﻿GEOLOGY 
  OF 
  THE 
  NORTH 
  CREEK 
  QUADRANGLE 
  23 
  

  

  MIXED 
  GNEISSES 
  

  

  In 
  the 
  areas 
  mapped 
  as 
  mixed 
  gneisses, 
  the 
  rocks 
  are 
  more 
  or 
  

   less 
  intimate 
  associations 
  of 
  the 
  various 
  Grenville, 
  syenite, 
  gran- 
  

   ite, 
  and 
  granite 
  porphyry 
  gneisses. 
  They 
  are 
  really 
  areas 
  of 
  Gren- 
  

   ville 
  which 
  have 
  been 
  all 
  cut 
  to 
  pieces, 
  and 
  in 
  some 
  cases 
  appar- 
  

   ently 
  partially 
  fused, 
  by 
  the 
  great 
  igneous 
  intrusives. 
  In 
  some 
  areas 
  

   true 
  Grenville 
  rocks 
  predominate; 
  in 
  others 
  true 
  igneous 
  rocks 
  pre- 
  

   vail 
  ; 
  while 
  in 
  still 
  others 
  the 
  most 
  common 
  rock 
  appears 
  to 
  be 
  of 
  

   intermediate 
  character 
  due 
  to 
  an 
  actual 
  melting 
  and 
  incorporation 
  

   of 
  Grenville 
  sediments 
  by 
  the 
  molten 
  intrusions. 
  Except 
  along 
  fault 
  

   lines, 
  these 
  mixed 
  rocks 
  everywhere 
  grade 
  into 
  either 
  true 
  Gren- 
  

   ville 
  or 
  syenite 
  or 
  granite 
  and 
  the 
  drawing 
  of 
  boundary 
  lines 
  is 
  

   largely 
  a 
  matter 
  of 
  personal 
  judgment. 
  Any 
  attempt 
  to 
  separate 
  

   the 
  various 
  members 
  of 
  these 
  mixed 
  gneiss 
  areas 
  would 
  be 
  un- 
  

   satisfactory 
  because 
  of 
  the 
  general 
  insufficiency 
  of 
  outcrops 
  and 
  

   the 
  small 
  scale 
  of 
  the 
  map. 
  

  

  There 
  are 
  many 
  places 
  within 
  the 
  quadrangle 
  where, 
  as 
  a 
  result 
  

   of 
  more 
  or 
  less 
  perfect 
  assimilation, 
  rocks 
  of 
  intermediate 
  com- 
  

   position 
  occur 
  on 
  both 
  small 
  and 
  large 
  scales. 
  One 
  and 
  one-third 
  

   miles 
  northeast 
  of 
  Kelm 
  mountain 
  and 
  near 
  the 
  map 
  edge 
  there 
  

   are 
  fine 
  illustrations 
  of 
  dark 
  Grenville 
  garnet 
  gneiss 
  inclusions 
  in 
  

   the 
  granite 
  porphyry, 
  the 
  inclusions 
  usually 
  grading 
  perfectly 
  

   through 
  zones 
  of 
  a 
  few 
  feet 
  into 
  the 
  granite. 
  The 
  intermediate 
  

   rock 
  is 
  coarse-grained, 
  very 
  garnetiferous, 
  and 
  not 
  so 
  porphyritic 
  

   as 
  the 
  true 
  granite 
  porphyry. 
  Similar 
  cases 
  of 
  local 
  assimilation 
  

   by 
  granite 
  porphyry, 
  granite 
  and 
  syenite 
  have 
  been 
  observed 
  at 
  

   other 
  places 
  within 
  the 
  quadrangle. 
  

  

  On 
  a 
  large 
  scale, 
  perhaps 
  the 
  best 
  examples 
  of 
  rocks 
  of 
  inter- 
  

   mediate 
  character 
  make 
  up 
  most 
  of 
  the 
  mixed 
  gneiss 
  area 
  just 
  

   east 
  of 
  Chestertown. 
  Thus 
  the 
  whole 
  top 
  of 
  Prospect 
  mountain 
  

   consists 
  of 
  gray, 
  fine-grained, 
  very 
  massive 
  rock 
  which 
  has 
  the 
  

   composition 
  of 
  a 
  biotite 
  granite. 
  This 
  rock 
  is 
  quite 
  homogeneous 
  

   except 
  for 
  occasional 
  patches 
  or 
  stringers 
  of 
  gray 
  Grenville 
  gneisses 
  

   which 
  are 
  fused 
  into 
  the 
  mass. 
  Passing 
  southward 
  and 
  southwest- 
  

   ward 
  down 
  the 
  mountain 
  side, 
  this 
  rock 
  grades 
  perfectly 
  into 
  a 
  

   medium-grained, 
  gray, 
  biotite 
  granite 
  which 
  contains 
  very 
  few 
  

   Grenville 
  inclusions, 
  and 
  this 
  rock, 
  in 
  turn, 
  grades 
  perfectly 
  into 
  

   the 
  typical 
  biotite 
  granite 
  porphyry 
  at 
  the 
  base 
  of 
  the 
  mountain. 
  

   Passing 
  westward 
  down 
  the 
  mountain 
  side, 
  however, 
  the 
  fine- 
  

   grained 
  granitic 
  rock 
  at 
  the 
  top 
  gradually 
  becomes 
  coarser 
  grained 
  

   and 
  contains 
  more 
  numerous 
  and 
  more 
  clearly 
  defined 
  inclusions 
  

  

  