﻿Pirsson 
  — 
  JSgirite 
  Granite 
  from 
  Miask, 
  Ural 
  Jits. 
  199 
  

  

  Aet. 
  XX. 
  — 
  On 
  ^Egirite 
  Granite 
  from 
  Miask, 
  Ural 
  Jits. 
  ; 
  

   by 
  L. 
  Y. 
  Pirsson. 
  

  

  Some 
  little 
  time 
  since, 
  there 
  was 
  received 
  in 
  tin's 
  laboratory 
  

   a 
  small 
  suite 
  of 
  rock 
  specimens 
  from 
  Krantz 
  in 
  Bonn 
  repre- 
  

   senting 
  the 
  mica-nephelite 
  syenite 
  from 
  Miask 
  in 
  the 
  Urals, 
  

   long 
  known 
  as 
  " 
  miascite 
  " 
  from 
  the 
  locality. 
  On 
  account 
  of 
  a 
  

   pressure 
  of 
  work 
  in 
  other 
  directions, 
  the 
  sections 
  from 
  these 
  

   rocks 
  have 
  been 
  only 
  recently 
  examined 
  for 
  classification 
  and 
  

   arrangement 
  It 
  was 
  at 
  once 
  seen 
  that 
  one 
  of 
  them 
  is 
  an 
  

   segirite 
  alkali 
  granite, 
  a 
  rare 
  and 
  interesting 
  type 
  of 
  rock. 
  

  

  Most 
  of 
  the 
  specimens 
  agree 
  well 
  with 
  the 
  descriptions 
  of 
  

   the 
  common 
  types 
  of 
  this 
  locality 
  given 
  by 
  several 
  authorities.* 
  

   They 
  are 
  light 
  gray 
  to 
  white 
  rocks 
  spotted 
  with 
  glittering 
  

   black 
  plates 
  of 
  biotite 
  often 
  collected 
  in 
  stringy 
  layers 
  so 
  that 
  

   the 
  rock 
  has 
  a 
  pronounced 
  gneissoid 
  character. 
  Under 
  the 
  

   microscope 
  the 
  sections 
  show 
  a 
  dark 
  mica, 
  probably 
  biotite, 
  

   alkali 
  feldspars 
  of 
  various 
  kinds, 
  albite, 
  microcline, 
  micro- 
  

   perthite, 
  cryptoperthite, 
  etc., 
  with 
  nephelite, 
  sodalite 
  and 
  can- 
  

   crinite. 
  They 
  frequently 
  indicate 
  that 
  the 
  gneissoid 
  structure 
  is 
  

   secondarily 
  induced 
  by 
  dynamic 
  pressures 
  from 
  the 
  modulatory 
  

   extinctions 
  and 
  broken 
  character 
  of 
  the 
  minerals. 
  

  

  The 
  specimen 
  which 
  is 
  the 
  occasion 
  of 
  this 
  note 
  is 
  a 
  light 
  

   colored 
  rock 
  of 
  much 
  the 
  texture 
  and 
  feeling 
  of 
  a 
  rather 
  firm 
  

   sandstone 
  of 
  medium 
  grain 
  ; 
  it 
  has 
  a 
  distinctly 
  saccharoidal 
  

   character. 
  It 
  does 
  not 
  possess 
  the 
  glittering 
  black 
  mica 
  of 
  the 
  

   other 
  types 
  but 
  is 
  spotted 
  with 
  small 
  grains 
  of 
  a 
  dull 
  black 
  

   ferro-magnesian 
  mineral 
  ; 
  these, 
  a 
  millimeter 
  or 
  less 
  in 
  diam- 
  

   eter, 
  are 
  not 
  regularly 
  sprinkled 
  through 
  the 
  rock, 
  but 
  are 
  

   gathered 
  in 
  clusters 
  or 
  small 
  areas, 
  where 
  they 
  are 
  closely 
  

   crowded, 
  while 
  the 
  intervening 
  spaces 
  are 
  quite 
  devoid 
  of 
  them 
  

   and 
  show 
  only 
  the 
  light 
  feldspathic 
  grains. 
  The 
  actual 
  amount 
  

   of 
  this, 
  the 
  only 
  dark 
  component, 
  is 
  small 
  compared 
  with 
  the 
  

   quartz 
  and 
  feldspar, 
  and 
  the 
  rock 
  is 
  therefore 
  distinctly 
  leuco- 
  

   cratic 
  in 
  character. 
  

  

  In 
  thin 
  section 
  the 
  minerals 
  observed 
  are 
  apatite, 
  zircon, 
  

   segirite, 
  albite,. 
  microcline, 
  orthoclase 
  and 
  quartz. 
  The 
  apatite 
  

   is 
  comparatively 
  rare 
  in 
  small 
  stout 
  prisms; 
  the 
  zircon 
  is 
  in 
  

   minute 
  well 
  crystallized 
  prisms 
  and 
  larger 
  anhedra 
  ; 
  it 
  is 
  more 
  

   abundant 
  than 
  apatite. 
  

  

  Albite 
  is 
  rather 
  abundant 
  and 
  well 
  twinned 
  according 
  to 
  the 
  

   albite 
  law, 
  not 
  according 
  to 
  the 
  carlsbad. 
  It 
  is 
  recognized 
  by 
  

   its 
  optical 
  relations 
  and 
  measurements 
  and 
  by 
  Becke's 
  method 
  

   on 
  contacts 
  with 
  quartz. 
  

  

  Microcline 
  is 
  seen 
  in 
  small 
  fragments 
  and 
  in 
  places 
  the 
  

   unstriated 
  alkali 
  feldspar 
  has 
  transitions 
  through 
  areas 
  of 
  undu- 
  

  

  * 
  Rose, 
  Reise 
  nach 
  dem 
  Ural, 
  vol. 
  ii. 
  p. 
  42, 
  1S42 
  : 
  Mouchketov 
  : 
  Rosenbusch, 
  

   Mass. 
  Gest. 
  3d 
  A.ufl 
  . 
  p. 
  173. 
  Ls96; 
  ZirkH. 
  Lehrb. 
  d. 
  Petr., 
  1894, 
  p. 
  415; 
  Kar- 
  

   pinsky, 
  Guide 
  VII 
  Intern. 
  Cong 
  , 
  No. 
  V 
  (Versant 
  orientale 
  de 
  l'Oural, 
  1897, 
  p. 
  21). 
  

  

  