﻿44 
  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  illustration 
  is 
  that 
  of 
  the 
  granite 
  gneiss 
  in 
  the 
  extreme 
  southeast 
  

   corner 
  of 
  the 
  Alexandria 
  sheet, 
  which 
  forms 
  a 
  wonderful 
  cliff 
  

   along 
  the 
  Indian 
  river. 
  

  

  In 
  quite 
  a 
  number 
  of 
  localities 
  syenitic 
  rocks 
  were 
  found, 
  always 
  

   of 
  trifling 
  extent, 
  and 
  with 
  field 
  relations 
  wholly 
  indeterminate. 
  

  

  At 
  the 
  west 
  end 
  of 
  the 
  upper 
  bridge 
  at 
  Theresa, 
  is 
  a 
  small 
  intru- 
  

   sion 
  of 
  gabbro, 
  which 
  is 
  but 
  little 
  mashed, 
  and 
  has 
  some 
  features 
  of 
  

   interest 
  in 
  that 
  it 
  recalls 
  the 
  anorthosites 
  and 
  gabbros 
  of 
  the 
  general 
  

   Adirondack 
  region, 
  and 
  is 
  the 
  only 
  representative 
  of 
  these 
  rocks 
  

   seen 
  here. 
  It 
  is 
  a 
  dark 
  colored 
  rock, 
  showing 
  numerous, 
  glittering, 
  

   lath-shaped 
  feldspars 
  up 
  to 
  an 
  inch 
  in 
  length, 
  on 
  broken 
  surfaces. 
  

   It 
  is 
  made 
  up 
  of 
  feldspar 
  (labradorite), 
  augite, 
  hypersthene 
  and 
  

   hornblende, 
  with 
  considerable 
  magnetite, 
  and 
  a 
  little 
  pyrite 
  and 
  

   apatite 
  as 
  accessories. 
  The 
  feldspar 
  constitutes 
  from 
  60 
  to 
  70;^ 
  of 
  

   the 
  rock. 
  In 
  composition 
  therefore 
  it 
  is 
  distinctly 
  a 
  gabbro, 
  

   though 
  with 
  more 
  abundant 
  feldspar 
  than 
  the 
  usual 
  Adirondack 
  

   gabbro. 
  Yet, 
  in 
  spite 
  of 
  the 
  coarsely 
  lath-shaped 
  feldspars 
  the 
  

   structure 
  is 
  more 
  nearly 
  that 
  of 
  a 
  gabbro 
  than 
  a 
  hyperite, 
  recall- 
  

   ing 
  in 
  this 
  respect 
  the 
  anorthosite-gabbros 
  farther 
  east. 
  

  

  Diabase. 
  Cutting 
  all 
  the 
  other 
  Precambric 
  rocks 
  of 
  the 
  region, 
  

   occasional 
  dikes 
  of 
  trap 
  rock 
  are 
  found. 
  The 
  fact 
  that 
  they 
  cut 
  all 
  

   the 
  other 
  rocks 
  shows 
  that 
  they 
  are 
  younger, 
  but 
  it 
  can 
  also 
  be 
  

   shown 
  that 
  they 
  are 
  much 
  younger 
  than 
  the 
  other 
  igneous 
  rocks, 
  

   though 
  nevertheless 
  older 
  than 
  the 
  Potsdam 
  sandstone. 
  They 
  

   are 
  found 
  only 
  in 
  the 
  form 
  of 
  dikes, 
  which 
  are 
  lava-filled 
  fissures 
  

   that 
  in 
  general 
  represent 
  plugged 
  channels 
  of 
  ascent 
  of 
  the 
  

   molten 
  rock, 
  leading 
  downward 
  to 
  some 
  source 
  of 
  suppl}^ 
  of 
  the 
  

   material, 
  and 
  tending 
  upward 
  toward 
  the 
  surface. 
  The 
  dikes 
  have 
  

   chilled 
  borders, 
  showing 
  that 
  the 
  inclosing 
  rocks 
  were 
  compara- 
  

   tively 
  cool 
  and 
  hence 
  at 
  no 
  great 
  depth 
  beneath 
  the 
  surface 
  at 
  

   the 
  time 
  of 
  solidification. 
  Furthermore 
  they 
  show 
  no 
  sign 
  of 
  

   having 
  undergone 
  the 
  kind 
  of 
  deformation 
  which 
  all 
  the 
  other 
  

   igneous 
  rocks 
  have 
  experienced 
  in 
  greater 
  or 
  less 
  degree, 
  a 
  kind 
  

   which 
  takes 
  place 
  only 
  at 
  considerable 
  depths. 
  Since 
  the 
  dia- 
  

   bases 
  cooled 
  much 
  nearer 
  the 
  surface 
  than 
  the 
  granites 
  and 
  

   syenites, 
  a 
  long 
  time 
  interval 
  of 
  surface 
  erosion 
  during 
  which 
  a 
  

   considerable 
  rock 
  thickness 
  was 
  worn 
  away 
  from 
  the 
  surface, 
  

   must 
  separate 
  the 
  two. 
  

  

  In 
  the 
  district 
  mapped 
  these 
  dikes 
  have 
  a 
  somewhat 
  unequal 
  

   distribution. 
  They 
  are 
  most 
  abundant 
  on 
  Grindstone 
  island, 
  seven 
  

   having 
  been 
  noted 
  there, 
  mostly 
  of 
  large 
  size, 
  none 
  of 
  them 
  less 
  

   than 
  20 
  feet 
  wide, 
  and 
  ranging 
  from 
  that 
  up 
  to 
  100 
  feet 
  in 
  the 
  case 
  

  

  