﻿T. 
  DAVIDSON 
  AND 
  W. 
  KING 
  ON 
  THE 
  TRIMERELLIDS. 
  141 
  

  

  rellids 
  were 
  in 
  existence, 
  limestones 
  are 
  no 
  more 
  than 
  exceptional 
  

   deposits 
  amongst 
  their 
  argillaceous 
  and 
  siliceous 
  associates. 
  On 
  the 
  

   contrary, 
  in 
  the 
  Cambro-Silurian 
  and 
  Silurian 
  Periods 
  (that 
  is, 
  during 
  

   the 
  role 
  of 
  the 
  Trimerellids), 
  as 
  ever 
  since, 
  depositional 
  phenomena 
  

   became 
  remarkably 
  characterized 
  by 
  calcareous 
  elaborations. 
  

  

  It 
  is 
  equally 
  noteworthy 
  that 
  the 
  Cambrian 
  life-system 
  is 
  but 
  

   poorly 
  represented 
  by 
  forms 
  in 
  whose 
  skeletons 
  calcium 
  compounds 
  

   are 
  essential 
  constituents. 
  Although 
  representing 
  groups 
  most 
  of 
  

   which 
  afterwards 
  included 
  members 
  possessing 
  a 
  massive 
  calcareous 
  

   framework, 
  the 
  Coelenterates, 
  Mollusks, 
  Echinoderms, 
  and 
  Crusta- 
  

   ceans 
  of 
  the 
  Cambrian 
  period 
  had 
  slender 
  skeletal 
  structures, 
  in 
  which 
  

   lime 
  was 
  not 
  particularly 
  abundant 
  ; 
  and 
  they 
  thus 
  stand 
  out 
  in 
  

   striking 
  contrast 
  to 
  their 
  massive 
  successors 
  of 
  subsequent 
  periods. 
  

  

  Doubtless 
  in 
  the 
  Cambro-Silurian 
  and 
  Silurian 
  seas 
  — 
  those 
  

   tenanted 
  by 
  the 
  Trimerellids 
  — 
  ordinary 
  marine 
  calcium 
  compounds 
  

   prevailed 
  as 
  at 
  present. 
  But, 
  in 
  view 
  of 
  what 
  has 
  already 
  been 
  

   stated, 
  are 
  we 
  to 
  suppose 
  that 
  such 
  compounds 
  did 
  not 
  occur 
  to 
  the 
  

   same 
  extent 
  in 
  Cambrian 
  seas 
  ? 
  Or 
  are 
  we 
  to 
  imagine 
  that 
  the 
  

   Cambrian 
  organisms 
  were 
  incapable 
  of 
  abstracting 
  lime 
  from 
  the 
  

   medium 
  they 
  lived 
  in 
  ? 
  As 
  few, 
  we 
  think, 
  will 
  be 
  found 
  to 
  enter- 
  

   tain 
  the 
  latter 
  idea, 
  it 
  may 
  be 
  dismissed 
  to 
  leave 
  room 
  for 
  the 
  con- 
  

   sideration 
  of 
  the 
  other 
  one. 
  

  

  It 
  is 
  well 
  known 
  that 
  the 
  Archaean 
  rocks 
  * 
  are 
  of 
  enormous 
  

   thickness. 
  In 
  the 
  British 
  Isles 
  they 
  consist 
  of 
  the 
  ordinary 
  meta- 
  

   morphics, 
  composed 
  of 
  silacid 
  minerals. 
  In 
  North 
  America, 
  especially 
  

   in 
  Canada, 
  where 
  they 
  apparently 
  comprise 
  two 
  (or 
  more) 
  uncon- 
  

   formable 
  series 
  — 
  Laurentian 
  and 
  Labradorian 
  — 
  they 
  include 
  both 
  

   silacid 
  and 
  silo-carbacid 
  rocks 
  f 
  . 
  The 
  " 
  zones 
  " 
  (" 
  bands," 
  or 
  

   " 
  masses 
  ") 
  of 
  " 
  Crystalline 
  limestone," 
  of 
  the 
  Canadian 
  Geological 
  

   Survey, 
  which 
  constitute 
  the 
  silo-carbacid 
  group, 
  attain 
  an 
  aggregate 
  

   thickness 
  of 
  some 
  thousands 
  of 
  feet 
  J. 
  

  

  The 
  Cambrian 
  rocks, 
  though 
  varying 
  in 
  certain 
  respects 
  in 
  

   different 
  regions, 
  agree 
  pretty 
  generally 
  in 
  their 
  lithological 
  features. 
  

  

  * 
  Dana, 
  rejecting 
  the 
  term 
  " 
  eozoic," 
  which 
  has 
  been 
  applied 
  to 
  them 
  " 
  to 
  

   indicate 
  the 
  morning 
  of 
  that 
  great 
  creatine 
  day 
  in 
  which 
  the 
  lower 
  forms 
  of 
  

   animal 
  life 
  were 
  introduced 
  upon 
  our 
  planet" 
  (Dawson), 
  distinguishes 
  the 
  

   Laurentian 
  and 
  associated 
  metamorphic 
  rocks, 
  older 
  than 
  the 
  primordials, 
  by 
  

   the 
  name 
  Archasan. 
  This 
  change 
  very 
  properly 
  does 
  not 
  reject 
  the 
  idea 
  that 
  

   undoubted 
  or 
  indisputable 
  organic 
  remains 
  may 
  yet 
  be 
  found 
  in 
  such 
  rocks 
  ; 
  

   for 
  it 
  is 
  quite 
  likely 
  that 
  the 
  term 
  eozoic 
  will 
  share 
  the 
  same 
  fate 
  as 
  that 
  of 
  

   azoic, 
  as 
  proposed 
  originally 
  for 
  the 
  Cambrians. 
  

  

  f 
  For 
  the 
  distinctive 
  characters 
  of 
  these 
  rock-divisions, 
  see 
  a 
  paper 
  in 
  the 
  

   Geological 
  Magazine, 
  January 
  1872, 
  entitled, 
  " 
  The 
  Microscopic 
  Characters 
  of 
  

   a 
  Silo-carbacid 
  rock 
  from 
  Ceylon 
  ; 
  and 
  their 
  Bearings 
  on 
  the 
  Methylotic 
  Origin 
  

   of 
  the 
  Laurentian 
  'limestones.' 
  " 
  

  

  \ 
  In 
  both 
  the 
  upper 
  and 
  lower 
  series 
  of 
  the 
  Archaean 
  rocks, 
  " 
  the 
  united 
  

   thickness 
  of 
  which 
  in 
  Canada 
  cannot 
  be 
  less 
  than 
  30,000 
  feet, 
  and 
  probably 
  

   much 
  exceeded 
  it," 
  there 
  are 
  several 
  zones 
  of 
  limestone. 
  Of 
  these 
  it 
  has 
  been 
  

   ascertained 
  that 
  three 
  at 
  least 
  belong 
  to 
  the 
  lower 
  series." 
  The 
  " 
  Zone 
  of 
  

   Grenville 
  limestone 
  [the 
  lowest 
  of 
  the 
  series] 
  is 
  in 
  some 
  places 
  1500 
  feet 
  

   thick." 
  At 
  Clarendon 
  the 
  Archrcan 
  rocks 
  have 
  a 
  "thickness 
  of 
  4000 
  feet, 
  

   about 
  two 
  thirds 
  of 
  which 
  (2666 
  feet) 
  consist 
  of 
  Crystalline 
  limestone." 
  — 
  Logan, 
  

   Geology 
  of 
  Canada, 
  p. 
  31, 
  1863. 
  

  

  Q. 
  J. 
  G, 
  S. 
  No. 
  118. 
  * 
  

  

  