﻿40 
  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  (see 
  plate 
  17 
  and 
  figure 
  12). 
  That 
  these 
  dike 
  rocks 
  were 
  intruded 
  

   after 
  the 
  great 
  pressure 
  and 
  uplift 
  of 
  the 
  region 
  is 
  shown 
  by 
  the 
  

   total 
  absence 
  of 
  metamorphism 
  or 
  alteration 
  of 
  any 
  kind 
  along 
  their 
  

   contact 
  lines. 
  The 
  fine-grained 
  texture 
  of 
  these 
  rocks, 
  often 
  with 
  

   borders 
  of 
  glass, 
  shows 
  that 
  they 
  must 
  have 
  cooled 
  close 
  to 
  the 
  sur- 
  

   face, 
  and 
  hence 
  it 
  is 
  evident 
  that 
  most 
  of 
  the 
  Precambric 
  erosion 
  of 
  

   the 
  region 
  had 
  been 
  accomplished 
  before 
  the 
  diabases 
  were 
  erupted. 
  

   Such 
  rocks 
  suggest 
  that 
  there 
  may 
  have 
  been 
  volcanic 
  activity 
  at 
  the 
  

   surface 
  but 
  no 
  positive 
  proof 
  for 
  such 
  activity 
  can 
  be 
  given 
  because, 
  

   if 
  such 
  volcanic 
  material 
  ever 
  existed, 
  every 
  trace 
  of 
  it 
  has 
  been 
  re- 
  

   moved 
  by 
  erosion. 
  The 
  diabases 
  have 
  been 
  observed 
  to 
  cut 
  the 
  

   pegmatites 
  and 
  hence 
  they 
  are- 
  not 
  only 
  the 
  younger 
  of 
  the 
  two, 
  but 
  

   they 
  must 
  take 
  rank 
  as 
  the 
  youngest 
  Precambric 
  rocks 
  in 
  the 
  State. 
  

   In 
  the 
  Adirondacks 
  the 
  pegmatites 
  are 
  very 
  widely 
  distributed 
  and 
  

   common, 
  while 
  the 
  diabases 
  are 
  most 
  abundant 
  in 
  the 
  northeast, 
  

   less 
  so 
  in 
  the 
  northwest 
  and 
  southeast, 
  and 
  nearly 
  absent 
  from 
  the 
  

   southwest. 
  

  

  