﻿274 
  PROCEEDINGS 
  OP 
  THE 
  GEOLOGICAL 
  SOCIETY. 
  

  

  An 
  important 
  question, 
  however, 
  remains 
  to 
  be 
  determined; 
  

   namely, 
  whether 
  the 
  remaining 
  eight 
  granites, 
  amounting 
  to 
  53^ 
  per 
  

   cent, 
  of 
  the 
  whole, 
  are 
  really 
  composed 
  of 
  these 
  four 
  minerals 
  or 
  not. 
  

  

  It 
  has 
  been 
  demonstrated, 
  mathematically, 
  that 
  they 
  might 
  be 
  

   formed 
  of 
  four 
  such 
  minerals, 
  having 
  the 
  oxygen-ratios 
  of 
  these 
  

   minerals 
  as 
  they 
  are 
  found 
  crystallizing 
  out 
  from 
  the 
  mass 
  of 
  the 
  

   Donegal 
  granites. 
  But 
  is 
  the 
  rock 
  really 
  made 
  up 
  of 
  only 
  four 
  

   minerals 
  or 
  not 
  ? 
  It 
  is 
  the 
  opinion 
  of 
  many 
  skilful 
  mineralogists, 
  

   that 
  the 
  composition 
  of 
  the 
  crystals 
  which 
  may 
  be 
  picked 
  out 
  of 
  a 
  

   granite 
  or 
  other 
  crystalline 
  rock 
  may 
  differ 
  essentially 
  from 
  the 
  

   composition 
  of 
  the 
  same 
  mineral 
  when 
  it 
  enters 
  into 
  the 
  constitu- 
  

   tion 
  of 
  the 
  general 
  rock-mass 
  ; 
  and 
  it 
  is 
  certainly 
  not 
  to 
  be 
  assumed 
  

   as 
  self-evident 
  that 
  a 
  rock 
  is 
  composed 
  of 
  those 
  minerals, 
  and 
  of 
  

   those 
  only, 
  which 
  may 
  be 
  seen 
  in 
  separate 
  crystals, 
  and 
  analyzed 
  

   as 
  such. 
  

  

  To 
  solve 
  this 
  interesting 
  problem 
  in 
  the 
  case 
  of 
  the 
  Donegal 
  

   granites, 
  we 
  must 
  call 
  to 
  our 
  aid 
  the 
  auxiliary 
  equations 
  not 
  yet 
  

   used, 
  depending 
  on 
  iron, 
  lime, 
  soda, 
  potash, 
  &c, 
  which 
  have 
  been 
  

   grouped 
  together 
  hitherto, 
  by 
  us, 
  in 
  the 
  equation 
  containing 
  the 
  

   oxygen 
  of 
  the 
  protoxides. 
  

  

  In 
  fact, 
  before 
  we 
  can 
  say 
  that 
  the 
  granites 
  are 
  really 
  made 
  of 
  

   the 
  four 
  minerals 
  whose 
  composition 
  is 
  given 
  in 
  Table 
  II., 
  we 
  must 
  

   be 
  certain 
  that 
  the 
  equations 
  of 
  condition 
  furnished 
  by 
  each 
  consti- 
  

   tuent 
  are 
  all 
  fulfilled, 
  as 
  well 
  as 
  those 
  furnished 
  by 
  the 
  oxygen- 
  

   ratios. 
  These 
  equations 
  of 
  condition 
  are 
  the 
  necessary 
  supplement 
  

   of 
  the 
  preceding 
  analysis, 
  which 
  has 
  only 
  proved 
  that 
  certain 
  gra- 
  

   nites 
  might 
  be 
  made 
  of 
  certain 
  minerals 
  with 
  given 
  mineralogical 
  

   formulae; 
  but 
  which 
  has 
  not 
  proved 
  that 
  they 
  might 
  be 
  made 
  of 
  

   certain 
  minerals 
  of 
  given 
  chemical 
  compositions. 
  

  

  The 
  method 
  of 
  using 
  these 
  test- 
  equations 
  will 
  be 
  readily 
  under- 
  

   stood 
  from 
  the 
  following 
  discussion 
  : 
  — 
  

  

  Granite 
  No. 
  I. 
  

  

  This 
  granite 
  might 
  be 
  formed 
  of 
  four 
  minerals 
  having 
  the 
  same 
  

   oxygen-ratios 
  as 
  the 
  minerals 
  of 
  Table 
  II., 
  but 
  it 
  cannot 
  be 
  formed 
  

   of 
  minerals 
  having 
  the 
  chemical 
  composition 
  of 
  those 
  in 
  Table 
  II., 
  

   because 
  it 
  fails 
  to 
  satisfy, 
  inter 
  alia, 
  the 
  potash 
  test. 
  

  

  The 
  quantity 
  of 
  potash 
  in 
  this 
  granite, 
  as 
  appears 
  from 
  Tables 
  II. 
  

   and 
  VI., 
  should 
  be 
  

  

  2-51x14-92 
  , 
  71-03x2-07 
  , 
  0-93x8-83 
  . 
  QOa 
  

   — 
  4- 
  = 
  L'vZb 
  ; 
  

  

  100 
  X 
  100 
  ^ 
  100 
  

  

  but 
  the 
  quantity 
  actually 
  found 
  by 
  analysis 
  was 
  4*66 
  (Table 
  I.) 
  ; 
  

   from 
  which 
  it 
  is 
  evident 
  that 
  the 
  calculated 
  and 
  observed 
  amounts 
  

   of 
  potash 
  cannot 
  be 
  made 
  to 
  agree, 
  and 
  therefore 
  that 
  this 
  granite 
  

   cannot 
  be 
  composed 
  of 
  the 
  four 
  minerals 
  of 
  Table 
  II. 
  

  

  This 
  granite 
  also 
  fails 
  to 
  satisfy 
  the 
  test-equations 
  derived 
  from 
  

   iron 
  and 
  lime. 
  It 
  will 
  be 
  found 
  on 
  trial 
  that 
  the 
  calculated 
  quan- 
  

   tity 
  of 
  iron, 
  like 
  that 
  of 
  potash, 
  is 
  too 
  small 
  ; 
  while 
  that 
  of 
  lime 
  

   will 
  be 
  found 
  too 
  great. 
  

  

  