﻿458 
  Darton 
  and 
  Kemp 
  — 
  Newly 
  Discovered 
  Dike 
  at 
  

  

  earlier 
  paper. 
  Dr. 
  Williams 
  also 
  found 
  abundant 
  inclusions 
  — 
  

   doubtless 
  Vanuxem's 
  "accretions" 
  — 
  even 
  of 
  the 
  acidic 
  crystal- 
  

   lines 
  on 
  which 
  the 
  sedimentary 
  series 
  must 
  rest. 
  It 
  is 
  evident, 
  

   however, 
  from 
  comparative 
  remarks 
  made 
  upon 
  the 
  peridotite 
  

   described 
  by 
  R. 
  N. 
  Brackett,* 
  from 
  Pike 
  Co., 
  Ark., 
  that 
  

   abundant 
  and 
  unaltered 
  little 
  augites 
  in 
  the 
  ground-mass 
  were 
  

   also 
  noted. 
  

  

  It 
  would 
  appear 
  that 
  in 
  some 
  respects 
  the 
  material 
  collected 
  

   by 
  Mr. 
  Darton 
  is 
  in 
  an 
  even 
  fresher 
  and 
  less 
  altered 
  state 
  than 
  

   any 
  yet 
  examined, 
  and 
  as 
  it 
  occurs 
  some 
  three 
  miles 
  from 
  the 
  

   former 
  locality, 
  a 
  few 
  additional 
  notes 
  are 
  not 
  out 
  of 
  place. 
  

   The 
  writer 
  is 
  fortunate 
  in 
  having 
  had 
  for 
  comparison 
  some 
  of 
  

   the 
  original 
  specimens 
  collected 
  by 
  Prof. 
  Oren 
  Root, 
  the 
  dis- 
  

   coverer 
  of 
  the 
  outcrop, 
  and 
  also 
  a 
  representative 
  set 
  of 
  pieces 
  

   from 
  Dr. 
  Williams's 
  collection, 
  given 
  him 
  by 
  the 
  latter 
  in 
  1889. 
  

   Comparisons 
  have 
  also 
  been 
  made 
  with 
  slides 
  of 
  some 
  other 
  

   allied 
  rocks, 
  as 
  indicated 
  below. 
  

  

  The 
  Dewitt 
  rock 
  belongs 
  to 
  the 
  porphyritic 
  type 
  of 
  Williams. 
  

   While 
  in 
  some 
  specimens 
  much 
  altered, 
  yet 
  in 
  others 
  it 
  contains 
  

   olivine, 
  as 
  fresh 
  and 
  unchanged 
  as 
  if 
  it 
  had 
  come 
  from 
  the 
  most 
  

   recent 
  of 
  basalts. 
  Almost 
  no 
  traces 
  of 
  serpentinization 
  are 
  

   present 
  in 
  some 
  of 
  the 
  slides. 
  In 
  addition 
  to 
  the 
  olivine, 
  

   whose 
  crystals 
  vary 
  from 
  l 
  mm 
  to 
  8 
  mm 
  in 
  diameter, 
  the 
  only 
  

   other 
  large 
  phenocrysts 
  are 
  biotite 
  and 
  one 
  or 
  two 
  crystals 
  of 
  

   augite. 
  In 
  the 
  ground-mass 
  are 
  innumerable 
  small 
  augites, 
  

   which 
  seem 
  to 
  have 
  made 
  it 
  up 
  in 
  largest 
  amount, 
  shreds 
  of 
  

   biotite, 
  magnetite, 
  apatite 
  and 
  perofskite. 
  It 
  is 
  probable 
  that 
  

   there 
  was 
  also 
  an 
  original 
  glass, 
  now 
  mostly 
  devitrih'ed 
  by 
  alter- 
  

   ation. 
  

  

  The 
  olivine 
  is 
  often 
  idiomorphic, 
  and 
  the 
  elongated, 
  lozenge- 
  

   shaped 
  cross-sections 
  are 
  common. 
  It 
  is 
  practically 
  colorless. 
  

   The 
  figure 
  given 
  on 
  page 
  142 
  of 
  Dr. 
  Williams's 
  paper, 
  would 
  

   answer 
  excellently 
  for 
  the 
  new 
  occurrence. 
  The 
  phenocrysts 
  

   of 
  biotite 
  are 
  smaller, 
  l 
  mm 
  being 
  the 
  general 
  diameter. 
  They 
  

   are 
  hexagonal, 
  and 
  the 
  outer 
  portions 
  are 
  thickly 
  set 
  with 
  

   included 
  grains 
  of 
  magnetite. 
  The 
  color 
  is 
  the 
  usual 
  rich 
  

   brown 
  of 
  the 
  biotite 
  in 
  basic 
  rocks, 
  and 
  there 
  is 
  a 
  slight 
  separa- 
  

   tion 
  of 
  the 
  optic 
  axes. 
  The 
  augite 
  is 
  a 
  rather 
  rare 
  pbenocryst, 
  

   but 
  two 
  or 
  three 
  crystals 
  having 
  been 
  seen 
  in 
  a 
  half 
  dozen 
  slides. 
  

   It 
  is, 
  however, 
  well 
  marked, 
  has 
  an 
  extinction 
  ranging 
  from 
  30°- 
  

   40°, 
  and 
  is 
  perfectly 
  fresh. 
  

  

  The 
  little 
  rods 
  of 
  augite 
  in 
  the 
  ground-mass 
  are 
  very 
  small, 
  

   •05 
  mm 
  or 
  less 
  in 
  diameter, 
  and 
  perhaps 
  twice 
  as 
  long. 
  They 
  are 
  

   faint 
  green, 
  have 
  a 
  high 
  extinction, 
  and 
  are 
  normal 
  in 
  their 
  

   properties. 
  The 
  ground-mass 
  is 
  practically 
  like 
  that 
  of 
  the 
  basic 
  

  

  * 
  This 
  Journal, 
  July, 
  1889, 
  p. 
  57, 
  second 
  paragraph 
  and 
  top 
  of 
  p. 
  59. 
  

  

  