﻿14 
  

  

  PROCEEDINGS 
  OF 
  THE 
  NATIONAL 
  MUSEUM 
  

  

  vol. 
  67 
  

  

  2. 
  Head 
  of 
  Mill 
  Gulch, 
  south 
  central 
  part 
  of 
  Uncompahgre 
  quad- 
  

   rangle. 
  Gabbro 
  inclusion 
  in 
  basalt. 
  Deep 
  reddish 
  brown 
  grains. 
  

   X 
  is 
  normal 
  to 
  cleavage 
  plates. 
  Extinction 
  parallel 
  to 
  cleavage. 
  

   2V=40° 
  estimated. 
  Optically 
  negative. 
  Dispersion 
  p<u 
  (strong). 
  

   Pleochroic. 
  Indices 
  a=1.724±.003. 
  0=1.745 
  ±.003. 
  y= 
  1.768 
  ±.003. 
  

   of— 
  Y 
  =.044. 
  

  

  3. 
  13 
  Pyroxene 
  latite, 
  Wicher 
  Mountain 
  knoll, 
  Pikes 
  Peak 
  quad- 
  

   rangle. 
  Reddish 
  brown 
  grains. 
  Optically—. 
  2V 
  large. 
  p<v 
  

   (strong). 
  X 
  normal 
  to 
  the 
  plates. 
  Indices 
  vary 
  somewhat 
  a?=1.71± 
  

   0.01. 
  0=1.74+0.01. 
  T 
  =1.76±0.01. 
  a— 
  Y 
  =.05. 
  

  

  4. 
  Uncompahgre 
  quadrangle, 
  Colorado. 
  In 
  thin 
  section 
  clear 
  pale 
  

   reddish 
  brown. 
  Optical 
  properties 
  vary 
  a 
  little. 
  Optically-)-. 
  2V 
  

   large. 
  p>v 
  (strong). 
  Faintly 
  pleochroic. 
  a 
  = 
  1.70±0.01. 
  (i=1.72± 
  

   0.01. 
  y=1.74±0.01. 
  a— 
  y=.04. 
  

  

  5. 
  La 
  Jara 
  Creek, 
  Conejos 
  quadrangle, 
  Colorado. 
  Bright 
  reddish 
  

   brown. 
  Optical 
  properties 
  vary 
  a 
  little. 
  Optically—. 
  2V=25° 
  to 
  

   45°. 
  p<u 
  (strong). 
  Pleochroic. 
  X 
  perpendicular 
  to 
  plates. 
  a= 
  

   1.674±0.0004. 
  p=1.710±0.004. 
  T 
  =1.718±0.004. 
  

  

  Table 
  of 
  optical 
  properties 
  of 
  iddingsite 
  

  

  " 
  About. 
  

  

  CHEMICAL 
  COMPOSITION 
  

  

  Iddingsite 
  has 
  not 
  heretofore 
  been 
  analyzed 
  because 
  of 
  its 
  mode 
  

   of 
  occurrence, 
  always 
  as 
  small 
  grains, 
  as 
  a 
  rock 
  constituent 
  which 
  

   made 
  the 
  obtaining 
  of 
  pure 
  material, 
  in 
  amount 
  sufficient 
  for 
  quan- 
  

   titative 
  chemical 
  examination, 
  exceedingly 
  difficult. 
  In 
  the 
  course 
  of 
  

  

  13 
  4, 
  5. 
  Larsen, 
  Esper 
  S., 
  U. 
  S. 
  Geol. 
  Survey 
  Bull. 
  670, 
  p. 
  91, 
  1921. 
  

  

  