﻿De 
  Witt, 
  near 
  Syracuse, 
  New 
  York. 
  459 
  

  

  dikes 
  called 
  monchiquite, 
  and 
  the 
  resemblance 
  is 
  very 
  close 
  in 
  

   this 
  respect 
  to 
  those 
  met 
  by 
  the 
  writer 
  on 
  Lake 
  Champlain.* 
  

  

  The 
  shreds 
  of 
  biotite 
  are 
  irregular 
  and 
  small. 
  It 
  is 
  not 
  certain 
  

   that 
  they 
  are 
  not, 
  in 
  large 
  part, 
  secondary. 
  One 
  vein 
  of 
  yel- 
  

   lowish-brown 
  biotite 
  was 
  found 
  running 
  across 
  a 
  thoroughly 
  

   serpentinized 
  olivine 
  crystal, 
  and 
  hence 
  must 
  have 
  been 
  second- 
  

   ary. 
  The 
  occurrence 
  casts 
  a 
  doubt 
  over 
  the 
  shreds 
  in 
  the 
  more 
  

   or 
  less 
  decomposed 
  ground-mass, 
  and 
  gives 
  ground 
  for 
  thinking 
  

   them 
  likewise 
  secondary. 
  The 
  magnetite 
  and 
  apatite 
  deserve 
  

   no 
  special 
  comment, 
  although 
  the 
  analysis 
  indicates 
  that 
  as 
  

   regards 
  the 
  former, 
  some 
  cliromite 
  is 
  also 
  present. 
  Dr. 
  Wil- 
  

   liams 
  came 
  to 
  the 
  conclusion 
  that 
  the 
  greater 
  part 
  of 
  the 
  black 
  

   opaque 
  grains 
  met 
  at 
  S^yracuse 
  were 
  chromite 
  ; 
  but 
  as 
  so 
  little 
  

   Cr 
  2 
  3 
  is 
  shown 
  by 
  the 
  analysis 
  of 
  the 
  Dewitt 
  material, 
  and 
  as 
  

   the 
  grains 
  are 
  quite 
  abundant, 
  and 
  the 
  rock 
  magnetic, 
  it 
  is 
  prob- 
  

   able 
  that 
  most 
  of 
  them 
  are 
  magnetite. 
  They 
  show 
  no 
  altera- 
  

   tion 
  to 
  leucoxene. 
  Xot 
  a 
  few 
  of 
  the 
  small 
  grains, 
  on 
  being 
  

   highly 
  magnified 
  in 
  a 
  strong 
  light, 
  are 
  seen 
  to 
  be 
  translucent 
  

   and 
  brown. 
  They 
  are 
  undoubtedly 
  perofskite, 
  and 
  a 
  close 
  par- 
  

   allel 
  to 
  the 
  occurrence 
  at 
  Syracuse. 
  The 
  translucency 
  was 
  not 
  

   detected 
  in 
  the 
  hasty 
  examination 
  made 
  b} 
  T 
  the 
  writer 
  prior 
  to 
  

   the 
  meeting 
  of 
  the 
  Geological 
  Society 
  of 
  America 
  in 
  Baltimore 
  

   last 
  December, 
  and 
  it 
  was 
  then 
  stated 
  verbally, 
  that 
  no 
  perof- 
  

   skite 
  had 
  been 
  detected. 
  The 
  minuteness 
  of 
  the 
  grains 
  and 
  their 
  

   high 
  refraction 
  led 
  to 
  this 
  erroneous 
  inference 
  from 
  study 
  with 
  

   low 
  powers. 
  The 
  web 
  of 
  apparently 
  devitrified 
  glass 
  in 
  which 
  

   these 
  small 
  crystals 
  of 
  the 
  ground-mass 
  are 
  caught, 
  is 
  an 
  unsat- 
  

   isfactory 
  subject 
  of 
  study. 
  Some 
  clear 
  patches 
  are 
  perfectly 
  

   isotropic, 
  while 
  others 
  show 
  irregular 
  spherulitic 
  crosses, 
  and 
  

   even 
  colors 
  of 
  the 
  first 
  order. 
  Where 
  the 
  network 
  of 
  small 
  

   augites 
  is 
  thick, 
  the 
  interstitial 
  masses 
  are 
  too 
  minute 
  to 
  be 
  sat- 
  

   isfactorily 
  studied. 
  Careful 
  search 
  was 
  made 
  for 
  melilite, 
  

   because 
  the 
  abundant 
  perofskite 
  and 
  the 
  interesting 
  occur- 
  

   rence 
  of 
  this 
  mineral 
  at 
  Manheim, 
  N. 
  Y., 
  described 
  by 
  0. 
  H. 
  

   Smyth, 
  Jr.,f 
  gave 
  some 
  ground 
  for 
  suspecting 
  it, 
  but 
  none 
  

   could 
  be 
  detected. 
  

  

  The 
  Dewitt 
  rock 
  might, 
  with 
  perfect 
  propriety, 
  be 
  called 
  a 
  

   picrite, 
  as 
  a 
  porphyritic 
  form 
  of 
  peridotite, 
  or 
  a 
  monchiquite 
  as 
  

   a 
  dike 
  rock 
  without 
  feldspar 
  and 
  containing 
  olivine, 
  there 
  

   being 
  no 
  real 
  need 
  for 
  both 
  these 
  names. 
  It 
  corresponds 
  to 
  

   picrite 
  as 
  used 
  by 
  Rosenbusch, 
  except 
  that 
  it 
  has 
  abundant 
  bio- 
  

   tite, 
  and 
  therefore 
  is 
  related 
  to 
  the 
  mica-peridotite 
  of 
  J. 
  S. 
  

   Diller,^: 
  from 
  the 
  very 
  similar 
  Flanary 
  dike 
  of 
  Crittenden 
  Co., 
  

   Ky., 
  but 
  biotite, 
  as 
  shown 
  by 
  a 
  comparison 
  of 
  slides, 
  is 
  less 
  

  

  * 
  Kemp 
  and 
  Marsters, 
  Bulletin 
  107, 
  U. 
  S. 
  Geological 
  Survey, 
  p. 
  33. 
  

   \ 
  This 
  Journal, 
  Aug., 
  1893, 
  p. 
  104. 
  

   \ 
  Ibid, 
  Oct., 
  1892, 
  p. 
  289. 
  

  

  