﻿51 
  f) 
  PROF. 
  F. 
  SCHMIDT 
  ON 
  THE 
  SILURIAN 
  STRATA 
  

  

  nificant 
  thin 
  layers 
  of 
  sandstone. 
  As 
  to 
  the 
  Lower 
  and 
  Upper 
  Silu- 
  

   rians, 
  there 
  cannot 
  be 
  any 
  question 
  as 
  to 
  their 
  representing 
  the 
  same 
  

   divisions 
  as 
  in 
  Scandinavia 
  and 
  the 
  British 
  Islands, 
  i. 
  e. 
  the 
  second 
  

   and 
  third 
  faunas 
  of 
  Barrande 
  ; 
  but 
  our 
  Cambrian 
  or 
  Primordial 
  Silu- 
  

   rian 
  I 
  admit 
  to 
  be 
  disputable, 
  because 
  of 
  its 
  peculiar 
  constitution. 
  

   ~No 
  Primordial 
  Trilobites 
  of 
  the 
  Paradoxicles 
  or 
  Olenus 
  group 
  are 
  found 
  

   with 
  us 
  — 
  indeed, 
  no 
  Trilobites 
  at 
  all, 
  only 
  Lingulidas, 
  some 
  Graptolites 
  

   (including 
  Dietyonema) 
  in 
  the 
  uppermost 
  stage, 
  and 
  some 
  doubtful 
  

   additional 
  organic 
  remains 
  (see 
  woodcuts 
  1-3 
  on 
  page 
  13 
  of 
  my 
  me- 
  

   moir 
  above 
  cited). 
  My 
  opinion 
  as 
  to 
  the 
  existence 
  of 
  the 
  Cambrian 
  

   in 
  our 
  country 
  is 
  founded 
  upon 
  the 
  identity 
  of 
  the 
  highest 
  Cambrian 
  

   (the 
  Dictyonema-shale) 
  and 
  the 
  lowest 
  Silurian 
  strata 
  in 
  Russia 
  and 
  

   in 
  Scandinavia, 
  and 
  upon 
  the 
  agreement 
  of 
  the 
  Scandinavian 
  geo- 
  

   logists 
  Linnarsson 
  and 
  Brogger, 
  who 
  both 
  visited 
  at 
  different 
  times 
  

   our 
  most 
  important 
  localities, 
  and 
  who 
  were 
  at 
  the 
  same 
  time 
  well 
  

   acquainted 
  with 
  the 
  geology 
  of 
  their 
  own 
  country. 
  

  

  Cambrian 
  Strata. 
  

  

  (Stage 
  A.) 
  

  

  Our 
  Cambrian 
  strata 
  are 
  visible 
  (as 
  I 
  mentioned 
  before) 
  only 
  in 
  the 
  

   cliffs 
  at 
  the 
  northern 
  border 
  of 
  our 
  system 
  and 
  in 
  corresponding 
  river- 
  

   valleys 
  from 
  the 
  S.E. 
  corner 
  of 
  the 
  Ladoga, 
  at 
  the 
  rivers 
  Walchow 
  

   and 
  Sjas, 
  to 
  the 
  cliffs 
  near 
  Baltischport 
  on 
  the 
  Gulf 
  of 
  Finland, 
  

   a 
  distance 
  of 
  about 
  500 
  versts. 
  Three 
  stages 
  of 
  these 
  strata 
  have 
  

   long 
  been 
  known 
  : 
  — 
  (1) 
  the 
  Blue 
  Clay, 
  (2) 
  the 
  Ungulite 
  Grit, 
  and 
  (3) 
  

   the 
  Dictyonema-a\a,te&, 
  All 
  these 
  stages 
  are 
  exposed 
  in 
  many 
  places 
  

   along 
  the 
  northern 
  cliffs 
  ; 
  but 
  their 
  thickness 
  (all 
  three 
  stages 
  taken 
  

   together), 
  as 
  visible 
  in 
  natural 
  sections, 
  does 
  not 
  exceed 
  100 
  feet. 
  Yet 
  

   the 
  artesian 
  wells 
  in 
  Reval 
  and 
  St. 
  Petersburg 
  have 
  shown 
  an 
  amount 
  

   of 
  about 
  000 
  feet 
  more, 
  the 
  Blue 
  Clay 
  reaching 
  a 
  thickness 
  of 
  300 
  

   feet, 
  and 
  the 
  underlying 
  iron-sandstones 
  about 
  the 
  same. 
  The 
  latter 
  

   repose 
  directly 
  upon 
  granite 
  in 
  the 
  artesian 
  well 
  at 
  St. 
  Petersburg 
  

   (Gen. 
  Helmersen, 
  Bullet, 
  de 
  l'Acad. 
  St. 
  Petersb. 
  1865, 
  p. 
  185). 
  In 
  

   the 
  upper 
  strata 
  of 
  the 
  Blue 
  Clay 
  some 
  strange 
  and 
  very 
  minute 
  

   fossils 
  have 
  been 
  found, 
  called 
  by 
  Pander 
  Platysolenites 
  (see 
  my 
  

   memoir, 
  p. 
  13, 
  woodcut, 
  fig. 
  1) 
  : 
  they 
  may 
  be 
  either 
  Annelide- 
  tubes 
  

   or 
  flattened 
  stalks 
  of 
  some 
  unknown 
  Cystidean 
  (as 
  they 
  show 
  a 
  

   distinct 
  articulation). 
  At 
  the 
  upper 
  edge 
  of 
  the 
  clay 
  we 
  often 
  

   detect 
  thin 
  layers 
  of 
  intercalated 
  sandstone 
  filled 
  with 
  green 
  agglo- 
  

   merated 
  corpuscles 
  resembling 
  casts 
  of 
  Eoraminifera 
  (my 
  memoir, 
  

   p. 
  13, 
  fig. 
  2). 
  The 
  so-called 
  Ungulite 
  Grit 
  has 
  an 
  average 
  thickness 
  

   of 
  50-60 
  feet 
  ; 
  its 
  lower 
  strata 
  pass 
  gradually 
  into 
  the 
  Blue 
  Clay 
  by 
  

   frequent 
  intercalations 
  of 
  clayey 
  bands. 
  In 
  these 
  intercalated 
  clay 
  

   beds 
  Dr. 
  Yolborth 
  has 
  found 
  some 
  doubtful 
  minute 
  Orthoceratites 
  

   (see 
  fig. 
  3 
  on 
  p. 
  13 
  of 
  my 
  memoir). 
  The 
  Ungulites 
  themselves 
  are 
  

   found 
  only 
  near 
  the 
  upper 
  edge 
  of 
  the 
  so-called 
  Ungulite 
  Grit. 
  

   These 
  real 
  Ungulite 
  layers 
  are 
  crowded 
  with 
  millions 
  of 
  separated 
  

   valves 
  of 
  Obolus 
  apollinis, 
  Eichw. 
  ( 
  Unyula, 
  Eichw.), 
  with 
  which 
  are 
  

   associated 
  in 
  some 
  places, 
  forming 
  distinct 
  layers, 
  Schmidtia 
  celata, 
  

  

  