﻿244 
  REPORT 
  UNITED 
  STATES 
  GEOLOGICAL 
  SURVEY. 
  

  

  There 
  are 
  three 
  distinct 
  groups 
  of 
  coal-bearing 
  rocks, 
  two 
  being 
  un- 
  

   doubtedly 
  Cretaceous, 
  and 
  the 
  other 
  belonging 
  either 
  in 
  the 
  top 
  of 
  No. 
  

   5 
  Cretaceous 
  or 
  in 
  the 
  lower 
  part 
  of 
  the 
  Laramie 
  Group. 
  The 
  Middle 
  

   Coal 
  Group 
  is 
  between 
  the 
  two 
  escarpments 
  of 
  the 
  Mesa 
  Verde, 
  and 
  the 
  

   Lower 
  in 
  the 
  upper 
  part 
  of 
  the 
  Dakota 
  Sandstones, 
  extending 
  up 
  into 
  the 
  

   Middle 
  Cretaceous 
  shales. 
  

  

  Besides 
  the 
  sedimentary 
  formations 
  there 
  are 
  in 
  this 
  district 
  five 
  dis- 
  

   tinct 
  trachytic 
  areas. 
  These 
  occur 
  in 
  small 
  mountain 
  groups 
  which 
  are 
  

   scattered 
  over 
  the 
  Cretaceous 
  plain. 
  They 
  are 
  the 
  San 
  Miguel, 
  the 
  La 
  

   Plata, 
  the 
  Late, 
  the 
  Carriso, 
  and 
  theAbajo 
  groups. 
  The 
  structure 
  is 
  

   somewhat 
  obscure, 
  although 
  not 
  greatly 
  complicated, 
  and 
  is 
  much 
  alike, 
  

   so 
  far 
  as 
  can 
  be 
  determined, 
  in 
  all 
  cases. 
  The 
  intrusive 
  matter 
  has 
  been 
  

   forced 
  up 
  through 
  crevices 
  in 
  the 
  superincumbent 
  sedimentary 
  rocks, 
  

   encroaching 
  as 
  it 
  rose 
  upon 
  the 
  less 
  firm 
  rocks, 
  sending 
  out 
  wedge-like 
  

   masses 
  between 
  the 
  yielding 
  strata, 
  .and 
  finally 
  resting, 
  so 
  far 
  as 
  can 
  be 
  

   observed, 
  in 
  greatest 
  bulk 
  among 
  the 
  easily-yielding 
  strata 
  of 
  the 
  Mid- 
  

   dle 
  Cretaceous. 
  

  

  In 
  most 
  cases 
  there 
  has 
  been 
  a 
  slight 
  elevation 
  of 
  the 
  surrounding 
  

   sedimentary 
  beds, 
  which 
  effect 
  is 
  probably 
  produced 
  partly 
  by 
  the 
  in- 
  

   trusion 
  of 
  sheets 
  and 
  wedges, 
  and 
  partly 
  by 
  the 
  upward 
  movement 
  of 
  

   the 
  fluid 
  or 
  plastic 
  mass. 
  Tne 
  structure 
  of 
  these 
  trachytic 
  mountains 
  is 
  

   fully 
  illustrated 
  in 
  Plates 
  XXXVI, 
  XLY, 
  and 
  XLVI. 
  

  

  The 
  distribution 
  of 
  these 
  groups, 
  as 
  well 
  as 
  the 
  positions 
  of 
  all 
  the' 
  

   important 
  topographical 
  features, 
  the 
  mesas, 
  the 
  caiions, 
  and 
  stream- 
  

   courses 
  of 
  our 
  district, 
  is 
  clearly 
  shown 
  in 
  the 
  accompanying 
  bird's- 
  

   eye 
  view 
  (Plate 
  XXXVI). 
  I 
  have 
  taken 
  pains 
  to 
  give 
  all 
  the 
  features 
  

   in 
  considerable 
  detail. 
  

  

  The 
  point 
  of 
  view 
  assumed 
  is 
  in 
  New 
  Mexico, 
  south 
  of 
  the 
  most 
  

   southerly 
  bend 
  of 
  the 
  Rio 
  San 
  Juan, 
  and 
  at 
  an 
  elevation 
  of 
  a 
  few 
  thou- 
  

   sand 
  feet 
  above 
  the 
  plain. 
  In 
  looking 
  to 
  the 
  north 
  from 
  such 
  a 
  point 
  a 
  

   fine 
  view 
  of 
  the 
  plateau 
  border 
  region 
  would 
  be 
  obtained. 
  The 
  San 
  Juan 
  

   Eiver 
  and 
  the 
  Carriso 
  Mountains 
  would 
  appear 
  in 
  the 
  foreground. 
  The 
  

   Mesa 
  Verde, 
  the 
  La 
  Plata, 
  and 
  Maucos 
  Valleys, 
  and 
  the 
  La 
  Plata 
  and 
  

   Late 
  Mountains 
  in 
  the 
  middle 
  distance, 
  and 
  the 
  Rio 
  Dolores, 
  the 
  San 
  

   Miguel, 
  the 
  Abajo, 
  the 
  La 
  Sal, 
  and 
  the 
  Henry 
  Mountains 
  in 
  the 
  dis- 
  

   tance. 
  The 
  appended 
  sections 
  will 
  be 
  understood 
  at 
  a 
  glance. 
  I 
  have 
  

   preferred 
  not 
  only 
  to 
  give 
  the 
  sections 
  across 
  my 
  district, 
  but 
  also 
  to 
  

   give, 
  on 
  a 
  reduced 
  scale, 
  their 
  continuations 
  to 
  the 
  east, 
  west, 
  north, 
  or 
  

   south, 
  as 
  the 
  case 
  may 
  be, 
  through 
  the 
  districts 
  examined 
  by 
  Dr. 
  Peale, 
  

   Dr. 
  Endlich, 
  Mr. 
  Gilbert, 
  and 
  others. 
  The 
  sections 
  are, 
  as 
  far 
  as 
  possi- 
  

   ble, 
  drawn 
  through 
  the 
  more 
  important 
  points. 
  The 
  lines 
  along 
  which 
  

   they 
  have 
  been 
  made 
  are 
  laid 
  down 
  on 
  the 
  accotnpanyiug 
  map 
  (Plate 
  

   XXXVII.) 
  

  

  In 
  this 
  part 
  of 
  Colorado 
  there 
  are 
  comparatively 
  few 
  well-marked 
  

   folds. 
  The 
  only 
  one 
  of 
  importance 
  that 
  crosses 
  my 
  district 
  extends 
  

   from 
  the 
  base 
  of 
  Sierra 
  La 
  Plata 
  down 
  into 
  New 
  Mexico, 
  maintaining 
  in 
  

   general 
  a 
  southwest 
  direction. 
  It 
  is 
  essentially 
  monoclinal 
  in 
  character", 
  

   the 
  depressed 
  side 
  being 
  to 
  the 
  east 
  and 
  southeast. 
  

  

  I 
  have 
  found 
  it 
  convenient, 
  in 
  reviewing 
  the 
  general 
  surface 
  geology 
  

   of 
  the 
  various 
  river-basins, 
  to 
  give 
  in 
  the 
  same 
  connection 
  such 
  topo- 
  

   graphic 
  and 
  other 
  details 
  as 
  seemed 
  especially 
  worthy 
  of 
  record. 
  

  

  