﻿REPORT 
  OF 
  THE 
  DIRECTOR 
  25 
  

  

  are 
  so 
  thick 
  and 
  extensive 
  that 
  the 
  outcrops 
  are 
  few. 
  The 
  locali- 
  

   ties 
  discussed 
  are 
  therefore 
  chiefly 
  confined 
  to 
  the 
  vicinity 
  of 
  New 
  

   York 
  city. 
  

  

  The 
  stratified' 
  crystalline 
  rocks 
  within 
  the 
  area 
  under 
  consideration 
  

   which 
  is 
  south 
  of 
  the 
  41° 
  parallel 
  belong 
  to 
  two 
  principal 
  divisions, 
  

   the 
  Precambrian 
  and 
  the 
  Paleozoic. 
  Of 
  the 
  Precambrian 
  only 
  

   one 
  member 
  can 
  be 
  recognized, 
  which 
  has 
  been 
  called 
  the 
  

   Fordham 
  gneiss. 
  Of 
  the 
  Palaeozoic 
  there 
  are 
  two 
  persistent 
  mem- 
  

   bers, 
  the 
  Inwood 
  limestone 
  and 
  Manhattan 
  schist, 
  and 
  a 
  third 
  of 
  local 
  

   and 
  slight 
  development, 
  the 
  Lowerre 
  quartzite, 
  which 
  underlies 
  the 
  

   limestone. 
  

  

  PRECAMBRIAN. 
  

  

  Fordham 
  Gneiss. 
  

  

  The 
  Fordham 
  gneiss, 
  named 
  from 
  the 
  former 
  town 
  of 
  that 
  name, 
  

   within 
  which 
  it 
  is 
  well 
  exposed, 
  is 
  a 
  gray 
  banded 
  gneiss 
  varying 
  much 
  

   in 
  the 
  composition 
  of 
  its 
  bands 
  or 
  layers, 
  which, 
  as 
  a 
  rule, 
  are 
  quite 
  

   thin, 
  rarely 
  exceeding 
  two 
  inches 
  in 
  thickness. 
  Some 
  of 
  these 
  arc 
  

   highly 
  quartzose 
  (PI. 
  IV.), 
  some 
  are 
  largely 
  composed 
  of 
  biotite 
  and 
  

   some 
  consist 
  of 
  pegmatite 
  or 
  granite 
  which 
  has 
  been 
  injected 
  parallel 
  

   to 
  the 
  regular 
  banding 
  of 
  the 
  gneiss. 
  Hornblende 
  is 
  an 
  occasional 
  

   constituent 
  of 
  this 
  rock 
  but, 
  though 
  highly 
  persistent 
  in 
  some 
  bands, 
  

   does 
  not 
  occur 
  over 
  large 
  areas 
  of 
  country. 
  Garnet 
  is 
  present 
  rarely 
  

   in 
  but 
  small 
  quantity. 
  

  

  As 
  the 
  schistosity 
  of 
  the 
  Fordham 
  gneiss 
  has 
  usually 
  a 
  very 
  steep 
  

   dip 
  the 
  exposures 
  of 
  this 
  rock 
  chiefly 
  show 
  cross 
  sections 
  of 
  the 
  

   banding. 
  

  

  It 
  is 
  difficult 
  to 
  give 
  this 
  rock 
  formation 
  a 
  systematic 
  name 
  which 
  

   exactly 
  indicates 
  its 
  age. 
  If 
  it 
  is 
  of 
  sedimentary 
  origin 
  it 
  may 
  be 
  

   called 
  Algonkian, 
  but 
  it 
  can 
  only 
  certainly 
  be 
  said 
  that 
  it 
  is 
  Precam- 
  

   brian. 
  

  

  The 
  Fordham 
  gneiss 
  forms 
  the 
  high 
  anticlinal 
  ridge 
  which 
  borders 
  

   the 
  New 
  York 
  shore 
  of 
  the 
  Hudson 
  River 
  from 
  Yonkers 
  southward 
  

   to 
  Spuyten 
  Duyvil 
  and 
  also 
  that 
  on 
  the 
  west 
  side 
  of 
  the 
  Bronx 
  valley. 
  

   The 
  former 
  ridge 
  terminates 
  on 
  the 
  south 
  at 
  Spuyten 
  Duyvil 
  and 
  

   does 
  not 
  reappear 
  on 
  Manhattan 
  Island. 
  The 
  latter 
  is 
  bifurcated 
  at 
  

  

  