﻿270 
  KEPORT 
  UNITED 
  STATES 
  GEOLOGICAL 
  SURVEY. 
  

  

  In 
  the 
  vicinity 
  of 
  Mount 
  Moss, 
  a 
  vast 
  number 
  of 
  sheets 
  and 
  masses 
  of 
  

   trachyte 
  have 
  been 
  forced 
  into 
  the 
  yielding 
  and 
  nearly 
  horizontal 
  Cre- 
  

   taceous 
  shales. 
  The 
  sheets 
  in 
  most 
  cases 
  follow 
  approximately 
  the 
  

   planes 
  of 
  bedding. 
  The 
  heavier 
  masses, 
  which 
  may 
  in 
  some 
  cases 
  

   be 
  outflows 
  from 
  vertical 
  dikes, 
  are 
  often 
  thrust 
  at 
  greatly 
  varying 
  

   angles 
  through 
  the 
  strata. 
  On 
  the 
  Mancos, 
  at 
  the 
  west 
  base 
  of 
  Hes- 
  

   perus 
  Peak, 
  there 
  is 
  a 
  heavy 
  mass 
  exposed, 
  on 
  the 
  upper 
  surfaces 
  of 
  

   which 
  the 
  shales 
  lie 
  — 
  somewhat 
  bent 
  and 
  distorted, 
  but 
  not 
  greatly 
  

   metamorphosed. 
  South 
  of 
  this, 
  and 
  probably 
  connected 
  with 
  it, 
  is 
  a 
  

   great 
  sloping 
  mass 
  of 
  trachyte, 
  the 
  relations 
  of 
  which 
  to 
  the 
  surround- 
  

   ing 
  strata 
  I 
  could 
  not 
  make 
  out, 
  excepting 
  that 
  it 
  rests 
  in 
  and 
  on 
  the 
  

   shales. 
  It 
  probably 
  connects 
  more 
  or 
  less 
  directly 
  with 
  the 
  trachyte 
  

   mass 
  of 
  the 
  mountains. 
  On 
  the 
  high 
  spur 
  which 
  extends 
  southwest 
  

   between 
  the 
  East 
  and 
  West 
  Mancos 
  is 
  a 
  high, 
  bald 
  point, 
  named 
  Helmet 
  

   Peak, 
  capped 
  with 
  trachyte, 
  which 
  probably 
  belongs 
  to 
  the 
  same 
  flow 
  

   that 
  caps 
  the 
  high 
  ridge 
  between 
  the 
  East 
  Mancos 
  and 
  the 
  La 
  Plata, 
  as 
  

   it 
  is 
  on 
  nearly 
  the 
  same 
  level. 
  The 
  ridge 
  that 
  extends 
  north 
  from 
  

   Mount 
  Moss 
  has 
  a 
  capping 
  of 
  trachyte 
  which 
  spreads 
  out 
  over 
  a 
  large 
  

   area 
  between 
  the 
  Mancos 
  and 
  South 
  Bear 
  Eiver 
  and 
  rises 
  occasionally 
  

   into 
  sharp 
  buttes. 
  The 
  intrusion 
  of 
  sheets 
  and 
  wedges, 
  however, 
  is 
  

   most 
  strikingly 
  illustrated 
  in 
  Mount 
  Hesperus. 
  Between 
  timber-line 
  

   and 
  the 
  summit 
  there 
  is 
  an 
  alternation 
  of 
  trachytic 
  sheets 
  with 
  the 
  shales, 
  

   in 
  such 
  regular 
  succession 
  that, 
  until 
  closely 
  examined, 
  I 
  imagined 
  the 
  

   whole 
  to 
  consist 
  of 
  the 
  sandstones 
  of 
  the 
  Upper 
  Cretaceous. 
  In 
  the 
  

   upper 
  1,500 
  feet 
  there 
  are 
  twelve 
  distinct 
  layers 
  of 
  trachyte 
  (see 
  plate 
  

   XLIV), 
  some 
  having 
  a 
  thickness 
  of 
  from 
  30 
  to 
  40 
  feet, 
  others 
  of 
  not 
  more 
  

   than 
  a 
  few 
  inches. 
  Between 
  these 
  the 
  shales, 
  which 
  ordinarily 
  are 
  

   soft, 
  black, 
  and 
  friable, 
  are 
  reduced 
  to 
  a 
  hard 
  gray 
  slate 
  that 
  weathers 
  

   into 
  angular 
  fragments, 
  and 
  frequently 
  into 
  symmetrical 
  slabs 
  and 
  

   blocks. 
  In 
  these 
  the 
  fossils 
  are 
  still 
  preserved, 
  but 
  much 
  crushed 
  and 
  

   often 
  nearly 
  obliterated. 
  

  

  Loweronthemountain-slope, 
  ledges 
  of 
  trachyte 
  occur 
  at 
  intervals, 
  indi- 
  

   cating 
  that 
  the 
  entire 
  mountain 
  has 
  been 
  thoroughly 
  penetrated 
  by 
  the 
  in- 
  

   trusive 
  matter. 
  On 
  the 
  west 
  slope, 
  by 
  way 
  of 
  which 
  we 
  approached 
  the 
  

   summit, 
  a 
  narrow 
  shoulder 
  occurs, 
  produced 
  by 
  a 
  mass 
  of 
  trachyte 
  that 
  

   seems 
  to 
  have 
  been 
  thrust 
  up 
  fiom 
  beneath, 
  probably 
  as 
  a 
  dike, 
  as 
  the 
  

   outcrop 
  can 
  be 
  traced 
  down 
  the 
  side 
  of 
  the 
  mountain 
  into 
  the 
  valley. 
  The 
  

   sheets 
  which 
  occur 
  in 
  the 
  body 
  of 
  the 
  mountain, 
  however, 
  evidently 
  radiate 
  

   from 
  the 
  region 
  of 
  the 
  Mount 
  Moss 
  group, 
  which 
  is 
  three-fourths 
  of 
  a 
  

   mile 
  to 
  the 
  east. 
  From 
  the 
  sentinel 
  gap 
  at 
  the 
  head 
  of 
  Boren 
  Gulch, 
  I 
  

   obtained 
  a 
  fair 
  view 
  of 
  the 
  south 
  ftice 
  of 
  Hesperus 
  or 
  Banded 
  Mountain, 
  

   and 
  could 
  see 
  plainly 
  the 
  interleaving 
  of 
  the 
  trachyte 
  with 
  the 
  shales. 
  

   (Plate 
  XLV, 
  Fig. 
  1.) 
  A 
  succession 
  of 
  wedges 
  of 
  the 
  former 
  penetrate 
  

   Mount 
  Hesperus, 
  while 
  the 
  included 
  layers 
  of 
  shale 
  extend 
  into 
  the 
  west 
  

   walls 
  of 
  Mount 
  Moss. 
  There 
  is 
  a 
  sort 
  of 
  dovetailing 
  produced 
  that 
  

   might 
  be 
  illustrated 
  by 
  setting 
  together 
  two 
  combs 
  so 
  that 
  the 
  teeth 
  

   should 
  alternate. 
  The 
  ends 
  of 
  the 
  shale 
  layers 
  are 
  much 
  distorted, 
  and 
  

   so 
  highly 
  metamorphosed 
  where 
  they 
  approach 
  the 
  mass 
  of 
  trachyte 
  

   that 
  it 
  is 
  difficult 
  to 
  say 
  how 
  far 
  they 
  extend, 
  but 
  the 
  sheets 
  of 
  trachyte 
  

   in 
  the 
  shales 
  are 
  perfectly 
  defined, 
  contrasting 
  with 
  the 
  shale 
  both 
  in 
  

   color 
  and 
  style 
  of 
  weathering. 
  The 
  sketch 
  I 
  present 
  is 
  not 
  as 
  good 
  

   as 
  I 
  would 
  like, 
  as 
  the 
  mountain 
  was 
  so 
  enveloped 
  in 
  clouds 
  that 
  the 
  de- 
  

   tails 
  could 
  not 
  be 
  thoroughly 
  studied. 
  The 
  north 
  side 
  of 
  the 
  mountain 
  

   presents 
  the 
  same 
  phenomena, 
  and 
  a 
  low 
  mountain 
  to 
  the 
  east, 
  and 
  im- 
  

   mediately 
  north 
  of 
  Mount 
  Moss 
  {b, 
  Plate 
  XLIV), 
  has 
  the 
  same 
  structure 
  ; 
  

   indeed, 
  the 
  various 
  sheets 
  of 
  trachyte 
  here 
  seen 
  may 
  be 
  simply 
  the 
  con- 
  

  

  