﻿426 
  Ransome 
  — 
  New 
  Occurrence 
  of 
  Nepheline 
  Syenite. 
  

  

  to 
  crystallize, 
  and 
  is 
  very 
  subordinate 
  in 
  amount. 
  It 
  is 
  some- 
  

   times 
  micropegmatitically 
  intergrown 
  with 
  a 
  small 
  amount 
  of 
  

   a 
  dusty 
  alkali 
  feldspar, 
  probably 
  orthoclase. 
  The 
  hypersthene 
  

   is 
  not 
  always 
  present, 
  and 
  its 
  place 
  is 
  sometimes 
  taken 
  by 
  

   hornblende, 
  as 
  in 
  a 
  specimen 
  from 
  Barbour 
  cfe 
  Ireland's 
  quarry. 
  

   The 
  structure 
  on 
  the 
  whole 
  is 
  granular 
  rather 
  than 
  ophitic, 
  and 
  

   the 
  name 
  gabbro 
  is 
  an 
  appropriate 
  designation 
  for 
  the 
  trap- 
  

   rock 
  as 
  a 
  whole. 
  

  

  The 
  Relation 
  of 
  the 
  Syenitlc 
  Rocks 
  to 
  the 
  Gabbro. 
  

  

  No 
  evidence 
  was 
  detected, 
  either 
  in 
  the 
  field 
  or 
  through 
  

   microscopical 
  investigation, 
  which 
  in 
  any 
  way 
  supports 
  the 
  

   hypothesis 
  that 
  the 
  alkali-rich 
  rocks 
  are 
  differentiated 
  facies 
  of, 
  

   or 
  immediately 
  derived 
  from 
  the 
  body 
  of 
  magma 
  which 
  cooled 
  

   as 
  gabbroitic 
  trap. 
  The 
  former 
  are 
  distinctly 
  different 
  from 
  

   the 
  trap 
  which 
  surrounds 
  them, 
  and 
  transitional 
  facies 
  appear 
  

   to 
  be 
  wholly 
  lacking. 
  The 
  trap 
  as 
  a 
  whole 
  show^s 
  rather 
  strik- 
  

   ing 
  uniformity 
  wherever 
  studied. 
  Specimens, 
  wherever 
  col- 
  

   lected, 
  are 
  characteristically 
  trap-like, 
  and 
  afford 
  no 
  suggestion 
  

   of 
  such 
  extreme 
  differentiation 
  as 
  would 
  be 
  necessary 
  for 
  the 
  

   production 
  of 
  a 
  nepheline 
  syenite, 
  a 
  mica 
  syenite, 
  or 
  a 
  horn- 
  

   blende 
  granite. 
  The 
  syenitic 
  masses, 
  therefore, 
  are 
  either 
  

   small 
  dike-like 
  intrusions 
  in 
  the 
  trap, 
  or 
  else 
  they 
  are 
  inclusions 
  

   in 
  the 
  latter, 
  floated 
  up 
  from 
  some 
  unknown 
  terrane 
  below, 
  

   through 
  w^hich 
  the 
  trap 
  broke. 
  The 
  exposures 
  were 
  not 
  suffi- 
  

   ciently 
  good 
  to 
  determine 
  which 
  of 
  these 
  hypotheses 
  is 
  the 
  true 
  

   one. 
  That 
  the 
  masses 
  are 
  dikes, 
  would 
  seem 
  to 
  be 
  most 
  

   inherently 
  probable. 
  Yet 
  dikes 
  might 
  be 
  expected 
  to 
  show 
  

   more 
  linear 
  persistency 
  than 
  could 
  here 
  be 
  detected, 
  although 
  

   the 
  exposures 
  were 
  poor 
  and 
  the 
  time 
  given 
  to 
  their 
  examina- 
  

   tion 
  somewhat 
  brief. 
  On 
  the 
  whole 
  it 
  seems 
  most 
  likel}^, 
  in 
  

   this 
  case, 
  that 
  the 
  masses 
  are 
  included 
  fragments, 
  caught 
  up 
  in 
  

   the 
  trap 
  magma 
  at 
  the 
  time 
  of 
  its 
  intrusion. 
  It 
  must 
  be 
  

   admitted, 
  however, 
  that 
  the 
  question 
  of 
  their 
  origin 
  is 
  still 
  an 
  

   open 
  one, 
  which 
  further 
  work, 
  with 
  other 
  and 
  better 
  exposures, 
  

   may 
  answer. 
  

  

  Washington, 
  D. 
  C, 
  June, 
  1899. 
  

  

  