﻿436 
  R. 
  A. 
  Daly 
  — 
  Geology 
  of 
  Pigeon 
  Point, 
  Minnesota. 
  

  

  or 
  perhaps 
  a 
  little 
  more, 
  with 
  an 
  average 
  of 
  about 
  160 
  meters. 
  

   Of 
  the 
  exposed 
  mass, 
  nearly 
  one-eighth 
  by 
  weight 
  is 
  red 
  rock 
  ; 
  

   the 
  remainder 
  is 
  gabbro, 
  associated 
  with 
  an 
  almost 
  negligible 
  

   proportion 
  of 
  intermediate 
  rock. 
  

  

  Nature 
  of 
  the 
  Magma 
  Differentiated. 
  — 
  If 
  the 
  red-rock 
  

   material 
  were 
  a 
  differentiate 
  of 
  a 
  purely 
  primary 
  magma, 
  

   and 
  if 
  the 
  actual 
  weight 
  ratio 
  of 
  red 
  rock 
  to 
  gabbro 
  is 
  given 
  by 
  

   the 
  surface 
  exposure, 
  the 
  composition 
  of 
  the 
  assumed 
  parent 
  

   solution 
  (neglecting 
  volatile 
  substances 
  which 
  have 
  escaped 
  from 
  

   the 
  sill 
  chamber) 
  may 
  be 
  roughly 
  calculated. 
  The 
  result 
  is 
  

   shown 
  in 
  Column 
  3 
  of 
  Table 
  II. 
  Columns 
  4, 
  5, 
  and 
  6 
  give 
  

   respectively 
  the 
  average 
  composition 
  of 
  diabase, 
  gabbro, 
  and 
  

   basalt 
  as 
  world 
  types. 
  

  

  

  

  

  Table 
  II. 
  

  

  

  

  

  

  i 
  

  

  2 
  

  

  3 
  

  

  4 
  

  

  5 
  

  

  6 
  

  

  Si0 
  2 
  

  

  49-88 
  

  

  72-42 
  

  

  52-70 
  

  

  50-12 
  

  

  48-24 
  

  

  48-78 
  

  

  ai 
  2 
  o 
  3 
  

  

  18-55 
  

  

  13-04 
  

  

  17-86 
  

  

  15-68 
  

  

  17-88 
  

  

  15-85 
  

  

  Fe 
  2 
  3 
  

  

  2-06 
  

  

  •68 
  

  

  1-89 
  

  

  4-55 
  

  

  3-16 
  

  

  5-37 
  

  

  FeO 
  

  

  8-37 
  

  

  2-49 
  

  

  7'64 
  

  

  6-73 
  

  

  5-95 
  

  

  6-34 
  

  

  MgO 
  

  

  5'77 
  

  

  •58 
  

  

  5-12 
  

  

  5-85 
  

  

  7-51 
  

  

  6-03 
  

  

  CaO 
  

  

  9-72 
  

  

  •66 
  

  

  8-59 
  

  

  8*80 
  

  

  1099 
  

  

  8'91 
  

  

  Na 
  2 
  

  

  2-59 
  

  

  3-44 
  

  

  2-70 
  

  

  2-95 
  

  

  2-55 
  

  

  3-18 
  

  

  K„0 
  

  

  •68 
  

  

  4-97 
  

  

  1 
  22 
  

  

  1-38 
  

  

  •89 
  

  

  1-63 
  

  

  1. 
  — 
  Analysis 
  of 
  mixed 
  powders 
  of 
  five 
  fresh 
  specimens 
  of 
  Pigeon 
  Point 
  

   gabbro. 
  

  

  2. 
  — 
  Analysis 
  of 
  mixed 
  powders 
  of 
  seven 
  specimens 
  of 
  granular 
  red 
  rock. 
  

  

  3. 
  — 
  Calculated 
  composition 
  of 
  the 
  mean 
  rock 
  of 
  the 
  sill, 
  giving 
  " 
  1 
  " 
  the 
  

   weight 
  of 
  seven 
  and 
  "2" 
  the 
  weight 
  of 
  unity. 
  

  

  4. 
  — 
  Mean 
  of 
  analyses 
  of 
  20 
  typical 
  diabases. 
  

  

  5. 
  — 
  Mean 
  of 
  analyses 
  of 
  41 
  typical 
  gabbros. 
  

  

  6. 
  — 
  Mean 
  of 
  analyses 
  of 
  161 
  typical 
  basalts. 
  

  

  The 
  calculated 
  mean 
  composition 
  of 
  the 
  sill 
  is 
  seen 
  to 
  be 
  not 
  

   far 
  from 
  that 
  of 
  the 
  average 
  diabase, 
  gabbro, 
  or 
  basalt 
  of 
  the 
  

   world. 
  The 
  result 
  would 
  be 
  little 
  affected 
  by 
  considering 
  also 
  

   the 
  volatile 
  matter 
  that 
  has 
  escaped 
  from 
  the 
  chamber. 
  Hence 
  

   one 
  must 
  seriously 
  entertain 
  the 
  hypothesis 
  that 
  the 
  red 
  rock 
  

   has 
  been 
  differentiated 
  from 
  a 
  gabbroid 
  or 
  diabasic 
  magma 
  

   essentially 
  like 
  that 
  represented 
  in 
  the 
  narrow, 
  and 
  therefore 
  

   quickly 
  chilled, 
  dikes 
  of 
  Keweenawan 
  age 
  in 
  Minnesota. 
  

  

  However, 
  there 
  are 
  two 
  chief 
  objections 
  to 
  that 
  view. 
  

   Though 
  red 
  rock 
  is 
  similarly 
  associated 
  with 
  gabbro 
  or 
  diabase 
  

   in 
  the 
  Logan 
  sills 
  farther 
  north, 
  and 
  in 
  the 
  gigantic 
  Duluth 
  

   laccolith 
  to 
  the 
  west, 
  many 
  of 
  the 
  thick 
  Keweenawan 
  injections 
  

   are 
  constituted 
  wholly 
  of 
  gabbro 
  or 
  diabase 
  without 
  any 
  red- 
  

   rock 
  phase. 
  In 
  other 
  regions, 
  many 
  comparatively 
  thick 
  sills 
  

   and 
  dikes, 
  of 
  dates 
  varying 
  from 
  the 
  pre-Cambrian 
  to 
  the 
  Terti- 
  

   ary, 
  contain 
  no 
  acid 
  differentiate 
  which 
  is 
  comparable 
  in 
  purity 
  

  

  