﻿166' 
  REPORT 
  UNITED 
  STATES 
  GEOLbGICAL 
  SURVEY. 
  

  

  kota 
  (a) 
  and 
  the 
  Colorado 
  groups 
  (b) 
  covered 
  by 
  a 
  thin 
  stratum 
  of 
  bas- 
  

   alt 
  (c) 
  ; 
  then 
  follows 
  the 
  quartzite 
  (fZ), 
  showmg 
  an 
  anticlinal 
  fold, 
  under 
  

   tbe 
  highest 
  point 
  of 
  the 
  mountain 
  composed 
  of 
  it. 
  Granite 
  (e) 
  coarse- 
  

   grained, 
  with 
  flesh-colored 
  orthoclase 
  and 
  white, 
  silvery 
  muscovite, 
  

   appears 
  on 
  the 
  upper 
  strata 
  of 
  the 
  quartzite. 
  Both 
  of 
  these 
  show 
  well- 
  

   detined, 
  regular 
  stratification. 
  On 
  the 
  west 
  side 
  the 
  dip 
  of 
  the 
  quartz- 
  

   ite 
  is 
  very 
  steep, 
  amounting 
  to 
  nearly 
  80°, 
  but 
  it 
  becomes 
  more 
  gentle 
  

   east 
  of 
  the 
  anticlinal 
  axis. 
  Conformable 
  with 
  it 
  there 
  is 
  the 
  dip 
  of 
  the 
  

   granite, 
  reaching 
  about 
  22°. 
  This 
  dip 
  is 
  continued, 
  with 
  slight 
  local 
  

   changes, 
  throughout 
  the 
  granitic 
  area 
  exposed, 
  and 
  is 
  particularly 
  notice- 
  

   able 
  in 
  the 
  ridge 
  upon 
  which 
  station 
  95 
  is 
  located. 
  Unconformable 
  with 
  

   the 
  metamorphic 
  strata, 
  though 
  also 
  dipping 
  east, 
  are 
  the 
  beds 
  of 
  tra- 
  

   chytic 
  conglomerate 
  (/). 
  They 
  show 
  an 
  inclination 
  of 
  6° 
  to 
  8° 
  east- 
  

   ward, 
  which 
  remains 
  constant 
  throughout 
  almost 
  the 
  entire 
  distance 
  

   that 
  they 
  show 
  any 
  exposures. 
  A 
  shallow 
  convex 
  fold 
  occurs 
  northeast 
  

   of 
  station 
  93, 
  which 
  is 
  located 
  at 
  an 
  altitude 
  of 
  11,214 
  feet 
  on 
  the 
  com- 
  

   pact 
  trachyte 
  (g) 
  covering 
  the 
  conglomerate. 
  Trachyte 
  continues, 
  worn 
  

   away 
  at 
  some 
  places 
  so 
  as 
  to 
  appear 
  thinner, 
  but 
  retaining 
  the 
  regular 
  

   succession 
  of 
  its 
  flows 
  and 
  its 
  constant 
  easterly 
  dip. 
  It 
  is 
  covered 
  by 
  

   the 
  basalt 
  (7^) 
  of 
  station 
  90, 
  which 
  reaches 
  a 
  thickness 
  of 
  200 
  to 
  250 
  

   feet. 
  From 
  the 
  small 
  plateau 
  formed 
  by 
  the 
  ]>rotecting 
  cap 
  of 
  basalt 
  

   the 
  trachyte 
  is 
  broken 
  away 
  again, 
  as 
  we 
  descend, 
  and 
  the 
  conglomerate 
  

   for 
  the 
  last 
  time 
  makes 
  its 
  appearance 
  on 
  the 
  banks 
  of 
  the 
  Conejos. 
  

   With 
  trachyte 
  underlying, 
  we 
  then 
  find 
  the 
  long 
  stretch 
  of 
  basalt 
  {() 
  in 
  

   San 
  Luis 
  Valley, 
  a 
  continuation 
  of 
  that 
  at 
  station 
  90. 
  Local 
  eruptions 
  

   of 
  small 
  extent, 
  such 
  as 
  station 
  99, 
  have 
  produced 
  hills 
  outside 
  of 
  the 
  

   mountains 
  proper, 
  but 
  they 
  arc 
  limited 
  in 
  number. 
  Although 
  even 
  

   here, 
  at 
  the 
  very 
  edge 
  of 
  the 
  great 
  valley, 
  the 
  easterly 
  dip 
  can 
  still 
  be 
  

   observed, 
  it 
  has 
  now 
  decreased 
  to 
  3° 
  and 
  4°. 
  On 
  account 
  of 
  the 
  even- 
  

   ness 
  of 
  the 
  flow, 
  the 
  uninterrupted 
  expanse 
  of 
  basalt, 
  we 
  find 
  a 
  very 
  

   level 
  plain 
  formed 
  in 
  this 
  region. 
  It 
  extends 
  for 
  some 
  distanc& 
  along 
  

   the 
  western 
  side 
  of 
  San 
  Luis 
  Valley, 
  and 
  is 
  quite 
  unique 
  in 
  its 
  character. 
  

   Evidently 
  the 
  volcanic 
  material 
  at 
  the 
  time 
  of 
  its 
  eruption 
  must 
  have 
  

   been 
  in 
  a 
  very 
  viscous 
  state, 
  which 
  accounts 
  for 
  its 
  present 
  regularity. 
  

   It 
  is 
  highly 
  probable 
  that 
  at 
  one 
  time 
  the 
  basalt 
  of 
  Sau 
  Luis 
  Valley 
  and 
  

   that 
  sloping 
  down 
  westward 
  from 
  station 
  94 
  and 
  then 
  continued 
  in 
  the 
  

   valley 
  below, 
  were 
  one 
  mass, 
  and 
  that 
  it 
  was 
  also 
  in 
  connection 
  with 
  the 
  

   basalt 
  of 
  station 
  88. 
  Lithologically 
  there 
  is 
  no 
  distinction 
  to 
  be 
  made, 
  

   and 
  the 
  arrangement 
  of 
  this 
  volcanic 
  rock, 
  topographically 
  considered, 
  

   speaks 
  for 
  the 
  former 
  connection. 
  

  

  One 
  main 
  stream 
  still 
  remains 
  before 
  we 
  reach 
  the 
  San 
  Juan 
  drainage. 
  

   Eio 
  Chama 
  heads 
  near 
  station 
  84, 
  and 
  flows 
  almost 
  due 
  south. 
  Its 
  main 
  

   tributary 
  is 
  Rio 
  Brazos, 
  from 
  the 
  east, 
  which 
  joins 
  it 
  at 
  the 
  town 
  of 
  ]N"u- 
  

   tritas. 
  The 
  Chama 
  heads 
  within 
  the 
  glacial 
  area 
  that 
  has 
  been 
  men- 
  

   tioned 
  as 
  existing 
  at 
  the 
  headwaters 
  of 
  the 
  Conejos. 
  From 
  station 
  84 
  

   the 
  glacier 
  bent 
  around 
  in 
  a 
  westerly 
  curve, 
  and 
  entered, 
  foiling 
  steeply, 
  

   the 
  cafion 
  of 
  the 
  Chama. 
  This 
  is 
  not 
  as 
  narrow 
  as 
  those 
  of 
  the 
  Conejos, 
  

   although 
  its 
  walls 
  are 
  composed 
  of 
  the 
  same 
  rocks. 
  Near 
  the 
  head 
  of 
  

   the 
  valley 
  metamorphic 
  schists 
  have 
  been 
  exposed 
  in 
  consequence 
  of 
  the 
  

   removal 
  of 
  the 
  superincumbent 
  trachytes 
  and 
  conglomerates. 
  The 
  ex- 
  

   posure 
  is 
  small 
  only, 
  and 
  the 
  schists 
  show 
  both 
  striation 
  and 
  polishing. 
  

   From 
  there 
  downward 
  the 
  glacier 
  has 
  deposited 
  large 
  moraines, 
  clay, 
  

   bowlders, 
  pebbles, 
  and 
  sand 
  ; 
  everything 
  is 
  thoroughly 
  mixed 
  in, 
  piled 
  up 
  

   in 
  small 
  hills 
  along 
  the 
  course 
  it 
  pursued. 
  At 
  the 
  upper 
  end 
  of 
  the 
  valley 
  

   the 
  appearance 
  of 
  the 
  surface 
  is 
  what 
  might 
  be 
  termed 
  "hummocky." 
  

   Lower 
  down 
  the 
  moraines 
  become 
  more 
  regular; 
  are 
  placed 
  either 
  par- 
  

  

  