﻿THE 
  TERTIARY 
  FORMATIONS 
  OF 
  VIRGINIA. 
  335 
  

  

  by 
  abrupt 
  or 
  violent 
  actions, 
  furnishes 
  another, 
  though 
  not 
  a 
  deci- 
  

   sive 
  argument 
  against 
  its 
  belonging 
  to 
  a 
  period 
  so 
  late 
  as 
  the 
  Older 
  

   Pliocene. 
  It 
  seems 
  reasonable 
  to 
  infer, 
  that 
  the 
  two 
  would 
  hardly 
  be 
  

   seen 
  resting 
  together 
  in 
  exact 
  conformability, 
  as 
  they 
  do, 
  had 
  they 
  been 
  

   separated 
  in 
  time 
  by 
  the 
  whole 
  interval 
  between 
  the 
  Eocene 
  and 
  the 
  

   Older 
  Pliocene, 
  during 
  which 
  the 
  surface 
  of 
  the 
  former 
  would 
  be 
  in 
  a 
  

   condition 
  to 
  undergo 
  changes 
  and 
  irregularities 
  nowhere 
  perceived 
  

   where 
  they 
  are 
  seen 
  in 
  contact.* 
  

  

  IV. 
  OF 
  THE 
  ORIGIN 
  OF 
  THE 
  DEPOSIT 
  OVERLYING 
  THE 
  MIOCENE 
  

  

  SHELL 
  MARL. 
  

  

  34. 
  It 
  is 
  not 
  easy, 
  in 
  the 
  present 
  state 
  of 
  our 
  information, 
  to 
  approx- 
  

   imate 
  to 
  the 
  precise 
  era 
  when 
  this 
  overlying 
  deposit 
  was 
  produced, 
  

   though 
  it 
  appears 
  to 
  have 
  had 
  a 
  date 
  perhaps 
  long 
  anterior 
  to 
  the 
  latest 
  

   superficial 
  diluvium 
  with 
  which 
  it 
  is 
  often 
  confounded. 
  We 
  infer 
  this 
  

   from 
  the 
  very 
  general 
  absence 
  of 
  all 
  those 
  signs 
  wnich 
  mark 
  a 
  trans- 
  

  

  * 
  In 
  a 
  recent 
  publication 
  (Silliman's 
  Journal, 
  vol. 
  28, 
  p. 
  106), 
  Mr 
  Conrad 
  has 
  attributed 
  

   to 
  a 
  portion 
  of 
  the 
  formation 
  here 
  under 
  discussion, 
  namely, 
  the 
  localities 
  of 
  Yorktown 
  

   and 
  the 
  James 
  river, 
  near 
  Smithfield, 
  a 
  date 
  still 
  more 
  recent 
  than 
  the 
  period 
  of 
  the 
  Older 
  

   Pliocene. 
  He 
  ranks 
  those 
  deposits, 
  together 
  with 
  another 
  at 
  Suffolk, 
  Virginia, 
  and 
  one 
  on 
  

   the 
  St 
  Mary's 
  river, 
  Maryland, 
  under 
  a 
  new 
  division, 
  Medial 
  Pliocene 
  ; 
  it 
  is 
  stated 
  at 
  the 
  same 
  

   time 
  that 
  the 
  recent 
  species 
  at 
  those 
  places 
  compose 
  about 
  thirty 
  per 
  cent. 
  A 
  subdivision 
  of 
  

   the 
  formation 
  as 
  it 
  occurs 
  in 
  Maryland, 
  characterized 
  by 
  Perna 
  maxillata 
  and 
  a 
  less 
  propor- 
  

   tion 
  of 
  recent 
  species, 
  is 
  referred 
  to 
  the 
  Older 
  Pliocene, 
  while 
  the 
  opinion 
  is 
  advanced 
  that 
  the 
  

   Miocene 
  is 
  probably 
  altogether 
  wanting. 
  Now 
  to 
  those 
  familiar 
  with 
  the 
  principles 
  of 
  the 
  

   new 
  nomenclature 
  of 
  the 
  Tertiary, 
  it 
  is 
  obvious 
  that 
  the 
  betis^ 
  so 
  styled, 
  the 
  Older 
  as 
  well 
  as 
  

   the 
  Medial 
  Pliocene, 
  are 
  entitled, 
  in 
  strictness, 
  to 
  the 
  appellation 
  of 
  Miocene 
  only. 
  

  

  To 
  confer 
  on 
  a 
  formation 
  the 
  name 
  Medial 
  Pliocene, 
  its 
  shells 
  should 
  contain 
  about 
  thirty 
  

   per 
  cent 
  extinct, 
  and 
  seventy 
  per 
  cent 
  recent, 
  and 
  not 
  the 
  converse. 
  We 
  believe, 
  moreover, 
  

   that 
  the 
  per-centage 
  of 
  recent 
  species 
  at 
  Yorktown 
  is 
  even 
  materially 
  less 
  than 
  thirty. 
  

  

  In 
  No. 
  3, 
  of 
  his 
  work 
  on 
  American 
  tertiary 
  shells, 
  issued 
  a 
  little 
  earlier 
  than 
  the 
  other 
  arti- 
  

   cle, 
  Mr 
  Conrad 
  adopts 
  a 
  somewhat 
  different 
  classification, 
  calling 
  the 
  several 
  localities 
  in 
  

   Virginia 
  and 
  Maryland, 
  Older 
  Pliocene, 
  as 
  before, 
  except 
  that 
  stratum 
  low 
  down 
  in 
  the 
  

   Maryland 
  formation 
  which 
  is 
  distinguished 
  by 
  the 
  Perna 
  maxillata, 
  and 
  this 
  he 
  denominates 
  

   Miocene. 
  For 
  reasons 
  before 
  stated, 
  namely, 
  the 
  small 
  per-centage 
  of 
  recent 
  species 
  through- 
  

   out 
  them 
  all, 
  we 
  believe 
  the 
  whole 
  together 
  to 
  have 
  been 
  produced 
  in 
  the 
  Miocene 
  epoch, 
  and 
  

   to 
  belong 
  to 
  one 
  formation 
  ; 
  and 
  we 
  have 
  been 
  led 
  into 
  this 
  note 
  in 
  the 
  sincere 
  wish 
  to 
  settle 
  

   the 
  question 
  of 
  the 
  age 
  of 
  this 
  division 
  of 
  our 
  Atlantic 
  Tertiary 
  formations, 
  lest 
  the 
  student 
  of 
  

   American 
  geology 
  be 
  disheartened 
  by 
  the 
  perplexity 
  which 
  grows 
  out 
  of 
  a 
  shifting 
  and 
  incon- 
  

   sistent 
  nomenclature. 
  

  

  