﻿274 
  REPORT 
  UNITED 
  STATES 
  GEOLOGICAL 
  SURVEY. 
  

  

  zontal 
  bed 
  of 
  trachyte 
  that 
  falls 
  off 
  in 
  rounded 
  bluffs. 
  It 
  seems 
  to 
  be 
  

   interbedded 
  between 
  the 
  Dakota 
  sandstones 
  and 
  the 
  Middle 
  Cretaceous 
  

   shales, 
  and 
  .rests 
  upon 
  the 
  upper 
  surface 
  of 
  the 
  former. 
  

  

  SIERRA 
  CARRISO. 
  

  

  In 
  the 
  extreme 
  southwestern 
  corner 
  of 
  our 
  district 
  and 
  chiefly 
  in 
  

   Arizona 
  is 
  the 
  interesting 
  group 
  of 
  mountains 
  known 
  as 
  the 
  Sierra 
  

   Cairiso. 
  Its 
  structure 
  is 
  somewLiat 
  more 
  clearly 
  defined 
  than 
  that 
  of 
  

   the 
  Late 
  group, 
  as 
  the 
  intruded 
  trachytes 
  occur 
  in 
  larger 
  and 
  more 
  com- 
  

   I)act 
  masses 
  and 
  the 
  surrounding 
  sedimentary 
  rocks 
  are 
  but 
  slightly 
  

   disturbed. 
  It 
  is 
  a 
  typical 
  example 
  of 
  the 
  eruptive 
  groups 
  of 
  this 
  part 
  

   of 
  the 
  Colorado 
  Plateau. 
  It 
  stands 
  alone, 
  an 
  island 
  in 
  the 
  midst 
  of 
  a 
  

   sedimentary 
  sea. 
  It 
  has 
  a 
  nucleus 
  of 
  its 
  own, 
  and 
  so 
  far 
  as 
  the 
  surface 
  

   is 
  concerned 
  is 
  independent 
  of 
  all 
  other 
  eruptive 
  masses. 
  The 
  masses 
  

   of 
  trachyte 
  were 
  not 
  poured 
  out 
  over 
  the 
  surface 
  of 
  the 
  country 
  but 
  

   lodged 
  between 
  the 
  sedimentary 
  strata, 
  producing 
  a 
  more 
  or 
  less 
  sym- 
  

   metrical 
  doming 
  of 
  those 
  beds 
  that 
  were 
  not 
  penetrated. 
  Beyond 
  the 
  

   immediate 
  vicinity 
  of 
  the 
  trachy 
  tic 
  masses 
  the 
  strata 
  remain 
  compara- 
  

   tively 
  undisturbed. 
  

  

  The 
  trachytes 
  are 
  now 
  found 
  chiefly 
  in 
  contact 
  with 
  the 
  Lower 
  Creta- 
  

   ceous 
  and 
  Jura-Trias 
  rocks, 
  for 
  the 
  reason 
  that 
  the 
  Middle 
  Cretaceous 
  

   shales, 
  in 
  which 
  a 
  large 
  part 
  of 
  the 
  trachytes 
  were 
  originally 
  deposited, 
  

   have 
  been 
  completely 
  carried 
  away, 
  leaving 
  only 
  small 
  fragments 
  im- 
  

   bedded 
  in 
  the 
  faces 
  and 
  upper 
  surfaces 
  of 
  the 
  trachyte. 
  The 
  opportu- 
  

   nities 
  for 
  study 
  are 
  unusually 
  fine, 
  but 
  my 
  time 
  was 
  very 
  limited, 
  and 
  

   I 
  will 
  not 
  attempt 
  a 
  closely 
  detailed 
  analysis 
  of 
  the 
  group. 
  The 
  trachytes 
  

   occupy 
  an 
  area 
  of 
  nearly 
  100 
  square 
  miles. 
  The 
  higher 
  points 
  rise 
  to 
  

   an 
  altitude 
  of 
  about 
  9,000 
  feet 
  above 
  sea 
  level, 
  and 
  some 
  2,000 
  feet 
  

   above 
  the 
  general 
  level 
  of 
  the 
  country. 
  The 
  northeast, 
  north, 
  and 
  north 
  

   west 
  faces 
  are 
  drained 
  by 
  the 
  San 
  Juan 
  and 
  its 
  tributaries, 
  including 
  

   Eed 
  Creek, 
  Navajo 
  Creek, 
  Arido 
  Creek, 
  Gothic 
  Creek, 
  and 
  th-c 
  Kio 
  De 
  

   Chelly. 
  In 
  apj)earance 
  it 
  is 
  a 
  rather 
  striking 
  group, 
  and 
  rises 
  in 
  the 
  east- 
  

   ern 
  and 
  central 
  portions 
  into 
  a 
  number 
  of 
  rugged 
  irregular 
  ridges 
  and 
  

   l>eaks, 
  but 
  on 
  the 
  north 
  and 
  west 
  is 
  bordered 
  by 
  remnants 
  of 
  a 
  high 
  ^able 
  

   land. 
  (See 
  sketch, 
  Plate 
  XLVIII.) 
  There 
  is 
  nearly 
  everywhere 
  a 
  sparse 
  

   growth 
  of 
  piiions 
  and 
  pines. 
  Good 
  pasturage 
  is 
  found 
  about 
  the 
  bases 
  

   as 
  well 
  as 
  on 
  the 
  interior 
  highlands. 
  Water 
  is 
  unusually 
  scarce. 
  We 
  

   observed 
  no 
  living 
  streams, 
  but 
  there 
  are 
  a 
  number 
  of 
  good 
  springs 
  along 
  

   the 
  north 
  and 
  east 
  bases. 
  

  

  In 
  the 
  central 
  part 
  of 
  the 
  group 
  is 
  a 
  high 
  summit 
  to 
  which 
  we 
  have 
  

   given 
  the 
  name 
  Pastora 
  Peak 
  (c, 
  Plate 
  XLVIII), 
  from 
  the 
  fact 
  that 
  it 
  

   overlooks 
  the 
  highland 
  meadows 
  in 
  which 
  the 
  Navajo 
  shepherds 
  keep 
  

   their 
  sheep. 
  This 
  peak 
  is 
  composed 
  of 
  a 
  brownish 
  gray 
  trachyte, 
  and 
  

   in 
  common 
  with 
  a 
  number 
  of 
  other 
  high 
  points 
  belongs 
  to 
  the 
  main 
  

   central 
  trachytic 
  mass 
  of 
  the 
  mountains. 
  Distinct 
  from 
  this 
  masR 
  and 
  

   lying 
  along 
  the 
  northern 
  and 
  western 
  flanks 
  are 
  two 
  high 
  table-lauds 
  

   (a 
  and 
  b), 
  capped 
  with 
  massive 
  sheets 
  of 
  trachyte. 
  It 
  will 
  be 
  seen 
  by 
  

   reference 
  to 
  the 
  sketch 
  that 
  these 
  masses 
  of 
  trachyte 
  rest 
  upon 
  a 
  

   heavy 
  series 
  of 
  sandstones, 
  and 
  do 
  not 
  connect 
  with 
  the 
  central 
  tra- 
  

   chytic 
  mass 
  in 
  any 
  way, 
  but 
  are 
  separated 
  from 
  it 
  by 
  the 
  series 
  of 
  

   sandstones. 
  The 
  remarkable 
  feature 
  is 
  that 
  portions 
  of 
  the 
  sandstones 
  

   are 
  apparently 
  arched 
  over 
  the 
  central 
  trachytic 
  mass, 
  as 
  may 
  be 
  seen 
  

   at 
  g 
  in 
  the 
  sketch, 
  and 
  that 
  the 
  massive 
  sheets 
  of 
  trachyte 
  that 
  cap 
  

   the 
  two 
  n:esas 
  are 
  also 
  flexed 
  with 
  the 
  sandstones 
  and 
  appear 
  as 
  if 
  

   they 
  might 
  at 
  one 
  time 
  have 
  formed 
  part 
  of 
  the 
  arch. 
  Unfortu- 
  

  

  