﻿HOLMES.] 
  TRACHYTES 
  OF 
  THE 
  SIERRA 
  CARRISO. 
  275 
  

  

  nately 
  my 
  observations 
  could 
  not 
  be 
  carried 
  to 
  the 
  southern 
  and 
  south- 
  

   eastern 
  slopes 
  of 
  the 
  mountains, 
  and 
  it 
  is 
  impossible 
  to 
  say 
  whether 
  the 
  

   arch 
  has 
  been 
  continuous 
  and 
  symmetrical 
  or 
  not. 
  On 
  the 
  north 
  these 
  

   two 
  remnants 
  seem 
  to 
  be 
  all 
  that 
  is 
  left 
  of 
  the 
  outer 
  sheet, 
  unless 
  a 
  small 
  

   fragment 
  of 
  trachyte 
  observed 
  resting 
  on 
  a 
  southwestern 
  space 
  may 
  

   belong 
  to 
  the 
  same. 
  It 
  does 
  not 
  appear 
  to 
  me 
  that 
  the 
  beds 
  of 
  sand- 
  

   stones 
  that 
  occur 
  in 
  the 
  arch 
  between 
  the 
  inner 
  mass 
  and 
  the 
  flexed 
  

   sheets 
  are 
  of 
  uniform 
  thickness. 
  Between 
  the 
  capping 
  of 
  the 
  North 
  

   Mesa 
  and 
  the 
  inner 
  mass 
  the 
  sandstones 
  are 
  nearly 
  pinched 
  out- 
  They 
  

   are 
  so 
  obscured 
  by 
  debris 
  that 
  I 
  could 
  not 
  determine 
  their 
  precise 
  re- 
  

   lations. 
  On 
  the 
  east 
  side 
  of 
  the 
  North 
  Mesa 
  the 
  bed 
  of 
  Navajo 
  Creek 
  

   has 
  cut 
  down 
  through 
  the 
  sandstones 
  and 
  exposed 
  in 
  the 
  walls 
  of 
  three 
  

   or 
  four 
  deep, 
  sharp 
  canons 
  (e, 
  e) 
  a 
  large 
  body 
  of 
  trachyte, 
  which 
  would 
  

   seem 
  to 
  belong 
  to 
  the 
  mass 
  which 
  in 
  the 
  center 
  of 
  the 
  group 
  rises 
  into 
  

   the 
  high 
  summits 
  east 
  of 
  Navajo 
  Creek. 
  West 
  of 
  the 
  North 
  Mesa 
  this 
  

   lower 
  mass 
  of 
  trachyte 
  is 
  exposed 
  in 
  the 
  canons 
  of 
  the 
  west 
  branches 
  

   of 
  Navajo 
  Creek, 
  (ff), 
  and 
  is 
  very 
  evidently 
  connected 
  with 
  the 
  central 
  

   mass. 
  

  

  East 
  of 
  North 
  Mesa, 
  in 
  the 
  faces 
  of 
  the 
  deep 
  valley 
  of 
  Navajo 
  Creek, 
  

   there 
  is 
  a 
  heavy 
  stratum 
  of 
  red 
  sandstones, 
  interbedded 
  with 
  the 
  tra- 
  

   chytes 
  and 
  dipping 
  away 
  from 
  the 
  interior 
  mass 
  at 
  a 
  high 
  angle. 
  The 
  

   red 
  sandstones 
  appear 
  also 
  in 
  a 
  number 
  of 
  places 
  in 
  the 
  midst 
  of 
  the 
  

   higher 
  summit 
  regions 
  of 
  the 
  eastern 
  part 
  of 
  the 
  group, 
  interbeded 
  with 
  

   sheets 
  of 
  trachyte. 
  In 
  figure 
  2, 
  plate 
  XLVIII, 
  I 
  give 
  the 
  section 
  exposed 
  

   on 
  the 
  east 
  side 
  of 
  the 
  upper 
  valley 
  of 
  Navajo 
  Creek. 
  The 
  conditions 
  here 
  

   suggest 
  the 
  idea 
  that 
  possibly 
  the 
  trachyte 
  exposed 
  at 
  e, 
  e, 
  f,f, 
  on 
  the 
  

   east 
  and 
  west 
  flanks 
  of 
  North 
  Mesa, 
  may 
  also 
  be 
  the 
  upper 
  surface 
  of 
  a 
  

   sheet 
  which 
  domes 
  over 
  the 
  Postiora 
  group, 
  and 
  in 
  turn 
  is 
  sUiCceeded 
  by 
  

   layers 
  of 
  red 
  sandstone. 
  

  

  In 
  East 
  Navajo 
  Creek 
  a 
  third 
  bed 
  of 
  trachyte 
  appears, 
  followed 
  by 
  a 
  

   second 
  stratum 
  of 
  red 
  sandstones. 
  This 
  suggests 
  the 
  possibility 
  of 
  a 
  

   sort 
  of 
  dome 
  within 
  dome 
  structure, 
  such 
  as 
  would 
  be 
  produced 
  by 
  the 
  

   uplifting 
  of 
  a 
  series 
  of 
  beds 
  in 
  which 
  there 
  had 
  been 
  horizontal 
  intru- 
  

   sions 
  of 
  trachytic 
  sheets, 
  or 
  even 
  by 
  the 
  intrusion 
  of 
  such 
  sheets 
  into 
  a 
  

   series 
  of 
  already 
  uplifted 
  sedimentary 
  strata. 
  

  

  I 
  am 
  not 
  inclined 
  to 
  think, 
  however, 
  that 
  the 
  movements 
  of 
  the 
  vol- 
  

   canic 
  matter 
  have 
  been 
  systematic 
  or 
  the 
  results 
  highly 
  uniform, 
  as 
  acci- 
  

   dental 
  causes 
  — 
  such 
  as 
  unevenly 
  yielding 
  strata, 
  diversity 
  of 
  dips, 
  and 
  

   presence 
  of 
  previously 
  deposited 
  volcanic 
  matter 
  — 
  must 
  often 
  greatly 
  

   influence 
  if 
  not 
  totally 
  govern 
  the 
  direction 
  of 
  the 
  flows, 
  the 
  thickness 
  

   of 
  the 
  sheets, 
  &c. 
  At 
  the 
  same 
  time 
  there 
  doubtless 
  are 
  laws 
  that 
  in 
  

   geueral 
  govern 
  the 
  movements 
  of 
  the 
  volcanic 
  products 
  and 
  tend 
  to 
  

   produce 
  uniform 
  results, 
  but 
  I 
  shall 
  not 
  attempt 
  their 
  discussion 
  here. 
  

  

  A 
  specimen 
  of 
  trachyte 
  from 
  West 
  Mesa 
  is 
  found 
  to 
  resemble 
  closely 
  

   in 
  appearance 
  and 
  composition 
  the 
  trachyte 
  of 
  other 
  groups 
  of 
  the 
  

   southwest. 
  It 
  has 
  a 
  bluish 
  white 
  paste, 
  which 
  contains 
  the 
  following 
  

   minerals 
  porphyritically 
  imbedded 
  : 
  fine 
  crystals 
  of 
  translucent 
  oligo- 
  

   c.ase, 
  minute 
  crystals 
  of 
  sanidite, 
  frequently 
  directly 
  associated 
  with 
  

   the 
  oligoclase, 
  small 
  crystals 
  of 
  biotite 
  (rare), 
  and 
  a 
  few 
  small 
  Inciosures 
  

   of 
  quartz. 
  

  

  In 
  Plate 
  XLIX, 
  I 
  present 
  a 
  sketch 
  of 
  a 
  remarkable 
  dike 
  that 
  occurs 
  on 
  

   the 
  eastern 
  branch 
  of 
  Navajo 
  Creek 
  at 
  its 
  exit 
  from 
  the 
  mountains. 
  It 
  

   rises 
  vertically 
  through 
  the 
  horizontal 
  sandstones 
  of 
  the 
  Lower 
  Dakota 
  

   group. 
  It 
  is 
  over 
  a 
  mile 
  in 
  length, 
  is 
  quite 
  straight, 
  and 
  has 
  a 
  north 
  

   and 
  south 
  strike. 
  A 
  very 
  remarkable 
  feature 
  of 
  this 
  dike 
  is 
  that 
  it 
  is 
  

   in 
  places 
  double. 
  Two 
  parallel 
  walls 
  of 
  trachyte 
  (?) 
  rise 
  through 
  the 
  

  

  