  has  not  been  definitely  ascertained,  but  it  is 
certainty  not  less  than  1,000  and  may  exceed  2,000  feet.  Fossils  are 
practically  wanting  in  these  rocks  in  this  part  of  the  valley,  but 
farther  north,  notably  at  several  localities  in  Pennsylvania  and  New 
Jersey,  sufficient  fossil  evidence  has  been  found  to  indicate  that 
probably  the  entire  division  is  of  Upper  Cambrian  age.  These  lime- 
stones are  underlain  by  a  quartzite  containing  Lower  Cambrian  fos- 
sils, so  that  although  the  two  formations  are  apparently  conformable, 
there  is  a  great  time  break  between  them. 
The  Cambrian  limestones  are  massive  bedded,  vary  from  dark 
gray  to  light  gray  or  light  blue  in  color,  and  are  nearly  always 
highly  magnesian  in  composition.  Toward  the  base  purple  or  sil- 
very shales  are  sometimes  seen,  but  as  a  rule  the  entire  formation 
is  one  of  heavily  bedded  magnesian  limestones.  The  following 
analysis  of  specimens  from  New  Jersey  localities,  copied  from  reports 
of  the  New  Jersey  Geological  Survey,  will  probably  apply  equally 
well  to  Virginia : 
Analyses  of  magnesian   Cambrian  limestone. 
Silica  (Si02) . . 
Alumina  ( A1.,03) 
Iron  oxide  (Fe203) .  _ 
Lime  (CaO) 
Magnesia  (MgO) 
Carbon  dioxide  (C02) 
8.0 
5.  3 
26.3 
17.4 
41.1 
2. 
4.9 
3. 
4. 
5. 
16.9 
8.8 
4.1 
6.5 
1.0 
.8 
1.6 
27.3 
28.3 
29.4 
30.  3 
14.6 
15.3 
17.8 
18.3 
44.8 
38.9 
42.8 
44.1 
2.0 
8.4 
32.4 
15.5 
42.5 
1.  Teapack,  N.  J. 
2.  Clinton,   Hunterdon  County,   N. 
li.  Annandale,  N.  J. 
4.   Asbury,  Warren  County,  N.  J. 
").    IVapack,  N.  J. 
6.   Pottersville,  Somerset  County,  N. 
