﻿PROCEEDINGS 
  OF 
  THE 
  NATIONAL 
  MUSEUM 
  

  

  vol. 
  67 
  

  

  associated 
  with 
  magmatic 
  cooling. 
  It 
  is, 
  therefore, 
  concluded 
  that 
  

   iddingsite 
  is 
  most 
  probably 
  a 
  deuteric 
  mineral 
  formed 
  in 
  the 
  pres- 
  

   ence 
  of 
  heat, 
  water, 
  and 
  gases 
  after 
  the 
  magma 
  has 
  reached 
  a 
  horizon 
  

   near 
  enough 
  the 
  surface 
  to 
  give 
  oxidizing 
  conditions. 
  The 
  magma 
  

   must 
  have 
  come 
  to 
  rest 
  before 
  iddingsite 
  formed 
  for 
  though 
  it 
  is 
  a 
  

   very 
  brittle 
  mineral 
  it 
  is 
  never 
  fractured, 
  or 
  distorted 
  by 
  flow. 
  

  

  A 
  similar 
  result 
  may 
  have 
  been 
  produced 
  in 
  other 
  ways. 
  Thus 
  it 
  

   is 
  quite 
  probable 
  that 
  the 
  heat 
  and 
  gases 
  given 
  off 
  by 
  one 
  lava 
  flow 
  

   would 
  have 
  a 
  metasomatic 
  action 
  on 
  a 
  previous 
  flow, 
  and 
  iddingsite 
  

   might 
  be 
  the 
  result 
  of 
  this 
  action. 
  It 
  is 
  doubted, 
  however, 
  if 
  this 
  

   effect 
  could 
  be 
  widespread, 
  and 
  it 
  could 
  not 
  produce 
  a 
  uniform 
  dis- 
  

   tribution 
  of 
  iddingsite 
  from 
  top 
  to 
  bottom 
  of 
  a 
  thick 
  flow. 
  

  

  A 
  comparison 
  of 
  the 
  chemical 
  composition 
  of 
  iddingsite 
  and 
  ser- 
  

   pentine 
  shows 
  how 
  different 
  are 
  the 
  processes 
  involved 
  in 
  the 
  devel- 
  

   opment 
  of 
  the 
  two 
  minerals. 
  

  

  Comparative 
  analyses 
  of 
  iddmg&ite, 
  serpentine, 
  and 
  olivine 
  

  

  (1) 
  Iddingsite 
  from 
  La 
  Jara 
  Creek, 
  Conejos 
  quadrangle. 
  Colo. 
  

  

  (2) 
  Serpentine 
  from 
  Fort 
  Henry, 
  New 
  York, 
  11 
  analysis 
  No. 
  19. 
  

  

  (3) 
  Serpentine 
  ideal 
  composition. 
  

  

  (4) 
  Olivine 
  from 
  Cerro 
  Mohera 
  near 
  Tres 
  Predias, 
  N. 
  M. 
  

  

  In 
  analyzed 
  serpentine 
  aluminum 
  peroxide 
  (ALQ.) 
  and 
  iron 
  

   peroxide 
  (Fe 
  2 
  O 
  a 
  ) 
  reach 
  a 
  maximum 
  of 
  G 
  per 
  cent, 
  and 
  a 
  variable 
  

   amount 
  of 
  iron 
  monoxide 
  (FeO) 
  replaces 
  magnesium 
  oxide 
  (MgO), 
  

   but- 
  no 
  serpentine 
  even 
  remotely 
  resembling 
  iddingsite 
  has 
  ever 
  been 
  

   described. 
  Serpentine 
  is 
  generally 
  believed 
  to 
  have 
  been 
  the 
  result 
  

   of 
  metasomatic 
  changes 
  at 
  some 
  depth 
  and 
  seldom, 
  if 
  ever, 
  the 
  result 
  

   of 
  surface 
  weathering, 
  and 
  yet 
  its 
  chemical 
  composition 
  is 
  not 
  very 
  

   different 
  from 
  that 
  of 
  the 
  olivine 
  from 
  which 
  it 
  is 
  derived. 
  The 
  

   changes 
  in 
  the 
  ratios 
  of 
  the 
  chemical 
  components 
  involved 
  in 
  the 
  

   derivation 
  of 
  serpentine 
  from 
  olivine 
  are 
  very 
  much 
  less 
  than 
  the 
  

   changes 
  in 
  ratio 
  when 
  iddin£site 
  is 
  derived 
  from 
  olivine. 
  It 
  is 
  also 
  

  

  11 
  Dana, 
  James 
  D., 
  Descriptive 
  mineralogy, 
  p. 
  672, 
  1909. 
  

  

  