﻿444 
  P. 
  A. 
  Daly 
  — 
  Geology 
  of 
  Pigeon 
  Point, 
  Minnesota. 
  

  

  friction 
  in 
  the 
  very 
  act 
  of 
  injection, 
  if 
  that 
  magma 
  were 
  

   initially 
  nearly 
  frozen 
  and 
  therefore 
  highly 
  viscous. 
  Super- 
  

   heat 
  to 
  the 
  extent 
  of 
  100° 
  or 
  perhaps 
  200° 
  C. 
  can 
  be 
  safely 
  

   assumed 
  for 
  the 
  melt 
  when 
  the 
  main 
  injection 
  took 
  place. 
  On 
  

   any 
  hypothesis 
  of 
  origins 
  this 
  original 
  magma 
  was 
  gabbroid 
  in 
  

   composition. 
  Its 
  temperature 
  at 
  the 
  time 
  of 
  emplacement 
  was 
  

   probably 
  at 
  1100° 
  C. 
  or 
  higher. 
  The 
  freezing 
  temperature 
  of 
  

   the 
  red 
  rock 
  is 
  not 
  known 
  ; 
  but, 
  on 
  account 
  of 
  its 
  richness 
  in 
  

   water, 
  was 
  doubtless 
  well 
  below 
  1000° 
  C, 
  if 
  not 
  as 
  low 
  as 
  

   800°. 
  Taking 
  the 
  masses 
  of 
  the 
  gabbro 
  and 
  red 
  rock 
  as 
  

   seven 
  to 
  one, 
  and 
  assuming 
  fair 
  values 
  for 
  the 
  latent 
  and 
  

   specific 
  heats 
  involved, 
  one 
  may 
  roughly 
  calculate 
  the 
  initial 
  

   superheat 
  required 
  for 
  the 
  assimilation 
  of 
  the 
  sediments 
  in 
  

   volume 
  sufficient 
  to 
  furnish 
  the 
  observed 
  amount 
  of 
  red 
  rock. 
  

   The 
  result 
  is 
  to 
  show 
  that 
  the 
  thermal 
  problem 
  is 
  not 
  so 
  

   portentous 
  as 
  it 
  is 
  thought 
  to 
  be, 
  in 
  such 
  cases, 
  by 
  certain 
  

   petrologists. 
  

  

  Assimilation 
  in 
  the 
  Sill 
  Conduit. 
  — 
  But 
  there 
  is 
  no 
  necessity 
  

   of 
  assuming 
  all 
  the 
  solution 
  or 
  fusion 
  of 
  the 
  Animikie 
  sedi- 
  

   ments, 
  corresponding 
  to 
  the 
  volume 
  of 
  the 
  red 
  rock, 
  to 
  have 
  

   taken 
  place 
  in 
  the 
  visible 
  sill 
  chamber 
  itself. 
  The 
  Pigeon 
  

   Point 
  eruptive 
  rose 
  through 
  nearly 
  the 
  whole 
  thickness 
  of 
  the 
  

   great 
  Animikie 
  series. 
  Those 
  rocks 
  are 
  quartzites 
  and 
  metar- 
  

   gillites 
  like 
  those 
  exposed 
  in 
  the 
  peninsula. 
  Some 
  solution 
  of 
  

   the 
  sediments 
  during 
  the 
  uprise 
  of 
  the 
  initial, 
  hot 
  magma 
  must 
  

   be 
  regarded 
  as 
  not 
  impossible. 
  The 
  sill 
  injection 
  may 
  have 
  

   been 
  a 
  single 
  act, 
  the 
  magma 
  rising, 
  through 
  one 
  or 
  more 
  

   simple 
  dike-passages, 
  from 
  the 
  earth's 
  deep 
  interior. 
  Or 
  the 
  

   magma 
  may 
  have 
  occupied 
  one 
  or 
  more 
  temporary 
  sill 
  chambers 
  

   before 
  reaching 
  the 
  horizon 
  of 
  the 
  visible 
  sill. 
  The 
  two 
  modes 
  

   of 
  injection 
  may 
  be 
  briefly 
  described 
  as 
  respectively 
  legato 
  and 
  

   staccato. 
  If 
  the 
  injection 
  were 
  staccato 
  in 
  quality, 
  some 
  assim- 
  

   ilation 
  in 
  the 
  temporarily 
  occupied 
  chambers 
  might 
  have 
  been 
  

   brought 
  about. 
  Material 
  then 
  dissolved 
  would 
  tend 
  to 
  be 
  

   mixed 
  through 
  the 
  original 
  gabbroid 
  magma, 
  because 
  of 
  the 
  

   later 
  movements 
  ; 
  the 
  final 
  separation 
  of 
  the 
  solute 
  occurring 
  

   in 
  the 
  chamber 
  Anally 
  occupied. 
  

  

  ^Whether 
  the 
  eruption 
  was 
  legato 
  or 
  staccato, 
  the 
  rising 
  gab- 
  

   broid 
  magma 
  doubtless 
  received 
  an 
  accession 
  of 
  water, 
  which 
  

   was 
  thermally 
  expelled 
  from 
  the 
  Animikie 
  sediments, 
  then 
  pos- 
  

   sibly 
  wetter 
  than 
  now. 
  Such 
  resurgent 
  water, 
  added 
  to 
  that 
  

   absorbed 
  from 
  xenoliths 
  and 
  main 
  contacts 
  in 
  the 
  visible 
  sill 
  

   chamber, 
  would 
  become 
  concentrated 
  at 
  the 
  sill 
  roof. 
  Through 
  

   the 
  consequent 
  depression 
  of 
  its 
  freezing 
  temperature, 
  the 
  

   magma 
  at 
  the 
  roof 
  was 
  not 
  quickly 
  chilled, 
  as 
  was 
  that 
  at 
  the 
  

   floor 
  ; 
  but 
  was 
  able 
  to 
  continue 
  stoping 
  and 
  also 
  some 
  marginal 
  

   solution 
  of 
  country 
  rock. 
  Stoping 
  and 
  vertical 
  currents 
  inci- 
  

  

  