﻿art, 
  7 
  THE 
  MINERAL 
  IDDINGSITE 
  ROSS 
  AND 
  SHANNON 
  11 
  

  

  represented 
  by 
  analysis 
  No. 
  5. 
  In 
  some 
  specimens 
  this 
  forms 
  at 
  the 
  

   core 
  and 
  in 
  others 
  as 
  a 
  border 
  around 
  the 
  crystalline 
  part. 
  It 
  seems 
  

   clear 
  that 
  the 
  two 
  types 
  of 
  material 
  were 
  not 
  the 
  result 
  of 
  different 
  

   conditions 
  during 
  formation 
  but 
  are 
  dependent 
  upon 
  variations 
  in 
  the 
  

   composition 
  of 
  the 
  olivine 
  from 
  which 
  they 
  were 
  derived. 
  

  

  SOUTH 
  ELK 
  CREEK, 
  COLO., 
  OCCURRENCE 
  

  

  The 
  rock 
  containing 
  the 
  iddingsite 
  represented 
  by 
  analysis 
  6 
  was 
  

   collected 
  at 
  the 
  cliffs 
  surrounding 
  a 
  cirque 
  at 
  the 
  head 
  of 
  South 
  Elk 
  

   Creek 
  in 
  the 
  southwest 
  part 
  of 
  the 
  Conejos 
  quadrangle, 
  Colorado. 
  

   The 
  rock 
  is 
  of 
  the 
  same 
  age 
  as 
  the 
  La 
  Jara 
  Creek 
  and 
  Gato 
  Creek 
  oc- 
  

   currences. 
  It 
  is 
  a 
  nearly 
  black 
  basalt 
  with 
  conspicuous 
  red 
  areas 
  of 
  

   iddingsite 
  reaching 
  a 
  maximum 
  diameter 
  of 
  2 
  millimeters. 
  The 
  min- 
  

   eral 
  composition 
  of 
  the 
  rocks 
  is 
  as 
  follows 
  : 
  

  

  Mode 
  of 
  basalt 
  from 
  South 
  Elk 
  Creek, 
  Colo. 
  

  

  Labradorite 
  52 
  

  

  Augite 
  20 
  

  

  Iddingsite 
  19 
  

  

  Magnetite 
  9 
  

  

  Total 
  100 
  

  

  GATO 
  CREEK, 
  COLO., 
  OCCURRENCE 
  

  

  The 
  iddingsite 
  represented 
  by 
  analysis 
  7 
  was 
  secured 
  from 
  a 
  dike 
  

   occurring 
  on 
  Gato 
  Creek, 
  2 
  miles 
  above 
  Tipton's 
  ranch 
  in 
  the 
  north 
  

   central 
  part 
  of 
  the 
  Conejos 
  quadrangle, 
  Colorado. 
  The 
  rock 
  is 
  a 
  

   fine-grained, 
  porous 
  gray 
  andesite 
  with 
  about 
  the 
  following 
  mineral 
  

   composition 
  : 
  

  

  Mode 
  of 
  andesite 
  from 
  Gato 
  Creek, 
  Colo. 
  

  

  Andesine 
  71 
  

  

  Augite 
  15 
  

  

  Iddingsite 
  10 
  

  

  Magnetite 
  1 
  4 
  

  

  Total 
  100 
  

  

  The 
  iddingsite 
  occurs 
  in 
  rounded 
  grains 
  about 
  0.5 
  millimeter 
  in 
  

   diameter 
  and 
  is 
  clearly 
  derived 
  from 
  olivine. 
  

  

  BRAZOS 
  RIVER, 
  N. 
  MEX., 
  OCCURRENCE 
  

  

  Analysis 
  No. 
  8 
  is 
  an 
  iddingsite 
  in 
  an 
  andesite 
  occurring 
  one-half 
  

   mile 
  east 
  of 
  the 
  Brazos 
  River 
  in 
  the 
  Rio 
  Ariba 
  County, 
  N. 
  Mex., 
  

   and 
  about 
  15 
  miles 
  south 
  of 
  Osier 
  on 
  the 
  Durango 
  branch 
  of 
  the 
  

   Denver 
  & 
  Rio 
  Grande 
  Railroad. 
  The 
  rock 
  appears 
  to 
  be 
  an 
  instru- 
  

   sive 
  sill 
  of 
  Miocene 
  age 
  that 
  forms 
  a 
  sheer 
  50-foot 
  ledge 
  at 
  this 
  place. 
  

   A 
  microscopic 
  study 
  showed 
  that 
  it 
  contained 
  a 
  red 
  material 
  de- 
  

   rived 
  from 
  olivine, 
  but 
  with 
  no 
  residual 
  olivine. 
  It 
  is 
  rather 
  coarse- 
  

  

  