﻿LAURENTIAN 
  LIMESTONES 
  OP 
  NORTH 
  AMERICA. 
  59 
  

  

  Canada. 
  The 
  crystals 
  of 
  apatite 
  in 
  Elmsleyand 
  Burgess, 
  Ontario 
  ? 
  

   rarely 
  present 
  sharp 
  or 
  well-defined 
  forms 
  ; 
  but 
  whether 
  lining 
  

   drusy 
  cavities, 
  or 
  imbedded 
  in 
  the 
  calcareous 
  veinstone, 
  present 
  

   rounded 
  or 
  sub-cylindrical 
  crystalline 
  masses, 
  while 
  the 
  pyroxene 
  

   and 
  sphene, 
  which 
  often 
  accompany 
  them, 
  preserve 
  the 
  sharpness 
  

   of 
  their 
  angles. 
  The 
  hypothesis 
  which 
  would 
  explain 
  by 
  igneous 
  

   fusion 
  this 
  rounding 
  of 
  the 
  angles, 
  is 
  evidently 
  untenable, 
  first, 
  

   because 
  the 
  more 
  fusible 
  species 
  show 
  no 
  signs 
  of 
  such 
  action, 
  

   and, 
  second, 
  because 
  the 
  carbonate 
  of 
  lime, 
  which 
  encloses 
  and 
  

   even 
  penetrates 
  the 
  rounded 
  quartz 
  crystals, 
  is 
  not 
  in 
  any 
  way 
  

   affected 
  at 
  the 
  surfaces 
  of 
  contact 
  as 
  it 
  would 
  have 
  been 
  by 
  fused 
  

   or 
  half-fused 
  quartz. 
  This 
  rounding 
  of 
  the 
  angles 
  of 
  certain 
  crys- 
  

   tals 
  appears 
  to 
  me 
  to 
  be 
  nothing 
  more 
  than 
  a 
  result 
  of 
  the 
  solvent 
  

   action 
  of 
  the 
  heated 
  watery 
  solutions, 
  from 
  which 
  the 
  minerals 
  

   of 
  these 
  veins 
  have 
  been 
  successively 
  deposited, 
  the 
  crytals 
  pre- 
  

   viously 
  formed 
  being 
  partially 
  redissolved 
  as 
  a 
  result 
  of 
  some 
  

   change 
  in 
  the 
  temperature, 
  or 
  in 
  the 
  chemical 
  constitution 
  of 
  the 
  

   solution. 
  Heated 
  solutions 
  of 
  alkaline 
  silicates, 
  as 
  shown 
  by 
  

   Daubree, 
  are 
  without 
  action 
  on 
  feldspars, 
  as 
  might 
  be 
  expected 
  

   from 
  the 
  fact, 
  observed 
  by 
  him, 
  of 
  the 
  production 
  of 
  crystals 
  of 
  

   feldspar 
  and 
  of 
  pyroxene 
  in 
  the 
  midst 
  of 
  such 
  solutions. 
  These 
  

   liquids 
  would, 
  however 
  doubtless 
  attack 
  and 
  dissolve 
  phosphate 
  

   of 
  lime, 
  which 
  is, 
  in 
  like 
  manner, 
  decomposed 
  by 
  solutions 
  of 
  alka- 
  

   line 
  carbonates, 
  and 
  these 
  latter 
  at 
  elevated 
  temperatures 
  attack 
  

   and 
  dissolve 
  crystallized 
  quartz. 
  

  

  The 
  regularity, 
  and 
  the 
  frequently 
  large 
  dimensions 
  of 
  the 
  

   crystals, 
  not 
  less 
  than 
  their 
  modes 
  of 
  association, 
  and 
  the 
  other 
  

   phenomena 
  just 
  mentioned, 
  serve 
  to 
  distinguish 
  the 
  minerals 
  

   of 
  these 
  veinstones 
  from 
  the 
  same 
  species 
  which 
  are 
  found 
  dis- 
  

   seminated 
  in 
  the 
  limestone 
  beds. 
  In 
  the 
  latter 
  case 
  they 
  some- 
  

   times 
  occur 
  in 
  small 
  distinct 
  crystals, 
  but 
  more 
  generally 
  in 
  

   rounded 
  irregular 
  grains, 
  which 
  present 
  a 
  marked 
  contrast 
  to 
  the 
  

   same 
  minerals 
  occuring 
  in 
  the 
  veins. 
  This 
  rounded 
  form 
  of 
  the 
  

   minerals 
  in 
  the 
  beds 
  of 
  limestone, 
  is 
  to 
  be 
  carefully 
  distinguished 
  

   from 
  the 
  rounding 
  of 
  the 
  crystals 
  in 
  the 
  veins 
  just 
  described, 
  

   although 
  the 
  two 
  phenomena 
  have 
  hitherto 
  been 
  confounded 
  by 
  

   those 
  who 
  have 
  written 
  upon 
  the 
  subject. 
  In 
  the 
  latter 
  case 
  the 
  

   rounding 
  is 
  by 
  no 
  means 
  constant, 
  and 
  is 
  confined 
  to 
  a 
  few 
  spe- 
  

   cies, 
  while 
  in 
  the 
  limestone 
  beds 
  it 
  will 
  be 
  found 
  that 
  a 
  rounded 
  

   form 
  characterizes 
  alike 
  apatite 
  and 
  quartz, 
  and 
  such 
  silicates 
  as 
  

   pyroxene, 
  b'ornblende, 
  serpentine 
  and 
  chondrodite. 
  The 
  rounded 
  

  

  