﻿GEOLOGY 
  OF 
  THOUSAND 
  ISLANDS 
  REGION 
  33 
  

  

  out 
  such 
  rocks 
  either 
  occur 
  along 
  granite-quartzite 
  contacts, 
  or 
  

   else 
  are 
  included 
  in 
  granite. 
  They 
  are 
  apt 
  to 
  show 
  close 
  set, 
  block 
  

   jointing, 
  like 
  the 
  quartzite. 
  They 
  have 
  been 
  found 
  only 
  in 
  asso- 
  

   ciation 
  with 
  the 
  granite 
  gneiss. 
  The 
  field 
  evidence 
  seems 
  to 
  us 
  

   strongly 
  indicative 
  of 
  the 
  fact 
  that 
  these 
  are 
  really 
  intermediate 
  

   rocks, 
  in 
  the 
  sense 
  that 
  they 
  represent 
  quartzites 
  in 
  various 
  stages 
  

   of 
  granitization 
  ; 
  that 
  the 
  quartzite 
  is 
  being 
  permeated, 
  soaked 
  and 
  

   even 
  digested 
  by 
  the 
  granite. 
  The 
  character 
  of 
  the 
  intermediate 
  

   rock, 
  the 
  shading 
  of 
  the 
  two 
  into 
  one 
  a;nother, 
  and 
  the 
  field 
  oc- 
  

   currence 
  of 
  the 
  intermediate 
  stages, 
  all 
  point 
  to 
  this 
  conclusion, 
  

   and 
  seem 
  incapable 
  of 
  explanation 
  on 
  any 
  other 
  hypothesis. 
  

  

  Amphibolitcs. 
  The 
  name 
  amphibolite 
  is 
  a 
  convenient, 
  compre- 
  

   hensive 
  term 
  for 
  a 
  group 
  of 
  rocks 
  of 
  gneissoid 
  habit 
  and 
  dark 
  color, 
  

   composed 
  essentially 
  of 
  hornblende 
  and 
  feldspars, 
  with 
  often 
  consid- 
  

   erable 
  amounts 
  of 
  biotite 
  or 
  pyroxenes, 
  and 
  with 
  accessory 
  minerals 
  

   of 
  which 
  magnetite 
  is 
  easily 
  chief, 
  and 
  quartz 
  and 
  garnet 
  of 
  fre- 
  

   quent 
  occurrence. 
  In 
  respect 
  to 
  origin, 
  the 
  rock 
  has 
  long 
  been 
  a 
  

   puzzling 
  one 
  since 
  apparently 
  identically 
  appearing 
  amphibolite 
  

   might 
  be 
  produced 
  by 
  metamorphism 
  from 
  either 
  igneous 
  or 
  from 
  

   sedimentary 
  rocks 
  of 
  the 
  proper 
  character. 
  In 
  a 
  multitude 
  of 
  locali- 
  

   ties 
  in 
  the 
  Adirondacks 
  it 
  has 
  been 
  shown 
  that 
  gabbro 
  intrusions 
  

   (whose 
  character 
  and 
  origin 
  is 
  rendered 
  certain 
  by 
  a 
  core 
  of 
  prac- 
  

   tically 
  unchanged 
  rock) 
  are 
  largely 
  changed 
  over 
  into 
  amphibolites, 
  

   every 
  step 
  in 
  the 
  process 
  being 
  open 
  to 
  inspection. 
  Similar 
  rela- 
  

   tions 
  have 
  been 
  shown 
  in 
  many 
  localities 
  in 
  all 
  continents. 
  

   Also 
  in 
  the 
  Adirondacks, 
  wherever 
  the 
  Grenville 
  series 
  is 
  exposed, 
  

   bands 
  of 
  amphibolite 
  of 
  varying 
  thickness 
  are 
  found 
  so 
  definitely 
  

   interstratified 
  with 
  other 
  Grenville 
  rocks 
  of 
  unquestioned 
  sedimen- 
  

   tary 
  nature, 
  that 
  there 
  seemed 
  no 
  escape 
  from 
  the 
  conclusion 
  that 
  

   the 
  rock 
  must 
  have 
  resulted 
  from 
  the 
  metamorphism 
  of 
  a 
  sedi- 
  

   ment; 
  and 
  amphibolite 
  of 
  such 
  origin 
  is 
  equally 
  of 
  world-wide 
  dis- 
  

   tribution. 
  In 
  addition 
  it 
  has 
  recently 
  been 
  shown 
  by 
  Adams 
  that 
  

   amphibolite 
  can 
  also 
  be 
  produced 
  on 
  a 
  large 
  scale 
  by 
  the 
  contact 
  

   action 
  of 
  granite 
  on 
  limestone. 
  Here 
  are 
  therefore 
  three 
  different 
  

   modes 
  of 
  origin, 
  and 
  the 
  rock 
  may 
  be 
  either 
  igneous, 
  sedimentary, 
  

   or 
  a 
  contact 
  rock. 
  Each 
  occurrence 
  of 
  the 
  rock 
  must 
  therefore 
  

   be 
  studied 
  by 
  itself, 
  in 
  so 
  far 
  as 
  its 
  origin 
  is 
  concerned. 
  Amphib- 
  

   olite 
  of 
  all 
  three 
  types 
  is 
  present 
  in 
  our 
  district. 
  

  

  Within 
  the 
  mapped 
  area 
  amphibolite 
  has 
  not 
  the 
  bulk 
  and 
  im- 
  

   portance 
  that 
  it 
  has 
  in 
  much 
  of 
  the 
  Precambric 
  district 
  adjacent. 
  

   There 
  is 
  much 
  of 
  it 
  present 
  as 
  inclusions 
  in 
  the 
  granite 
  gneiss 
  

   bathyliths 
  and 
  stocks, 
  inclusions 
  of 
  much 
  variation 
  in 
  size 
  and 
  in 
  

  

  