﻿THE 
  TERTIARY 
  FORMATIONS 
  OP 
  VIRGINIA. 
  325 
  

  

  The 
  shelly 
  matter 
  has 
  entirely 
  disappeared, 
  and 
  the 
  casts 
  lie 
  loosely 
  in 
  

   the 
  cavities 
  produced 
  by 
  the 
  removal 
  of 
  the 
  shells, 
  entirely 
  distinct 
  

   from 
  each 
  other, 
  and 
  covered 
  by 
  a 
  film 
  of 
  oxide 
  of 
  iron. 
  The 
  layer 
  

   beneath, 
  consisting 
  of 
  bluish-green 
  silicious 
  clay, 
  is 
  full 
  of 
  well 
  pre- 
  

   served 
  Pectens, 
  Pernas, 
  and 
  a 
  variety 
  of 
  other 
  shells. 
  

  

  12. 
  In 
  general, 
  the 
  various 
  species 
  of 
  shells 
  are 
  found 
  associated 
  in 
  

   colonies 
  or 
  groups, 
  but 
  as 
  in 
  the 
  case 
  of 
  banks 
  of 
  recent 
  shells, 
  these 
  

   colonies 
  contain 
  many 
  scattered 
  specimens, 
  differing 
  from 
  the 
  general 
  

   contents 
  of 
  the 
  group. 
  The 
  two 
  species 
  of 
  Chama, 
  the 
  C. 
  congregata 
  

   and 
  C. 
  corticosa, 
  which 
  are 
  found 
  in 
  almost 
  every 
  deposit 
  of 
  shells 
  in 
  

   this 
  region, 
  in 
  many 
  cases 
  form 
  extensive 
  beds, 
  with 
  a 
  very 
  small 
  ad- 
  

   mixture 
  of 
  other 
  genera. 
  The 
  best 
  agricultural 
  marl, 
  of 
  a 
  purely 
  cal- 
  

   careous 
  nature, 
  which 
  is 
  used 
  in 
  lower 
  Virginia, 
  is 
  derived 
  from 
  these 
  

   beds 
  of 
  Chama, 
  the 
  friable 
  texture 
  of 
  the 
  shell 
  upon 
  exposure 
  to 
  the 
  air 
  

   rendering 
  this 
  species 
  of 
  marl 
  more 
  easy 
  of 
  application 
  to 
  land, 
  and 
  

   more 
  prompt 
  in 
  its 
  ameliorating 
  effects. 
  Crassatellos 
  often 
  form 
  an 
  

   extensive 
  deposit, 
  and 
  the 
  large 
  Pectens 
  occur 
  in 
  continuous 
  layers 
  of 
  

   considerable 
  depth 
  and 
  extent. 
  The 
  different 
  species 
  of 
  Area, 
  Arte- 
  

   mis, 
  Crepidula, 
  &c, 
  present 
  a 
  similar 
  arrangement. 
  Even 
  those 
  shells 
  

   which 
  are 
  of 
  comparatively 
  rare 
  occurrence, 
  are 
  usually 
  found 
  in 
  little 
  

   groups. 
  Thus 
  the 
  Isocardia 
  fraterna 
  is 
  found, 
  to 
  the 
  extent 
  of 
  a 
  dozen 
  

   or 
  twenty, 
  closely 
  packed 
  together. 
  This 
  gregarious 
  assemblage 
  of 
  

   shells 
  of 
  the 
  same 
  species 
  is 
  what 
  would 
  naturally 
  be 
  anticipated 
  in 
  

   the 
  absence 
  of 
  violent 
  agencies 
  during 
  or 
  after 
  their 
  deposition, 
  and 
  

   furnishes 
  another 
  very 
  striking 
  proof 
  of 
  the 
  comparatively 
  tranquil 
  

   condition 
  of 
  the 
  sea 
  or 
  estuary 
  in 
  which 
  they 
  were 
  allowed 
  to 
  accu- 
  

   mulate. 
  

  

  13. 
  Disposition 
  of 
  the 
  fossils. 
  

  

  In 
  nearly 
  all 
  the 
  vertical 
  sections 
  of 
  the 
  deposit 
  we 
  are 
  now 
  describ- 
  

   ing, 
  a 
  series 
  of 
  beds 
  or 
  strata 
  may 
  be 
  observed, 
  each 
  distinguished 
  by 
  

   the 
  predominance 
  of 
  one 
  or 
  more 
  species, 
  and 
  the 
  order 
  of 
  superposi- 
  

   tion 
  of 
  these 
  beds 
  frequently 
  continues 
  without 
  interruption 
  for 
  some 
  

   distance. 
  It 
  does 
  not 
  appear, 
  however, 
  that 
  in 
  localities 
  remote 
  from 
  

   each 
  other 
  the 
  arrangement 
  of 
  the 
  shells 
  is 
  always 
  alike, 
  although 
  in 
  

   many 
  instances 
  there 
  appears 
  to 
  be 
  a 
  striking 
  correspondence. 
  In 
  a 
  

   majority 
  of 
  cases 
  in 
  the 
  neighbourhood 
  of 
  Williamsburg 
  the 
  upper 
  layer 
  

  

  VOL. 
  V. 
  4 
  G 
  

  

  