﻿110 
  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  down 
  the 
  feldspar 
  into 
  granular 
  aggregates, 
  it 
  seldom 
  shows 
  any 
  

   well 
  developed 
  schistosity. 
  Near 
  the 
  ore 
  the 
  gneiss 
  is 
  seamed 
  

   through 
  and 
  through 
  by 
  pegmatite 
  of 
  lighter 
  color 
  and 
  is 
  also 
  

   penetrated 
  by 
  a 
  fine-grained 
  granite 
  like 
  that 
  found 
  on 
  Birch 
  hill. 
  

   Bands 
  of 
  somewhat 
  darker 
  appearance 
  are 
  not 
  infrequently 
  inter- 
  

   calated 
  in 
  the 
  gneiss. 
  They 
  are 
  apt 
  to 
  be 
  more 
  hornblendic 
  than 
  

   the 
  surrounding 
  rock 
  and 
  are 
  probably 
  to 
  be 
  interpreted 
  as 
  masses 
  

   of 
  the 
  hornblende 
  schist, 
  which 
  is 
  described 
  on 
  a 
  following 
  page, 
  

   that 
  have 
  been 
  penetrated 
  by, 
  and, 
  to 
  a 
  greater 
  or 
  less 
  extent, 
  

   incorporated 
  with 
  the 
  augite 
  gneiss. 
  The 
  origin 
  of 
  the 
  latter 
  rock 
  

   is 
  believed 
  to 
  be 
  igneous; 
  in 
  physical 
  character 
  and 
  mineralogy 
  

   there 
  is 
  a 
  close 
  accord 
  with 
  the 
  plutonic 
  rocks 
  such 
  as 
  are 
  found 
  

   among 
  the 
  basal 
  formations 
  of 
  the 
  Adirondacks. 
  That 
  it 
  belongs 
  

   probably 
  to 
  the 
  older 
  series 
  of 
  this 
  class 
  is 
  indicated 
  by 
  its 
  cata- 
  

   clastic 
  texture 
  along 
  with 
  the 
  presence 
  of 
  similar 
  intrusives 
  in 
  the 
  

   vicinity 
  that 
  have 
  been 
  but 
  little 
  affected 
  by 
  dynamic 
  agencies. 
  

   Though 
  the 
  chemical 
  constitution 
  of 
  the 
  gneiss 
  has 
  not 
  been 
  

   determined 
  by 
  analysis, 
  there 
  would 
  seem 
  to 
  be 
  little 
  doubt 
  that 
  

   the 
  relative 
  proportions 
  of 
  the 
  constituents 
  agree 
  with 
  those 
  of 
  the 
  

   acid 
  igneous 
  class, 
  varying 
  from 
  syenite 
  to 
  a 
  low-quartz 
  granite. 
  

  

  This 
  gneiss 
  underlies 
  the 
  greater 
  part 
  of 
  the 
  area 
  about 
  Lyon 
  

   Mountain. 
  It 
  constitutes 
  most 
  of 
  the 
  high 
  ridge 
  east 
  of 
  the 
  ore 
  

   bodies 
  as 
  well 
  as 
  the 
  projecting 
  spurs, 
  and 
  probably 
  extends 
  

   beneath 
  the 
  drift-covered 
  valley 
  to 
  the 
  north 
  and 
  west. 
  It 
  forms 
  

   the 
  walls 
  in 
  most 
  of 
  the 
  mine 
  openings 
  and 
  is 
  invariably 
  closely 
  

   associated 
  with 
  the 
  ore. 
  

  

  Granitic 
  gneisses. 
  On 
  the 
  top 
  of 
  Lyon 
  mountain 
  a 
  coarse 
  quartz- 
  

   feldspar 
  rock 
  of 
  slightly 
  gneissoid 
  appearance 
  is 
  exposed 
  over 
  an 
  

   area 
  that 
  can 
  not 
  be 
  accurately 
  delimited, 
  though 
  it 
  is 
  probably 
  

   small. 
  Judging 
  from 
  surface 
  indications 
  it 
  extends 
  a 
  few 
  hundred 
  

   feet 
  down 
  the 
  slopes, 
  which 
  are 
  thickly 
  strewn 
  with 
  its 
  boulders, 
  

   but 
  no 
  contacts 
  were 
  found. 
  The 
  rock 
  has 
  the 
  composition 
  of 
  an 
  

   acid 
  granite, 
  with 
  which 
  it 
  is 
  allied 
  so 
  closely 
  in 
  texture 
  and 
  field 
  

   structures 
  as 
  well, 
  that 
  little 
  doubt 
  of 
  its 
  igneous 
  nature 
  can 
  exist. 
  

   Its 
  mineralogy 
  is 
  simple, 
  feldspar 
  and 
  quartz 
  forming 
  almost 
  the 
  

   whole 
  mass. 
  The 
  latter 
  mineral 
  occurs 
  in 
  flattened 
  lenses 
  and 
  

   spindles 
  which 
  have 
  a 
  common 
  orientation 
  and 
  give 
  the 
  somewhat 
  

   indefinite 
  gneissoid 
  appearance 
  observable 
  on 
  weathered 
  surfaces. 
  

   Microcline 
  predominates 
  over 
  orthoclase, 
  the 
  two 
  feldspars 
  repre- 
  

   sented. 
  Both 
  show 
  commonly 
  a 
  microperthitic 
  habit. 
  Of 
  the 
  

   ferromagnesian 
  minerals 
  there 
  are 
  a 
  few 
  scattered 
  grains 
  of 
  green 
  

   augite 
  and 
  small 
  shreds 
  of 
  biotite. 
  In 
  outcrop 
  the 
  rock 
  exhibits 
  a 
  

  

  