﻿REPORT 
  OF 
  THE 
  DIRECTOR 
  2^ 
  

  

  than 
  that 
  it 
  was 
  posterior 
  to 
  the 
  deposition 
  of 
  the 
  Manhattan 
  schists 
  

   and 
  therefore 
  post-Hudson 
  River, 
  and 
  prior 
  to 
  at 
  least 
  a 
  part 
  of 
  the 
  

   dynamic 
  disturbance 
  and 
  crumpHng 
  of 
  these 
  rocks 
  with 
  which 
  the 
  

   intrusives 
  have 
  become 
  schistose 
  and 
  even 
  crumpled. 
  The 
  igneous 
  

   rocks 
  which 
  occur 
  in 
  the 
  pre-Cambrian 
  and 
  Palaeozoic 
  within 
  the 
  

   region 
  south 
  of 
  the 
  41st 
  parallel 
  of 
  latitude 
  may 
  be 
  classified 
  as 
  

  

  follows 
  : 
  

  

  Yonkers 
  gneiss 
  

  

  Granites, 
  red 
  and 
  grey 
  

   Pegmatite 
  dykes, 
  very 
  coarse 
  

   Harrison 
  diorite 
  

   Amphibolites 
  and 
  pyroxenites 
  

   Serpentines, 
  derived 
  from 
  basic 
  intrusives. 
  

  

  YONKERS 
  GNEISS. 
  

  

  In 
  an 
  article 
  on 
  the 
  Metamorphic 
  Strata 
  of 
  Southeastern 
  New 
  

   York*, 
  the 
  writer 
  called 
  attention 
  to 
  a 
  reddish 
  gneiss 
  which 
  appeared 
  

   to 
  be 
  the 
  lowest 
  Stratum 
  in 
  that 
  terrane. 
  From 
  the 
  microscopic 
  

   structure 
  of 
  this 
  rock, 
  studied 
  at 
  certain 
  localities, 
  and 
  from 
  its 
  appar- 
  

   ent 
  relations 
  to 
  the 
  overlying 
  gray 
  gneiss, 
  the 
  conclusion 
  was 
  formed 
  

   at 
  that 
  time 
  that 
  it 
  was 
  a 
  metamorphosed 
  sedimentary 
  rock. 
  More 
  

   extended 
  observations 
  on 
  this 
  formation 
  made 
  during 
  the 
  summer 
  of 
  

   1 
  89 
  1 
  showed 
  that 
  it 
  was 
  not 
  uniformly 
  persistent 
  as 
  a 
  basal 
  member 
  in 
  

   southern 
  Westchester 
  county, 
  and 
  that 
  it 
  was 
  not 
  limited 
  to 
  the 
  axes 
  

   of 
  the 
  eroded 
  anticlinals. 
  The 
  fact 
  that 
  it 
  was 
  overlain 
  by 
  a 
  varying 
  

   thickness 
  of 
  the 
  gray 
  gneiss 
  was 
  noticed 
  by 
  the 
  writer 
  at 
  an 
  early 
  date 
  

   but 
  was 
  attributed 
  to 
  unequal 
  repetition 
  of 
  the 
  gray 
  gneiss 
  by 
  folding. 
  

   Later 
  investigations 
  showed 
  that 
  a 
  rock 
  of 
  the 
  same 
  composition 
  

   occurred 
  frequently 
  as 
  an 
  intrusive 
  either 
  in 
  veins 
  and 
  dikes 
  or 
  in. 
  

   bosses 
  like 
  the 
  one 
  at 
  Sparta. 
  

  

  The 
  Yonkers 
  gneiss 
  is 
  technically 
  a 
  gnieissoid 
  granite. 
  (PI. 
  V.) 
  

   It 
  is 
  a 
  well 
  foliated 
  rock 
  consisting 
  of 
  quartz, 
  reddish 
  orthoclase 
  and 
  

   biotite 
  with 
  a 
  little 
  plagioclase. 
  It 
  is 
  plainly 
  intrusive 
  in 
  the 
  Ford- 
  

   ham 
  gneiss 
  and 
  has 
  become 
  completely 
  schistose. 
  

  

  *Am. 
  Jour. 
  Sci. 
  III. 
  Vol. 
  XXXIX, 
  p. 
  389. 
  

  

  