﻿LAURENTIAN 
  LIMESTONES 
  OP 
  NORTH 
  AMERICA. 
  93 
  

  

  scales 
  between 
  the 
  laminae 
  of 
  mica 
  crystals, 
  or 
  ^lse 
  forming 
  solid 
  

   masses 
  in 
  the 
  vein. 
  These 
  masses, 
  when 
  pure, 
  are 
  generally 
  made 
  

   of 
  broad 
  and 
  thick 
  laminae, 
  the 
  edges 
  of 
  which, 
  in 
  some 
  cases 
  at 
  

   least, 
  are 
  at 
  right 
  angles 
  to 
  the 
  sides 
  of 
  the 
  vein. 
  In 
  some 
  cases 
  

   a 
  large 
  vein 
  will 
  carry 
  two 
  or 
  more 
  bands 
  or 
  layers 
  of 
  pure 
  or 
  

   nearly 
  pure 
  graphite, 
  separated 
  from 
  each 
  other 
  and 
  from 
  the 
  

   wall-rock 
  by 
  feldspar, 
  pyroxene 
  or 
  quartz. 
  Occasionally 
  the 
  

   graphite 
  found 
  in 
  these 
  veins 
  is 
  finely 
  granular, 
  or 
  like 
  that 
  from 
  

   Warrensburgh, 
  New 
  York, 
  breaks 
  easily 
  into 
  rectangular 
  masses, 
  

   which 
  exhibit 
  on 
  certain 
  of 
  the 
  fractured 
  surfaces 
  a 
  peculiar 
  

   finely 
  waved 
  aspect, 
  due 
  to 
  a 
  structure 
  which 
  may 
  be 
  described 
  

   as 
  consisting 
  of 
  layers 
  of 
  a 
  millimeter 
  or 
  less 
  in 
  thickness, 
  tolera- 
  

   bly 
  regular, 
  and 
  made 
  up 
  of 
  minute 
  and 
  narrow 
  lamellae, 
  arranged 
  

   at 
  right 
  angles 
  to 
  the 
  layers, 
  and 
  presenting 
  a 
  fibrous 
  or 
  columnar 
  

   aspect 
  when 
  broken 
  across. 
  When 
  the 
  fracture 
  is 
  with 
  the 
  layers, 
  

   and 
  thus 
  exposes 
  only 
  the 
  ends 
  of 
  the 
  lamellae, 
  a 
  granular 
  surface 
  

   is 
  presented. 
  Fractures 
  at 
  right 
  angles 
  with 
  the 
  layers 
  show 
  an 
  

   undulating 
  surface, 
  recalling 
  that 
  of 
  certain 
  waved 
  maple 
  woods, 
  

   and 
  due 
  to 
  the 
  fact 
  that 
  the 
  fibres 
  of 
  the 
  successive 
  layers 
  are 
  not 
  

   quite 
  parallel 
  with 
  each 
  other. 
  This 
  Laurentian 
  graphite, 
  accord- 
  

   ing 
  to 
  Prof. 
  C. 
  F. 
  Chandler's 
  analysis, 
  consists 
  of 
  carbon, 
  64.06 
  ; 
  

   carbonate 
  of 
  lime, 
  32.90 
  ; 
  the 
  remaining 
  three 
  per 
  cent 
  being 
  

   chiefly 
  silica 
  and 
  oxyd 
  of 
  iron. 
  The 
  carbonate 
  of 
  lime 
  is 
  invisibly 
  

   diffused 
  through 
  the 
  mass, 
  which 
  effervesces 
  freely 
  with 
  acids. 
  It 
  

   is 
  not 
  in 
  any 
  way 
  connected 
  with 
  the 
  peculiar 
  waved 
  structure, 
  

   since 
  the 
  graphite 
  from 
  the 
  famous 
  mine 
  of 
  Marinski, 
  in 
  the 
  Gov- 
  

   ernment 
  of 
  Irkutsk 
  in 
  Siberia, 
  which 
  presents 
  a 
  structure 
  precisely 
  

   similar, 
  contains 
  no 
  carbonate 
  of 
  lime, 
  and 
  only 
  small 
  quantities 
  of 
  

   earthy 
  impurities, 
  amounting, 
  according 
  to 
  Dumas, 
  to 
  3.7 
  per 
  cent 
  

   of 
  the 
  purest 
  specimens. 
  

  

  The 
  Laurentian 
  graphites, 
  then, 
  besides 
  their 
  visibly 
  present 
  

   foreign 
  minerals, 
  may 
  contain 
  finely 
  disseminated 
  impurities, 
  which 
  

   detract 
  from 
  their 
  economical 
  value, 
  and 
  can 
  only 
  be 
  detected 
  by 
  

   analysis. 
  

  

  A 
  Laurentian 
  graphite 
  from 
  Patterson, 
  New 
  Jersey, 
  crystallized 
  

   in 
  broad 
  lamellae, 
  gave 
  to 
  Prof. 
  Chandler, 
  21.0 
  per 
  cent 
  of 
  pyrites, 
  

   finely 
  disseminated 
  between 
  the 
  laminae. 
  This 
  graphite, 
  which* 
  by 
  

   exposure 
  becomes 
  covered 
  with 
  an 
  efflorescence 
  of 
  sulphate 
  of 
  

   iron, 
  gave 
  also 
  portions 
  of 
  silica, 
  alumina 
  and 
  lime, 
  apparently 
  

   derived 
  from 
  some 
  mineral 
  like 
  scapolite, 
  disseminated 
  through 
  

  

  