﻿EXDucH.] 
  SANGEE 
  DE 
  CRISTO 
  VOLCANICS. 
  127 
  

  

  that 
  time 
  the 
  Carboniferous 
  strata 
  had 
  already 
  been 
  subjected 
  to 
  dis- 
  

   turbances 
  and 
  to 
  erosion, 
  (compare 
  section 
  III,) 
  so 
  that 
  at 
  a 
  number 
  of 
  

   places 
  the 
  shales 
  are 
  found 
  to 
  be 
  bedded 
  unconformably 
  with 
  the 
  under- 
  

   lying 
  red 
  sandstones. 
  This 
  is 
  but 
  in 
  accordance 
  with 
  observations 
  made 
  

   on 
  the 
  Pacific 
  side 
  of 
  the 
  Rocky 
  Mountains,* 
  where 
  Carboniferous 
  beds 
  

   were 
  apparently 
  overlying 
  Cretaceous. 
  In 
  that 
  particular 
  instance 
  the 
  

   strike 
  and 
  dip 
  of 
  the 
  strata 
  of 
  both 
  was 
  the 
  same, 
  thus 
  making 
  the 
  

   deception 
  more 
  perfect. 
  

  

  On 
  the 
  west 
  side 
  of 
  the 
  Sangre 
  de 
  Cristo 
  Range 
  no 
  Cretaceous 
  was 
  

   observed 
  along 
  its 
  entire 
  length. 
  It 
  seems 
  strange 
  that 
  this 
  region 
  

   should 
  so 
  couipletely 
  have 
  escaped 
  all 
  invasion 
  during 
  that 
  period, 
  but 
  

   can 
  readily 
  be 
  explained 
  by 
  the 
  assumption 
  that 
  the 
  points 
  of 
  inlet, 
  

   which 
  would 
  be 
  looked 
  for 
  at 
  P5^ncho 
  and 
  Veta 
  Passes, 
  and 
  at 
  the 
  exit 
  

   of 
  the 
  Rio 
  Grande 
  from 
  San 
  Luis 
  Valley, 
  were 
  already 
  at 
  too 
  high 
  an 
  

   elevation 
  to 
  be 
  overflowed. 
  With 
  the 
  exception 
  of 
  a 
  curious 
  drift 
  con- 
  

   glomerate, 
  deposited 
  most 
  likely 
  by 
  water 
  and 
  ice, 
  I 
  have 
  observed 
  in 
  

   San 
  Luis 
  Valley 
  no 
  sedimentary 
  formation 
  younger 
  than 
  the 
  Carboni- 
  

   ferous. 
  In 
  chapter 
  IV, 
  while 
  speaking 
  of 
  the 
  Post-Cretaceous 
  forma- 
  

   tion 
  I 
  shall 
  have 
  occasion 
  to 
  refer 
  to 
  some 
  of 
  its 
  younger 
  members, 
  which 
  

   do 
  not 
  occur 
  in 
  the 
  district 
  here 
  treated 
  of. 
  

  

  YOLCANIC 
  FORMATIONS. 
  

  

  Although 
  there 
  are 
  no 
  extensive 
  volcanic 
  areas 
  in 
  this 
  district, 
  those 
  

   we 
  do 
  find 
  are 
  of 
  great 
  interest, 
  not 
  -so 
  much 
  locally, 
  as 
  on 
  account 
  of 
  

   their 
  peculiar 
  geological 
  genesis. 
  So 
  far 
  as 
  could 
  be 
  observed, 
  nothing 
  

   but 
  trachyte, 
  dolerite, 
  and 
  basalt 
  occur 
  among 
  the 
  volcanic 
  rocks 
  

   composing 
  the 
  various 
  groups 
  and 
  dikes. 
  Mention 
  has 
  already 
  been 
  

   made 
  of 
  tlie 
  mountains 
  near 
  the 
  eastern 
  entrance 
  of 
  Sangre 
  de 
  Cristo 
  

   Pass. 
  On 
  account 
  of 
  their 
  color 
  partly, 
  partly 
  on 
  account 
  of 
  their 
  shape, 
  

   two 
  of 
  them 
  have 
  receiv^ed 
  the 
  name 
  of 
  " 
  Sheep 
  Mountains," 
  while 
  the 
  

   others, 
  lower 
  in 
  elevation 
  and 
  less 
  prominent 
  in 
  their 
  Ibrms, 
  have 
  not 
  

   been 
  named 
  as 
  yet. 
  There 
  are 
  at 
  that 
  locality 
  six 
  volcanic 
  areas, 
  all 
  

   belonging 
  to 
  one 
  system, 
  though 
  separated 
  at 
  present 
  by 
  the 
  effects 
  of 
  

   erosion. 
  Lithologically 
  they 
  are 
  similar 
  or 
  identical. 
  Some 
  basalt 
  is 
  

   found 
  on 
  the 
  Lower 
  Huerfano, 
  forming 
  small 
  tables, 
  the 
  terminations 
  of 
  

   huge 
  dikes. 
  The 
  Spanish 
  Peaks, 
  the 
  volcanic 
  portion 
  of 
  which 
  covers 
  

   but 
  a 
  small 
  area, 
  are 
  very 
  prominent 
  and 
  of 
  importance 
  in 
  the 
  volcanic 
  

   system 
  of 
  the 
  region. 
  Near 
  the 
  headwaters 
  of 
  Rios 
  Culebra 
  and 
  Cos- 
  

   tilla 
  a 
  large 
  portion 
  of 
  the 
  higher 
  land 
  is 
  covered 
  by 
  trachyte, 
  some 
  

   of 
  which 
  extends 
  down 
  into 
  the 
  valleys. 
  Besides 
  these 
  volcanic 
  deposits 
  

   there 
  are 
  innumerable 
  dikes 
  in 
  the 
  district, 
  traversing 
  mostly 
  the 
  Creta- 
  

   ceous, 
  sometimes 
  the 
  Carboniferous 
  formations. 
  

  

  A.— 
  Volca7iie 
  areas. 
  — 
  As 
  such 
  I 
  mean 
  to 
  designate 
  the 
  more 
  massive 
  

   portions 
  of 
  volcanic 
  material, 
  in 
  contradistinction 
  to 
  the 
  dikes 
  proper. 
  

   Station 
  4 
  is 
  located 
  at 
  an 
  elevation 
  of 
  10,705 
  feet, 
  on 
  the 
  summit 
  of 
  one 
  

   of 
  the 
  more 
  northerly 
  mountains 
  belonging 
  to 
  the 
  Huerfano 
  group, 
  near 
  

   the 
  pass. 
  Resting 
  immediately 
  upon 
  the 
  sandstones 
  of 
  the 
  Dakota 
  

   group, 
  the 
  strata 
  — 
  if 
  we 
  can 
  speak 
  of 
  siich^ 
  — 
  have 
  a 
  southwesterly 
  dip. 
  

   I 
  do 
  not 
  regard 
  the 
  differences, 
  x>artly 
  in 
  color, 
  i^artly 
  in 
  structure, 
  that 
  

   can 
  here 
  be 
  observed, 
  either 
  as 
  strata 
  in 
  the 
  true 
  sense 
  or 
  as 
  volcanic 
  flows, 
  

   but 
  assume 
  that, 
  at 
  the 
  succeeding 
  stages 
  of 
  cooling, 
  the 
  volcanic 
  mass, 
  

   in 
  this 
  instance, 
  resolved 
  itself 
  into 
  physically 
  different 
  rock, 
  so 
  that 
  

   to-day, 
  upon 
  weathering, 
  the 
  effect 
  observed 
  is 
  similar 
  to 
  that 
  which 
  

   would 
  have 
  been 
  produced 
  by 
  stratification. 
  The 
  trachyte 
  composing 
  

  

  * 
  Kepoit 
  United 
  States 
  Geological 
  and 
  Geographical 
  Survey 
  1874, 
  page 
  218. 
  

  

  