﻿GEOLOGY 
  OF 
  THE 
  NORTH 
  CREEK 
  QUADRANGLE 
  39 
  

  

  fine 
  grained 
  along 
  the 
  borders. 
  It 
  breaks 
  through 
  both 
  Grenville 
  

   and 
  granite 
  gneisses 
  and 
  the 
  contacts 
  are 
  everywhere 
  perfectly 
  sharp, 
  

   there 
  being 
  no 
  evidence 
  whatever 
  of 
  contact 
  metamorphism. 
  A 
  num- 
  

   ber 
  of 
  small 
  tongues, 
  from 
  I 
  inch 
  to 
  3 
  or 
  4 
  feet 
  wide, 
  branch 
  off 
  

   the 
  large 
  dike 
  and 
  extend 
  as 
  much 
  as 
  25 
  or 
  30 
  feet 
  into 
  the 
  country 
  

   rock. 
  One 
  of 
  these 
  branches 
  cuts 
  a 
  pegmatite 
  dike 
  and 
  another 
  

   cuts 
  Grenville 
  limestone. 
  This 
  large 
  dike 
  strikes 
  across 
  the 
  folia- 
  

   tion 
  almost 
  at 
  right 
  angles. 
  

  

  One 
  and 
  one-half 
  miles 
  southeast 
  of 
  Johnsburg 
  a 
  diabase 
  dike, 
  

   2y 
  2 
  feet 
  wide 
  and 
  60 
  feet 
  long, 
  cuts 
  Grenville 
  quartzite 
  parallel 
  to 
  

   the 
  foliation. 
  All 
  of 
  this 
  rock 
  is 
  fine 
  grained 
  but 
  exceptionally 
  so 
  

   at 
  the 
  contacts, 
  and 
  on 
  one 
  side 
  an 
  inch 
  wide 
  zone 
  of 
  basaltic 
  glass 
  

   or 
  obsidian 
  is 
  perfectly 
  developed 
  with 
  some 
  very 
  small 
  tongues 
  of 
  

   glass 
  extending 
  into 
  the 
  country 
  rock. 
  

  

  A 
  typical 
  diabase 
  dike 
  4 
  feet 
  wide 
  cuts 
  the 
  gabbro 
  stock 
  three- 
  

   quarters 
  of 
  a 
  mile 
  south 
  of 
  Warner 
  pond. 
  The 
  dike 
  has 
  fine 
  grained 
  

   borders, 
  sharp 
  contacts 
  against 
  the 
  gabbro, 
  and 
  is 
  clearly 
  traceable 
  

   for 
  150 
  feet 
  or 
  more. 
  

  

  In 
  all 
  eleven 
  diabase 
  dikes 
  were 
  found, 
  being 
  well 
  scattered 
  over 
  

   the 
  quadrangle. 
  Most 
  of 
  them 
  cut 
  across 
  the 
  foliation 
  of 
  the 
  coun- 
  

   try 
  rock 
  at 
  high 
  angles, 
  thus 
  differing 
  from 
  the 
  gabbros, 
  and 
  they 
  

   probably 
  have 
  been 
  forced 
  up 
  along 
  joint 
  planes. 
  In 
  nine 
  of 
  the 
  

   eleven 
  occurrences 
  the 
  dikes 
  strike 
  northeast 
  and 
  southwest 
  which 
  

   is 
  quite 
  the 
  rule 
  for 
  such 
  dikes 
  in 
  the 
  eastern 
  Adirondacks. 
  So 
  far 
  

   as 
  can 
  be 
  determined, 
  these 
  dikes 
  all 
  come 
  up 
  vertically 
  through 
  

   the 
  country 
  rock. 
  

  

  Megascopic 
  and 
  microscopic 
  features 
  

  

  The 
  diabase 
  is 
  a 
  very 
  dark 
  bluish 
  gray 
  to 
  almost 
  black 
  rock 
  which, 
  

   in 
  all 
  exposures, 
  is 
  hard 
  and 
  fresh 
  except 
  for 
  the 
  immediate 
  surface 
  

   which 
  is 
  often 
  weathered 
  to 
  reddish 
  brown. 
  

  

  The 
  granularity 
  and 
  texture 
  vary 
  from 
  glassy 
  to 
  very 
  fine 
  grained 
  

   to 
  medium 
  grained 
  diabasic, 
  the 
  finer 
  grained 
  rock 
  being 
  wholly 
  

   confined 
  to 
  the 
  borders 
  and 
  the 
  diabasic 
  texture 
  nearly 
  always 
  being 
  

   just 
  visible 
  to 
  the 
  naked 
  eye 
  in 
  the 
  typical 
  medium 
  grained 
  rock. 
  

   Except 
  for 
  the 
  above 
  named 
  differences 
  the 
  diabase 
  shows 
  no 
  facies 
  

   whatever 
  visible 
  to 
  the 
  naked 
  eye, 
  and 
  this 
  again 
  is 
  in 
  marked 
  con- 
  

   trast 
  with 
  the 
  gabbros. 
  The 
  diabase 
  is 
  wholly 
  devoid 
  of 
  any 
  meta- 
  

   morphism 
  and 
  inclusions 
  of 
  country 
  rock 
  are 
  never 
  found. 
  The 
  

   only 
  minerals 
  recognizable 
  by 
  the 
  naked 
  eye 
  are 
  the 
  tiny 
  feldspar 
  

   laths 
  and 
  an 
  occasional 
  pyrite 
  speck. 
  

  

  