﻿GEOLOGY 
  OF 
  THOUSAND 
  ISLANDS 
  REGION 
  29 
  

  

  calcite. 
  Toward 
  the 
  edges, 
  however, 
  the 
  rock 
  becomes 
  much 
  less 
  

   pure, 
  and 
  at 
  times 
  the 
  same 
  thmg 
  happens 
  m 
  the 
  near 
  vicmity 
  of 
  

   the 
  igneous 
  intrusions, 
  large 
  and 
  small, 
  which 
  repeatedly 
  cut 
  through 
  

   it. 
  This 
  is 
  by 
  no 
  means 
  the 
  invariable 
  rule 
  however. 
  In 
  the 
  case 
  

   of 
  the 
  thin 
  limestone 
  bands 
  which 
  occur 
  in 
  the 
  general 
  schist 
  series 
  

   [pi. 
  2] 
  there 
  is 
  much 
  less 
  pure 
  limestone, 
  since 
  these 
  bands 
  show 
  

   the 
  same 
  impure 
  borders 
  as 
  does 
  the 
  thick 
  belt, 
  leaving 
  only 
  a 
  

   small 
  central 
  thickness 
  of 
  the 
  purer 
  rock. 
  In 
  this 
  pure 
  rock 
  oc- 
  

   casional 
  graphite 
  scales, 
  flakes 
  of 
  brown 
  mica 
  (phlogopite) 
  and 
  oc- 
  

   casional 
  small 
  crystals 
  of 
  white 
  pyroxene 
  (diopside) 
  are 
  the 
  usual 
  

   accessory 
  minerals 
  and 
  in 
  very 
  small 
  amount. 
  Others 
  occur^ 
  but 
  

   very 
  sparingly. 
  These 
  rocks 
  must 
  originally 
  have 
  been 
  extremely 
  

   pure 
  limestones, 
  slightly 
  contaminated 
  with 
  organic 
  matter, 
  which 
  

   now 
  appears 
  in 
  the 
  form 
  of 
  graphite. 
  

  

  The 
  impure 
  limestone 
  of 
  the 
  area 
  is 
  owing 
  to 
  two 
  distinct 
  causes. 
  

   Certain 
  thin 
  bands 
  of 
  impure 
  limestone 
  in 
  the 
  schist 
  series, 
  and 
  the 
  

   impure 
  borders 
  of 
  the 
  otherwise 
  pure 
  bands 
  seem 
  unquestionably 
  

   owing 
  to 
  original 
  deposit 
  as 
  shaly 
  or 
  sandy 
  limestones, 
  forming 
  

   gradations 
  between 
  the 
  pure 
  rock 
  and 
  the 
  overlying 
  and 
  underlying 
  

   shales 
  and 
  sandstones. 
  Hence 
  on 
  recrystallization 
  a 
  much 
  smaller 
  

   percentage 
  of 
  calcite 
  and 
  a 
  much 
  larger 
  one 
  of 
  other 
  minerals 
  has 
  

   resulted. 
  The 
  other 
  cause 
  is 
  the 
  interaction 
  of 
  the 
  limestone 
  with 
  

   igneous 
  rocks, 
  producing 
  what 
  are 
  known 
  as 
  contact 
  rocks, 
  in 
  which 
  

   certain 
  added 
  ingredients 
  are 
  supplied 
  to 
  the 
  limestones 
  from 
  the 
  

   igneous 
  rocks 
  and 
  react 
  with 
  the 
  limestone 
  to 
  form 
  minerals 
  which 
  

   thus 
  have 
  a 
  mixed 
  origin. 
  Such 
  contact 
  rocks 
  are 
  thus 
  limited 
  to' 
  

   the 
  near 
  vicinity 
  of 
  the 
  igneous 
  rocks. 
  

  

  The 
  two 
  most 
  common 
  kinds 
  of 
  impure 
  limestone 
  of 
  the 
  first 
  

   type 
  in 
  the 
  region 
  are 
  the 
  quartzose 
  limestones, 
  and 
  the 
  pyroxenic 
  

   limestones. 
  Much 
  of 
  the 
  marginal 
  limestone 
  seems 
  to 
  have 
  been 
  

   sandy, 
  and 
  even 
  to 
  have 
  contained 
  thin 
  layers 
  of 
  fairly 
  pure 
  sand- 
  

   stone. 
  This 
  has 
  recrystallized 
  as 
  quartz, 
  partly 
  in 
  fine 
  grain, 
  form- 
  

   ing 
  a 
  mosaic 
  w'ith 
  the 
  calcite, 
  and 
  partly 
  coarser 
  and 
  in 
  films 
  and 
  

   patches 
  in 
  the 
  limestone. 
  Each 
  mineral 
  at 
  times 
  contains 
  inclu- 
  

   sions 
  of 
  the 
  other, 
  they 
  evidently 
  recrystallized. 
  together, 
  and 
  the 
  

   quartz 
  evidently 
  had 
  the 
  stronger 
  crystallizing 
  force. 
  There 
  is 
  a 
  

   considerable 
  amount 
  of 
  limestone 
  in 
  the 
  area 
  which 
  is 
  a 
  calcite- 
  

   quartz 
  rock, 
  with 
  little 
  or 
  no 
  admixture 
  with 
  other 
  minerals. 
  

  

  Even 
  more 
  common 
  is 
  the 
  pyroxenic 
  limestone, 
  where 
  the 
  cal- 
  

   cite 
  is 
  accompanied 
  by 
  a 
  greater 
  or 
  less 
  amount 
  of 
  a 
  white 
  or 
  a 
  

   light 
  green 
  pyroxene. 
  This 
  is 
  prone 
  to 
  alter 
  to 
  serpentine, 
  a 
  dull 
  

   green, 
  greasy 
  to 
  earthy 
  looking 
  mineral, 
  producing 
  a 
  mottled 
  green 
  

  

  