﻿SILURIAN 
  ROCKS 
  OF 
  SCANDINAVIA. 
  325 
  

  

  Trimicleus-shales, 
  contain 
  Turrilepas, 
  Trinucleus, 
  and 
  Remopleurides, 
  

   whilst 
  the 
  first 
  and 
  last 
  also 
  contain 
  Agnostus 
  trinodus. 
  These 
  are 
  

   absent, 
  or 
  occur 
  very 
  rarely 
  in 
  the 
  intermediate 
  Cystidean 
  limestone 
  

   and 
  in 
  the 
  hard 
  limestones. 
  

  

  But 
  the 
  shallower-water 
  beds 
  are 
  more 
  interesting 
  as 
  illustrating 
  

   migrations. 
  

  

  In 
  a 
  paper 
  by 
  Dr. 
  Hicks 
  (Q. 
  J. 
  G. 
  S. 
  Nov. 
  1875, 
  p. 
  557), 
  the 
  

   author 
  concludes 
  that 
  a 
  Precambrian 
  continent 
  extended 
  over 
  Europe, 
  

   with 
  a 
  general 
  inclination 
  to 
  the 
  south-west, 
  and 
  that 
  as 
  this 
  continent 
  

   was 
  gradually 
  submerged, 
  migrations 
  of 
  marine 
  faunas 
  took 
  place 
  

   from 
  the 
  south-west, 
  where 
  ocean 
  areas 
  prevailed. 
  

  

  The 
  deep-water 
  faunas 
  certainly 
  bear 
  out 
  this 
  view 
  ; 
  but 
  the 
  

   migration 
  of 
  shallow-water 
  forms 
  seems 
  to 
  have 
  taken 
  place 
  along 
  

   the 
  coast-lines, 
  and 
  to 
  have 
  proceeded 
  much 
  more 
  slowly 
  than 
  that 
  

   of 
  the 
  deep-water 
  forms 
  ; 
  so 
  that 
  it 
  becomes 
  much 
  more 
  difficult 
  to 
  

   correlate 
  shallow-water 
  deposits 
  by 
  their 
  faunas 
  than 
  to 
  correlate 
  

   those 
  formed 
  in 
  deep 
  water. 
  This 
  is 
  easily 
  accounted 
  for, 
  as 
  the 
  

   deep-water 
  deposits 
  were 
  of 
  a 
  uniform 
  character 
  over 
  large 
  areas, 
  so 
  

   that 
  animals 
  could 
  migrate 
  widely, 
  and 
  still 
  meet 
  with 
  suitable 
  con- 
  

   ditions, 
  whereas 
  the 
  inhabitants 
  of 
  the 
  shallower 
  seas 
  would 
  require 
  

   some 
  time, 
  and 
  also 
  undergo 
  considerable 
  change, 
  whilst 
  passing 
  

   from 
  one 
  kind 
  of 
  sediment 
  to 
  another. 
  

  

  Dr. 
  Tornqvist 
  (Ofv. 
  af 
  Kongl. 
  Y.-Akad. 
  Porh. 
  1879, 
  No. 
  2, 
  p. 
  70) 
  

   considers 
  that 
  during 
  Cambrian 
  and 
  Silurian 
  times, 
  Sweden 
  alternately 
  

   underwent 
  an 
  invasion 
  of 
  shallow- 
  water 
  forms 
  from 
  Eussia, 
  and 
  of 
  

   deep-water 
  forms 
  from 
  Britain. 
  Dr. 
  Hicks's 
  opinion 
  of 
  the 
  direction 
  

   taken 
  by 
  the 
  deep-water 
  faunas 
  seems, 
  however, 
  more 
  correct, 
  whilst 
  

   the 
  direction 
  taken 
  by 
  the 
  shallow-water 
  forms 
  seems 
  to 
  have 
  varied 
  

   considerably, 
  according 
  as 
  they 
  were 
  influenced 
  by 
  widespread 
  move- 
  

   ments 
  or 
  by 
  more 
  local 
  ones. 
  

  

  In 
  later 
  Cambrian 
  times, 
  the 
  shallow-water 
  forms 
  seem 
  to 
  have 
  

   moved 
  along 
  the 
  coast-line 
  in 
  a 
  general 
  W.S.W. 
  direction 
  from 
  

   Eussia 
  to 
  Britain. 
  The 
  Orthoceras-limestone 
  of 
  Sweden, 
  correlated 
  

   with 
  the 
  Arenig, 
  contains 
  large 
  Asaplii, 
  which 
  do 
  not 
  occur 
  in 
  Britain 
  

   until 
  Lower-Bala 
  times. 
  

  

  The 
  Cystidean 
  limestone 
  contains 
  Cystideans, 
  such 
  as 
  EcliinospTim- 
  

   rites 
  aurantium, 
  and 
  Phacopes 
  of 
  the 
  subgenus 
  Chasmops, 
  which 
  are 
  

   found 
  in 
  somewhat 
  earlier 
  beds 
  in 
  Eussia, 
  but 
  first 
  occur 
  in 
  

   Wales 
  in 
  Middle-Bala 
  beds, 
  whilst, 
  as 
  I 
  have 
  before 
  pointed 
  out 
  

   (Q. 
  J. 
  G. 
  S. 
  May 
  1880, 
  p. 
  279), 
  the 
  Cystideans 
  did 
  not 
  reach 
  the 
  

   Lake-district 
  until 
  Upper-Bala 
  times. 
  These 
  migrations 
  seem 
  to 
  

   be 
  simply 
  due 
  to 
  the 
  remoteness 
  of 
  Britain 
  from 
  land 
  in 
  Arenig 
  and 
  

   Lower-Bala 
  times 
  ; 
  for 
  there 
  are 
  no 
  shallow-water 
  forms 
  of 
  Arenig 
  

   age, 
  and 
  very 
  few 
  of 
  Lower-Bala 
  age, 
  in 
  the 
  southern 
  parts 
  of 
  

   Britain. 
  

  

  In 
  the 
  earlier 
  Silurian 
  times, 
  however, 
  a 
  change 
  seems 
  to 
  have 
  

   taken 
  place 
  in 
  the 
  direction 
  of 
  migration, 
  owing 
  to 
  the 
  fact 
  that 
  

   widespread 
  upheaval 
  was 
  accompanied 
  by 
  a 
  greater 
  local 
  upheaval 
  

   in 
  Britain, 
  so 
  that 
  this 
  became 
  a 
  centre 
  of 
  dispersal 
  in 
  May-Hill 
  

   times, 
  from 
  which 
  shallow- 
  water 
  forms 
  migrated. 
  The 
  coast-line 
  

   Q.J.G.S. 
  No. 
  151. 
  z 
  

  

  