﻿518 
  PKOF. 
  F. 
  SCHMIDT 
  ON 
  THE 
  SILURIAN 
  STRATA 
  

  

  limestone 
  and 
  slaty 
  rocks. 
  With 
  us 
  the 
  whole 
  of 
  the 
  Lower 
  Silurian 
  

   (with 
  the 
  exception 
  of 
  the 
  very 
  lowest 
  stage) 
  is 
  formed 
  of 
  limestones 
  

   only, 
  deposited 
  in 
  one 
  and 
  the 
  same 
  ancient 
  ocean, 
  without 
  any 
  re- 
  

   markable 
  change 
  of 
  physical 
  conditions, 
  and 
  without 
  any 
  interruption 
  

   in 
  the 
  evolution 
  of 
  animal 
  life. 
  Hence 
  it 
  will 
  be 
  possible 
  with 
  us 
  to 
  

   follow 
  the 
  minute 
  changes 
  produced 
  by 
  time 
  in 
  the 
  form 
  of 
  marine 
  

   organisms 
  (mutations), 
  especially 
  as 
  our 
  widespread 
  zones 
  enable 
  us 
  

   at 
  the 
  same 
  time 
  to 
  study 
  the 
  variations 
  of 
  the 
  same 
  organisms 
  in 
  

   space. 
  

  

  Stage 
  B 
  (B 
  1, 
  Glauconite 
  Sand 
  ; 
  B 
  2, 
  Glauconite 
  Limestone 
  ; 
  B 
  3, 
  

   Orihoceratite 
  Limestone). 
  

  

  We 
  enumerate 
  five 
  principal 
  stages 
  in 
  our 
  Lower 
  Silurian, 
  distin- 
  

   guished 
  by 
  the 
  letters 
  B, 
  C, 
  D, 
  E, 
  and 
  F 
  ; 
  but 
  several 
  of 
  these 
  can 
  

   be 
  subdivided 
  into 
  two 
  or 
  more 
  substages. 
  For 
  example, 
  the 
  first 
  

   stage, 
  B, 
  might 
  be 
  regarded 
  as 
  composed 
  of 
  two 
  or 
  three 
  different 
  

   stages 
  ; 
  they 
  have 
  been 
  united 
  mainly 
  as 
  a 
  matter 
  of 
  convenience. 
  

  

  B 
  1, 
  Glauconite 
  Sand. 
  — 
  The 
  first 
  subdivision, 
  B 
  1, 
  is 
  the 
  so-called 
  

   Greensand, 
  or 
  Glauconite 
  Sand. 
  It 
  has 
  a 
  thickness 
  of 
  from 
  1 
  to 
  10 
  

   feet, 
  and 
  covers 
  the 
  Dictyonema-sl&te 
  everywhere 
  in 
  the 
  northern 
  

   cliffs 
  of 
  Esthonia. 
  and 
  in 
  the 
  prolongation 
  of 
  these 
  cliffs 
  throughout 
  

   the 
  government 
  of 
  St. 
  Petersburg, 
  as 
  far 
  as 
  the 
  lower 
  Wolchow. 
  

   The 
  lower 
  part 
  of 
  this 
  subdivision 
  is 
  comparatively 
  sandy, 
  the 
  upper 
  

   somewhat 
  clayey. 
  The 
  predominating 
  green 
  colour 
  is 
  produced 
  by 
  

   agglomerated 
  grains 
  similar 
  to 
  those 
  formerly 
  mentioned 
  as 
  occurring 
  

   in 
  the 
  sandy 
  layers 
  at 
  the 
  top 
  of 
  the 
  Blue 
  Clay. 
  These 
  grains 
  were 
  

   regarded 
  by 
  Ehrenberg 
  as 
  casts 
  of 
  Foraminifera 
  and 
  small 
  Pteropoda 
  ; 
  

   and 
  he 
  has 
  described 
  and 
  named 
  several 
  of 
  them 
  in 
  the 
  'Monats- 
  

   berichte' 
  of 
  the 
  Berlin 
  Academy, 
  1861. 
  In 
  the 
  same 
  layers, 
  principally 
  

   in 
  the 
  clayey 
  ones, 
  are 
  found 
  the 
  famous 
  Conodonts 
  of 
  Pander 
  

   (they 
  begin 
  here, 
  but 
  are 
  found 
  also 
  in 
  higher 
  stages), 
  the 
  real 
  zoo- 
  

   logical 
  position 
  of 
  which 
  has 
  hitherto 
  been 
  uncertain 
  ; 
  of 
  other 
  

   fossils 
  are 
  found 
  only 
  Obolus 
  siluricus, 
  Eichw., 
  a 
  peculiar 
  Sipho- 
  

   notreta, 
  and 
  a 
  rather 
  variable 
  Lingula, 
  reminding 
  us, 
  by 
  its 
  

   somewhat 
  pentagonal 
  form, 
  of 
  Lingulella 
  Davisi, 
  M'Coy 
  (see 
  my 
  

   memoir, 
  p. 
  17, 
  fig. 
  5). 
  In 
  geological 
  position 
  our 
  Glauconite 
  

   Sand 
  corresponds 
  exactly 
  to 
  the 
  Ceratopyge-sta.ge 
  of 
  Scandinavian 
  

   geologists, 
  although 
  no 
  traces 
  of 
  Trilobites 
  are 
  found. 
  The 
  syste- 
  

   matic 
  position 
  of 
  the 
  Glauconite 
  Sand, 
  occurring, 
  as 
  it 
  does, 
  between 
  

   the 
  Dictyonema-sl&te 
  and 
  the 
  Glauconite 
  Limestone 
  (the 
  Lower 
  Ortho- 
  

   ceratite-limestone 
  of 
  Sweden) 
  will 
  not 
  allow 
  of 
  any 
  other 
  paralielization. 
  

   In 
  England 
  the 
  Upper 
  Tremadoc 
  may 
  be 
  the 
  corresponding 
  stage 
  ; 
  

   yet 
  Lingula 
  Davisii 
  commences 
  its 
  range 
  there 
  at 
  a 
  much 
  earlier 
  date. 
  

   B 
  2, 
  Glauconite 
  Limestone. 
  — 
  The 
  second 
  substage, 
  B 
  2, 
  of 
  our 
  stage 
  

   B 
  is 
  the 
  so-called 
  Glauconite 
  Limestone. 
  It 
  attains 
  a 
  thickness 
  of 
  

   12-40 
  feet, 
  the 
  thickness 
  increasing 
  to 
  the 
  eastward. 
  The 
  uppermost 
  

   layers 
  of 
  the 
  clayey 
  beds 
  of 
  the 
  Greensand 
  become 
  impregnated 
  with 
  

   carbonate 
  of 
  lime, 
  and 
  are 
  in 
  this 
  way 
  intimately 
  linked 
  to 
  the 
  Glau- 
  

   conite 
  Limestone. 
  The 
  same 
  glauconitic 
  grains 
  do 
  exist 
  in 
  the 
  lime- 
  

   stone 
  ; 
  but 
  they 
  are 
  more 
  scattered, 
  and 
  the 
  limestone 
  becomes 
  in 
  con- 
  

  

  