﻿GEOLOGY 
  OF 
  THE 
  NORTH 
  CREEK 
  QUADRANGLE 
  35 
  

  

  and 
  assimilation 
  hypothesis 
  to 
  the 
  solution 
  of 
  the 
  problem. 
  For 
  

   full 
  discussions 
  of 
  this 
  hypothesis 
  the 
  reader 
  is 
  referred 
  to 
  Daly's 
  

   original 
  papers. 
  1 
  Some 
  of 
  the 
  more 
  fundamental 
  principles 
  are 
  as 
  

   follows: 
  Batholithic 
  (or 
  stock) 
  magmas 
  have 
  reached 
  their 
  present 
  

   positions 
  chiefly 
  by 
  the 
  successive 
  engulfment 
  of 
  blocks 
  of 
  country 
  

   rock 
  (xenoliths) 
  stoped 
  or 
  broken 
  out 
  of 
  the 
  roof 
  and 
  walls 
  of 
  the 
  

   magma 
  chamber; 
  the 
  xenoliths 
  become 
  immersed 
  and 
  dissolved 
  in 
  

   the 
  depths 
  of 
  the 
  original 
  magma 
  with 
  the 
  formation 
  of 
  a 
  second- 
  

   ary 
  magma; 
  when 
  the 
  magma 
  becomes 
  very 
  viscous 
  (due 
  to 
  cool- 
  

   ing) 
  the 
  xenoliths 
  neither 
  sink 
  nor 
  become 
  dissolved. 
  

  

  Only 
  a 
  summary 
  of 
  the 
  application 
  of 
  these 
  principles 
  to 
  the 
  

   North 
  Creek 
  gabbros 
  will 
  here 
  be 
  given, 
  the 
  writer 
  having 
  more 
  

   fully 
  discussed 
  this 
  matter 
  in 
  a 
  recent 
  paper. 
  2 
  

  

  We 
  have 
  shown 
  that 
  the 
  gabbro 
  stocks 
  are 
  of 
  the 
  pluglike 
  or 
  pipe- 
  

   like 
  form 
  with 
  practically 
  vertical 
  boundaries. 
  Such 
  igneous 
  masses 
  

   were 
  not 
  intruded 
  by 
  simply 
  displacing 
  or 
  pushing 
  aside 
  the 
  country 
  

   rock, 
  but 
  rather 
  there 
  was 
  a 
  process 
  of 
  replacement. 
  Thus 
  the 
  

   mode 
  of 
  occurrence 
  of 
  these 
  stocks 
  furnishes 
  strong 
  evidence 
  in 
  

   favor 
  of 
  magmatic 
  stoping 
  as 
  an 
  important 
  factor 
  in 
  the 
  intrusion. 
  

  

  The 
  very 
  presence 
  of 
  the 
  inclusions 
  as 
  xenoliths 
  proves 
  that 
  the 
  

   process 
  of 
  stoping 
  or 
  rifting 
  off 
  blocks 
  from 
  the 
  chamber 
  vault 
  

   actually 
  did 
  take 
  place 
  to 
  some 
  extent 
  at 
  least, 
  and 
  this 
  when 
  the 
  

   magma 
  had 
  cooled 
  to 
  a 
  highly 
  viscous 
  condition 
  and 
  hence 
  had 
  com- 
  

   paratively 
  little 
  power 
  to 
  stope 
  and 
  too 
  low 
  a 
  temperature 
  to 
  assimi- 
  

   late 
  the 
  blocks. 
  Thus 
  the 
  occurrence 
  of 
  such 
  xenoliths 
  is 
  quite 
  in 
  

   harmony 
  with 
  Daly's 
  hypothesis. 
  

  

  The 
  writer 
  believes 
  that 
  the 
  more 
  acidic 
  patches 
  or 
  masses 
  (al- 
  

   ready 
  described) 
  within 
  the 
  gabbro 
  stocks 
  are 
  evidence 
  of 
  chemical 
  

   change 
  within 
  the 
  intrusive 
  magma 
  due 
  to 
  solution 
  or 
  partial 
  solu- 
  

   tion 
  and 
  diffusion 
  of 
  blocks 
  of 
  country 
  rock. 
  In 
  such 
  cases 
  the 
  

   magma 
  was 
  just 
  hot 
  enough 
  to 
  melt 
  or 
  partially 
  melt 
  and 
  only 
  par- 
  

   tially 
  diffuse 
  the 
  blocks 
  of 
  country 
  rock. 
  

  

  Five 
  or 
  six 
  of 
  the 
  stocks 
  are 
  composed 
  wholly 
  of 
  rocks 
  more 
  acidic 
  

   than 
  the 
  typical 
  gabbro. 
  In 
  the 
  earlier 
  stage 
  of 
  very 
  active 
  intru- 
  

   sion 
  the 
  invading 
  magma 
  was 
  more 
  thoroughly 
  molten 
  and 
  as 
  the 
  

   blocks 
  of 
  country 
  rock 
  were 
  stoped 
  off 
  they 
  sank 
  in 
  the 
  magma 
  and 
  

   became 
  thoroughly 
  dissolved 
  and 
  diffused. 
  Since 
  the 
  country 
  rock 
  

   was 
  nearly 
  always 
  syenite, 
  granite, 
  or 
  gneiss 
  the 
  magma 
  became 
  more 
  

   and 
  more 
  acidic. 
  

  

  1 
  Amer. 
  Jour. 
  Sci. 
  1903, 
  15:269-98; 
  Amer. 
  Jour. 
  Sci. 
  1903, 
  16:107-26; 
  Amer. 
  

   Jour. 
  Sci. 
  1908, 
  26:17-50. 
  

  

  2 
  Jour, 
  of 
  Geol. 
  1913, 
  21:160-80. 
  

  

  2 
  

  

  