﻿F. 
  W. 
  Clar'ke 
  — 
  Constitution 
  of 
  Tourmaline. 
  121 
  

  

  SiO,. 
  B2O3 
  found. 
  B0O3 
  calc. 
  DifE. 
  

  

  Snarum 
  35-64 
  9-93 
  10'40 
  — 
  1-4'7 
  

  

  Mursinka 
  34-88 
  8-94 
  10-17 
  —1-23 
  

  

  Buckwoith 
  35-50 
  8-34 
  10-35 
  —2-01 
  

  

  Brazil 
  37-05 
  9-09 
  10-81 
  —1-72 
  

  

  It 
  would 
  hardly 
  be 
  wise 
  to 
  dismiss 
  these 
  variations 
  as 
  due 
  to 
  

   errors, 
  especially 
  when 
  the 
  summation 
  of 
  the 
  analyses 
  is 
  good, 
  

   and 
  the 
  analysts 
  are 
  known 
  to 
  be 
  trustworthy. 
  Such 
  errors 
  on 
  

   the 
  part 
  of 
  either 
  Jannasch 
  or 
  Eis^gs 
  would 
  be 
  almost 
  incred- 
  

   ible, 
  and 
  I 
  am, 
  therefore, 
  inclined 
  to 
  believe 
  that 
  the 
  analyses 
  

   are 
  good, 
  and 
  that 
  we 
  should 
  seek 
  a 
  cause 
  for 
  the 
  variations. 
  

   In 
  my 
  scheme 
  of 
  formulation 
  the 
  bivalent 
  group 
  of 
  atoms 
  

   =A1— 
  BO2 
  occurs. 
  Keplace 
  this 
  in 
  part 
  by 
  the 
  similar 
  groups 
  

   = 
  Al— 
  OH 
  and 
  = 
  Al— 
  F, 
  and 
  the 
  variations 
  are 
  accounted 
  for. 
  

   This 
  supposition 
  satisfies 
  the 
  analyses 
  completely, 
  and 
  covers 
  

   the 
  ground. 
  It 
  is 
  in 
  accord 
  with 
  all 
  the 
  evidence, 
  even 
  though 
  

   its 
  validity 
  may 
  not 
  be 
  definitely 
  proved. 
  By 
  its 
  application 
  

   to 
  the 
  discussion 
  of 
  the 
  analyses 
  the 
  divergencies 
  between 
  the 
  

   calculated 
  composition 
  and 
  the 
  composition 
  as 
  found, 
  can 
  be 
  

   notably 
  diminished. 
  

  

  But 
  although 
  the 
  formulae 
  which 
  I 
  have 
  adopted 
  serve 
  to 
  

   express 
  the 
  composition 
  of 
  all 
  tourmalines, 
  they 
  still 
  leave 
  

   room 
  for 
  alternatives. 
  Penfield 
  and 
  Foote, 
  as 
  well 
  as 
  myself, 
  

   assume 
  that 
  tourmaline 
  is 
  a 
  mixed 
  salt 
  containing 
  distinct 
  boric 
  

   and 
  silicic 
  radicles. 
  Future 
  investigation 
  may 
  prove 
  that 
  it 
  is 
  

   really 
  derived 
  from 
  a 
  complex 
  boro-silicic 
  acid, 
  as 
  yet 
  unknown 
  ; 
  

   and 
  the 
  same 
  conception 
  may 
  be 
  true 
  of 
  other 
  species, 
  such 
  as 
  

   axinite, 
  datolite, 
  danburite, 
  cappelenite, 
  etc. 
  A 
  series 
  of 
  boro- 
  

   silicic 
  acids 
  is 
  theoretically 
  conceivable 
  ; 
  and 
  until 
  this 
  ques- 
  

   tion 
  has 
  been 
  considered, 
  the 
  constitution 
  of 
  all 
  the 
  minerals 
  

   above-mentioned 
  must 
  be 
  regarded 
  as 
  unsettled. 
  

  

  Washington, 
  April 
  19, 
  1899. 
  

  

  U. 
  S. 
  Geological 
  Survey. 
  

  

  Am. 
  Joue. 
  Sci. 
  — 
  Fourth 
  Series, 
  Yol. 
  YIII, 
  No. 
  44.— 
  August, 
  1899. 
  

  

  