﻿390 
  REPORT 
  UNITED 
  STATES 
  GEOLOOICAL 
  SURVEY. 
  

  

  sandy 
  shales 
  and 
  marls 
  of 
  this 
  group 
  weather 
  very 
  much 
  like 
  the 
  for- 
  

   mation 
  of 
  the 
  'bad-lands.'" 
  

  

  Professor 
  Newberry* 
  regards 
  these 
  marls, 
  as 
  well 
  as 
  the 
  over- 
  

   lying 
  sandstones, 
  as 
  Cretaceous. 
  Cope 
  is 
  positive 
  in 
  his 
  identification 
  

   as 
  Eocene, 
  however, 
  and 
  by 
  comparing 
  carefully 
  the 
  descriptions, 
  &c., 
  

   given 
  by 
  Kewberry 
  and 
  Cope 
  with 
  my 
  own 
  observations, 
  T 
  do 
  not 
  doubt 
  

   that 
  we 
  have 
  on 
  the 
  Lower 
  Animas 
  the 
  same 
  formation 
  in 
  which 
  Cope 
  

   found 
  (p. 
  89) 
  " 
  a 
  lower 
  molar 
  of 
  Bafhnodony 
  A 
  section 
  given 
  by 
  ]S"ew- 
  

   berry, 
  taken 
  near 
  his 
  camp, 
  (46,)t 
  shows 
  the 
  same 
  general 
  arraniiement 
  

   and 
  physical 
  character 
  that 
  has 
  been 
  noticed 
  by 
  Cope, 
  Holmes, 
  and 
  

   myself. 
  Farther 
  east 
  these 
  variegated 
  marls 
  gradually 
  change 
  into 
  

   shales 
  and 
  sandstones, 
  so 
  that 
  they 
  are 
  no 
  longer 
  characteristic. 
  

   Above 
  them 
  we 
  find 
  in 
  our 
  section 
  about 
  1,000 
  feet 
  of 
  yellow 
  to 
  

   brown 
  sandstones 
  and 
  shales 
  {/); 
  As 
  a 
  rule 
  the 
  beds 
  of 
  sandstone 
  are 
  

   heavy, 
  weathering 
  massively, 
  but 
  they 
  frequently 
  show 
  but 
  small 
  thick- 
  

   ness 
  and 
  are 
  interstratifled 
  with 
  yellow 
  and 
  greyish 
  shales. 
  In 
  some 
  of 
  

   the 
  shales 
  indications 
  of 
  coal 
  may 
  be 
  observed, 
  but 
  nowhere 
  through- 
  

   out 
  the 
  San 
  Juan 
  region 
  was 
  any 
  vein 
  found 
  that 
  would 
  have 
  been 
  suf- 
  

   fiiciently 
  large 
  or 
  of 
  good 
  quality 
  to 
  be 
  worked. 
  

  

  All 
  the 
  lower 
  caiBions 
  of 
  the 
  San 
  Juan 
  drainage, 
  and 
  that 
  of 
  the 
  river 
  

   itself, 
  are 
  formed 
  by 
  this 
  series 
  of 
  sandstones 
  and 
  others 
  superincum- 
  

   bent. 
  Over 
  the 
  entire 
  region 
  which 
  they 
  cover, 
  they 
  are 
  uniform, 
  both 
  

   in 
  occurrence 
  and 
  in 
  lithological 
  character. 
  Their 
  very 
  small 
  dip 
  to 
  

   the 
  south, 
  2° 
  to 
  4°, 
  and 
  their 
  total 
  thickness 
  of 
  about 
  3,000 
  feet, 
  enaljles 
  

   them 
  to 
  extend 
  over 
  a 
  large 
  area 
  of 
  country, 
  so 
  long 
  as 
  neither 
  of 
  these 
  

   conditions 
  are 
  seriously, 
  changed. 
  Farther 
  south 
  than 
  our 
  investiga- 
  

   tions 
  carried 
  us, 
  Cope 
  has 
  discovered 
  higher 
  Tertiary 
  beds 
  that 
  yielded 
  

   vast 
  numbers 
  of 
  fossils 
  of 
  unique 
  interest.^ 
  Within 
  the 
  borders 
  of 
  our 
  

   district 
  we 
  found 
  none, 
  excepting 
  some 
  poorly 
  preserved 
  leaves 
  and 
  

   silicified 
  wood. 
  

  

  In 
  its 
  highest 
  members 
  (so 
  far 
  as 
  seen 
  by 
  us) 
  the 
  sandstone-shale 
  

   series 
  becomes 
  more 
  changeable. 
  Interstrata 
  of 
  shales 
  and 
  marls 
  oc- 
  

   cur, 
  mostly 
  yellow 
  and 
  grey, 
  containing 
  coaled 
  remains 
  of 
  plants, 
  and, 
  

   in 
  several 
  instances, 
  finely 
  developed 
  beds 
  of 
  selenite. 
  

  

  VOLCANICS. 
  

  

  Volcanic 
  outcrops 
  are 
  but 
  local 
  in 
  this 
  region 
  of 
  the 
  Cretaceous 
  and 
  

   Tertiary. 
  A 
  number 
  of 
  dikes 
  were 
  observed, 
  besides 
  the 
  capping 
  of 
  the 
  

   Cerro 
  del 
  ISTavajo. 
  The 
  latter 
  is 
  composed 
  of 
  a 
  black, 
  vesicular 
  basalt, 
  

   about 
  250 
  feet 
  in 
  thickness. 
  

  

  Fo» 
  the 
  dikes 
  no 
  prevalent 
  direction 
  could 
  be 
  established. 
  Many 
  of 
  

   them 
  trend 
  west 
  of 
  north, 
  but 
  others 
  show 
  directions 
  totally 
  different. 
  

   Station 
  41 
  is 
  located 
  on 
  basalt 
  that 
  evidently 
  came 
  up 
  through 
  a 
  chim- 
  

   ney-like 
  fissure 
  and 
  formed 
  a 
  mound 
  about 
  400 
  feet 
  high, 
  on 
  the 
  Colorado 
  

   shales. 
  Other 
  dikes 
  show 
  the 
  same 
  characteristics 
  that 
  are 
  elsewhere 
  

   observed. 
  They 
  stand 
  out 
  prominently, 
  if 
  they 
  have 
  penetrated 
  sand- 
  

   stones 
  or 
  sandy 
  shales, 
  because 
  these 
  yield 
  readily 
  to 
  erosion. 
  When 
  

   we 
  find 
  them 
  in 
  tough 
  shales, 
  they 
  have 
  not 
  unfrequently 
  given 
  rise 
  

   to 
  the 
  formation 
  of 
  hogback-shaped 
  hills. 
  This 
  is 
  due 
  mainly 
  to 
  the 
  

   fact 
  that 
  the 
  shales 
  on 
  either 
  side 
  of 
  the 
  dike 
  have 
  been 
  hardened 
  

  

  * 
  Report 
  of 
  Exploring 
  Expedition 
  of 
  J. 
  M. 
  Macomb, 
  18r)9-1876, 
  p. 
  80. 
  

  

  t 
  Annual 
  Report 
  of 
  Explorations 
  and 
  Surveys 
  West 
  of 
  the 
  One 
  Hundredth 
  Meridian, 
  

   Appendix 
  44, 
  1875, 
  p. 
  111. 
  

  

  t 
  Since 
  writing 
  the 
  above 
  I 
  have 
  seen 
  Professor 
  Cope, 
  and 
  comparing 
  notes, 
  we 
  

   have 
  established 
  the 
  two 
  horizons 
  of 
  variegated 
  marls. 
  The 
  lower 
  one 
  on 
  the 
  Animas, 
  

   which 
  extends 
  south, 
  is 
  devoid 
  of 
  fossils, 
  while 
  the 
  upper, 
  not 
  reaching 
  northward 
  

   into 
  our 
  district, 
  furnishes 
  numerous 
  most 
  interesting 
  species. 
  — 
  E. 
  

  

  