﻿BNDLicH.] 
  SAN 
  LUIS 
  VALLEY 
  VOLGA 
  NICS. 
  145 
  

  

  ent 
  ago, 
  that 
  volcanic 
  eruptions 
  of 
  such 
  magnitude 
  as 
  those 
  we 
  here 
  

   observe 
  must 
  have 
  been 
  preceded, 
  accompanied, 
  and 
  followed 
  by 
  vol- 
  

   canic 
  earthquakes. 
  And 
  again, 
  the 
  observations 
  during 
  the 
  present 
  

   century 
  have 
  shown 
  us 
  results 
  therefrom 
  that 
  compare 
  admirably 
  with 
  

   those 
  noticed 
  in 
  San 
  Luis 
  Valley. 
  I 
  regard 
  it 
  as 
  ill-advised 
  to 
  explain 
  

   the 
  presence 
  of 
  the 
  canons 
  or 
  ^' 
  cracks 
  " 
  in 
  any 
  other 
  way, 
  though 
  the 
  

   theory 
  of 
  contraction 
  upon 
  cooling 
  of 
  the 
  material 
  might 
  produce 
  

   analogous 
  results. 
  Some 
  of 
  the 
  river-bottoms 
  are 
  much 
  broader 
  than 
  

   that 
  of 
  the 
  San 
  Antonio, 
  broad 
  enough 
  to 
  contain 
  human 
  habitations 
  

   and 
  small 
  "farms." 
  This 
  is 
  probably 
  due 
  in 
  part 
  to 
  the 
  gradual 
  ero- 
  

   sion 
  of 
  the 
  vertical 
  walls, 
  which, 
  slowly 
  receding, 
  increased 
  the 
  dis- 
  

   tance 
  between 
  each 
  other, 
  and 
  furnished 
  material 
  for 
  the 
  formation 
  

   of 
  silt 
  and 
  soil. 
  

  

  Quite 
  a 
  considerable 
  area 
  is 
  covered 
  by 
  this 
  basalt 
  ; 
  northward 
  it 
  ex- 
  

   tends 
  to 
  Rio 
  Alamosa, 
  reaching 
  that 
  river 
  opposite 
  station 
  101. 
  Prom 
  

   there 
  its 
  boundary 
  runs 
  in 
  a 
  southeasterly 
  direction, 
  crossing 
  Rio 
  Cone- 
  

   jos 
  about 
  25 
  miles 
  above 
  its 
  junction 
  with 
  the 
  Grande. 
  Ten 
  miles 
  

   north 
  of 
  TJte 
  Peak 
  (station 
  107) 
  it 
  crosses 
  the 
  Rio 
  Grande, 
  and 
  from 
  

   there 
  continues 
  southward, 
  including 
  Ute 
  Peak 
  and 
  the 
  lower 
  hills 
  near 
  

   the 
  Rio 
  Colorado. 
  Between 
  Rio 
  Conejos, 
  after 
  it 
  has 
  changed 
  its 
  course 
  

   into 
  a 
  northeasterly 
  one, 
  and 
  the 
  Rio 
  Grande, 
  there 
  are 
  a 
  number 
  of 
  

   table 
  shaped 
  bluff's, 
  rising 
  to 
  a 
  relative 
  elevation 
  of 
  1,400 
  feet 
  (9,200 
  

   feet 
  above 
  sea-levei). 
  They 
  are 
  composed 
  of 
  black, 
  vesicular 
  basalt, 
  the 
  

   strata 
  of 
  which 
  show 
  a 
  general 
  westerly 
  dip 
  of 
  a 
  few 
  degrees. 
  A 
  num- 
  

   ber 
  of 
  small 
  knolls 
  are 
  located 
  nearer 
  the 
  Conejos, 
  separated 
  from 
  each 
  

   other 
  by 
  an 
  accumulation 
  of 
  diluvial 
  or 
  alluvial 
  deposits. 
  Near 
  the 
  Rio 
  

   Grande, 
  however, 
  basalt 
  is 
  no 
  longer 
  the 
  only 
  material 
  building 
  up 
  the 
  

   bluffs. 
  Trachyte 
  forms 
  the 
  lower 
  portion, 
  while 
  the 
  former 
  shows 
  itself 
  

   as 
  a 
  capping 
  of 
  the 
  higher 
  portions. 
  Station 
  105 
  is 
  located 
  on 
  one 
  of 
  these- 
  

   trachytic 
  hills, 
  west 
  of 
  the 
  river, 
  about 
  400 
  feet 
  above 
  it. 
  At 
  this 
  point 
  

   the 
  trachyte 
  crosses 
  the 
  Rio 
  Grande, 
  extending 
  itself 
  in 
  a 
  row 
  of 
  bluffs 
  

   for 
  about 
  eight 
  miles 
  to 
  the 
  northeast. 
  Here, 
  too, 
  the 
  highest 
  portions 
  

   are 
  capped 
  by 
  basalt, 
  black 
  and 
  vesicular. 
  Trachyte, 
  from 
  station 
  105, 
  

   shows 
  a 
  partly 
  crystalline, 
  partly 
  amorphous 
  paste, 
  which 
  contains 
  

   small 
  crystals 
  of 
  a 
  black 
  biotite, 
  crystals 
  of 
  a 
  colorless 
  sanidite, 
  reaching 
  

   a 
  length 
  of 
  5 
  millimeters, 
  and 
  dark-green 
  portions, 
  probably 
  chloritic. 
  Its 
  

   color 
  is 
  a 
  dark, 
  dull 
  greyish-brown, 
  becoming 
  a 
  little 
  lighter 
  upon 
  weath- 
  

   ering 
  than 
  on 
  the 
  fresh 
  fracture. 
  It 
  is 
  a 
  very 
  compact 
  rock, 
  producing, 
  

   almost, 
  the 
  impression 
  as 
  if 
  cooled 
  under 
  heavy 
  pressure. 
  

  

  Following 
  from 
  there 
  down 
  the 
  Rio 
  Grande, 
  we 
  pass 
  for 
  about 
  10 
  miles 
  

   through 
  fine 
  drift-sand, 
  containing 
  occasionally 
  a 
  small 
  patch 
  of 
  pebbles, 
  

   but 
  as 
  soon 
  as 
  the 
  basalt 
  is 
  reached 
  the 
  river 
  caiions, 
  and 
  its 
  bottom 
  is 
  no 
  

   longer 
  accessible 
  to 
  either 
  man 
  or 
  beast. 
  The 
  same 
  can 
  be 
  said 
  of 
  Rio 
  

   Costilla, 
  which 
  empties 
  into 
  Rio 
  Grande 
  opposite 
  the 
  north 
  side 
  of 
  Ute 
  

   Peak. 
  Again, 
  the 
  same 
  characteristics 
  that 
  have 
  been 
  given 
  when 
  

   speaking 
  of 
  the 
  San 
  Antonio 
  hold 
  good. 
  Great 
  regularity 
  of 
  the 
  canon- 
  

   walls, 
  identity 
  of 
  the 
  strata 
  opposite 
  each 
  other, 
  no 
  evidence 
  of 
  the 
  

   caiion's 
  having 
  been 
  produced 
  by 
  erosion, 
  and 
  no 
  certainty 
  as 
  to 
  whether 
  

   the 
  rivers 
  flow 
  over 
  basalt 
  in 
  positu. 
  Ute 
  Peak, 
  similar 
  in 
  shape 
  to 
  

   Mount 
  San 
  Antonio, 
  rises 
  2,200 
  feet 
  above 
  the 
  valley 
  to 
  an 
  altitude 
  of 
  

   9,664 
  feet. 
  It 
  is 
  composed 
  entirely 
  of 
  basalt, 
  which 
  shows 
  varieties 
  

   similar 
  to 
  those 
  from 
  San 
  Antonio, 
  the 
  black 
  vesicular 
  one 
  predominat- 
  

   ing. 
  Following 
  from 
  Ute 
  Peak 
  southward 
  along 
  the 
  edge 
  of 
  the 
  basalt,, 
  

   we 
  find 
  it 
  bordered 
  by 
  drift-sand, 
  partly 
  of 
  metamorphic, 
  but 
  mainly 
  of 
  

   volcanic 
  origin. 
  Commencing 
  a 
  little 
  north 
  of 
  Rio 
  Colorado, 
  and 
  trav- 
  

   eling 
  northward 
  along 
  the 
  eastern 
  edge 
  of 
  San 
  Luis 
  Valley, 
  it 
  will 
  be 
  

   10 
  a 
  s 
  

  

  