﻿504: 
  Lane 
  and 
  Keller 
  — 
  Notes 
  on 
  Michigan 
  Minerals. 
  

  

  not 
  true 
  and 
  we 
  are 
  told 
  to 
  strike 
  it 
  out 
  in 
  the 
  errata, 
  but 
  the 
  

   statement 
  may 
  have 
  arisen 
  from 
  the 
  fact 
  that 
  ft 
  is 
  at 
  a 
  much 
  

   greater 
  angle 
  to 
  the 
  basal 
  cleavage 
  than 
  #, 
  as 
  we 
  believe 
  that 
  

   Sanger 
  was 
  still 
  at 
  work 
  on 
  the 
  chloritoids 
  when 
  he 
  was 
  inter- 
  

   rupted 
  by 
  his 
  last 
  illness, 
  and 
  never 
  put 
  his 
  work 
  in 
  final 
  shape. 
  

   For, 
  as 
  we 
  have 
  seen 
  in 
  our 
  material, 
  sections 
  which 
  show 
  a 
  

   change 
  from 
  blue 
  to 
  yellow 
  are 
  those 
  which 
  have 
  the 
  greatest 
  

   extinction, 
  whereas 
  according 
  to 
  the 
  orientation 
  given 
  in 
  the 
  

   Physiography 
  they 
  should 
  have 
  no 
  angle 
  of 
  extinction. 
  The 
  

   ottrelite 
  from 
  Ottrez, 
  however, 
  shows 
  no 
  marked 
  difference 
  of 
  

   color 
  in 
  the 
  different 
  twin 
  lamellae 
  parallel 
  to 
  the 
  basis. 
  La- 
  

   croix 
  merely 
  remarks 
  that 
  the 
  extinctions 
  are 
  longitudinal 
  but 
  

   much 
  dispersed. 
  

  

  The 
  triclinic 
  character 
  of 
  chloritoid 
  seems 
  assured, 
  for: 
  (1), 
  

   sections 
  showing 
  the 
  pleochroism 
  of 
  ft 
  and 
  f, 
  i. 
  e. 
  perpendicu- 
  

   lar 
  to 
  a, 
  have 
  a 
  large 
  and 
  dispersed 
  angle 
  of 
  extinction. 
  Con- 
  

   sequently 
  if 
  monoclinic, 
  a 
  must 
  be 
  parallel 
  to 
  the 
  orthodiago- 
  

   nal 
  b, 
  inasmuch 
  as 
  ft 
  and 
  y 
  are 
  inclined 
  to 
  the 
  basal 
  cleavage. 
  

   Then 
  lateral 
  cleavage 
  fragments 
  showing 
  the 
  pleochroism 
  from 
  

   blue 
  to 
  yellow 
  should 
  also 
  exhibit 
  the 
  directly 
  perpendicular 
  

   emergence 
  of 
  a 
  negative 
  bisectrix. 
  This 
  is 
  not 
  the 
  case. 
  

  

  (2.) 
  The 
  lateral 
  cleavages 
  have 
  not 
  a 
  corresponding 
  sym- 
  

   metry. 
  

  

  (3.) 
  In 
  a 
  monoclinic 
  mineral 
  the 
  emergence 
  of 
  an 
  axial 
  image 
  

   from 
  a 
  fragment 
  due 
  to 
  a 
  solitary 
  perfect 
  cleavage 
  cannot 
  be 
  

   doubly 
  oblique,, 
  but 
  when 
  the 
  axial 
  image 
  is 
  in 
  the 
  shape 
  

   of 
  a 
  cross, 
  one 
  arm 
  must 
  pass 
  through 
  the 
  center 
  of 
  the 
  field 
  

   of 
  view. 
  Or, 
  which 
  amounts 
  to 
  the 
  same 
  thing, 
  when 
  the 
  

   mineral 
  is 
  in 
  the 
  position 
  of 
  extinction 
  with 
  parallel 
  light, 
  on 
  

   changing 
  to 
  convergent 
  light, 
  without 
  disturbing 
  the 
  mineral, 
  

   the 
  axial 
  image 
  must 
  be 
  that 
  of 
  a 
  cross 
  with 
  one 
  bar 
  extending 
  

   directly 
  across 
  the 
  field 
  of 
  view. 
  In 
  this 
  case, 
  although 
  the 
  

   possible 
  effect 
  of 
  twinning 
  makes 
  observations 
  on 
  basal 
  sec- 
  

   tions 
  the 
  least 
  trustworthy, 
  it 
  is 
  nevertheless 
  pretty 
  certain 
  that 
  

   the 
  above 
  condition 
  is 
  not 
  fulfilled. 
  

  

  The 
  various 
  apparent 
  lateral 
  cleavage 
  lines 
  are 
  numerous. 
  

   Lacroix 
  gives 
  a 
  third 
  cleavage 
  bisecting 
  the 
  acute 
  angle 
  be- 
  

   tween 
  the 
  two 
  better 
  ones, 
  while 
  Rosenbusch 
  describes 
  it 
  as 
  

   bisecting 
  the 
  obtuse 
  angle. 
  Traces 
  of 
  both 
  seem 
  to 
  occur, 
  or 
  

   rather 
  it 
  seems 
  as 
  if 
  lines 
  corresponding 
  both 
  to 
  "druckfigur" 
  

   and 
  "schlagfigur 
  " 
  occurred. 
  Obviously 
  however 
  in 
  crystals 
  

   made 
  up 
  of 
  twin 
  lamellae 
  as 
  these 
  so 
  often 
  are, 
  cleavage 
  lines 
  

   are 
  liable 
  to 
  pass, 
  or 
  be 
  imposed, 
  from 
  one 
  lamella 
  to 
  the 
  next 
  

   at 
  an 
  angle 
  of 
  about 
  60° 
  from 
  their 
  proper 
  direction. 
  

  

  § 
  6. 
  The 
  discussion 
  of 
  the 
  paragenesis 
  and 
  occurrence 
  must 
  

   be 
  left 
  till 
  later. 
  Suffice 
  it 
  to 
  say 
  that 
  the 
  Champion 
  chlori- 
  

   toid 
  occurs 
  in 
  a 
  schist 
  which 
  bears 
  a 
  general 
  resemblance 
  to 
  the 
  

  

  