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  CONTRIBUTIONS 
  TO 
  THE 
  GEOLOGY 
  OF 
  

  

  and 
  hollow 
  Fissurella 
  are 
  found 
  in 
  perfect 
  preservation 
  in 
  many 
  places. 
  

   The 
  state 
  of 
  the 
  shells 
  seems 
  to 
  depend 
  chiefly 
  upon 
  the 
  mechanical 
  

   texture 
  and 
  chemical 
  character 
  of 
  the 
  materials 
  with 
  which 
  they 
  are 
  

   mixed, 
  and 
  of 
  which 
  the 
  overlying 
  stratum 
  is 
  composed. 
  In 
  the 
  moist 
  

   blue 
  clay 
  the 
  shells 
  are 
  generally 
  found 
  in 
  a 
  very 
  soft 
  condition. 
  In 
  

   a 
  highly 
  ferruginous 
  clayey 
  bed 
  they 
  are 
  found 
  either 
  partially 
  or 
  en^ 
  

   tirely 
  dissolved, 
  and 
  beautiful 
  casts 
  remain 
  in 
  their 
  stead. 
  

  

  11. 
  In 
  many 
  places 
  entire 
  banks 
  occur, 
  composed 
  of 
  casts 
  of 
  Chama 
  

   and 
  other 
  shells, 
  sometimes 
  separate, 
  sometimes 
  cemented 
  together 
  so 
  

   as 
  to 
  form 
  a 
  species 
  of 
  rock. 
  These 
  appearances 
  occur 
  chiefly 
  near 
  

   the 
  surface, 
  and 
  when 
  the 
  soil 
  is 
  porous 
  and 
  ferruginous. 
  The 
  casts 
  

   thus 
  formed 
  often 
  consist 
  chiefly 
  of 
  carbonate 
  of 
  lime, 
  and 
  in 
  many 
  

   specimens 
  as 
  much 
  as 
  eighty 
  per 
  cent 
  of 
  this 
  substance 
  is 
  found. 
  

   Casts 
  of 
  this 
  kind 
  belong 
  mostly 
  to 
  the 
  smaller 
  shells, 
  and 
  by 
  far 
  the 
  

   most 
  common 
  are 
  of 
  the 
  Chama 
  congregata. 
  These, 
  as 
  already 
  stated, 
  

   are 
  often 
  found 
  nearly 
  or 
  quite 
  empty, 
  and 
  we 
  may, 
  therefore, 
  con- 
  

   ceive, 
  that 
  as 
  the 
  matter 
  of 
  the 
  shell 
  in 
  an 
  extensive 
  bank 
  of 
  Chamas 
  

   is 
  gradually 
  dissolved, 
  the 
  water 
  charged 
  with 
  carbonate 
  of 
  lime 
  enters 
  

   the 
  cavities, 
  and 
  slowly 
  deposits 
  the 
  carbonate 
  mixed 
  with 
  fine 
  parti- 
  

   cles 
  of 
  clay 
  and 
  sand. 
  Thus 
  by 
  degrees 
  the 
  cavities 
  are 
  filled. 
  In 
  

   the 
  mean 
  time 
  the 
  shell 
  disappears, 
  frequently 
  leaving 
  on 
  the 
  surface 
  

   of 
  the 
  cast 
  a 
  chalky 
  covering, 
  like 
  the 
  decomposed 
  inner 
  film 
  of 
  shelly 
  

   matter. 
  In 
  support 
  of 
  this 
  explanation 
  it 
  may 
  be 
  added, 
  that 
  in 
  many 
  

   casts 
  beautiful 
  crystals 
  of 
  carbonate 
  of 
  lime 
  are 
  found, 
  forming 
  a 
  por- 
  

   tion 
  of 
  the 
  cast, 
  and 
  having 
  the 
  appearance 
  of 
  Dog-tooth 
  Spar. 
  In 
  

   some 
  cases 
  the 
  shelly 
  matter 
  appears 
  to 
  have 
  been 
  dissolved, 
  and 
  its 
  

   place 
  supplied 
  by 
  the 
  crystallized 
  carbonate, 
  encrusting 
  the 
  earth 
  for- 
  

   merly 
  contained 
  within 
  the 
  shell. 
  Sometimes, 
  too, 
  a 
  thin 
  film 
  of 
  

   oxide 
  of 
  iron 
  surrounds 
  the 
  cast, 
  showing 
  very 
  distinctly 
  all 
  the 
  mark- 
  

   ings 
  of 
  the 
  inner 
  surface 
  of 
  the 
  shell. 
  In 
  many 
  localities, 
  presenting 
  a 
  

   series 
  of 
  beds 
  differing 
  in 
  composition, 
  the 
  shells 
  will 
  be 
  found 
  perfect 
  

   in 
  some 
  of 
  them, 
  while 
  in 
  others 
  immediately 
  above 
  or 
  below 
  only 
  

   casts 
  remain. 
  Thus 
  at 
  the 
  College 
  Mill, 
  about 
  one 
  mile 
  from 
  Wil- 
  

   liamsburg, 
  the 
  upper 
  fossiliferous 
  layer 
  is 
  a 
  yellow 
  silicious 
  sand, 
  con- 
  

   taining 
  perfect 
  shells. 
  Below 
  this 
  is 
  a 
  brown 
  ferruginous 
  clay, 
  filled 
  

   with 
  the 
  most 
  beautiful 
  casts 
  of 
  Chama, 
  Pectunculus, 
  Turritella, 
  &c. 
  

  

  