﻿ENDLicH.] 
  SAN 
  JUAN 
  EEGION 
  FOX 
  HILLS 
  GROUP. 
  187 
  

  

  rests 
  a 
  heavy 
  bed 
  of 
  sandstone 
  (e), 
  a 
  small 
  portion 
  of 
  which 
  appears 
  to 
  

   crop 
  out 
  east 
  of 
  the 
  station. 
  In 
  that 
  direction 
  the 
  dip 
  of 
  the 
  strata 
  

   gradually 
  decreases, 
  until 
  eventually, 
  at 
  the 
  immediate 
  base 
  of 
  the 
  mount- 
  

   ains, 
  it 
  inclines 
  westward 
  again, 
  and 
  is 
  then 
  soon 
  hidden 
  from 
  sight 
  

   by 
  the 
  overlying 
  trachyte 
  (/). 
  Whether 
  this 
  anticlinal 
  fold 
  is 
  in 
  direct 
  

   connection 
  with 
  that 
  given 
  in 
  Section 
  X 
  I 
  am 
  unable 
  to 
  say, 
  but 
  pre- 
  

   sume 
  that 
  a 
  connection 
  could 
  be 
  traced, 
  subject 
  probably 
  to 
  many 
  local 
  

   variations 
  of 
  the 
  angle 
  of 
  dip. 
  

  

  Viewing 
  this 
  group 
  as 
  a 
  whole 
  from 
  the 
  Piedra 
  southeastward 
  to 
  the 
  

   Navajo, 
  we 
  find 
  that 
  it 
  must 
  be 
  regarded, 
  stratigraphically, 
  as 
  a 
  very 
  

   low 
  anticlinal 
  fold, 
  the 
  axis 
  of 
  which 
  runs 
  east 
  of 
  north. 
  At 
  the 
  same 
  

   time 
  the 
  gentle 
  southerly 
  dip 
  of 
  the 
  sedimentary 
  strata 
  throughout 
  that 
  

   region 
  does 
  not 
  disappear, 
  but 
  combining 
  with 
  the 
  easterly 
  and 
  westerly 
  

   dips 
  on 
  either 
  side 
  of 
  the 
  axis, 
  produces 
  an 
  effect 
  that 
  at 
  first 
  is 
  decidedly 
  

   puzzling. 
  Local 
  dips 
  also 
  greatly 
  affect 
  the 
  youngest 
  Cretaceous 
  group, 
  

   the 
  Fox 
  Hills 
  series, 
  which 
  will 
  be 
  discussed 
  further 
  on. 
  

  

  Southeast 
  of 
  station 
  G3 
  is 
  the 
  valley 
  of 
  the 
  Chama 
  and 
  its 
  tributaries. 
  

   It 
  is 
  formed 
  by 
  shales 
  of 
  the 
  Colorado 
  group. 
  These 
  are 
  covered 
  mostly 
  

   by 
  the 
  Fox 
  Hills 
  sandstones, 
  which 
  gave 
  occasion 
  for 
  the 
  formation 
  of 
  

   the 
  high 
  bluffs 
  to 
  the 
  south 
  and 
  southwest. 
  Near 
  the 
  settlements 
  in 
  the 
  

   valley 
  small 
  tables 
  or 
  mesas 
  occur 
  where 
  basalt 
  overlies 
  the 
  shales. 
  

   They 
  are 
  of 
  limited 
  extent^ 
  and 
  fall 
  beyond 
  the 
  southern 
  border 
  of 
  our 
  

   district. 
  

  

  Here 
  the 
  area 
  of 
  the 
  Colorado 
  group 
  ceases, 
  so 
  far 
  as 
  contained 
  in 
  

   the 
  section 
  to 
  which 
  this 
  chapter 
  is 
  devoted. 
  A 
  comparatively 
  arbi- 
  

   trary 
  division 
  of 
  the 
  entire 
  Cretaceous 
  formation 
  had 
  to 
  be 
  made 
  in 
  

   order 
  to 
  arrive 
  at 
  any 
  acceptable 
  classification 
  whatever. 
  Owing 
  to 
  

   the 
  gradation 
  of 
  sandstones 
  into 
  shales 
  and 
  marls 
  and 
  vice 
  versa, 
  the 
  

   five 
  established 
  Cretaceous 
  groups 
  of 
  Meek 
  and 
  Hayden 
  could 
  no 
  longer 
  

   be 
  recognized, 
  and 
  a 
  different 
  application 
  of 
  the 
  group-names 
  was 
  neces- 
  

   sary. 
  So 
  far 
  as 
  the 
  names 
  apj^lied 
  to 
  the 
  three 
  groups 
  are 
  concerned, 
  

   they 
  are 
  parallel 
  to 
  those 
  of 
  other 
  regions, 
  and 
  indicate 
  as 
  nearly 
  as 
  

   possible 
  the 
  same 
  geological 
  ages. 
  

  

  Fox 
  Hills 
  group. 
  — 
  Resting 
  conformably 
  upon 
  the 
  Colorado 
  shales, 
  we 
  

   find 
  the 
  shales 
  and 
  sandstones 
  of 
  the 
  Fox 
  Hills 
  group. 
  They 
  cover 
  

   quite 
  an 
  extensive 
  area 
  along 
  the 
  northern 
  drainage 
  of 
  the 
  San 
  Juan, 
  

   which 
  area 
  is 
  terminated 
  on 
  the 
  north 
  and 
  east 
  by 
  the 
  appearance 
  of 
  

   lower 
  Cretaceous, 
  on 
  the 
  south 
  by 
  Tertiary 
  beds. 
  On 
  the 
  Animas 
  a 
  good 
  

   section 
  of 
  all 
  the 
  strata, 
  from 
  the 
  Carboniferous 
  upward, 
  is 
  exposed. 
  We 
  

   find 
  there 
  that 
  the 
  hogback-iidge 
  upon 
  which 
  station 
  44 
  of 
  1874 
  is 
  located 
  

   is 
  formed 
  by 
  a 
  yellow 
  sandstone 
  belonging 
  to 
  the 
  Fox 
  Hills. 
  South 
  of 
  this 
  

   a 
  number 
  of 
  lower 
  hogbacks, 
  composed 
  of 
  Cretaceous 
  shales 
  and 
  sand- 
  

   stones, 
  occur, 
  until 
  we 
  reach 
  the 
  base 
  of 
  the 
  Puerco 
  marls. 
  A 
  section 
  

   (Section 
  XII) 
  taken 
  from 
  station 
  36 
  to 
  station 
  42 
  will 
  give 
  some 
  idea 
  of 
  the 
  

   arrangement 
  of 
  the 
  strata. 
  Underlying 
  the 
  Colorado 
  group 
  are 
  the 
  yel- 
  

   low 
  to 
  white 
  Dakota 
  sandstones 
  (a). 
  Above 
  them 
  follow 
  1,000 
  feet 
  of 
  

   dark 
  grey 
  shales 
  (b), 
  containing 
  two 
  interstrata 
  of 
  sandstone 
  (c). 
  These 
  

   shales 
  reach 
  a 
  greater 
  vertical 
  development 
  as 
  we 
  proceed 
  eastward, 
  

   and 
  remain 
  either 
  the 
  same 
  or 
  deci 
  ease 
  as 
  we 
  go 
  farther 
  west. 
  Above 
  the 
  

   second 
  sandstone 
  700 
  feet 
  of 
  lighter 
  grey 
  shales 
  (e) 
  set 
  in, 
  belonging 
  still 
  

   to 
  the 
  Colorado 
  group. 
  They 
  are 
  capped 
  on 
  a 
  small 
  hill 
  north 
  of 
  station 
  

   42 
  by 
  400 
  to 
  600 
  feet 
  of 
  yellow 
  argillaceous 
  sandstone 
  (/), 
  the 
  base 
  of 
  

   the 
  Fox 
  Hills. 
  Passing 
  southward 
  over 
  this 
  rise, 
  we 
  descend 
  into 
  the 
  

   valley 
  of 
  the 
  Rio 
  Nutria, 
  which 
  runs 
  in 
  the 
  lower 
  shales. 
  Facing 
  north- 
  

   ward 
  is 
  a 
  bluff' 
  about 
  1,400 
  feet 
  high. 
  At 
  its 
  foot 
  we 
  find 
  the 
  same 
  

   sandstone 
  (/), 
  overlaid 
  by 
  900 
  feet 
  of 
  yellowish-grey 
  shales 
  {g). 
  They 
  

  

  