﻿LAURENTIAN 
  LIMESTONES 
  OF 
  NORTH 
  AMERICA. 
  71 
  

  

  apatite, 
  magnesian 
  mica, 
  pyroxene, 
  brown 
  tourmaline, 
  and 
  other 
  

   minerals. 
  Drusy 
  cavities, 
  in 
  the 
  veins 
  of 
  mingled 
  calcite 
  and 
  

   apatite 
  in 
  Burgess, 
  are 
  sometimes 
  lined 
  with 
  large 
  crystals 
  of 
  dog- 
  

   tooth 
  spar. 
  We 
  have 
  already 
  insisted 
  on 
  page 
  48 
  upon 
  the 
  fact, 
  

   which 
  is 
  there 
  for 
  the 
  first 
  time 
  pointed 
  out, 
  that 
  it 
  is 
  these 
  highly 
  

   calcareous 
  veinstones 
  which 
  have 
  given 
  rise, 
  in 
  North 
  America 
  at 
  

   least, 
  to 
  the 
  widely-spread 
  notion 
  of 
  the 
  eruptive 
  origin 
  of 
  crystal- 
  

   line 
  limestones. 
  

  

  Of 
  calcite 
  as 
  a 
  bed-mineral, 
  constituting 
  great 
  stratified 
  masses 
  

   of 
  limestone 
  in 
  the 
  Laurentian 
  series, 
  it 
  is 
  not 
  necessary 
  here 
  to 
  

   speak. 
  It 
  is, 
  however, 
  to 
  be 
  remarked 
  that 
  in 
  these 
  limestones, 
  

   as 
  in 
  those 
  of 
  later 
  periods, 
  we 
  have 
  now 
  evidence 
  that 
  portions 
  

   of 
  the 
  carbonate 
  of 
  lime 
  once 
  belonged 
  to 
  living 
  organisms, 
  as 
  is 
  

   shown 
  in 
  the 
  calcareous 
  skeletons 
  of 
  the 
  Eozoon. 
  These, 
  though 
  

   sometimes 
  preserved 
  by 
  injection 
  with 
  silicates, 
  appear 
  in 
  other 
  

   cases 
  with 
  their 
  tubes 
  and 
  canals 
  filled 
  with 
  carbonate 
  of 
  lime, 
  

   evidently 
  — 
  like 
  the 
  silicates 
  — 
  a 
  chemical 
  deposit, 
  and 
  there 
  is 
  no 
  

   doubt 
  that 
  a 
  part 
  of 
  these 
  limestones, 
  like 
  those 
  of 
  more 
  recent 
  

   formations, 
  have 
  been 
  directly 
  precipitated 
  by 
  chemical 
  reactions 
  

   from 
  the 
  waters 
  of 
  the 
  ocean. 
  The 
  often 
  repeated 
  assertion 
  that 
  

   organic 
  life 
  has 
  built 
  up 
  all 
  the 
  great 
  limestone 
  formations, 
  is 
  based 
  

   upon 
  a 
  fallacy, 
  for 
  animals 
  have 
  no 
  power 
  to 
  generate 
  carbonate 
  

   of 
  lime. 
  Although 
  many 
  invertebrate 
  animals 
  construct 
  calcareous 
  

   skeletons, 
  which 
  form 
  a 
  great 
  part 
  of 
  the 
  limestone 
  of 
  the 
  earth's 
  

   crust, 
  the 
  pre 
  existence 
  of 
  this 
  carbonate 
  of 
  lime 
  is 
  one 
  of 
  the 
  

   conditions 
  necessary 
  to 
  their 
  growth, 
  and, 
  as 
  I 
  have 
  elsewhere 
  

   shown, 
  owes 
  its 
  origin 
  to 
  chemical 
  reactions 
  which 
  are 
  still 
  going 
  

   on 
  in 
  the 
  ocean's 
  waters, 
  and 
  which 
  have 
  in 
  past 
  times 
  given 
  rise 
  

   directly 
  to 
  limestone 
  strata, 
  in 
  which 
  the 
  occurrence 
  of 
  shells 
  and 
  

   corals 
  is 
  only 
  accidental 
  [Geol. 
  Can., 
  pages 
  575, 
  631). 
  

  

  Dolomite. 
  — 
  As 
  already 
  pointed 
  out 
  in 
  the 
  Geology 
  of 
  Canada, 
  

   pages 
  24 
  and 
  592, 
  large 
  beds 
  of 
  the 
  Laurentian 
  limestones 
  are 
  

   magnesian, 
  and 
  sometimes 
  have 
  the 
  composition 
  of 
  true 
  dolomites. 
  

   These 
  dolomites 
  and 
  magnesian 
  limestones 
  have 
  been 
  found 
  to 
  con- 
  

   tain 
  serpentine, 
  hornblende 
  (tremolite), 
  apatite, 
  quartz 
  and 
  small 
  

   portions 
  of 
  mica, 
  and 
  they 
  may, 
  perhaps, 
  in 
  different 
  localities, 
  

   include 
  all 
  those 
  mineral 
  species 
  which 
  have 
  been 
  indicated 
  as 
  

   belonging 
  to 
  the 
  limestone 
  strata. 
  It 
  is 
  to 
  be 
  remarked 
  that 
  the 
  

   calcareous 
  skeleton 
  of 
  Eozoon 
  Oanadense, 
  which 
  is 
  carbonate 
  of 
  

   ime, 
  is 
  found, 
  in 
  specimens 
  from 
  Burgess, 
  replaced 
  by 
  dolomite. 
  

  

  