﻿GEOLOGY 
  OF 
  THE 
  NORTH 
  CREEK 
  QUADRANGLE 
  2^] 
  

  

  which 
  have 
  been 
  loosely 
  described 
  as 
  bosses, 
  and 
  so 
  rendered 
  in 
  

   ideal 
  sections, 
  are 
  doubtless 
  of 
  laccolitic 
  or 
  other 
  stratiform 
  shape; 
  

   some, 
  not 
  of 
  the 
  largest 
  dimensions, 
  appear 
  to 
  have 
  a 
  pluglike 
  form, 
  

   with 
  more 
  or 
  less 
  vertical 
  boundaries." 
  The 
  small 
  stocks 
  or 
  bosses 
  

   of 
  the 
  North 
  Creek 
  region 
  are 
  certainly 
  of 
  this 
  pluglike 
  or 
  pipelike 
  

   form 
  as 
  shown 
  by 
  the 
  very 
  character 
  of 
  their 
  eroded 
  cross-sections 
  

   and 
  also 
  by 
  the 
  vertical 
  contacts 
  with 
  the 
  country 
  rock. 
  Among 
  the 
  

   many 
  fine 
  contacts 
  which 
  came 
  under 
  the 
  writer's 
  observation, 
  not 
  

   a 
  single 
  exception 
  to 
  the 
  rule 
  of 
  vertical 
  or 
  practically 
  vertical 
  con- 
  

   tacts 
  was 
  noted. 
  

  

  In 
  most 
  cases 
  the 
  long 
  axes 
  of 
  the 
  stocks 
  lie 
  parallel 
  to 
  the 
  folia- 
  

   tion 
  of 
  the 
  inclosing 
  rock, 
  though 
  there 
  are 
  some 
  notable 
  exceptions. 
  

   It 
  would 
  therefore 
  seem 
  that 
  the 
  molten 
  intrusives 
  generally 
  followed 
  

   the 
  lines 
  of 
  least 
  resistance 
  but, 
  even 
  in 
  these 
  cases, 
  the 
  broad 
  ends 
  

   of 
  the 
  stocks 
  cut 
  sharply 
  across 
  the 
  foliation 
  bands, 
  sometimes 
  for 
  

   a 
  distance 
  of 
  several 
  hundred 
  yards. 
  Such 
  a 
  phenomenon 
  is 
  well 
  

   exhibited 
  at 
  the 
  south 
  end 
  of 
  the 
  large 
  stock 
  just 
  south 
  of 
  Mountain 
  

   Spring 
  lake 
  where 
  a 
  big 
  quarry 
  has 
  been 
  opened 
  up 
  along 
  the 
  contact. 
  

  

  The 
  gabbro 
  stocks 
  are 
  not 
  at 
  all 
  uniformly 
  distributed 
  over 
  the 
  

   area 
  of 
  the 
  quadrangle, 
  the 
  largest 
  number 
  being 
  confined 
  to 
  a 
  nearly 
  

   north-south 
  belt 
  with 
  a 
  width 
  of 
  from 
  3 
  to 
  5 
  miles 
  and 
  extending 
  

   through 
  the 
  middle 
  of 
  the 
  quadrangle. 
  This 
  belt 
  roughly 
  corre- 
  

   sponds 
  to 
  the 
  general 
  strike 
  of 
  the 
  foliation. 
  A 
  secondary 
  belt, 
  

   about 
  1 
  mile 
  wide 
  and 
  5 
  miles 
  long 
  near 
  the 
  middle 
  eastern 
  bound- 
  

   ary 
  of 
  the 
  sheet, 
  contains 
  a 
  dozen 
  small 
  stocks. 
  With 
  a 
  single 
  small 
  

   exception 
  the 
  whole 
  western 
  side 
  of 
  the 
  quadrangle 
  is 
  devoid 
  of 
  

   gabbro 
  masses. 
  In 
  the 
  northeastern 
  portion 
  a 
  few 
  stocks 
  occur, 
  but 
  

   they 
  may 
  really 
  belong 
  to 
  some 
  other 
  belt 
  not 
  yet 
  mapped. 
  Thus 
  

   we 
  see 
  that 
  the 
  gabbro 
  intrusions 
  were 
  limited 
  to 
  rather 
  well-defined 
  

   areas 
  or 
  belts. 
  

  

  Among 
  these 
  gabbro 
  stocks 
  four 
  types 
  of 
  occurrence 
  are 
  especially 
  

   noteworthy 
  as 
  follows: 
  (1) 
  The 
  normal, 
  dark, 
  basic 
  gabbro 
  with 
  

   diabasic 
  texture 
  and 
  usually 
  homogeheous 
  throughout; 
  (2) 
  gabbro 
  

   chiefly 
  of 
  the 
  normal 
  type 
  but 
  with 
  irregular 
  patches 
  or 
  masses 
  of 
  

   lighter 
  colored 
  rocks 
  of 
  syenitic 
  or 
  dioritic 
  make-up, 
  these 
  patches 
  

   blending 
  with 
  the 
  normal 
  gabbro; 
  (3) 
  the 
  whole 
  stock 
  made 
  up 
  of 
  

   lighter 
  .colored, 
  more 
  acidic 
  rock; 
  and 
  (4) 
  any 
  one 
  of 
  the 
  above 
  

   types 
  with 
  blocks 
  or 
  inclusions 
  of 
  the 
  country 
  rock. 
  These 
  four 
  

   types 
  are 
  all 
  primary 
  variations. 
  Examples 
  of 
  the 
  last 
  three 
  types 
  

   will 
  be 
  given 
  later. 
  

  

  