﻿320 
  PROCEEDINGS 
  OF 
  THE 
  GEOLOGICAL 
  SOCIETY. 
  [Apr. 
  13, 
  

  

  on 
  either 
  side 
  of 
  Six-mile 
  Canon, 
  from 
  Virginia 
  City 
  to 
  the 
  desert- 
  

   plains 
  of 
  Carson 
  Valley, 
  we 
  find 
  porphyry 
  with 
  occasional 
  belts 
  of 
  

   greenstone, 
  and 
  heavy 
  ledges 
  of 
  quartz 
  vein-stone. 
  The 
  sides 
  of 
  this 
  

   canon 
  are 
  very 
  rugged 
  and 
  abrupt, 
  and 
  the 
  porphyry 
  forms 
  sharp 
  

   serrated 
  spurs 
  and 
  ridges, 
  with 
  overhanging 
  cliffs. 
  This 
  is 
  also 
  the 
  

   case 
  at 
  intervals 
  along 
  Gold 
  Canon, 
  especially 
  at 
  Silver 
  City 
  and 
  the 
  

   narrow 
  passage 
  called 
  the 
  Devil's 
  Gate, 
  where 
  two 
  opposite 
  spurs 
  

   nearly 
  close 
  the 
  canon, 
  leaving 
  scarcely 
  room 
  for 
  a 
  roadway. 
  

  

  These 
  porphyries 
  weather 
  very 
  unequally 
  : 
  in 
  some 
  places 
  the 
  

   cliffs 
  are 
  surrounded 
  by 
  a 
  talus 
  of 
  small 
  angular 
  fragments, 
  often 
  

   laminated; 
  in 
  others 
  the 
  fragments 
  are 
  solid 
  rounded 
  blocks, 
  or 
  

   long 
  oval-shaped 
  masses. 
  Some 
  of 
  the 
  larger 
  outcrops 
  on 
  Mount 
  

   Davidson 
  have 
  a 
  rude 
  columnar 
  structure, 
  with 
  the 
  columns 
  tapering 
  

   to 
  a 
  blunt 
  rounded 
  point, 
  much 
  like 
  the 
  porphyries 
  of 
  Western 
  

   Texas, 
  about 
  Fort 
  Davis. 
  In 
  general 
  there 
  is 
  no 
  distinct 
  trend, 
  nor 
  

   any 
  signs 
  of 
  bedding 
  in 
  these 
  masses 
  ; 
  but 
  cleavage-planes 
  appear 
  to 
  

   extend 
  in 
  many 
  different 
  directions, 
  and 
  are 
  much 
  more 
  developed 
  

   at 
  some 
  points 
  than 
  at 
  others. 
  Careful 
  observation, 
  however, 
  will 
  

   show, 
  here 
  and 
  there, 
  what 
  appear 
  to 
  be 
  traces 
  of 
  original 
  stratifi- 
  

   cation, 
  and 
  an 
  obscure 
  granular 
  structure, 
  like 
  that 
  of 
  a 
  partially 
  

   changed 
  sandstone, 
  and 
  at 
  other 
  points 
  finer 
  and 
  more 
  compact 
  

   varieties, 
  such, 
  for 
  example, 
  as 
  rise 
  up 
  into 
  laminated 
  fragments, 
  

   which 
  may 
  once 
  have 
  been 
  clay-slate. 
  A 
  large 
  portion 
  of 
  the 
  porphyry 
  

   is 
  quartzose, 
  the 
  siliceous 
  grains 
  being 
  glassy, 
  and 
  without 
  distinct 
  

   forms 
  of 
  crystallization. 
  The 
  weathered 
  outcrops 
  are 
  nearly 
  all 
  of 
  a 
  

   light 
  drab 
  or 
  yellowish 
  colour, 
  with 
  a 
  reddish 
  or 
  rose 
  tinge. 
  Below 
  

   the 
  water-level 
  in 
  the 
  mines 
  on 
  the 
  Cornstock 
  Lode, 
  the 
  colour 
  is 
  

   generally 
  olive 
  -green, 
  or 
  greyish 
  green 
  of 
  various 
  shades. 
  

  

  An 
  outcrop 
  of 
  porphyry, 
  different 
  from 
  these 
  and 
  very 
  interesting 
  

   in 
  its 
  character, 
  occurs 
  in 
  the 
  midst 
  of 
  the 
  granite 
  of 
  the 
  Sierra 
  

   Nevada, 
  between 
  Carson 
  and 
  Bigler 
  Lake, 
  on 
  the 
  New 
  Lake 
  Road. 
  

   It 
  is 
  not 
  very 
  extensive, 
  but 
  is 
  composed 
  of 
  distinct 
  and 
  often 
  

   well-formed 
  crystals 
  of 
  felspar 
  a 
  quarter 
  of 
  an 
  inch 
  or 
  more 
  in 
  

   length, 
  and 
  of 
  crystals 
  of 
  black 
  mica, 
  in 
  hexagonal 
  prisms, 
  imbedded 
  

   in 
  a 
  lilac-coloured 
  matrix. 
  The 
  outer 
  portions 
  are 
  comparatively 
  

   soft, 
  and 
  the 
  crystals 
  of 
  mica 
  and 
  felspar 
  may 
  be 
  detached 
  by 
  the 
  

   aid 
  of 
  a 
  hammer. 
  Mr. 
  Pampelly 
  has 
  seen 
  a 
  porphyry 
  similar 
  to 
  

   this 
  in 
  the 
  Silver 
  Region 
  of 
  Arizona, 
  thus 
  affording 
  the 
  second 
  point 
  

   of 
  similarity 
  noted 
  between 
  the 
  rocks 
  of 
  the 
  two 
  regions. 
  

  

  The 
  rocks 
  forming 
  the 
  mountains 
  on 
  the 
  southern 
  side 
  of 
  the 
  

   Carson 
  Desert, 
  and 
  on 
  the 
  right 
  bank 
  of 
  the 
  river, 
  are 
  similar 
  to 
  those 
  

   around 
  Virginia 
  City 
  and 
  Mount 
  Davidson, 
  at 
  least 
  on 
  the 
  north 
  

   side 
  ; 
  but 
  they 
  have 
  not 
  been 
  much 
  explored. 
  At 
  one 
  place, 
  near 
  

   where 
  Colonel 
  Whitmans 
  discovered 
  coal, 
  I 
  observed 
  a 
  semi-meta- 
  

   morphic 
  porphyritic 
  conglomerate 
  similar 
  to 
  that 
  seen 
  between 
  

   Carson 
  and 
  Chalk 
  Hill. 
  There 
  are 
  also 
  regular 
  strata 
  of 
  clay- 
  

   shales 
  and 
  of 
  sandstones, 
  with 
  bituminous 
  shales 
  and 
  beds 
  of 
  brown 
  

   coal, 
  not 
  fully 
  opened 
  at 
  the 
  time 
  of 
  my 
  visit. 
  These 
  strata 
  are 
  

   probably 
  Tertiary. 
  

  

  The 
  formations 
  which 
  have 
  been 
  successively 
  noticed 
  are 
  shown 
  

  

  