﻿508 
  Lane 
  and 
  Sharpless 
  — 
  Notes 
  on 
  Michigan 
  Minerals. 
  

  

  due 
  to 
  such 
  molecular 
  or 
  submicroscopic 
  twinning, 
  for 
  such 
  a 
  

   structure 
  would 
  produce 
  a 
  rhombic 
  symmetry. 
  Then 
  antho- 
  

   phyllite 
  and 
  griinerite 
  would 
  be 
  related, 
  as 
  orthoclase 
  (submi- 
  

   croscopic 
  microcline) 
  and 
  albite, 
  or 
  the 
  two 
  kinds 
  of 
  natrolite. 
  

  

  Considering 
  the 
  unfortunate 
  ambiguity 
  of 
  the 
  word 
  cum- 
  

   mingtonite, 
  its 
  hitherto 
  more 
  imperfect 
  optical 
  description, 
  

   and 
  the 
  greater 
  length 
  of 
  the 
  word, 
  it 
  seems 
  preferable 
  to 
  

   denote 
  the 
  allied 
  Michigan 
  amphiboles 
  as 
  griinerite, 
  pending 
  

   further 
  investigation. 
  Inasmuch 
  as 
  they 
  are 
  concomitants 
  of 
  

   the 
  iron 
  ores, 
  it 
  seems 
  the 
  more 
  proper 
  to 
  lay 
  stress 
  on 
  the 
  

   Fe 
  4 
  Si 
  4 
  12 
  molecules. 
  

  

  3. 
  RlEBECKITE 
  OR 
  CrOOIDOLITE. 
  [L.] 
  

  

  Our 
  knowledge 
  of 
  this 
  group 
  of 
  amphiboles 
  is 
  rapidly 
  in- 
  

   creasing, 
  but 
  it 
  has 
  not 
  yet 
  been 
  so 
  frequently 
  observed 
  that 
  a 
  

   new 
  occurrence 
  is 
  devoid 
  of 
  interest. 
  I 
  have 
  observed 
  it 
  as 
  a 
  

   secondary 
  fibrous 
  growth 
  on 
  the 
  primary 
  hornblende 
  of 
  a 
  

   syenite.* 
  It 
  occurs 
  much 
  as 
  those 
  fibers 
  do, 
  that 
  we 
  often 
  see 
  

   in 
  more 
  basic 
  rocks 
  growing 
  out 
  from 
  patches 
  of 
  uralite 
  into 
  

   the 
  adjacent 
  feldspar, 
  and 
  it 
  is 
  worth 
  noting 
  that 
  uralite 
  patches 
  

   are 
  often 
  most 
  bluish 
  at 
  the 
  margin. 
  The 
  growths 
  I 
  have 
  

   noticed 
  answer 
  precisely 
  to 
  those 
  described 
  by 
  Cross,f 
  and 
  

   verify 
  his 
  observations, 
  as 
  I 
  can 
  testify 
  from 
  a 
  personal 
  exam- 
  

   ination 
  of 
  his 
  sections, 
  which 
  he 
  kindly 
  afforded 
  me. 
  The 
  

   vertical 
  axis 
  and 
  orientation 
  are 
  parallel 
  to 
  those 
  of 
  the 
  original 
  

   hornblende, 
  but 
  the 
  angle 
  of 
  the 
  + 
  extinction 
  is 
  very 
  large, 
  

   somewhere 
  about 
  75°, 
  above, 
  to 
  the 
  front, 
  so 
  that 
  as 
  Cross 
  

   notes 
  the 
  nearest 
  extinction 
  is 
  on 
  the 
  other 
  side 
  of 
  the 
  vertical 
  

   axis 
  from 
  that 
  of 
  common 
  hornblende. 
  The 
  pleochroism 
  is, 
  — 
  

   a 
  blue 
  to 
  greenish 
  blue; 
  /? 
  violet 
  or 
  reddish 
  blue; 
  y 
  yellow. 
  

   The 
  bi-refraction 
  is 
  weak. 
  The 
  fibers 
  are 
  often 
  separated 
  from 
  

   the 
  dark 
  green 
  hornblende 
  by 
  a 
  sharp 
  crystallographic 
  line. 
  

   At 
  other 
  times 
  they 
  seem 
  to 
  mingle 
  and 
  compensate, 
  forming 
  

   an 
  isotropic 
  band. 
  

  

  Michigan 
  Mining 
  School, 
  Houghton, 
  May 
  23d, 
  1891. 
  

  

  *No. 
  583 
  of 
  the 
  Mich. 
  State 
  Coll. 
  ; 
  325 
  paces 
  N.. 
  975 
  paces 
  W., 
  of 
  the 
  S.E.. 
  

   corner 
  of 
  Sec. 
  17, 
  T. 
  49, 
  R. 
  25, 
  

  

  fThis 
  Journal, 
  xxxix, 
  1890, 
  p. 
  359. 
  

  

  