﻿l8 
  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  ularly 
  then 
  more 
  rapidly 
  increases 
  in 
  gneissoid 
  structure 
  till 
  at 
  

   a 
  distance 
  of 
  from 
  3 
  to 
  5 
  miles 
  from 
  the 
  limestone 
  a 
  region 
  

   of 
  red 
  hornblende 
  gneiss 
  is 
  reached. 
  . 
  . 
  West 
  of 
  Natural 
  

   Bridge 
  in 
  Jefferson 
  county 
  the 
  syenite 
  unquestionably 
  passes 
  

   over 
  into 
  a 
  very 
  perfect 
  red 
  gneiss. 
  . 
  . 
  Both 
  in 
  the 
  field 
  

   and 
  under 
  the 
  microscope 
  the 
  gradual 
  change 
  of 
  the 
  rock 
  can 
  

   be 
  followed 
  through 
  every 
  step." 
  The 
  same 
  sort 
  of 
  a 
  change 
  

   has 
  been 
  observed 
  by 
  Gushing 
  in 
  the 
  Tupper 
  Lake 
  syenite.^ 
  

  

  Syenite-Grenville 
  mixed 
  gneisses 
  

  

  A 
  considerable 
  portion 
  of 
  the 
  known 
  Precambric 
  rock 
  area 
  

   has 
  been 
  represented 
  upon 
  the 
  geologic 
  map 
  as 
  being 
  made 
  up 
  

   of 
  various 
  mixed 
  gneisses. 
  These 
  gneisses 
  include 
  a 
  great 
  many 
  

   rock 
  varieties 
  which 
  make 
  up 
  a 
  heterogeneous 
  mass. 
  After 
  a 
  

   careful 
  study 
  of 
  these 
  gneisses 
  the 
  w^riter 
  is 
  fully 
  convinced 
  that, 
  

   for 
  most 
  part 
  at 
  least, 
  they 
  represent 
  a 
  more 
  or 
  less 
  intricate 
  

   mixture 
  of 
  the 
  syenite 
  and 
  Grenville 
  rocks. 
  Some 
  of 
  these 
  

   gneisses 
  are 
  admittedly 
  of 
  uncertain 
  origin. 
  Certain 
  small 
  

   patches 
  within 
  these 
  areas 
  are 
  undoubtedly 
  rather 
  pure 
  Gren- 
  

   ville, 
  while 
  others 
  seem 
  to 
  be 
  pure 
  syenite, 
  but 
  the 
  small 
  scale 
  

   of 
  the 
  map 
  does 
  not 
  permit 
  these 
  to 
  be 
  separately 
  shown. 
  Some- 
  

   times 
  the 
  syenitic 
  and 
  sometimes 
  the 
  Grenville 
  facies 
  predomi- 
  

   nate. 
  These 
  rocks 
  are 
  everywhere 
  thoroughly 
  gneissoid 
  and 
  

   they 
  are 
  generally 
  well 
  banded 
  except 
  in 
  some 
  of 
  the 
  more 
  syen- 
  

   itic 
  masses. 
  The 
  " 
  leaf 
  gneiss 
  " 
  structure 
  is 
  at 
  times 
  well 
  developed. 
  

  

  It 
  has 
  already 
  been 
  stated 
  that 
  the 
  syenite 
  is 
  intrusive 
  into 
  

   and 
  younger 
  than 
  the 
  Grenville 
  and 
  that 
  the 
  Grenville 
  areas 
  

   must 
  be 
  regarded 
  as 
  large 
  inclusions. 
  A 
  study 
  of 
  the 
  syenite- 
  

   Grenville 
  mixed 
  gneisses 
  furnishes 
  convincing 
  evidence 
  of 
  the 
  

   same 
  kind. 
  Actual 
  inclusions 
  of 
  undoubted 
  Grenville 
  may 
  occa- 
  

   sionally 
  be 
  seen 
  within 
  the 
  syenitic 
  masses. 
  Such 
  inclusions 
  

   may 
  be 
  seen 
  in 
  the 
  vicinity 
  of 
  Lyons 
  Falls; 
  J^ 
  mile 
  north 
  and 
  i 
  

   mile 
  east 
  of 
  Port 
  Leyden; 
  i'>^ 
  miles 
  above 
  the 
  mouth 
  of 
  Miller 
  

   brook; 
  i 
  mile 
  above 
  the 
  mouth 
  of 
  Fall 
  brook, 
  etc. 
  As 
  already 
  

   suggested, 
  the 
  writer 
  is 
  of 
  the 
  opinion 
  that 
  the 
  Grenville 
  was 
  

   often 
  either 
  partially 
  or 
  w^holly 
  incorporated 
  into 
  the 
  syenite 
  

   by 
  fusion 
  when 
  the 
  latter 
  came 
  up 
  in 
  a 
  molten 
  condition. 
  Vari- 
  

   ous 
  rock 
  types 
  formed 
  in 
  this 
  way 
  would 
  depend 
  partly 
  upon 
  

   the 
  degree 
  of 
  metamorphism 
  and 
  partly 
  upon 
  the 
  character 
  of 
  

   the 
  Grenville. 
  The 
  masses 
  of 
  rather 
  syenitic 
  looking 
  rocks 
  which 
  

  

  ' 
  N. 
  Y. 
  State 
  Mus. 
  Bui. 
  95, 
  p.323. 
  

  

  