﻿136 
  REPOET 
  UNITED 
  STATES 
  GEOLOGICAL 
  SURVEY. 
  

  

  nortbern 
  and 
  western 
  sides 
  of 
  the 
  luonntains. 
  East 
  and 
  south 
  they 
  pen- 
  

   etrate 
  the 
  Cretaceous 
  and 
  Post-Cretaceous 
  (lignitic) 
  beds. 
  Although 
  the 
  

   system 
  of 
  dikes 
  does 
  not 
  continue' 
  into 
  this 
  latter 
  group 
  to 
  any 
  great 
  

   extent, 
  a 
  number 
  of 
  dikes 
  belonging 
  to 
  the 
  same 
  age 
  as 
  the 
  others 
  were 
  

   found, 
  showing 
  also 
  the 
  same 
  lithological 
  character. 
  It 
  is 
  evident, 
  

   therefore, 
  that 
  the 
  time 
  for 
  the 
  eraption 
  of 
  all 
  this 
  volcanic 
  material 
  

   must 
  be 
  placed 
  at 
  a 
  period 
  subsequent 
  to 
  the 
  deposition 
  of 
  the 
  Lignitic 
  

   group. 
  This, 
  so 
  far 
  as 
  1 
  am 
  informed, 
  will 
  agree 
  with 
  the 
  relative 
  age 
  

   accepted 
  for 
  analogous 
  eruptions 
  in 
  other 
  regions 
  of 
  Colorado. 
  Dr. 
  A. 
  

   C. 
  Peale 
  will 
  publish 
  in 
  a 
  Bulletin 
  for 
  1877 
  a 
  thorough 
  synopsis 
  of 
  all 
  

   occurrences 
  referable 
  to 
  this 
  class 
  of 
  volcanic 
  rocks, 
  and 
  in 
  that 
  publi- 
  

   cation 
  will 
  furnish 
  a 
  digest 
  of 
  all 
  the 
  principal 
  features 
  thus 
  far 
  ob- 
  

   served 
  in 
  connection 
  therewiih. 
  

  

  DEIFT. 
  

  

  There 
  are 
  found 
  in 
  this 
  district 
  no 
  very 
  extensive 
  drift 
  areas, 
  save 
  

   that 
  on 
  the 
  west 
  side 
  of 
  Sangre 
  de 
  Cristo 
  Range, 
  south 
  of 
  Fort 
  Garland. 
  

   Adjoining 
  the 
  trachytes 
  of 
  the 
  station 
  114 
  group, 
  we 
  find 
  that 
  the 
  bluffs 
  

   skirting 
  the 
  mountains 
  are 
  continued 
  northward. 
  In 
  form 
  and 
  general 
  

   character 
  they 
  resemble 
  the 
  volcanic 
  bluffs, 
  but 
  upon 
  examination 
  prove 
  

   to 
  consist 
  of 
  drift 
  of 
  a 
  peculiar 
  nature. 
  Stations 
  115, 
  117, 
  and 
  118 
  were 
  

   located 
  on 
  it. 
  In 
  my 
  field-notes 
  I 
  have 
  designated 
  it 
  as 
  ''compact 
  drift,'' 
  

   on 
  account 
  of 
  its 
  congiomeritic 
  nature. 
  Large 
  and 
  small 
  bowldeis 
  and 
  

   pebbles 
  have 
  been 
  cemented 
  loosely 
  by 
  quartz-sand, 
  and 
  clay, 
  and 
  form 
  

   the 
  bluffs 
  that 
  rise 
  a 
  thousand 
  feet 
  above 
  the 
  level 
  of 
  the 
  valley. 
  At 
  

   the 
  southern 
  end 
  of 
  the 
  outcrop 
  of 
  this 
  curious 
  material 
  metamorphic 
  

   and 
  some 
  trachytic 
  rocks, 
  originating 
  in 
  the 
  Sangre 
  de 
  Cristo 
  Range, 
  

   compose 
  the 
  drift. 
  It 
  could 
  not 
  be 
  determined 
  which 
  of 
  the 
  two 
  pre- 
  

   dominated. 
  Bowlders 
  of 
  all 
  sizes, 
  weighiitg 
  from 
  a 
  ton 
  down 
  to 
  the 
  

   smallest 
  pebbles, 
  occur. 
  All 
  of 
  them 
  are 
  well 
  water-worn, 
  but 
  show 
  

   striation 
  only 
  in 
  very 
  rare 
  instances. 
  Station 
  117, 
  9,583 
  feet 
  above 
  

   sea-level, 
  shows 
  these 
  features 
  very 
  well. 
  Gradual 
  aqueous 
  erosion 
  has 
  

   loosened 
  a 
  large 
  quantity 
  of 
  the 
  bowldei^s 
  and 
  pebbles, 
  and 
  has 
  deposited 
  

   them 
  in 
  the 
  valleys 
  of 
  the 
  adjoining 
  streams, 
  where 
  they 
  greatly 
  im- 
  

   pede 
  the 
  study 
  of 
  the 
  strata 
  underlying. 
  

  

  Immediately 
  east 
  of 
  this 
  north 
  to 
  south 
  exposure 
  of 
  compact 
  drift 
  we 
  

   find 
  the 
  red 
  Carboniferous 
  sandstones, 
  which 
  are 
  in 
  part 
  covered 
  by 
  the 
  

   ibrmer. 
  The 
  bluffs 
  continue 
  northward, 
  until 
  within 
  about 
  6 
  miles 
  of 
  

   Fort 
  Garland. 
  Here 
  the 
  composition 
  of 
  the 
  drift 
  changes. 
  Instead 
  of 
  

   only 
  metamorphic 
  rocks 
  and 
  trachytes 
  being 
  found 
  among 
  the 
  erratic 
  

   material, 
  basalt 
  and 
  dolerite 
  also 
  occur. 
  Station 
  118 
  is 
  located 
  on 
  one 
  

   of 
  these 
  hills, 
  and 
  it 
  was 
  a 
  matter 
  of 
  some 
  difficulty 
  to 
  get 
  the 
  animals 
  

   up 
  hill 
  on 
  account 
  the 
  innumerable 
  bowlders, 
  all 
  round, 
  that 
  covered 
  

   the 
  side 
  of 
  the 
  bluff. 
  So 
  far 
  as 
  could 
  be 
  determined, 
  a 
  thin 
  layer 
  of 
  

   basalt 
  about 
  150 
  feet 
  in 
  thickness 
  covers 
  a 
  number 
  of 
  these 
  small 
  i)luffs. 
  

   It 
  seems 
  strange, 
  at 
  first 
  sight, 
  that 
  basalt 
  should 
  cover 
  drift 
  that 
  is 
  

   composed, 
  in 
  part, 
  of 
  the 
  same 
  volcanic 
  rock. 
  Considering, 
  however, 
  

   over 
  how 
  very 
  long 
  a 
  time 
  the 
  period 
  allotted 
  to 
  each 
  one 
  of 
  the 
  volcanic 
  

   formations 
  must 
  have 
  extended, 
  there 
  is 
  no 
  reason 
  why 
  not 
  a 
  large 
  mass 
  

   of. 
  basalt 
  or 
  any 
  other 
  volcanic 
  material 
  should 
  have 
  become 
  rigid, 
  

   should 
  have 
  been 
  eroded 
  and 
  even 
  partly 
  carried 
  away 
  before 
  the 
  next 
  

   flow 
  took 
  place.* 
  

  

  * 
  In 
  some 
  of 
  the 
  Liassic 
  formations 
  of 
  Southern 
  Germany 
  it 
  is 
  not 
  a 
  very 
  rare 
  thing 
  to 
  

   find 
  the 
  pctrifi(x] 
  water-worn 
  fragments 
  of 
  Ammonites, 
  &c., 
  imbedded 
  in 
  the 
  limestones 
  of 
  

   the 
  very 
  strata 
  of 
  which 
  they 
  are 
  considered 
  to 
  be 
  characteristic 
  in 
  perfect 
  specimens. 
  

  

  