﻿146 
  REPORT 
  UNITED 
  STATES 
  GEOLOGICAL 
  SURVEY. 
  

  

  observed 
  that 
  metamorphic 
  granite 
  and 
  some 
  schist 
  rocks 
  compose 
  the 
  

   base 
  of 
  the 
  foot-hills, 
  very 
  nearly 
  as 
  far 
  up 
  as 
  to 
  the 
  point 
  where 
  Eio 
  

   Costilla 
  leaves 
  the 
  mountains. 
  Having 
  crossed 
  this 
  river, 
  the 
  San 
  Pe- 
  

   dro 
  Mesa, 
  a 
  basaltic 
  table, 
  which 
  has 
  been 
  mentioned 
  above, 
  is 
  reached. 
  

   East 
  of 
  it 
  is 
  an 
  equally 
  long, 
  narrow 
  valley, 
  having 
  a 
  drift-bottom, 
  and 
  

   containing 
  several 
  Mexican 
  settlements. 
  This 
  table 
  continues, 
  striking 
  

   nearly 
  north 
  until 
  it 
  reaches 
  Rio 
  Cnlebra, 
  a 
  large 
  branch 
  of 
  which 
  

   follows 
  along 
  the 
  inclosed 
  valley. 
  At 
  the 
  entrances 
  of 
  most 
  of 
  the 
  

   streams 
  into 
  the 
  valley 
  may 
  be 
  found 
  large 
  deposits 
  of 
  bowlders 
  and 
  

   pebbles, 
  located 
  in 
  such 
  a 
  way 
  as 
  to 
  obstruct 
  the 
  direct 
  passage 
  of 
  the 
  

   water, 
  thus. 
  forcing 
  it 
  to 
  turn 
  either 
  to 
  the 
  right 
  or 
  to 
  the 
  left. 
  Eocks 
  

   composing 
  the 
  foot-hills 
  are 
  most 
  frequently 
  met 
  with 
  in 
  this 
  drift. 
  

  

  Having 
  given 
  an 
  idea 
  of 
  the 
  distribution 
  of 
  the 
  volcanic 
  formations 
  

   in 
  San 
  Luis 
  Valley, 
  it 
  becomes 
  incumbent 
  to 
  consider 
  their 
  correlation. 
  

   Trachyte 
  is 
  the 
  older 
  of 
  the 
  two 
  volcanic 
  rocks 
  under 
  discussion. 
  It 
  is 
  

   possible 
  that 
  the 
  trachytic 
  group, 
  through 
  the 
  center 
  of 
  which 
  the 
  Eio 
  

   Grande 
  finds 
  its 
  way, 
  may 
  be 
  but 
  a 
  remnant 
  of 
  the 
  great 
  mass 
  30 
  to 
  40 
  

   miles 
  farther 
  west. 
  It 
  seems 
  difficult, 
  however, 
  in 
  case 
  this 
  view 
  should 
  

   be 
  maintained, 
  to 
  account 
  for 
  the 
  disappearance 
  of 
  all 
  that 
  enormous 
  

   mass 
  of 
  material 
  that 
  must 
  have 
  formed 
  the 
  connection 
  between 
  the 
  

   two. 
  East 
  of 
  the 
  group 
  there 
  is 
  the 
  trachytic 
  mass 
  of 
  station 
  114 
  con- 
  

   tinuing, 
  in 
  the 
  San 
  Pedro 
  mesa, 
  where 
  it 
  underlies 
  basalt. 
  It 
  seems 
  

   highly 
  probable, 
  therefore, 
  that 
  at 
  one 
  time 
  the 
  two 
  were 
  in 
  connec- 
  

   tion. 
  I 
  regard 
  this 
  outcrop 
  as 
  the 
  continuation 
  of 
  the 
  eastern 
  

   trachytic 
  area, 
  the 
  connection 
  between 
  the 
  two 
  having 
  been 
  broken. 
  

   From 
  the 
  points 
  mentioned, 
  the 
  flows 
  extended 
  principally 
  in 
  a 
  south- 
  

   westerly 
  direction. 
  It 
  is, 
  to 
  a 
  certain 
  extent, 
  different 
  with 
  the 
  basalt. 
  

   We 
  have 
  two 
  mountains, 
  San 
  Antonio 
  and 
  TJte, 
  situated 
  but 
  a 
  short 
  

   distance 
  from 
  each 
  other, 
  both 
  presenting 
  a 
  very 
  similar 
  appearance, 
  

   and 
  both 
  composed 
  of 
  the 
  same 
  volcanic 
  material. 
  We 
  have, 
  further- 
  

   more, 
  the 
  uniform 
  flow 
  of 
  basaltic 
  lava 
  west 
  and 
  northwest 
  of 
  these 
  two 
  

   mountains, 
  and 
  have 
  isolated 
  patches 
  of 
  it 
  to 
  the 
  north 
  and 
  northeast. 
  

   Probably 
  a 
  considerable 
  area 
  is 
  covered 
  by 
  it 
  farther 
  south, 
  but 
  our 
  ex- 
  

   plorations 
  did 
  not 
  take 
  us 
  there. 
  Comparing 
  the 
  altitudes 
  of 
  San 
  An- 
  

   tonio, 
  Ute, 
  and 
  the 
  basaltic 
  plateau 
  of 
  station 
  104, 
  we 
  find 
  the 
  differ- 
  

   ence 
  between 
  Antonio 
  and 
  Ute, 
  1,219 
  feet, 
  and 
  between 
  Ute 
  and 
  station 
  

   104, 
  516 
  feet, 
  the 
  last 
  named 
  being 
  about 
  1,500 
  feet 
  above 
  the 
  valley 
  

   there. 
  Assumiiig 
  that 
  we 
  regard 
  Ute 
  and 
  Antonio 
  as 
  the 
  two 
  main 
  

   points 
  of 
  outflow 
  for 
  that 
  immediate 
  region, 
  we 
  have 
  a 
  thickness 
  of 
  

   1,500 
  feet 
  of 
  volcanic 
  rocks 
  to 
  account 
  for 
  in 
  the 
  neighborhood 
  of 
  Ute. 
  

   In 
  speaking 
  of 
  the 
  104 
  plateau 
  above, 
  the 
  general 
  westerly 
  dip 
  has 
  to 
  

   be 
  noticed, 
  while 
  the 
  basalt, 
  forming 
  the 
  western 
  side 
  of 
  the 
  valley, 
  dips 
  

   a 
  little 
  north 
  of 
  east. 
  Adding 
  to 
  this 
  former 
  dip 
  the 
  fact 
  that 
  the 
  east- 
  

   ern 
  boundary 
  of 
  the 
  basalt 
  closely 
  follows 
  a 
  straight 
  line 
  nearly 
  south 
  

   of 
  station 
  104; 
  adding, 
  furthermore, 
  that 
  with 
  the 
  exception 
  of 
  the 
  ba- 
  

   salt 
  between 
  Eios 
  Costilla 
  and 
  Culebra, 
  none 
  was 
  found, 
  I 
  have 
  arrived 
  

   at 
  the 
  conclusion 
  that, 
  at 
  the 
  time 
  of 
  the 
  basaltic 
  eruption, 
  the 
  westerly 
  

   dip 
  of 
  the 
  older 
  volcanics 
  in 
  question 
  did 
  already 
  exist, 
  and 
  that 
  its 
  ex- 
  

   istence 
  caused 
  an 
  almost 
  complete 
  cessation 
  of 
  the 
  flow 
  toward 
  the 
  

   east. 
  The 
  one 
  exception 
  to 
  it 
  is 
  San 
  Pedro 
  mesa, 
  which 
  was 
  probably 
  

   formed 
  by 
  an 
  arm 
  of 
  the 
  flowing 
  lava 
  extending 
  from 
  the 
  southwest. 
  

   Neither 
  San 
  Antonio 
  nor 
  Ute 
  show 
  any 
  craters 
  on 
  their 
  summits, 
  in 
  evi- 
  

   dence 
  that 
  the 
  eruption 
  was 
  a 
  massive 
  one. 
  To 
  this 
  species 
  we 
  are 
  by 
  

   far 
  more 
  accustomed 
  in 
  Sontliern 
  Colorado 
  than, 
  to 
  any 
  other. 
  

  

  The 
  influence 
  of 
  the 
  dip 
  oi 
  the 
  basalt 
  can 
  be 
  readily 
  observed 
  in 
  the 
  

   course 
  of 
  the 
  Eio 
  Conejbs. 
  Flowing 
  first 
  in 
  an 
  easterly 
  direction, 
  after 
  

  

  