﻿700 
  Forty-seventh 
  Report 
  on 
  the 
  State 
  Museum. 
  

  

  granite. 
  The 
  same 
  end 
  might 
  be 
  sometimes 
  accomplished 
  by 
  the 
  

   gneiss, 
  when 
  the 
  structure 
  was 
  favorable. 
  

  

  At 
  the 
  same 
  time 
  the 
  solutions 
  of 
  iron, 
  calcium, 
  and 
  magnesium 
  

   sulphate, 
  and 
  of 
  sulphuric 
  acid 
  would 
  afford 
  a 
  very 
  powerful 
  

   agent 
  for 
  the 
  alteration 
  of 
  the 
  granite, 
  or 
  gneiss, 
  whose 
  action 
  

   would 
  be 
  limited 
  to 
  the 
  vicinity 
  of 
  the 
  ore 
  deposits. 
  The 
  accom- 
  

   panying 
  analyses 
  give 
  an 
  idea 
  of 
  what 
  this 
  chemical 
  change 
  has 
  

   been. 
  Its 
  extent 
  can 
  be 
  appreciated 
  only 
  by 
  considering 
  the 
  fact 
  

   that 
  the 
  original 
  granite 
  was 
  very 
  acid, 
  containing 
  practically 
  no 
  

   ferro-magnesian 
  constituents. 
  That 
  so 
  profound 
  an 
  alteration 
  

   indicates 
  the 
  action 
  of 
  some 
  special 
  agent 
  will 
  hardly 
  be 
  ques- 
  

   tioned. 
  The 
  wide 
  variation 
  in 
  composition 
  of 
  the 
  two 
  samples 
  is 
  

   striking, 
  but 
  no 
  more 
  than 
  might 
  result 
  from 
  variations 
  in 
  the 
  

   complicated 
  processes 
  of 
  alteration. 
  The 
  Old 
  Sterling 
  rock 
  has, 
  

   perhaps, 
  the 
  most 
  remarkable 
  composition, 
  considering 
  its 
  origin, 
  

   and 
  points 
  clearly 
  to 
  ferruginous 
  and 
  magnesian 
  solutions 
  as 
  the 
  

   agent 
  of 
  change. 
  As 
  shown 
  by 
  analysis 
  I, 
  it 
  has 
  lost 
  silica 
  and 
  

   alkalies, 
  and 
  has 
  taken 
  up 
  water, 
  magnesia 
  and 
  a 
  remarkable 
  

   amount 
  of 
  iron. 
  In 
  the 
  case 
  of 
  the 
  Caledonia 
  rock 
  the 
  changes 
  

   are 
  less 
  extensive, 
  though 
  still 
  marked. 
  Hydration, 
  loss 
  of 
  silica 
  

   and 
  relative 
  gain 
  of 
  alumina 
  are 
  shown, 
  while 
  there 
  is 
  a 
  notable 
  

   retention 
  of 
  alkali. 
  This 
  is 
  unusual 
  as 
  the 
  alkalies 
  are 
  .generally 
  

   among 
  the 
  first 
  constituents 
  removed 
  in 
  the 
  alteration 
  of 
  such 
  

   rocks. 
  No 
  cause 
  for 
  the 
  exception 
  in 
  the 
  present 
  case 
  has 
  yet 
  

   been 
  found. 
  

  

  The 
  altered 
  rocks 
  show 
  much 
  evidence 
  of 
  having 
  been 
  subjected 
  

   to 
  pressure, 
  with 
  resulting 
  shearing. 
  That 
  this* 
  has 
  aided 
  the 
  

   process 
  of 
  alteration, 
  particularly 
  by 
  crushing 
  the 
  constituents, 
  

   the 
  facts 
  seem 
  to 
  indicate. 
  Thus, 
  at 
  the 
  Caledonia 
  mine 
  where 
  

   the 
  quartz 
  has 
  almost 
  entirely 
  disappeared, 
  the 
  rock 
  is 
  decidedly 
  

   slaty, 
  which 
  has 
  given 
  rise 
  to 
  the 
  idea 
  that 
  it 
  is 
  an 
  altered 
  sedi- 
  

   ment. 
  It 
  is 
  probable 
  that 
  the 
  rock 
  movements 
  producing 
  the 
  

   slickensides 
  at 
  the 
  Old 
  Sterling 
  mine 
  have, 
  in 
  large 
  part, 
  resulted 
  

   from 
  change 
  of 
  bulk 
  attendant 
  upon 
  chemical 
  alteration, 
  as 
  sug- 
  

   gested 
  by 
  Diller* 
  in 
  the 
  case 
  of 
  serpentine. 
  

  

  As 
  to 
  the 
  mineralogical 
  affinities 
  of 
  the 
  two 
  rocks, 
  the 
  Cale- 
  

   donia 
  variety 
  is 
  related 
  to 
  muscovite 
  in 
  composition, 
  and 
  Dana 
  

   gives 
  an 
  analysis 
  of 
  it 
  among 
  the 
  varieties 
  of 
  pinite. 
  The 
  Old 
  

  

  * 
  J. 
  S. 
  Diller, 
  Geology 
  of 
  the 
  Lassen 
  Peak 
  District, 
  8th 
  Ann. 
  Report, 
  U. 
  S. 
  GK 
  S., 
  I, 
  p. 
  401. 
  

  

  