﻿6 
  PROCEEDINGS 
  OE 
  THE 
  NATIONAL 
  MUSEUM 
  vol.. 
  67 
  

  

  In 
  many 
  of 
  the 
  rocks 
  studied 
  the 
  relation 
  of 
  fresh 
  olivine 
  and 
  

   iddingsite 
  present 
  pecularities 
  that 
  appear 
  to 
  give 
  a 
  clew 
  as 
  to 
  the 
  

   mode 
  of 
  origin. 
  The 
  presence 
  of 
  small 
  grains 
  of 
  ground-mass 
  

   olivine 
  remaining 
  nearly 
  fresh 
  in 
  the 
  presence 
  of 
  large 
  phenoerysts 
  

   that 
  have 
  been 
  completely 
  changed 
  to 
  iddingsite 
  suggests 
  that 
  the 
  

   processes 
  involved 
  in 
  the 
  change 
  are 
  partly 
  dependent 
  on 
  the 
  original 
  

   composition 
  of 
  olivine. 
  The 
  basalt 
  of 
  the 
  Hinsdale 
  volcanic 
  series 
  

   of 
  the 
  Rio 
  Grande 
  Valley 
  of 
  northern 
  New 
  Mexico 
  has 
  been 
  traced 
  

   for 
  80 
  miles, 
  and 
  wherever 
  observed 
  it 
  shows 
  cores 
  of 
  iddingsite 
  

   surrounded 
  by 
  fresh 
  olivine 
  (see 
  pi. 
  1, 
  fig. 
  2, 
  and 
  pi. 
  2, 
  figs. 
  5, 
  6). 
  

   Similarly, 
  rocks 
  from 
  many 
  other 
  sources 
  show 
  a 
  very 
  distinct 
  zonal 
  

   relationship 
  in 
  the 
  development 
  of 
  the 
  iddingsite. 
  It 
  seems 
  very 
  

   difficult 
  to 
  explain 
  such 
  relationships 
  on 
  the 
  basis 
  of 
  weathering, 
  

   especially 
  as 
  these 
  phenomena 
  are 
  characteristic 
  of 
  single 
  flows 
  or 
  

   single 
  groups 
  of 
  flows 
  over 
  wide 
  areas. 
  On 
  the 
  other 
  hand, 
  these 
  

   facts 
  suggest 
  very 
  strongly 
  that 
  the 
  alteration 
  was 
  partly 
  dependent 
  

   upon 
  zonal 
  variations 
  in 
  the 
  original 
  olivine 
  from 
  which 
  the 
  idding- 
  

   site 
  was 
  derived. 
  This 
  led 
  to 
  an 
  investigation 
  of 
  the 
  olivines 
  of 
  

   iddingsite-bearing 
  rocks. 
  The 
  basalt 
  of 
  Cerro 
  Mohera, 
  New 
  Mexico, 
  

   is 
  of 
  the 
  same 
  age 
  and 
  type 
  as 
  that 
  giving 
  rise 
  to 
  the 
  sharp 
  zones 
  of 
  

   olivine 
  around 
  iddingsite 
  shown 
  (pi. 
  1, 
  fig. 
  2, 
  and 
  pi. 
  2, 
  figs. 
  5, 
  6), 
  

   but 
  is 
  itself 
  little 
  altered. 
  A 
  careful 
  study 
  of 
  the 
  optical 
  properties 
  

   of 
  this 
  olivine 
  showed 
  that 
  the 
  index 
  of 
  refraction 
  for 
  [3 
  varied 
  from 
  

   n=1.711 
  to 
  n=1.722, 
  and 
  the 
  optical 
  character 
  varied 
  from 
  -\- 
  to 
  — 
  , 
  

   indicating 
  an 
  appreciable 
  variation 
  in 
  the 
  proportion 
  of 
  iron 
  silicate 
  

   (Fe 
  2 
  Si0 
  4 
  ) 
  in 
  the 
  olivine 
  molecule. 
  These 
  facts, 
  supported 
  as 
  they 
  

   are 
  by 
  the 
  mineral 
  relationships, 
  seem 
  to 
  show 
  that 
  the 
  formation 
  

   of 
  iddingsite 
  from 
  olivine 
  is 
  partly 
  dependent 
  upon 
  the 
  chemical 
  

   composition 
  of 
  the 
  olivine. 
  

  

  Iddingsite 
  is 
  confined 
  almost 
  exclusively 
  to 
  extrusive 
  or 
  hypa- 
  

   byssal 
  rocks 
  and 
  is 
  practically 
  absent 
  from 
  deep-seated 
  rocks, 
  but 
  

   if 
  iddingsite 
  were 
  derived 
  from 
  olivine 
  by 
  ordinary 
  weathering 
  

   there 
  is 
  no 
  reason 
  why 
  it 
  should 
  not 
  occur 
  in 
  abyssal 
  rocks. 
  The 
  

   restriction 
  in 
  occurrence 
  shows 
  that 
  specialized 
  conditions 
  are 
  re- 
  

   quired 
  for 
  the 
  formation 
  of 
  the 
  mineral 
  and 
  that 
  these 
  conditions 
  

   are 
  most 
  often 
  realized 
  in 
  a 
  cooling 
  extrusive. 
  This 
  restriction 
  in 
  

   occurrence 
  and 
  the 
  relationships 
  described 
  indicate 
  that 
  the 
  develop- 
  

   ment 
  of 
  iddingsite 
  is 
  definitely 
  associated 
  with 
  magmas 
  that 
  cooled 
  

   near 
  the 
  surface. 
  

  

  In 
  discussing 
  iddingsite, 
  Iddings 
  8 
  says: 
  

  

  There 
  remained 
  in 
  the 
  portion 
  (iddingsite) 
  subjected 
  to 
  acid, 
  well 
  developed, 
  

   nearly 
  opaque 
  octahedrons, 
  most 
  likely 
  picotite. 
  

  

  Biddings, 
  Joseph 
  P., 
  U. 
  S. 
  Geol. 
  Survey, 
  Mono. 
  20, 
  p. 
  390. 
  

  

  