﻿9. 
  PROCEEDINGS 
  OF 
  THE 
  NATIONAL 
  MUSEUM 
  vol.67 
  

  

  been 
  studied: 
  and 
  conclusions 
  as 
  to 
  the 
  genesis 
  of 
  iddingsite 
  have 
  

   been 
  reached. 
  This 
  paper 
  is 
  presented 
  in 
  the 
  belief 
  that 
  the 
  available 
  

   data 
  on 
  the 
  composition, 
  and 
  a 
  detailed 
  study 
  of 
  the 
  origin 
  of 
  this 
  

   long 
  discussed 
  mineral 
  will 
  prove 
  to 
  be 
  of 
  interest, 
  as 
  the 
  results 
  

   are 
  distinctly 
  at 
  -variance 
  with 
  previous 
  views 
  about 
  iddingsite. 
  

  

  REVIEW 
  OF 
  PREVIOUS 
  INVESTIGATIONS 
  OF 
  IDDINGSITE 
  

  

  In 
  a 
  discussion 
  of 
  the 
  rocks 
  of 
  the 
  Eureka 
  district, 
  Nevada, 
  Id- 
  

   dings 
  3 
  says 
  of 
  the 
  mineral 
  later 
  named 
  iddingsite: 
  "There 
  com- 
  

   mences 
  from 
  the 
  surface 
  and 
  from 
  fractures 
  (in 
  olivine) 
  as 
  in 
  the 
  

   ordinary 
  process 
  a 
  fibration, 
  not 
  in 
  directions 
  always 
  normal 
  to 
  the 
  

   surface, 
  but 
  in 
  lines 
  parallel 
  throughout 
  the 
  crystal, 
  and 
  parallel 
  also 
  

   to 
  some 
  direction 
  in 
  the 
  plane 
  of 
  the 
  more 
  perfect 
  cleavage- 
  The 
  

   fibers 
  have 
  a 
  light 
  yellow 
  color 
  at 
  first, 
  which 
  deepens 
  into 
  a 
  red- 
  

   dish 
  brown 
  or 
  blood 
  red 
  as 
  the 
  decomposition 
  proceeds; 
  they 
  polarize 
  

   light 
  brilliantly 
  and 
  show 
  parallel 
  extinction 
  and 
  sometimes 
  faint 
  

   pleochroism. 
  The 
  resultant 
  mineral 
  is 
  evidently 
  not 
  a 
  compound 
  

   aggregate, 
  but 
  a 
  crystallographic 
  individual, 
  with 
  parallel 
  orienta- 
  

   tion 
  in 
  all 
  its 
  parts, 
  for 
  the 
  extinction 
  of 
  light 
  is 
  the 
  same 
  through- 
  

   out, 
  and 
  the 
  interference 
  figure 
  is 
  that 
  of 
  a 
  doubly 
  refracting 
  crys- 
  

   tal." 
  Iddings 
  observed 
  occasional 
  well 
  crystallized 
  hexagonal 
  plates 
  

   in 
  the 
  less 
  altered 
  olivine 
  and 
  found 
  that 
  on 
  treatment 
  with 
  hot 
  

   concentrated 
  hydrochloric 
  acid 
  the 
  mineral 
  lost 
  color 
  without 
  chang- 
  

   ing 
  its 
  optical 
  properties. 
  This 
  induced 
  him 
  to 
  think 
  that 
  : 
  " 
  The 
  

   substance 
  is 
  in 
  this 
  case 
  a 
  nearly 
  colorless 
  micaceous 
  mineral, 
  colored 
  

   red 
  by 
  iron 
  oxide." 
  He 
  concludes: 
  

  

  That 
  the 
  mineral 
  is 
  a 
  foliated, 
  crystallized 
  form 
  of 
  serpentine 
  seems 
  prob- 
  

   able 
  from 
  the 
  fact 
  that 
  most 
  of 
  the 
  basalts 
  are 
  so 
  fresh, 
  with 
  the 
  decomposi- 
  

   tion 
  of 
  tbe 
  olivine 
  frequently 
  confined 
  to 
  the 
  weathered 
  surface, 
  that 
  a 
  very 
  

   radical 
  change 
  is 
  not 
  likely 
  to 
  have 
  taken 
  place, 
  and 
  that 
  simple 
  hydration 
  

   and 
  oxidation 
  of 
  a 
  very 
  ferruginous 
  olivine 
  would 
  supply 
  all 
  the 
  chemical 
  

   elements 
  necessary 
  to 
  transform 
  it 
  into 
  anhydrous 
  unsilicate 
  of 
  magnesia 
  and 
  

   ferric 
  iron; 
  besides 
  which 
  is 
  the 
  fact 
  that 
  the 
  optical 
  properties 
  of 
  the 
  min- 
  

   eral 
  in 
  question 
  correspond 
  to 
  those 
  given 
  by 
  Miller 
  for 
  thermophyllite. 
  

  

  Describing 
  the 
  " 
  Potlach 
  pseudomorphs 
  after 
  olivin 
  • 
  " 
  in 
  the 
  Car- 
  

   boniferous 
  tuffs 
  and 
  dolorite 
  of 
  Derbyshire, 
  Arnold-Bemrose 
  * 
  says: 
  

  

  The 
  plane 
  of 
  the 
  optic 
  axes 
  is 
  at 
  right 
  angles 
  to 
  the 
  length 
  of 
  the 
  original 
  

   crystal, 
  the 
  angle 
  between 
  the 
  optic 
  axes 
  is 
  very 
  small, 
  and 
  the 
  double 
  refrac- 
  

   tion 
  negative. 
  As 
  a 
  rule 
  the 
  pseudomorphs 
  behave 
  as 
  a 
  crystallographic 
  in- 
  

   dividual, 
  and 
  not 
  as 
  an 
  aggregate. 
  The 
  traces 
  of 
  the 
  cleavage 
  are 
  generally 
  

   parallel 
  to 
  the 
  length 
  of 
  the 
  crystal. 
  * 
  * 
  * 
  When 
  mounted, 
  the 
  thin 
  Hakes 
  

   appeal- 
  brown 
  or 
  brownish-yellow 
  by 
  transmitted 
  light. 
  In 
  convergent 
  light 
  

   they 
  show 
  a 
  biaxial 
  figure, 
  with 
  a 
  small 
  angle 
  between 
  the 
  axes 
  and 
  negative 
  

   double 
  refraction. 
  They 
  are 
  sometimes 
  almost 
  uniaxial. 
  When 
  a 
  fragment 
  does 
  

  

  :1 
  [ddings, 
  Joseph 
  P., 
  Appendix 
  H, 
  Mono. 
  20, 
  1". 
  s. 
  Geol. 
  Survey, 
  pp. 
  388-300, 
  1892. 
  

   ' 
  Arnold-Bemrose, 
  II. 
  H., 
  Quart. 
  Joui-n. 
  Geol. 
  Soc, 
  p. 
  603, 
  London, 
  1804. 
  

  

  