﻿2S 
  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  MANHATTAN 
  SCHIST. 
  

  

  This 
  formation 
  covers 
  a 
  larger 
  area 
  than 
  any 
  other 
  within 
  the 
  

   limits 
  of 
  the 
  Harlem 
  sheet, 
  and 
  is 
  the 
  uppermost 
  of 
  the 
  crystalline 
  

   groups. 
  The 
  rock 
  is 
  essentially 
  a 
  mixture 
  of 
  biotite 
  and 
  quartz, 
  fre- 
  

   quently 
  containing 
  enough 
  orthoclase 
  to 
  give 
  it 
  the 
  composition 
  of 
  a 
  

   gneiss. 
  The 
  principal 
  accessory 
  is 
  garnet, 
  which 
  occurs 
  in 
  crystals 
  

   varying 
  from 
  one-sixteenth 
  to 
  one-quarter 
  of 
  an 
  inch 
  in 
  diameter. 
  

   Occasionally 
  much 
  larger 
  crystals 
  are 
  found. 
  Fibrolite, 
  kyanite 
  and 
  

   staurolite 
  are 
  also 
  frequent 
  accessories. 
  The 
  Manhattan 
  schist 
  has 
  

   a 
  marked 
  schistosity 
  which 
  is 
  frequently 
  nearly 
  parallel 
  to 
  the 
  bed- 
  

   ding, 
  though 
  not 
  always. 
  

  

  The 
  aspect 
  of 
  this 
  formation 
  is 
  intimately 
  afifected 
  by 
  numerous 
  

   igneous 
  intrusions 
  and 
  injections 
  of 
  granitic 
  and 
  basic 
  material, 
  

   which, 
  in 
  some 
  places, 
  are 
  so 
  numerous 
  as 
  to 
  predominate 
  over 
  the 
  

   schist. 
  The 
  small 
  masses 
  are 
  for 
  the 
  most 
  part 
  parallel 
  to 
  the 
  schis- 
  

   tosity, 
  though 
  in 
  part, 
  oblique 
  to 
  it. 
  The 
  larger 
  areas 
  usually 
  have 
  

   their 
  longer 
  diameters 
  parallel 
  to 
  the 
  strike 
  of 
  the 
  schistosity. 
  They 
  

   are 
  most 
  abundant 
  near 
  the 
  shores 
  of 
  Long 
  Island 
  Sound. 
  

  

  As 
  the 
  geologic 
  map 
  shows, 
  in 
  southern 
  Westchester 
  

   county, 
  the 
  Manhattan 
  schist 
  is 
  the 
  prevaihng 
  rock 
  east 
  

   of 
  the 
  limestone 
  valley 
  in 
  which 
  lies 
  the 
  New 
  York 
  and 
  

   Harlem 
  Railroad. 
  This 
  eastern 
  area 
  is 
  closely 
  folded 
  and 
  its 
  bedding 
  

   planes 
  are 
  mostly 
  on 
  edge. 
  It 
  terminates 
  at 
  its 
  southern 
  extremity 
  

   in 
  a 
  closely 
  pressed 
  synclinal 
  fold, 
  pitching 
  northward, 
  which 
  crosses 
  

   Randall's 
  Island 
  and 
  Ward's 
  Island 
  and 
  ends 
  at 
  Little 
  Mill 
  Rock 
  in 
  

   Hell 
  Gate. 
  Flood 
  Rock, 
  which 
  was 
  removed 
  in 
  the 
  improvement 
  of 
  

   Hell 
  Gate 
  channel, 
  was 
  part 
  of 
  this 
  synclinal. 
  On 
  Mill 
  Rock 
  the 
  

   ' 
  schist 
  is 
  much 
  injected 
  with 
  amphibolite 
  and 
  pegmatite. 
  

  

  The 
  Manhattan 
  schist 
  is 
  also 
  the 
  prevailing 
  rock 
  on 
  New 
  York 
  

  

  Island. 
  

  

  IGNEOUS 
  ROCKS. 
  

  

  Under 
  this 
  head 
  are 
  classified 
  those 
  rocks 
  which 
  are 
  clearly 
  intru- 
  

   sive 
  in 
  the 
  Fordham 
  gneiss, 
  the 
  Inwood 
  limestone 
  and 
  the 
  Manhat- 
  

   tan 
  schist. 
  

  

  So 
  far 
  as 
  we 
  know, 
  they 
  belong 
  to 
  one 
  general 
  period 
  of 
  igneous 
  

   activity, 
  the 
  time 
  of 
  which 
  can 
  not 
  be 
  stated 
  with 
  greater 
  exactness 
  

  

  