﻿182 
  EEPOET 
  UNITED 
  STATES 
  GEOLOGICAL 
  SUEVEY. 
  

  

  - 
  A 
  short 
  distance 
  southeast 
  of 
  station 
  36 
  the 
  Dakota 
  sandstone 
  is 
  en- 
  

   tirely 
  covered 
  by 
  trachorheites. 
  We 
  do 
  not 
  again 
  find 
  it 
  along 
  the 
  

   westernedgeof 
  the 
  mountains, 
  until 
  we 
  reach, 
  farther 
  south, 
  the 
  canons 
  

   that 
  have 
  cut 
  down 
  deeply, 
  removing 
  the 
  superincumbent 
  beds 
  and 
  ex- 
  

   posing 
  the 
  Lower 
  Cretaceous. 
  Unless 
  local 
  d 
  isturbances, 
  of 
  small 
  extent 
  

   only, 
  have 
  taken 
  place, 
  we 
  find 
  the 
  dip 
  of 
  these 
  sedimentaries 
  to 
  be 
  

   away 
  from 
  the 
  mountains. 
  The 
  sandstone, 
  which 
  is 
  comparatively 
  

   yielding 
  to 
  active 
  erosive 
  influences, 
  forms 
  narrow 
  canons 
  with 
  steep 
  

   walls. 
  At 
  many 
  localities, 
  besides 
  the 
  one 
  under 
  discussion, 
  this 
  may 
  

   be 
  regarded 
  as 
  a 
  regularly-recurring 
  feature. 
  Although 
  in 
  many 
  in- 
  

   stances, 
  of 
  course, 
  erosion 
  by 
  flowing 
  water, 
  or 
  by 
  glaciers, 
  has 
  widened 
  

   the 
  valley 
  composed 
  of 
  or 
  walled 
  in 
  by 
  these 
  sandstones, 
  it 
  is 
  not 
  the 
  

   rule, 
  but 
  rather 
  an 
  exception. 
  But 
  a 
  short 
  distance 
  from 
  the 
  mountains, 
  

   the 
  Colorado 
  shales 
  regularly 
  set 
  in. 
  

  

  Traveling 
  along 
  the 
  base 
  of 
  the 
  mountains 
  in 
  a 
  southerly 
  direction, 
  

   we 
  no 
  longer 
  find 
  any 
  outcrops 
  of 
  the 
  Dakota 
  sandstones. 
  They 
  are 
  

   either 
  covered 
  by 
  the 
  volcanic 
  rocks, 
  or 
  by 
  the 
  avalanchial 
  and 
  glacial 
  

   drift 
  immediately 
  at 
  the 
  western 
  foot 
  of 
  the 
  Sawatch 
  Range. 
  On 
  a 
  north- 
  

   ern 
  tributary 
  of 
  Rio 
  Blanco 
  we 
  met 
  with 
  the 
  first 
  exposure 
  again. 
  It 
  was 
  

   found 
  in 
  a 
  small 
  caSon 
  leading 
  up 
  toward 
  station 
  77, 
  and 
  there 
  the 
  yel- 
  

   low 
  sandstones 
  were 
  exposed 
  along 
  the 
  bed 
  of 
  the 
  creek. 
  No 
  local 
  dis- 
  

   turbance 
  was 
  noticed, 
  and 
  the 
  beds 
  had 
  the 
  prevailing 
  general 
  dip 
  toward 
  

   the 
  southwest. 
  Following 
  up 
  Rio 
  Blanco 
  we 
  fiud 
  the 
  same 
  outcrop 
  

   again, 
  appearing 
  in 
  its 
  upper 
  valley. 
  At 
  these 
  localities 
  the 
  sandstones 
  

   cover 
  but 
  a 
  small 
  area, 
  as 
  they 
  are 
  soon 
  hidden 
  from 
  sight 
  by 
  the 
  vol- 
  

   canic 
  beds 
  on 
  the 
  east 
  and 
  the 
  shales 
  of 
  the 
  Colorado 
  group 
  on 
  the 
  west. 
  

   A 
  more 
  extensive 
  exposure 
  can 
  be 
  observed 
  in 
  the 
  upper 
  valley 
  of 
  the 
  

   ISTavajo. 
  A 
  prominent 
  ridge. 
  Sierra 
  del 
  Navajo, 
  forms 
  a 
  detached 
  spur 
  

   of 
  the 
  main 
  range, 
  and 
  having 
  a 
  trend 
  north 
  to 
  south, 
  permits 
  the 
  forma- 
  

   tion 
  of 
  a 
  valley 
  between 
  itself 
  and 
  the 
  range. 
  In 
  this 
  valley 
  the 
  Dakota 
  

   sandstone 
  flanksjts 
  edge, 
  while 
  near 
  the 
  center 
  it 
  is 
  overlaid 
  by 
  Colorado 
  

   shales. 
  An 
  anticlinal 
  fold, 
  occurring 
  a 
  short 
  distance 
  farther 
  off 
  toward 
  

   the 
  southwest, 
  has 
  changed 
  the 
  dip, 
  making 
  it 
  slightly 
  to 
  the 
  east, 
  or 
  very 
  

   nearly 
  horizontal, 
  instead 
  of 
  westward. 
  After 
  leaving 
  this 
  narrow 
  valley, 
  

   the 
  sandstone 
  soon 
  disai)pears 
  under 
  the 
  volcanics. 
  We 
  again 
  notice 
  it 
  

   in 
  the 
  upper 
  valley 
  of 
  the 
  Chama. 
  At 
  its 
  exit 
  from 
  the 
  mountains 
  this 
  

   riv^er 
  flows 
  through 
  a 
  narrow 
  caiion 
  with 
  vertical 
  walls 
  for 
  a 
  short 
  dis- 
  

   tance. 
  This 
  is 
  formed 
  by 
  Dakota 
  sandstone. 
  From 
  there 
  it 
  extends 
  

   northward, 
  covered 
  on 
  either 
  side 
  by 
  trachyte, 
  until 
  its 
  outcrop 
  is 
  lost 
  

   under 
  the 
  morainal 
  deposits 
  of 
  Conejos 
  glacier. 
  So 
  far 
  as 
  could 
  be 
  ob- 
  

   served, 
  the 
  dip 
  was 
  a 
  westerly 
  one, 
  although 
  slight. 
  Professor 
  Stev^en- 
  

   son* 
  mentions 
  Carboniferous 
  strata 
  " 
  in 
  an 
  almost 
  vertical 
  position" 
  as 
  

   occurring 
  on 
  the 
  trail 
  from 
  Tierra 
  Amarilla 
  to 
  Conejos. 
  I 
  did 
  not 
  pass 
  

   over 
  the 
  trail, 
  but 
  from 
  analogy 
  with 
  the 
  exposures 
  farther 
  north, 
  along 
  

   the 
  range, 
  I 
  should 
  feel 
  inclined 
  to 
  question 
  the 
  identification 
  as 
  Car- 
  

   boniferous. 
  

  

  In 
  our 
  district 
  there 
  is 
  but 
  one 
  more 
  outcrop 
  of 
  the 
  Dakota 
  group. 
  

   This 
  occurs 
  west 
  of 
  station 
  94, 
  between 
  the 
  Chama 
  and 
  the 
  Brazos. 
  

   Dipping 
  steeply 
  from 
  the 
  high 
  quartzitic 
  mass 
  upon 
  which 
  the 
  station 
  

   was 
  located, 
  the 
  sandstones 
  fall 
  off 
  toward 
  the 
  Tierra 
  Amarilla 
  Valley. 
  

   They 
  are 
  covered 
  in 
  part 
  by 
  basalt, 
  which 
  again 
  appears 
  on 
  small 
  tables 
  

   in 
  the 
  valley 
  itself. 
  The 
  road 
  from 
  Puenta 
  to 
  Nutritas 
  passes 
  over 
  such 
  

   basalt, 
  although 
  there 
  it 
  covers 
  shales 
  of 
  the 
  Colorado 
  group. 
  Only 
  for 
  

   a 
  short 
  distance, 
  flanking 
  the 
  mountains 
  and 
  then 
  extending 
  down 
  along 
  

   the 
  river-beds, 
  does 
  the 
  Dakota 
  sandstone 
  there 
  appear 
  within 
  the 
  limits 
  

  

  * 
  Explorations 
  and 
  Surveys 
  West 
  of 
  the 
  One 
  Hundredth 
  Meridian, 
  1875, 
  p. 
  375. 
  

  

  