538 
CONTRIBUTIONS    TO    ECONOMIC    GEOLOGY,  1904. 
I' BULL.  2<;o. 
The  result  is  that  here  the  argillaceous  limestones  resemble  those  of 
the  Lehigh  district  more  than  at  any  other  point  in  the  valley. 
Analyses  of  these  rocks  are  given  under  the  detailed  discussion 
of  localities. 
-MARTINSBURG    SHALES. 
The  highest  formation  in  the  valley  proper,  geologically  speaking, 
is  a  great  series  of  gray,  light-brown,  or  black  shales,  varying  in 
thickness  from  1,000  to  1,500  feet.  When  the  Trenton  limestones 
underlie  the  shales  the  passage  from  the  one  formation  to  the  other 
is  often  so  gradual  that  no  marked  distinction  can  be  observed. 
Even  when  the  shales  rest  upon  formations  older  than  the  Trenton 
argillaceous  limestones  their  loAver  beds  are  often  quite  calcareous 
and  may  include  thin  layers  of  impure  limestone.  Although  the 
calcareous  portion  of  the  shales  may  be  found  to  burn  into  a  cement 
when  mixed  with  other  material,  it  is  probable  that  the  main  value 
of  this  series  will  rest  in  the  noncalcareous  portion  for  mixture  with 
high-lime  argillaceous  rock.  The  following  analyses  show  the  com- 
position of  the  lower  calcareous  part  and  also  of  the  higher,  more 
typical  shales. 
Analyses  of  Martinsburg  shales. 
Silica  (Si02) 68.00 
Alumina  ( A1203) 14. 40 
Iron  oxide  (Fe203) 5.  40 
Lime  (CaO) 2.  68 
Magnesia  (MgO) 1.  51 
Alkalies  (K20,  Na,0)  .11 
Carbon  dioxide  (C02) 2. 30 
Water  ( H20 ) ,  2. 70 
I 
56.  60 
21.00 
5.65 
3.42 
2.30 
.50 
2.20 
3.00 
22.51 
10.54 
3.26 
33. 24 
1.54 
29.0^ 
1.  Typical  shales,  1  mile  northwest  of  Colemanville.  N.  J.     Geology  New  Jersey,  1808, 
p.  136. 
2.  Typical  shales,  Delaware  Water  Gap,  N.  J.     Geology  New  Jersey,  1868,  p.  136. 
3.  Calcareous  shales  1J  miles  east  of  Staunton,  Va.     Booth,  Garret,  and  Blair,  chemists. 
CALCAREOUS    MARLS. 
Small  deposits  of  calcareous  tufa  have  been  noticed  in  various  parts 
of  the  valley,  and  these,  if  favorably  located  in  sufficient  quantity, 
would  undoubtedly  be  of  much  value  in  cement  manufacture.  Of 
more  importance,  however,  are  deposits  of  calcareous  fresh- water  marl 
which  have  been  found  scattered  throughout  this  region.  In  certain 
portions  of  Staunton,  as  has  been  noted  by  Mr.  Catlett,  the  founda- 
tions of  the  houses  are  cut  in  marl  10  or  12  feet  deep.     The  surface 
