﻿LAURENTIAN 
  LIMESTONES 
  OF 
  NORTH 
  AMERICA. 
  61 
  

  

  as 
  granitic 
  dykes, 
  there 
  are 
  other 
  fissures 
  which 
  have, 
  by 
  a 
  slow 
  

   deposition 
  from 
  solutions, 
  been 
  filled 
  with 
  the 
  constituent 
  minerals 
  

   of 
  granite, 
  constituting 
  true 
  granitic 
  veinstones, 
  which, 
  unlike 
  the 
  

   granitic 
  dykes, 
  are 
  often 
  rich 
  in 
  foreign 
  minerals. 
  These 
  aggre- 
  

   gates 
  pass 
  by 
  gradations 
  into 
  the 
  pyroxenic 
  and 
  calcareous 
  vein- 
  

   stones 
  already 
  noticed. 
  It 
  is 
  from 
  not 
  knowing 
  this 
  distinction 
  

   that 
  Durocher, 
  Fournet 
  and 
  others 
  have 
  perplexed 
  themselves 
  with 
  

   strange 
  hypotheses 
  in 
  attempting 
  to 
  explain 
  the 
  phenomena 
  pre- 
  

   sented 
  by 
  the 
  associations 
  and 
  juxtapositions 
  of 
  mineral 
  species 
  in 
  

   granitic 
  veinstones, 
  which 
  they 
  imagined 
  to 
  have 
  been 
  formed, 
  

   like 
  granitic 
  dykes, 
  by 
  the 
  consolidation 
  of 
  a 
  fused 
  or 
  pasty 
  mass, 
  

   instead 
  of 
  being 
  the 
  result 
  of 
  a 
  slow 
  . 
  deposition 
  from 
  solution. 
  

   For 
  convenience 
  of 
  definition, 
  I 
  have 
  elsewhere 
  distinguished 
  these 
  

   veinstones 
  by 
  the 
  title 
  of 
  endogenous 
  rocks, 
  as 
  describing 
  the 
  con- 
  

   ditions 
  of 
  their 
  formation. 
  The 
  intrusive 
  dykes, 
  on 
  the 
  other 
  

   hand, 
  I 
  have 
  called 
  exotic, 
  and 
  the 
  sedimentary 
  strata, 
  indigenous 
  

   rocks. 
  

  

  . 
  As 
  to 
  the 
  conditions 
  under 
  which 
  these 
  various 
  minerals 
  have 
  

   been 
  crystallized, 
  the 
  beautiful 
  researches 
  of 
  Sorby 
  furnish 
  us 
  

   considerable 
  light. 
  The 
  limestones, 
  from 
  Soinma, 
  near 
  Naples, 
  

   afford, 
  in 
  a 
  finely 
  crystallized 
  state, 
  the 
  greater 
  number 
  of 
  the 
  

   mineral 
  species 
  met 
  with 
  in 
  the 
  Laurentian 
  limestones 
  of 
  North 
  

   America, 
  and 
  the 
  crystals 
  of 
  hornblende, 
  idocrase 
  and 
  orthoclase 
  

   from 
  that 
  locality 
  contain 
  small 
  cavities, 
  often 
  of 
  microscopic 
  

   dimensions, 
  partially 
  filled 
  with 
  water, 
  holding 
  in 
  solution 
  alka- 
  

   line 
  chlorides, 
  sulphates 
  and 
  carbonates. 
  As 
  these 
  cavities 
  were 
  

   filled 
  with 
  liquid 
  during 
  the 
  formation 
  of 
  the 
  crystal, 
  the 
  subse- 
  

   quent 
  cooling 
  has 
  produced 
  a 
  partial 
  vacuum 
  ; 
  this 
  is 
  again 
  filled 
  

   on 
  heating 
  the 
  crystal 
  to 
  the 
  temperature 
  at 
  which 
  it 
  was 
  formed, 
  

   which 
  in 
  this 
  way 
  may 
  be 
  approximately 
  determined. 
  Mr. 
  Sorby 
  

   found, 
  by 
  this 
  method, 
  that 
  the 
  hornblende, 
  idocrase 
  and 
  feldspar 
  

   from 
  the 
  limestones 
  of 
  Somma 
  must 
  have 
  been 
  crystallized 
  at 
  from 
  

   360° 
  to 
  380° 
  Centigrade, 
  a 
  temperature 
  equal 
  to 
  that 
  of 
  low 
  red- 
  

   ness. 
  The 
  crystals 
  from 
  the 
  granitic 
  veins 
  of 
  Cornwall, 
  including 
  

   quartz, 
  mica, 
  orthoclase 
  and 
  oxyd 
  of 
  tin, 
  all 
  of 
  which 
  contain 
  cavi- 
  

   ties 
  holding 
  watery 
  solutions, 
  have 
  shown, 
  in 
  like 
  manner, 
  to 
  Mr. 
  

   Sorby, 
  that 
  these 
  minerals 
  must 
  have 
  been 
  deposited 
  at 
  tempera- 
  

   tures 
  approaching 
  those 
  deduced 
  for 
  the 
  minerals 
  from 
  the 
  crystal- 
  

   line 
  limestones 
  of 
  Somma, 
  or 
  from 
  200° 
  to 
  340° 
  Centigrade 
  (from 
  

   392° 
  to 
  644° 
  Fahrenheit) 
  (Quar. 
  Jour. 
  Geol. 
  Soc, 
  London, 
  xiv, 
  

   453). 
  He 
  thence 
  concludes 
  that 
  these 
  minerals 
  have 
  crystallized 
  

  

  