﻿Geology. 
  457 
  

  

  These 
  lectures 
  were 
  given 
  between 
  Wednesday, 
  April 
  25, 
  and 
  

   Thursday, 
  May 
  3. 
  They 
  were 
  delivered 
  in 
  English 
  and 
  before 
  

   good 
  audiences 
  ; 
  a 
  number 
  of 
  visiting 
  geologists 
  from 
  Washington 
  

   and 
  elsewhere 
  being 
  present. 
  In 
  his 
  first 
  two 
  lectures 
  the 
  speaker 
  

   said 
  essentially 
  as 
  follows, 
  after 
  referring 
  to 
  the 
  particular 
  cir- 
  

   cumstances 
  under 
  which 
  the 
  course 
  of 
  lectures 
  was 
  given 
  : 
  

  

  There 
  has 
  been 
  evolution 
  in 
  classification, 
  from 
  the 
  early 
  pre- 
  

   Linnean 
  and 
  Linnean 
  systems 
  of 
  botany 
  and 
  zoology 
  to 
  the 
  nat- 
  

   ural 
  systems 
  of 
  the 
  present 
  day. 
  Classification 
  of 
  petrography 
  

   should 
  (or 
  will) 
  follow 
  the 
  same 
  course. 
  In 
  the 
  case 
  of 
  rocks 
  there 
  

   is 
  a 
  constant 
  effort 
  to 
  reach 
  a 
  natural 
  system, 
  i. 
  e., 
  a 
  genetic 
  one. 
  

   The 
  aim 
  of 
  classification 
  is 
  to 
  group 
  objects 
  by 
  their 
  essential 
  

   characters. 
  What 
  are 
  then 
  the 
  essential 
  characters 
  in 
  rocks 
  ? 
  

   Primarily 
  they 
  are 
  geological 
  bodies, 
  and 
  therefore 
  their 
  classifi- 
  

   cation 
  should 
  be 
  based 
  on 
  geological 
  observations 
  and 
  geological 
  

   relationships. 
  Rocks 
  produced 
  by 
  the 
  same 
  processes 
  should 
  be 
  

   grouped 
  together. 
  The 
  name 
  diorite 
  is 
  essentially 
  genetic 
  be- 
  

   cause 
  the 
  idea 
  is 
  implied 
  that 
  it 
  is 
  igneous, 
  next 
  because 
  the 
  place 
  

   of 
  consolidation 
  is 
  indicated, 
  and 
  finally 
  because 
  it 
  is 
  understood 
  

   that 
  it 
  must 
  have 
  originated 
  primarily 
  from 
  the 
  magma. 
  

  

  The 
  present 
  system 
  is 
  founded 
  largely 
  on 
  accidental 
  characters 
  ; 
  

   thus, 
  for 
  example, 
  minettes 
  and 
  leucite-basanites 
  are 
  chemically 
  

   identical, 
  but 
  are 
  separated 
  and 
  put 
  in 
  different 
  families. 
  The 
  

   present 
  system 
  is 
  not 
  a 
  natural 
  but 
  an 
  artificial 
  one, 
  comparable 
  

   to 
  the 
  Linnean 
  system 
  of 
  botany. 
  

  

  Petrographical 
  classification 
  began 
  in 
  1811 
  with 
  Hatiy, 
  and 
  

   was 
  based 
  on 
  mineral 
  composition. 
  This 
  has 
  predominated 
  ever 
  

   since 
  and 
  the 
  consequence 
  is 
  that 
  like 
  rocks 
  are 
  separated 
  and 
  

   unlike 
  put 
  together, 
  so 
  for 
  example 
  the 
  granites 
  and 
  granitites, 
  

   aagite-andesites 
  and 
  hornblende-andesites 
  are 
  separated, 
  while 
  

   nephelinite 
  embraces 
  many 
  different 
  rocks. 
  A 
  purely 
  mineral 
  

   basis 
  is 
  quite 
  insufficient. 
  In 
  1875 
  Lossing, 
  in 
  a 
  memoir 
  on 
  the 
  

   Bodegang 
  in 
  the 
  Hartz, 
  established 
  the 
  connection 
  between 
  quartz- 
  

   porphyry 
  and 
  granite. 
  This 
  was 
  of 
  the 
  highest 
  importance. 
  He 
  

   showed 
  that 
  both 
  were 
  only 
  structural 
  modifications 
  of 
  the 
  same 
  

   magma, 
  and 
  therefore 
  the 
  structure 
  was 
  a 
  function 
  of 
  the 
  condi- 
  

   tions 
  of 
  the 
  genesis 
  of 
  the 
  rock. 
  Structure 
  has 
  since 
  then 
  been 
  

   maintained 
  by 
  Rosenbusch 
  and 
  others 
  as 
  the 
  main 
  basis 
  of 
  classi- 
  

   fication. 
  Thus 
  Rosenbusch's 
  three 
  main 
  groups 
  (abyssal, 
  dike 
  

   and 
  effusive 
  rocks) 
  are 
  well 
  founded 
  types, 
  characterized 
  by 
  

   special 
  structures. 
  It 
  is 
  urged 
  by 
  opponents 
  that 
  this 
  is 
  purely 
  

   hypothetical, 
  but 
  this 
  is 
  based 
  on 
  a 
  misapprehension, 
  for 
  Rosen- 
  

   busch's 
  classification 
  is 
  not 
  founded 
  on 
  hypothesis, 
  but 
  on 
  geolog- 
  

   ical 
  observation 
  of 
  facts, 
  which 
  have 
  been 
  made 
  once 
  for 
  all. 
  

   Exact 
  definitions 
  are 
  of 
  course 
  very 
  hard 
  to 
  give, 
  but 
  this 
  is 
  

   always 
  so 
  in 
  nature, 
  where 
  there 
  are 
  no 
  sharp 
  limits. 
  While 
  the 
  

   same 
  chemically, 
  granite, 
  quartz-porphyry 
  and 
  liparite 
  are 
  differ- 
  

   ent 
  structurally 
  and 
  genetically 
  and 
  a 
  system 
  of 
  classification 
  

   should 
  express 
  these 
  differences. 
  

  

  