﻿3§ 
  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  PEGMATITE 
  

  

  The 
  most 
  interesting 
  thing 
  about 
  the 
  pegmatite 
  is 
  its 
  distribution, 
  

   because 
  it 
  is 
  very 
  commonly 
  directly 
  associated 
  with 
  the 
  gabbro 
  

   masses. 
  Many 
  times 
  the 
  pegmatite, 
  in 
  the 
  form 
  of 
  dikes 
  or 
  veins, 
  

   may 
  be 
  seen 
  cutting 
  the 
  gabbro 
  (see 
  plate 
  4) 
  and 
  hence 
  the 
  younger 
  

   age 
  of 
  the 
  pegmatite. 
  This 
  direct 
  association 
  of 
  the 
  very 
  acid 
  

   pegmatite 
  with 
  basic 
  gabbro 
  and 
  its 
  age 
  intermediate 
  between 
  the 
  

   basic 
  intrusives 
  — 
  gabbro 
  and 
  diabase 
  — 
  are 
  rather 
  anomalous 
  fea- 
  

   tures 
  for 
  which 
  the 
  writer 
  can 
  offer 
  no 
  explanation. 
  

  

  Among 
  other 
  places 
  where 
  pegmatite 
  may 
  be 
  seen 
  cutting 
  the 
  

   gabbro 
  are: 
  (1) 
  at 
  the 
  top 
  of 
  Hackensack 
  mountain; 
  (2) 
  i 
  1 
  /^ 
  miles 
  

   south-southeast 
  of 
  Potter 
  mountain; 
  (3) 
  ij4 
  miles 
  a 
  little 
  east 
  of 
  

   north 
  of 
  The 
  Glen; 
  (4) 
  1 
  mile 
  south-southeast 
  of 
  The 
  Glen; 
  (5) 
  

   ij/3 
  miles 
  northeast 
  of 
  Pottersville; 
  (6) 
  i 
  2 
  /s 
  miles 
  southeast 
  of 
  Ches- 
  

   tertown; 
  (7) 
  2f/ 
  2 
  miles 
  southeast 
  of 
  Chestertown; 
  and 
  (8) 
  2^4 
  

   miles 
  south-southeast 
  of 
  Chestertown. 
  At 
  most 
  of 
  these 
  places 
  the 
  

   gabbro 
  is 
  shot 
  through 
  with 
  small 
  pegmatite 
  veins. 
  At 
  the 
  fourth 
  

   named 
  locality 
  one 
  dike 
  is 
  50 
  feet 
  long 
  and 
  25 
  feet 
  wide 
  and 
  very 
  

   rich 
  in 
  big 
  orthoclase 
  and 
  albite 
  feldspar 
  crystals. 
  At 
  the 
  seventh 
  

   named 
  locality 
  a 
  small 
  pegmatite 
  dike 
  contains 
  fine 
  crystals 
  of 
  biotite, 
  

   mnscovite 
  and 
  black 
  tourmaline. 
  At 
  the 
  sixth 
  and 
  eighth 
  named 
  

   localities 
  there 
  are 
  pegmatite 
  dikes 
  50 
  to 
  100 
  feet 
  long 
  with 
  books 
  

   of 
  muscovite 
  up 
  to 
  5 
  or 
  6 
  inches 
  across 
  at 
  the 
  eighth 
  locality. 
  

  

  Large 
  pegmatite 
  dikes 
  are 
  not 
  common 
  away 
  from 
  the 
  gabbro, 
  

   there 
  being 
  but 
  two 
  examples 
  worthy 
  of 
  mention 
  namely: 
  just 
  east 
  

   of 
  the 
  old 
  garnet 
  mine 
  south 
  of 
  Daggett 
  pond 
  where 
  there 
  are 
  many 
  

   poorly 
  formed 
  black 
  tourmaline 
  crystals 
  up 
  to 
  6 
  inches 
  long, 
  and 
  one- 
  

   quarter 
  of 
  a 
  mile 
  above 
  the 
  mouth 
  of 
  Mill 
  creek 
  where 
  there 
  is 
  a 
  

   dike 
  200 
  feet 
  long 
  and 
  40 
  feet 
  wide 
  in 
  granitic 
  syenite. 
  

  

  DIABASE 
  

  

  Mode 
  of 
  occurrence 
  and 
  distribution 
  

  

  In 
  striking 
  contrast 
  with 
  the 
  neighboring 
  gabbro, 
  the 
  diabase 
  

   'invariably 
  occurs 
  in 
  typical 
  dikes 
  which 
  have 
  clearly 
  broken 
  through 
  

   narrow 
  fissures 
  in 
  the 
  country 
  rock. 
  They 
  vary 
  in 
  length 
  from 
  

   20 
  or 
  30 
  feet 
  to 
  200 
  yards, 
  and 
  in 
  width 
  from 
  5^ 
  inches 
  to 
  40 
  feet. 
  

  

  The 
  chief 
  features 
  of 
  occurrence 
  are 
  brought 
  out 
  in 
  the 
  following 
  

   description 
  of 
  the 
  largest 
  dike 
  of 
  the 
  region 
  which 
  lies 
  at 
  the 
  western 
  

   base 
  of 
  Heath 
  mountain 
  or 
  3 
  miles 
  west-northwest 
  of 
  Warrensburg. 
  

   This 
  dike 
  has 
  a 
  maximum 
  width 
  of 
  40 
  feet 
  and 
  a 
  length 
  of 
  200 
  

   yards. 
  It 
  is 
  fine 
  to 
  medium 
  grained 
  toward 
  the 
  interior 
  and 
  very 
  

  

  