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  Scientific 
  Intelligence. 
  

  

  Structure 
  is 
  very 
  sensitive, 
  and 
  gives 
  the 
  history 
  of 
  a 
  rock, 
  

   either 
  alone 
  or 
  with 
  the 
  help 
  of 
  geological 
  observations. 
  (The 
  

   lecturer 
  then 
  spoke 
  of 
  different 
  kinds 
  of 
  metamorphism, 
  and 
  

   showed 
  that 
  structure 
  could 
  distinguish 
  between 
  the 
  different 
  

   kinds.) 
  The 
  essential 
  criterion 
  of 
  the 
  various 
  metamorphic 
  rocks 
  

   of 
  granitic 
  character 
  is 
  genesis, 
  not 
  structure 
  or 
  mineral 
  compo- 
  

   sition. 
  There 
  is 
  a 
  distinction 
  between 
  the 
  cataclastic 
  and 
  pro- 
  

   toclastic 
  structures, 
  the 
  latter 
  being 
  a 
  structure 
  like 
  cataclastic, 
  

   but 
  produced 
  during 
  solidification, 
  and 
  hence 
  is 
  primary. 
  These 
  

   should 
  be 
  distinguished 
  in 
  a 
  system 
  of 
  classification. 
  Structure 
  

   only 
  indicates 
  that 
  the 
  rock 
  has 
  been 
  pressed: 
  geological 
  observa- 
  

   tion 
  should 
  do 
  more, 
  and 
  indicate 
  whether 
  during 
  or 
  after 
  solidi- 
  

   fication. 
  

  

  Classification 
  must 
  go 
  back 
  to 
  geological 
  observations 
  as 
  a 
  

   fundamental 
  criterion. 
  

  

  In 
  regard 
  to 
  the 
  porphyritic 
  structure 
  it 
  is 
  believed 
  that 
  this 
  

   indicates 
  interrupted 
  movement 
  in 
  the 
  magma. 
  

  

  The 
  finer 
  distinctions 
  and 
  splitting 
  up 
  of 
  "sack 
  (catch-all) 
  

   names 
  " 
  will 
  separate 
  types 
  now 
  together. 
  That 
  it 
  is 
  possible 
  to 
  

   found 
  a 
  system 
  of 
  classification 
  on 
  abyssal, 
  hypabyssal 
  and 
  effu- 
  

   sive 
  rocks 
  is 
  proved 
  by 
  Rosenbusch. 
  For 
  many 
  years 
  petrog- 
  

   raphy 
  was 
  a 
  science 
  of 
  specimens 
  and 
  thin 
  sections, 
  bnt 
  it 
  should 
  

   be 
  more 
  than 
  this 
  and 
  should 
  be 
  living. 
  Of 
  all 
  criteria 
  geolog- 
  

   ical 
  observation 
  is 
  first, 
  since 
  it 
  gives 
  the 
  history 
  and 
  changes 
  a 
  

   dead 
  science 
  to 
  a 
  living 
  one, 
  showing 
  evolution 
  of 
  one 
  rock 
  from 
  

   another. 
  

  

  In 
  his 
  second 
  lecture 
  the 
  subject 
  was 
  continued 
  essentially 
  as 
  

   follows 
  : 
  It 
  is 
  well 
  known 
  that 
  rocks 
  of 
  different 
  chemical 
  and 
  

   mineral 
  composition 
  occur 
  together, 
  so 
  that 
  they 
  are 
  genetically 
  

   related. 
  It 
  is 
  only 
  within 
  the 
  last 
  few 
  years, 
  from 
  elaborate 
  

   studies 
  made 
  by 
  different 
  petrographers, 
  that 
  an 
  explanation 
  of 
  

   differentiation 
  has 
  been 
  brought 
  out. 
  As 
  the 
  result 
  of 
  these 
  

   studies 
  the 
  various 
  kinds 
  of 
  differentiated 
  masses 
  may 
  be 
  classi- 
  

   fied 
  as 
  follows 
  : 
  

  

  1. 
  Composite 
  dikes. 
  

  

  2. 
  Complementary 
  dikes. 
  

  

  3. 
  Constitutional 
  (e. 
  g., 
  marginal) 
  facies. 
  

  

  4. 
  Connected 
  occurrences 
  (e. 
  g., 
  leucocratic 
  with 
  melanocratic 
  

   rocks). 
  

  

  5. 
  Dike 
  associations 
  (Ganggefolge). 
  

  

  6. 
  Petrographical 
  provinces. 
  

  

  7. 
  Original 
  earth 
  magma, 
  from 
  which 
  others 
  are 
  derived. 
  (The 
  

   latter 
  is 
  probable, 
  but 
  the 
  arguments 
  in 
  favor 
  of 
  it 
  cannot 
  be 
  dis- 
  

   cussed 
  here.) 
  

  

  Classification 
  should 
  be 
  based 
  in 
  the 
  first 
  instance 
  on 
  chemical 
  

   composition. 
  Geological 
  observations 
  prove 
  the 
  essential 
  connec- 
  

   tion 
  of 
  the 
  chemical 
  composition 
  of 
  igneous 
  rocks. 
  The 
  essen- 
  

   tial 
  features 
  of 
  classes 
  1-6 
  are 
  geological 
  and 
  independent 
  of 
  

   theory. 
  On 
  these 
  facts 
  a 
  system 
  of 
  classification 
  should 
  be 
  based. 
  

  

  