﻿Becker 
  — 
  Some 
  Queries 
  on 
  Rock 
  Differentiation. 
  21 
  

  

  Art. 
  III. 
  — 
  Some 
  Queries 
  on 
  Rock 
  Differentiation; 
  by 
  

   Geo. 
  F. 
  Becker. 
  

  

  Hypothesis 
  of 
  differentiation. 
  — 
  As 
  I 
  understand 
  the 
  theory 
  

   of 
  what 
  is 
  called 
  the 
  differentiation 
  of 
  rock 
  magmas, 
  now 
  so 
  

   generally 
  held 
  by 
  lithologists, 
  its 
  outlines 
  may 
  be 
  expressed 
  in 
  

   the 
  following 
  terms 
  : 
  In 
  some 
  extensive 
  districts 
  the 
  massive 
  

   rocks 
  are 
  found 
  to 
  possess 
  similarities 
  of 
  composition, 
  and 
  such 
  

   rocks 
  have 
  been 
  called 
  consanguineous. 
  This 
  consanguinity 
  

   might 
  be 
  accounted 
  for 
  by 
  supposing 
  an 
  originally 
  homogene- 
  

   ous 
  magma 
  to 
  have 
  undergone 
  partial 
  segregation 
  into 
  fluid 
  

   portions 
  of 
  distinctly 
  different 
  yet 
  allied 
  composition, 
  prior 
  to 
  

   eruption, 
  and 
  this 
  is 
  the 
  process 
  called 
  differentiation. 
  It 
  is 
  

   also 
  known 
  by 
  observation 
  in 
  the 
  laboratory, 
  that 
  if 
  a 
  more 
  or 
  

   less 
  complex, 
  homogeneous 
  solution 
  be 
  exposed 
  to 
  certain 
  

   physical 
  conditions, 
  segregation 
  into 
  distinct 
  portions 
  may 
  take 
  

   place. 
  It 
  is 
  hence 
  inferred 
  that 
  such 
  is 
  actually 
  the 
  history 
  of 
  

   consanguineous 
  rocks.* 
  By 
  a 
  slight 
  extension 
  of 
  this 
  inference 
  

   most 
  massive 
  rocks 
  are 
  regarded 
  as 
  resulting 
  by 
  differentiation 
  

   from 
  a 
  generalized 
  magma. 
  

  

  The 
  existence 
  of 
  distinct 
  though 
  allied 
  rocks 
  locally 
  asso- 
  

   ciated 
  with 
  transitional 
  varieties 
  is 
  undisputed. 
  The 
  validity 
  

   of 
  the 
  explanation 
  offered 
  by 
  the 
  modern 
  school 
  for 
  these 
  

   occurrences 
  is 
  another 
  matter. 
  The 
  hypothesis 
  of 
  differentia- 
  

   tion 
  is 
  extremely 
  attractive 
  and 
  if 
  it 
  were 
  substantiated 
  would 
  

   lead 
  to 
  a 
  well 
  organized 
  system 
  of 
  rock 
  investigation. 
  It 
  may 
  

   also 
  be 
  correct 
  ; 
  but 
  there 
  are 
  respects 
  in 
  which 
  it 
  appears 
  to 
  

   be 
  in 
  need 
  of 
  much 
  explanation, 
  and 
  it 
  does 
  not 
  seem 
  certain 
  

   that 
  the 
  fundamental 
  postulate 
  of 
  originally 
  homogeneous 
  

   magmas 
  of 
  vast 
  volume 
  is 
  well 
  established. 
  

  

  Possible 
  modes 
  of 
  segregation. 
  — 
  The 
  segregationf 
  of 
  a 
  

   homogeneous 
  fluid 
  into 
  distinguishable 
  portions 
  under 
  the 
  

   influence 
  of 
  varying 
  temperature 
  or 
  pressure 
  may 
  take 
  place 
  

   by 
  different 
  methods. 
  An 
  increase 
  or 
  decrease 
  in 
  the 
  concen- 
  

   tration 
  of 
  certain 
  components 
  may 
  occur 
  in 
  the 
  cooler 
  part 
  of 
  

   the 
  fluid 
  or 
  in 
  that 
  portion 
  which 
  is 
  under 
  greatest 
  pressure. 
  

   There 
  are 
  also 
  cases 
  in 
  which 
  solutions 
  which 
  are 
  homogeneous 
  

  

  * 
  The 
  early 
  literature 
  bearing 
  on 
  this 
  subject, 
  together 
  with 
  fresh 
  contributions, 
  

   is 
  given 
  in 
  Mr. 
  Arnold 
  Hague's 
  work 
  on 
  the 
  Geology 
  of 
  the 
  Eureka 
  district, 
  U. 
  S. 
  

   G-eol. 
  Survey, 
  Mon. 
  20, 
  1892, 
  p. 
  267 
  et 
  seq. 
  Some 
  of 
  Prof. 
  J. 
  P. 
  Iddings' 
  papers 
  

   bearing 
  on 
  the 
  subject 
  are: 
  the 
  Crystallization 
  of 
  Igneous 
  Rocks, 
  1889; 
  Electric 
  

   peak 
  and 
  Sepulchre 
  mountain, 
  1891 
  ; 
  the 
  Origin 
  of 
  Igneous 
  Rocks. 
  1892, 
  etc. 
  

   The 
  latest 
  of 
  Prof. 
  W. 
  C. 
  Brogger's 
  contributions 
  is 
  : 
  die 
  Eruptionsfolge 
  der 
  iriad- 
  

   schen 
  Eruptivgesteiue 
  bei 
  Predazzo, 
  1895. 
  

  

  f 
  The 
  term 
  differentiation 
  is 
  ambiguous. 
  Tn 
  the 
  older, 
  and 
  as 
  it 
  seems 
  to 
  me 
  

   the 
  more 
  proper 
  sense, 
  differentiation 
  is 
  the 
  discrimination 
  of 
  existing 
  differences. 
  

   One 
  differentiates 
  lime-soda 
  feldspars 
  by 
  their 
  angles 
  of 
  extinction. 
  

  

  