﻿290 
  E. 
  0. 
  Hovey 
  — 
  Acid 
  Dike 
  in 
  the 
  Triassic 
  area. 
  

  

  4:a 
  cannot 
  be 
  offshoots 
  from 
  No. 
  5. 
  on 
  account 
  of 
  the 
  difference 
  

   in 
  microscopical 
  structure 
  and 
  mineral 
  ogical 
  composition. 
  

  

  Petrography. 
  — 
  The 
  hand 
  specimen 
  from 
  dike 
  No. 
  4 
  repre- 
  

   sents 
  the 
  whole 
  breadth 
  of 
  the 
  dike 
  at 
  the 
  place 
  from 
  which 
  it 
  

   was 
  taken 
  and 
  is 
  from 
  4 
  to 
  4'5 
  cm 
  wide. 
  The 
  texture 
  is 
  apha- 
  

   nitic 
  at 
  the 
  sides 
  and 
  somewhat 
  coarser 
  in 
  the 
  middle, 
  some 
  of 
  

   the 
  larger 
  feldspar 
  phenocrysts 
  and 
  many 
  areas 
  of 
  calcite 
  and 
  

   chlorite 
  being 
  discernible 
  with 
  an 
  ordinary 
  hand 
  lens. 
  Under 
  

   the 
  microscope 
  the 
  gradual 
  increase 
  in 
  coarseness 
  from 
  wall 
  to 
  

   center 
  is 
  well 
  shown. 
  The 
  rock 
  consists 
  essentially 
  of 
  feld- 
  

   spar. 
  There 
  are 
  two 
  sets 
  of 
  phenocrysts 
  of 
  feldspar. 
  One 
  is 
  

   made 
  up 
  of 
  relatively 
  large 
  tabular, 
  cuboidal 
  and 
  lath-shaped 
  

   twinned 
  crystals 
  which 
  tend 
  to 
  form 
  groups 
  of 
  twos 
  and 
  threes. 
  

   The 
  angles 
  of 
  extinction 
  measured 
  on 
  the 
  twinning 
  trace, 
  18° 
  

   to 
  19° 
  on 
  each 
  side, 
  and 
  the 
  predominance 
  of 
  soda 
  shown 
  by 
  

   the 
  chemical 
  analysis 
  -indicate 
  the 
  probability 
  of 
  these 
  being 
  

   albite. 
  The 
  other 
  and 
  by 
  far 
  the 
  larger 
  set 
  of 
  feldspar 
  pheno- 
  

   crysts 
  consists 
  of 
  minute 
  acicular 
  crystals, 
  which 
  are 
  scattered 
  

   very 
  thickly 
  through 
  the 
  section 
  where 
  it 
  cuts 
  the 
  middle 
  of 
  

   the 
  dike, 
  but 
  become 
  less 
  numerous 
  toward 
  the 
  sides. 
  These 
  

   minute 
  crystals 
  seem 
  to 
  be 
  simple 
  (not 
  twinned) 
  and 
  are 
  

   referred 
  to 
  anorthoclase, 
  on 
  account 
  of 
  the 
  chemical 
  analysis. 
  

   They 
  show 
  a 
  tendency 
  toward 
  prolongation 
  of 
  the 
  corners, 
  pro- 
  

   ducing 
  forms 
  not 
  unlike 
  those 
  of 
  orthoclase 
  described 
  by 
  J. 
  P. 
  

   Tddings* 
  from 
  the 
  rhyolite 
  of 
  Pinto 
  Peak, 
  in 
  the 
  Eureka 
  dis- 
  

   trict, 
  Nevada, 
  in 
  which 
  the 
  extensions 
  do 
  not 
  diverge. 
  

  

  The 
  relations 
  of 
  the 
  two 
  sets 
  of 
  feldspar 
  phenocrysts 
  seem 
  

   to 
  indicate 
  that 
  the 
  groups 
  of 
  relatively 
  large 
  crystals 
  (albite) 
  

   are 
  older 
  than 
  the 
  others. 
  The 
  albite 
  areas 
  also 
  show 
  a 
  few 
  

   inclusions 
  and 
  some 
  alteration. 
  

  

  The 
  ferro-magnesian 
  minerals 
  are 
  conspicuous 
  by 
  their 
  

   absence 
  from 
  the 
  thin 
  sections 
  studied, 
  but 
  the 
  areas 
  of 
  sec- 
  

   ondary 
  calcite 
  and 
  chlorite 
  for 
  the 
  most 
  part 
  have 
  definite 
  out- 
  

   lines 
  which 
  strongly 
  suggest 
  the 
  original 
  presence 
  of 
  pheno- 
  

   crysts 
  of 
  pyroxene 
  (angite) 
  in 
  the 
  rock. 
  In 
  some 
  instances 
  

   these 
  areas 
  are 
  partly 
  penetrated 
  by 
  the 
  acicular 
  crystals 
  of 
  

   feldspar, 
  somewhat 
  after 
  the 
  manner 
  of 
  diabase 
  angite 
  and 
  

   feldspar. 
  

  

  The 
  base 
  or 
  groundmass 
  is 
  cryptocrystalline 
  and 
  is 
  made 
  up 
  

   for 
  the 
  most 
  part 
  of 
  crystals 
  like 
  the 
  smaller 
  feldspar 
  pheno- 
  

   crysts. 
  SphaBrocrystalline 
  structure 
  was 
  observed 
  in 
  some 
  

   places. 
  Some 
  areas 
  are 
  not 
  resolved 
  by 
  a 
  No. 
  7 
  Fuess 
  objec- 
  

   tive, 
  but 
  may 
  be 
  devitrified 
  glass, 
  as 
  they 
  affect 
  polarized 
  light. 
  

  

  Chemistry. 
  — 
  A 
  chemical 
  analysis 
  of 
  this 
  peculiar 
  dike 
  was 
  

   very 
  kindly 
  made 
  for 
  the 
  writer 
  by 
  Dr. 
  H. 
  S. 
  Washington 
  with 
  

  

  * 
  Hague, 
  Geology 
  of 
  the 
  Eureka 
  District, 
  U. 
  S. 
  G-. 
  S., 
  Mon 
  xx, 
  p. 
  378, 
  pi. 
  iii, 
  

   fig. 
  U, 
  1892. 
  

  

  