﻿200 
  Pirsson 
  — 
  JEgirite 
  Granite 
  from 
  MiasJc, 
  Ural 
  Mts. 
  

  

  latory 
  or 
  rolling 
  extinction 
  into 
  those 
  of 
  undoubted 
  microcline 
  

   twinning. 
  The 
  inference 
  is 
  not 
  far 
  that 
  this 
  twinning 
  has 
  

   been 
  produced 
  by 
  pressure, 
  and 
  the 
  microcline 
  is 
  therefore 
  not 
  

   an 
  original 
  constituent 
  but 
  a 
  paramorphic 
  one. 
  

  

  The 
  segirite 
  has 
  the 
  usual 
  properties 
  of 
  this 
  mineral 
  and 
  is 
  

   present 
  in 
  distinct 
  but 
  not 
  well 
  crystallized 
  grains. 
  

  

  The 
  quartz 
  is 
  recognized 
  by 
  its 
  being 
  a 
  mineral 
  of 
  low 
  

   refraction 
  and 
  birefringence, 
  colorless, 
  without 
  cleavage 
  and 
  of 
  

   positive 
  uniaxial 
  character. 
  It 
  occurs 
  in 
  two 
  ways, 
  in 
  the 
  

   formless 
  irregular 
  areas 
  between 
  other 
  more 
  automorphic 
  con- 
  

   stituents 
  common 
  to 
  granitic 
  rocks, 
  and 
  in 
  clear 
  round 
  grains 
  

   embedded 
  in 
  the 
  feldspars 
  and 
  appearing 
  like 
  the 
  corrosion 
  

   quartz 
  of 
  the 
  French 
  petrographers. 
  Everywhere 
  the 
  quartz 
  

   shows 
  optical 
  strains 
  and 
  broken 
  undulatory 
  extinctions, 
  indica- 
  

   tive 
  of 
  its 
  having 
  been 
  subjected 
  to 
  severe 
  pressure 
  ; 
  its 
  total 
  

   amount 
  in 
  the 
  rock 
  is 
  considerable. 
  

  

  . 
  The 
  structure 
  indicates 
  also 
  that 
  the 
  rock 
  has 
  been 
  under 
  

   dynamic 
  pressure 
  in 
  that 
  the 
  constituents 
  appear 
  broken, 
  

   angular 
  and 
  with 
  at 
  times 
  a 
  distinct 
  approach 
  to 
  the 
  " 
  mortar 
  " 
  

   structure. 
  Aside 
  from 
  this 
  the 
  structure 
  is 
  nearest 
  that 
  of 
  an 
  

   aplitic 
  granite. 
  

  

  Classification. 
  — 
  From 
  what 
  has 
  been 
  said 
  it 
  is 
  clear 
  that 
  the 
  

   rock 
  is 
  an 
  segirite 
  granite 
  or 
  taking 
  into 
  account 
  its 
  sugar 
  

   granular 
  structure 
  and 
  relative 
  paucity 
  of 
  dark 
  minerals, 
  per- 
  

   haps 
  better 
  an 
  segirite 
  aplite. 
  Its 
  alkaline 
  character 
  points 
  

   clearly 
  to 
  its 
  being 
  a 
  differentiation 
  product 
  of 
  acid 
  oxyphyric, 
  

   leucocratic 
  character 
  of 
  the 
  alkaline 
  magmas 
  at 
  Miask, 
  and 
  it 
  

   would 
  seem 
  probable 
  that 
  it 
  is 
  from 
  a 
  dike 
  possibly 
  of 
  some 
  

   size 
  which 
  has 
  suffered 
  from 
  orogenetic 
  movements. 
  

  

  In 
  the 
  literature 
  of 
  the 
  Miask 
  area 
  accessible* 
  to 
  the 
  writer 
  

   there 
  is 
  no 
  mention 
  of 
  this 
  type, 
  and 
  as 
  it 
  is 
  a 
  very 
  interesting 
  

   and 
  important 
  one 
  in 
  several 
  ways, 
  the 
  author 
  has 
  thought 
  

   well 
  to 
  call 
  attention 
  to 
  it, 
  and 
  also 
  with 
  the 
  hope 
  that 
  more 
  

   may 
  be 
  learned 
  of 
  its 
  geologic 
  and 
  genetic 
  relations. 
  

  

  Quartzose 
  igneous 
  rocks 
  containing 
  segirite 
  are 
  known 
  but 
  

   are 
  still 
  rare. 
  They 
  are 
  known 
  chiefly 
  in 
  the 
  finer-grained 
  and 
  

   porphyritic 
  textures 
  such 
  as 
  the 
  grorudites 
  of 
  the 
  Christiania 
  

   region, 
  the 
  Bearpaw 
  Mts., 
  Montana 
  and 
  in 
  the 
  rockallite 
  of 
  

   Rockall 
  Island, 
  and 
  the 
  granite 
  porphyry 
  of 
  the 
  Judith 
  Mts. 
  

   In 
  alkaline 
  granites 
  the 
  iron-bearing 
  component 
  is 
  usually 
  a 
  

   hornblende, 
  such 
  as 
  riebeckite 
  and 
  segirite 
  I 
  have 
  been 
  able 
  to 
  

   find 
  mentioned 
  but 
  once 
  in 
  the 
  literature 
  by 
  Broggerf 
  in 
  a 
  

   granite 
  near 
  Drammen 
  in 
  South 
  Norway. 
  

  

  *Rose, 
  "Reise 
  nach 
  dem 
  Ural. 
  1842, 
  vol. 
  ii; 
  Guide 
  des 
  Excur., 
  VII, 
  Cong. 
  Geol. 
  

   Inter., 
  No. 
  V, 
  Karpinsky. 
  Karpinsky 
  mentions 
  two 
  or 
  three 
  others 
  in 
  Russian 
  

   journals. 
  

  

  f 
  Spaltenverwerfungen 
  Langesund-Skien, 
  Nyt. 
  Mag. 
  for 
  Naturvid, 
  vol. 
  xxviii, 
  

   p. 
  40. 
  

  

  Sheffield 
  Laboratory 
  of 
  Mineralogy 
  and 
  Petrography, 
  

  

  Yale 
  University, 
  New 
  Haven, 
  January, 
  1900. 
  

  

  