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  CONTRIBUTIONS 
  TO 
  THE 
  GEOLOGY 
  OF 
  

  

  similar 
  dark 
  bluish-green 
  argillaceous 
  sand 
  is 
  observed, 
  enclosing 
  fre- 
  

   quently 
  a 
  great 
  number 
  and 
  variety 
  of 
  shells. 
  

  

  The 
  very 
  general 
  existence 
  of 
  the 
  lower 
  stratum, 
  here 
  described, 
  

   forms 
  an 
  interesting 
  and 
  prominent 
  feature 
  in 
  the 
  geology 
  of 
  the 
  Mio- 
  

   cene 
  Tertiary 
  districts, 
  as 
  well 
  of 
  eastern 
  Virginia 
  as 
  of 
  Maryland. 
  

   Throughout 
  all 
  the 
  upper 
  fossiliferous 
  strata, 
  as 
  well 
  as 
  in 
  the 
  argilla- 
  

   ceous 
  beds 
  just 
  mentioned, 
  will 
  be 
  found 
  disseminated, 
  greenish-black 
  

   grains 
  of 
  silicate 
  of 
  iron 
  and 
  potash, 
  identical 
  with 
  those 
  already 
  de- 
  

   scribed 
  as 
  existing 
  in 
  the 
  stratum 
  immediately 
  overlying 
  the 
  shells, 
  

   and 
  having 
  the 
  same 
  form 
  and 
  composition 
  with 
  the 
  granules 
  contained 
  

   very 
  abundantly 
  in 
  an 
  older 
  Formation, 
  both 
  in 
  this 
  country 
  and 
  Eu- 
  

   rope. 
  In 
  some 
  beds 
  of 
  the 
  marl, 
  or 
  shells, 
  these 
  particles 
  so 
  abound 
  as 
  

   to 
  give 
  a 
  very 
  decided 
  colour 
  to 
  the 
  whole 
  mass. 
  In 
  specimens 
  from 
  

   James 
  City 
  and 
  York 
  counties, 
  as 
  much 
  as 
  thirty-five 
  per 
  cent 
  of 
  the 
  

   green 
  sand 
  has 
  been 
  found, 
  and 
  occasionally 
  shells 
  are 
  seen 
  filled 
  with 
  

   this 
  substance 
  almost 
  alone. 
  

  

  7. 
  The 
  surface 
  of 
  the 
  strata 
  containing 
  shells 
  is 
  usually 
  irregular. 
  

   Sometimes 
  it 
  rises 
  abruptly, 
  in 
  the 
  form 
  of 
  a 
  hillock 
  ; 
  then 
  it 
  is 
  scooped 
  

   out 
  into 
  depressions 
  of 
  a 
  few 
  feet 
  in 
  depth. 
  These 
  irregularities, 
  how- 
  

   ever, 
  are 
  apparently 
  of 
  two 
  kinds 
  ; 
  the 
  one 
  the 
  original 
  form 
  of 
  the 
  deposit, 
  

   the 
  other 
  produced 
  by 
  denuding 
  action 
  upon 
  the 
  surface. 
  Thus 
  in 
  many 
  

   places 
  the 
  same 
  stratum 
  may 
  be 
  remarked 
  rising 
  with 
  more 
  or 
  less 
  

   abruptness 
  ; 
  then 
  again 
  descending, 
  and 
  perhaps 
  preserving 
  a 
  nearly 
  

   horizontal 
  line 
  for 
  some 
  distance, 
  marked 
  at 
  its 
  upper 
  surface 
  by 
  a 
  

   clear 
  and 
  unbroken 
  outline, 
  and 
  presenting 
  no 
  indication 
  of 
  violent 
  

   abrasion 
  from 
  above. 
  In 
  other 
  places, 
  and 
  this 
  is 
  a 
  very 
  frequent 
  oc- 
  

   currence, 
  deep 
  and 
  irregular 
  furrows 
  and 
  cavities 
  are 
  seen, 
  such 
  as 
  

   would 
  naturally 
  arise 
  from 
  the 
  action 
  of 
  the 
  currents 
  and 
  eddies 
  of 
  a 
  

   large 
  mass 
  of 
  water 
  in 
  rapid 
  movement. 
  

  

  8. 
  Having 
  thus 
  given 
  an 
  account 
  of 
  the 
  nature 
  and 
  arrangement 
  of 
  

   the 
  strata 
  overlying 
  the 
  shells, 
  as 
  well 
  as 
  those 
  in 
  which 
  they 
  are 
  im- 
  

   bedded, 
  we 
  will 
  now 
  describe 
  the 
  general 
  condition 
  and 
  disposition 
  in 
  

   which 
  the 
  shells 
  occur. 
  

  

  9. 
  Condition 
  of 
  the 
  shells 
  in 
  the 
  tertiary 
  deposits. 
  

  

  In 
  general, 
  the 
  state 
  of 
  the 
  shells, 
  and 
  their 
  arrangement 
  in 
  the 
  earth, 
  

   are 
  such 
  as 
  to 
  indicate 
  their 
  tranquil 
  deposition 
  at 
  the 
  spots 
  in 
  which 
  

  

  