﻿PBALB-J 
  EEUPTIVE 
  KOCKS 
  SIERRA 
  LA 
  SAL. 
  95 
  

  

  fer 
  considerably 
  from 
  each 
  other 
  in 
  detail, 
  but 
  there 
  was 
  a 
  general 
  re- 
  

   semblance. 
  All 
  were 
  more 
  or 
  less 
  porphyritic, 
  and 
  in 
  some 
  places 
  their 
  

   trachytic 
  character 
  was 
  undoubted. 
  In 
  the 
  absence 
  of 
  any 
  microscopic 
  

   or 
  chemical 
  analyses 
  of 
  the 
  rocks, 
  I 
  included 
  them 
  all 
  under 
  the 
  general 
  

   heading 
  of 
  porphyritic 
  trachyte. 
  In 
  the 
  Sierra 
  la 
  Sal, 
  we 
  have 
  rocks 
  

   similar 
  to 
  those 
  of 
  Mount 
  Marcellina 
  and 
  other 
  groups 
  west 
  of 
  the 
  Elk 
  

   Mountains. 
  As 
  their 
  mode 
  of 
  occurrence 
  is 
  the 
  same, 
  I 
  have 
  adopted 
  

   the 
  same 
  name. 
  It 
  is, 
  of 
  course, 
  provisional, 
  as 
  the 
  rocks 
  have 
  never 
  

   been 
  critically 
  examined. 
  All 
  the 
  specimens 
  collected 
  on 
  the 
  Sierra 
  la 
  

   Sal 
  were 
  lost, 
  and 
  therefore 
  no 
  complete 
  description 
  of 
  them 
  can 
  be 
  

   given. 
  The 
  following 
  remarks 
  are 
  given 
  simply 
  as 
  a 
  sketch, 
  as 
  the 
  

   western 
  side 
  of 
  the 
  mountains 
  was 
  not 
  visited, 
  and 
  the 
  limited 
  time 
  we 
  

   had 
  to 
  examine 
  this 
  interesting 
  locality 
  precludes 
  our 
  giving 
  as 
  detailed 
  

   a 
  description 
  as 
  I 
  could 
  wish. 
  Enough, 
  however, 
  was 
  seen 
  to 
  leave 
  no 
  

   doubt 
  as 
  to 
  the 
  structure 
  of 
  the 
  mountains, 
  which 
  is 
  the 
  same 
  as 
  that 
  of 
  

   Mount 
  Marcellina 
  and 
  its 
  surrounding 
  groups, 
  (see 
  Eeport 
  for 
  1874, 
  pp. 
  

   163, 
  168,) 
  and 
  the 
  Elk 
  Mountains, 
  viz, 
  eruptive. 
  By 
  this, 
  I 
  mean 
  that 
  

   the 
  sedimentary 
  strata 
  have 
  been 
  lifted 
  up 
  by 
  eruptive 
  rock, 
  which 
  has 
  

   broken 
  through 
  them 
  in 
  some 
  places, 
  and 
  in 
  others 
  is 
  seen 
  only 
  as 
  the 
  

   result 
  of 
  subsequent 
  erosion. 
  In 
  the 
  Eeports 
  for 
  1873 
  and 
  1874, 
  the 
  

   eruptive 
  character 
  of 
  the 
  Elk 
  Mountains 
  is 
  frequently 
  referred 
  to.* 
  In 
  

   that 
  region 
  there 
  is 
  so 
  much 
  complication, 
  the 
  sedimentaries 
  being 
  over- 
  

   turned 
  and 
  penetrated 
  by 
  dikes 
  in 
  every 
  direction, 
  that 
  it 
  is 
  only 
  when 
  

   we 
  glance 
  at 
  the 
  range 
  generally 
  that 
  we 
  begin 
  to 
  comprehend 
  its 
  struct- 
  

   ure. 
  In 
  the 
  isolated 
  groups 
  southwest 
  of 
  the 
  Elk 
  Mountains, 
  described 
  

   in 
  the 
  Report 
  for 
  1874, 
  (pp. 
  163-168,) 
  there 
  is 
  more 
  simplicity, 
  the 
  struct- 
  

   ure 
  being 
  much 
  more 
  apparent. 
  Even 
  there, 
  however, 
  there 
  are 
  diffi- 
  

   culties 
  met 
  with 
  in 
  the 
  study 
  of 
  the 
  mountains, 
  arising 
  from 
  the 
  fact 
  

   that 
  the 
  line 
  of 
  junction 
  between 
  the 
  volcanic 
  rocks 
  and 
  the 
  surround- 
  

   ing 
  sedimentaries 
  is 
  generally 
  concealed 
  by 
  the 
  debris 
  from 
  the 
  peaks. 
  

   There 
  are 
  also 
  a 
  number 
  of 
  centers 
  of 
  eruption 
  close 
  to 
  each 
  other, 
  and 
  

   numerous 
  dikes 
  radiating 
  from 
  the 
  main 
  masses. 
  

  

  In 
  the 
  Sierra 
  la 
  Sal 
  the 
  structure 
  is 
  comparatively 
  simple, 
  and 
  there 
  

   . 
  can 
  be 
  no 
  doubt 
  of 
  the 
  eruptive 
  character 
  of 
  the 
  mountains. 
  

  

  As 
  a 
  group, 
  the 
  mountains 
  are 
  isolated, 
  and 
  rise 
  for 
  5,000 
  to 
  6,000 
  feet 
  

   above 
  the 
  general 
  level 
  of 
  the 
  surrounding 
  country. 
  As 
  we 
  approach 
  

   the 
  peaks, 
  sedimentary 
  beds 
  rise, 
  and 
  form 
  the 
  lower 
  outstanding 
  

   eastern 
  peaks, 
  dipping 
  steeply 
  to 
  the 
  eastward. 
  At 
  one 
  place 
  there 
  is 
  a 
  

   capping 
  of 
  sedimentaries 
  on 
  the 
  summit 
  of 
  the 
  peak, 
  showing 
  that 
  they 
  

   once 
  extended 
  over 
  the 
  mountains. 
  The 
  general 
  trend 
  of 
  the 
  mount- 
  

   ains 
  is 
  about 
  north 
  and 
  south 
  as 
  a 
  whole, 
  but 
  we 
  find 
  that 
  they 
  are 
  

   separable 
  into 
  three 
  groups, 
  which 
  seem 
  to 
  mark 
  three 
  centers 
  of 
  great- 
  

   est 
  uplift. 
  There 
  are 
  three 
  high 
  areas 
  of 
  volcanic 
  rock 
  separated 
  by 
  

   saddles 
  of 
  sedimentary 
  rocks, 
  beneath 
  which 
  the 
  volcanic 
  rocks 
  may 
  

   be 
  connected. 
  I 
  shall 
  take 
  up 
  these 
  groups 
  in 
  order, 
  naming 
  them 
  

   northern, 
  middle, 
  and 
  southern. 
  

  

  Northern 
  group. 
  — 
  This 
  group 
  contains 
  the 
  largest 
  number 
  of 
  peaks 
  

   and 
  is 
  nearest 
  in 
  resemblance 
  to 
  a 
  range 
  trending 
  approximately 
  north- 
  

   west 
  and 
  southeast. 
  At 
  the 
  north 
  end, 
  on 
  the 
  east 
  side, 
  the 
  red 
  beds 
  

   (Trias 
  %) 
  are 
  seen 
  rising 
  up 
  and 
  forming 
  outlying 
  peaks, 
  dipping 
  steeply 
  

   to 
  the 
  eastward. 
  At 
  the 
  south 
  end, 
  near 
  station 
  67, 
  is 
  a 
  gray 
  sandstone 
  

   (probably 
  Dakota 
  group) 
  reaching 
  nearly 
  to 
  the 
  summit 
  of 
  the 
  station. 
  

   The 
  saddle 
  south 
  of 
  the 
  group 
  has 
  an 
  elevation 
  of 
  10,800 
  feet, 
  and 
  is 
  

   probably 
  underlaid 
  by 
  Lower 
  Cretaceous 
  sandstones, 
  which 
  extend 
  

  

  'Report 
  United 
  States 
  Geological 
  Survey 
  1873, 
  pp. 
  59-69, 
  247-261; 
  Report 
  United 
  

   States 
  Geological 
  Survey 
  1874, 
  pp. 
  55, 
  63, 
  70. 
  

  

  