﻿LAURENTIAN 
  LIMESTONES 
  OP 
  NORTH 
  AMERICA. 
  57 
  

  

  nation 
  of 
  the 
  locality, 
  it 
  will 
  be 
  found 
  in 
  the 
  latter 
  case 
  that 
  the 
  

   deposit 
  occurs 
  in 
  a 
  fissure 
  cutting 
  the 
  stratification, 
  and 
  as 
  well- 
  

   defined 
  walls. 
  

  

  A 
  banded 
  arrangement 
  of 
  the 
  mineral 
  contents 
  is 
  often 
  very 
  

   well 
  marked. 
  Thus, 
  while 
  the 
  walls 
  may 
  be 
  coated 
  with 
  crystal- 
  

   line 
  hornblende, 
  or 
  with 
  phlogopite, 
  the 
  body 
  of 
  the 
  vein 
  will 
  be 
  

   filled 
  with 
  apatite, 
  in 
  the 
  midst 
  of 
  which 
  may 
  be 
  found 
  a 
  mass 
  

   of 
  crystalline 
  orthoclase, 
  or 
  of 
  loganite, 
  occupying 
  the 
  centre 
  of 
  

   the 
  vein. 
  In 
  other 
  instances, 
  portions 
  of 
  the 
  vein 
  will 
  be 
  occupied 
  

   by 
  crystals 
  of 
  apatite, 
  pyroxene, 
  or 
  phlogopite, 
  imbedded 
  in 
  cal- 
  

   careous 
  spar, 
  which, 
  in 
  some 
  other 
  part 
  of 
  the 
  breadth 
  of 
  the 
  vein, 
  

   or 
  in 
  its 
  prolongation, 
  will 
  so 
  far 
  predominate 
  as 
  to 
  give 
  to 
  the 
  

   mass 
  the 
  aspect 
  of 
  a 
  coarsely 
  crystalline 
  lamellar 
  limestone. 
  

   Most 
  of 
  the 
  well 
  crystallized 
  minerals 
  described 
  by 
  observers, 
  

   both 
  on 
  this 
  continent 
  and 
  in 
  Europe, 
  as 
  occuring 
  in 
  crystalline 
  

   limestones, 
  appear 
  to 
  be 
  derived 
  from 
  calcareous 
  veins 
  like 
  those 
  

   just 
  described. 
  

  

  In 
  like 
  manner 
  I 
  have 
  described 
  localities 
  of 
  crystallized 
  apatite 
  

   as 
  occuring 
  in 
  beds 
  of 
  limestones 
  in 
  Burgess, 
  Ontario, 
  where 
  a 
  

   subsequent 
  examination 
  (while 
  confirming 
  the 
  existence 
  of 
  this 
  

   mineral 
  in 
  the 
  limestone 
  beds 
  of 
  that 
  region) 
  has 
  shown, 
  neverthe- 
  

   less, 
  that 
  the 
  workable 
  deposits 
  are 
  with 
  few, 
  if 
  any 
  y 
  exceptions 
  

   confined 
  to 
  the 
  veinstones. 
  

  

  From 
  a 
  lithological 
  point 
  of 
  view, 
  there 
  cannot 
  be 
  any 
  objection 
  

   to 
  extending 
  the 
  name 
  of 
  limestone 
  to 
  these 
  calcareous 
  veinstones 
  ; 
  

   but 
  geologically, 
  it 
  becomes 
  important 
  to 
  discriminate 
  between 
  

   them 
  and 
  those 
  great 
  masses 
  of 
  limestone 
  which 
  are 
  sedimentary 
  

   deposits. 
  

  

  That 
  these 
  deposits 
  of 
  mineral 
  matter, 
  occupying 
  fissures 
  in 
  the 
  

   stratified 
  rocks, 
  are 
  not 
  intrusive 
  veins 
  or 
  dykes, 
  but 
  have 
  been 
  

   formed 
  by 
  gradual 
  deposition 
  or 
  accretion, 
  is 
  shown 
  by 
  the 
  banded 
  

   arrangement 
  parallel 
  to 
  the 
  walls, 
  just 
  noticed. 
  Further 
  evidence 
  

   of 
  this 
  origin 
  is 
  seen 
  in 
  the 
  manner 
  in 
  which 
  the 
  various 
  minerals 
  

   surround 
  or 
  incrust 
  each 
  other. 
  Thus, 
  small 
  prisms 
  of 
  apatite 
  are 
  

   enclosed 
  in 
  large 
  crystals 
  of 
  phlogopite, 
  in 
  spinel, 
  and 
  even 
  in 
  

   massive 
  apatite 
  ; 
  crystals 
  or 
  crystalline 
  masses 
  of 
  calcite 
  are 
  

   imbedded 
  in 
  apatite 
  and 
  in 
  quartz, 
  and 
  well-defined 
  crystals 
  of 
  

   hornblende 
  (p'argasite) 
  occur 
  imbedded 
  in 
  others 
  of 
  pyroxene. 
  In 
  

   another 
  example, 
  small 
  crystals 
  of 
  hornblende 
  are 
  implanted 
  on 
  a 
  

   large 
  crystal 
  of 
  pyroxene, 
  and 
  both 
  of 
  these 
  are, 
  in 
  their 
  turn, 
  

   incrusted 
  by 
  small 
  prisms 
  of 
  epidote. 
  This 
  latter 
  crystal 
  was 
  

  

  