﻿460 
  Darton 
  and 
  Kemp 
  — 
  Newly 
  Discovered 
  Dike 
  at 
  

  

  abundant 
  at 
  Dewitt, 
  while 
  augite, 
  even 
  in 
  the 
  ground-mass, 
  is 
  

   absent 
  in 
  the 
  Kentucky 
  occurrence. 
  It 
  is 
  practically 
  the 
  same 
  

   as 
  the 
  peridotite 
  of 
  Pike 
  Co., 
  Ark., 
  referred 
  to 
  above, 
  and 
  both 
  

   Williams 
  in 
  the 
  original 
  papers 
  on 
  the 
  Syracuse 
  occurrence, 
  

   and 
  Brackett 
  in 
  the 
  one 
  earlier 
  cited, 
  on 
  the 
  Arkansas 
  expos- 
  

   ure, 
  were 
  abundantly 
  justified 
  in 
  placing 
  these 
  rocks 
  with 
  

   Lewis' 
  kimberlite* 
  from 
  South 
  Africa. 
  The 
  writer 
  has 
  com- 
  

   pared 
  the 
  Dewitt 
  rock 
  with 
  slides 
  of 
  all 
  the 
  American 
  related 
  

   dikes, 
  and 
  with 
  others 
  of 
  the 
  dike 
  in 
  the 
  De 
  Beers 
  mine 
  of 
  

   South 
  Africa. 
  It 
  is 
  practically 
  the 
  same 
  rock 
  as 
  the 
  last, 
  

   except 
  that 
  in 
  the 
  specimens 
  at 
  hand, 
  the 
  latter 
  appears 
  to 
  

   have 
  had 
  a 
  glassy 
  ground-mass 
  now 
  devitrified, 
  which 
  lacks 
  

   augite. 
  The 
  writer 
  is 
  in 
  thorough 
  sympathy 
  with 
  the 
  growing 
  

   opinion, 
  that 
  rocks 
  should 
  be 
  classified 
  on 
  texture, 
  and, 
  broadly^ 
  

   speaking, 
  into 
  granitoid, 
  porphyritic 
  and 
  glassy 
  groups: 
  that 
  

   dikes 
  should 
  be 
  referred 
  to 
  their 
  nearest 
  granitoid 
  or 
  porphy- 
  

   ritic 
  relatives, 
  and 
  called 
  by 
  their 
  names. 
  Taking 
  plutonic 
  

   rocks 
  as 
  practically 
  the 
  granitoid, 
  and 
  volcanic 
  as 
  the 
  porphy- 
  

   ritic, 
  the 
  Dewitt 
  rock 
  is 
  a 
  basaltic 
  dike 
  of 
  the 
  same 
  composi- 
  

   tion 
  and 
  texture 
  as 
  limburgite, 
  and 
  should 
  be 
  called 
  limburgite, 
  

   even 
  if 
  it 
  is 
  not 
  a 
  surface 
  flow. 
  It 
  would 
  probably 
  simplify 
  

   matters 
  in 
  a 
  commendable 
  degree 
  if 
  all 
  the 
  other 
  names 
  of 
  

   feldspar- 
  free, 
  olivine-bearing 
  dikes 
  with 
  a 
  glassy 
  ground-mass, 
  

   be 
  allowed 
  to 
  drop 
  out 
  of 
  use, 
  and 
  if 
  in 
  this 
  and 
  other 
  similar 
  

   cases, 
  large 
  resemblances, 
  rather 
  than 
  small 
  differences, 
  were 
  

   brought 
  out 
  in 
  our 
  nomenclature. 
  

  

  At 
  Mr. 
  Darton's 
  request, 
  the 
  following 
  analysis 
  was 
  made 
  of 
  

   the 
  Dewitt 
  rock, 
  in 
  the 
  laboratory 
  of 
  the 
  IT. 
  S. 
  Geological 
  

   Survey, 
  by 
  Dr. 
  H. 
  Stokes. 
  With 
  it 
  are 
  placed 
  analyses 
  of 
  the 
  

   Syracuse 
  serpentine, 
  made 
  by 
  T. 
  S. 
  Huntf 
  in 
  1858, 
  and 
  of 
  the 
  

   mica-peridotite 
  from 
  Crittenden 
  Co., 
  Ky., 
  made 
  by 
  W. 
  F. 
  

   Hillebrand 
  for 
  J. 
  S. 
  Diller.J 
  Although 
  an 
  analysis 
  of 
  the 
  badly- 
  

   weathered 
  Ithaca 
  dike 
  was 
  made 
  for 
  the 
  writer, 
  and 
  published, 
  

   it 
  is 
  here 
  omitted, 
  because 
  it 
  is 
  clearly 
  untrustworthy, 
  the 
  high 
  

   A1 
  2 
  3 
  and 
  low 
  MgO, 
  being 
  unlikely. 
  

  

  Mr. 
  Darton 
  collected 
  a 
  coarsely 
  crystalline 
  rock, 
  which 
  

   occurred 
  with 
  the 
  fragments 
  of 
  peridotite. 
  In 
  thin 
  sections 
  it 
  

   is 
  seen 
  to 
  contain 
  brown, 
  basaltic, 
  quite 
  idiomorphic 
  hornblende, 
  

   plagioclase, 
  one 
  large 
  untwinned 
  feldspar 
  with 
  parallel 
  extinc- 
  

   tion, 
  apparently 
  orthoclase, 
  and 
  many 
  quite 
  large 
  bits 
  of 
  mag- 
  

   netite. 
  This 
  is 
  probably 
  one 
  of 
  the 
  syenitic 
  accretions 
  of 
  Van- 
  

   uxem, 
  and 
  an 
  inclusion 
  of 
  wall 
  rock 
  in 
  the 
  peridotite, 
  brought 
  

   up 
  from 
  great 
  depth. 
  It 
  may 
  be, 
  it 
  should 
  also 
  be 
  stated, 
  a 
  

   drift-boulder, 
  as 
  it 
  was 
  found 
  with 
  the 
  loose, 
  blasted 
  peridotite, 
  

  

  *Geol. 
  Mag., 
  1887, 
  22. 
  

  

  f 
  This 
  Journal, 
  Sept., 
  1858, 
  237. 
  

  

  \ 
  Ibid., 
  Oct., 
  1892, 
  288. 
  

  

  