﻿26 
  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  syenite 
  or 
  granite 
  and 
  Grenville 
  gneisses 
  in 
  about 
  equal 
  amounts. 
  

   There 
  are 
  few 
  suggestions 
  of 
  assimilation, 
  the 
  igneous 
  and 
  sedimen- 
  

   tary 
  rocks 
  generally 
  retaining 
  their 
  characteristic 
  features. 
  

   The 
  other 
  mixed 
  gneiss 
  areas 
  require 
  no 
  special 
  mention. 
  

  

  THE 
  GABBRO 
  AND 
  ITS 
  DERIVATIVES 
  

  

  Because 
  of 
  the 
  large 
  number 
  of 
  exposures, 
  mode 
  of 
  occurrence, 
  

   excellent 
  outcrops, 
  frequent 
  contacts 
  against 
  the 
  country 
  rocks, 
  

   and 
  the 
  remarkable 
  variations 
  in 
  composition 
  and 
  appearance 
  the 
  

   gabbros 
  are 
  of 
  unusual 
  interest 
  and 
  will 
  be 
  described 
  in 
  considerable 
  

   detail. 
  

  

  Mode 
  of 
  occurrence 
  

  

  The 
  gabbro 
  and 
  its 
  derivatives 
  nearly 
  always 
  occur 
  in 
  the 
  form 
  

   of 
  small 
  stocks 
  or 
  bosses 
  rather 
  than 
  as 
  true 
  dikes, 
  their 
  length 
  

   ranging 
  from 
  30 
  to 
  40 
  feet 
  to 
  about 
  a 
  mile, 
  and 
  with 
  widths 
  up 
  to 
  

   three-fourths 
  of 
  a 
  mile. 
  Sixty-one 
  separate 
  gabbro 
  bodies 
  were 
  

   found 
  and 
  are 
  shown 
  on 
  the 
  geologic 
  map. 
  In 
  spite 
  of 
  the 
  detailed 
  

   field 
  work 
  a 
  few 
  others 
  have 
  probably 
  escaped 
  the 
  writer's 
  notice. 
  

  

  The 
  ground 
  plan, 
  as 
  represented 
  on 
  the 
  geologic 
  map, 
  is 
  almost 
  

   invariably 
  elliptical, 
  though 
  sometimes 
  approaching 
  the 
  circular. 
  

   When 
  the 
  contact 
  with 
  the 
  country 
  rock 
  is 
  carefully 
  traced 
  out 
  it 
  

   is 
  commonly 
  found 
  to 
  be 
  sharp 
  and 
  shows 
  smooth 
  or 
  flowing 
  out- 
  

   lines 
  against 
  the 
  country 
  rock. 
  In 
  only 
  two 
  or 
  three 
  cases 
  do 
  the 
  

   gabbro 
  masses 
  approach 
  the 
  true 
  dikelike 
  form, 
  and 
  in 
  each 
  of 
  these 
  

   cases 
  fine-grained 
  tongues 
  were 
  found 
  to 
  extend 
  into 
  the 
  surround- 
  

   ing 
  rock. 
  One 
  stock, 
  one 
  and 
  one-third 
  miles 
  southeast 
  of 
  Chester- 
  

   town, 
  shows 
  several 
  such 
  tongues, 
  one 
  of 
  them 
  (1 
  to 
  6 
  inches 
  wide) 
  

   clearly 
  cutting 
  the 
  granite 
  porphyry 
  for 
  30 
  feet. 
  Other 
  and 
  smaller 
  

   gabbro 
  dikes 
  at 
  the 
  summit 
  of 
  Hackensack 
  mountain, 
  and 
  1 
  mile 
  

   south 
  of 
  South 
  Horicon 
  show 
  a 
  number 
  of 
  such 
  fine-grained 
  tongues. 
  

  

  At 
  one 
  place 
  in 
  the 
  dike 
  or 
  boss 
  which 
  crosses 
  the 
  road 
  1 
  % 
  miles 
  

   a 
  little 
  west 
  of 
  south 
  of 
  South 
  Horicon, 
  fairly 
  coarse-grained 
  gabbro 
  

   is 
  in 
  sharp 
  contact 
  (for 
  6 
  or 
  8 
  feet) 
  with 
  fine-grained 
  gabbro, 
  the 
  

   latter 
  becoming 
  coarser 
  grained 
  again 
  away 
  from 
  the 
  contact. 
  This 
  

   suggests 
  a 
  second 
  intrusion 
  of 
  the 
  gabbro 
  after 
  the 
  first 
  but 
  after 
  

   the 
  first 
  had 
  cooled. 
  

  

  It 
  is 
  a 
  striking 
  fact 
  that 
  in 
  spite 
  of 
  many 
  excellent 
  contacts 
  which 
  

   were 
  observed, 
  such 
  dikelike 
  tongues 
  are 
  so 
  rare. 
  As 
  Harker 
  says 
  l 
  : 
  

   "Although 
  most 
  of 
  the 
  bodies 
  of 
  granite 
  and 
  other 
  plutonic 
  rocks 
  

  

  1 
  Natural 
  History 
  of 
  Igneous 
  Rocks, 
  1909. 
  p 
  86. 
  

  

  