﻿ENDLicH] 
  SAN 
  LUIS 
  VALLEY 
  BASALT. 
  143 
  

  

  find 
  Oil 
  the 
  east 
  side 
  specimens 
  representing 
  the 
  formations 
  of 
  the 
  

   Sangre 
  de 
  Cristo 
  Eauge, 
  but 
  they, 
  too, 
  cease 
  as 
  soon 
  as 
  the 
  valley 
  

   is 
  fairly 
  reached. 
  At 
  the 
  western 
  entrance 
  of 
  Mosco 
  Pass 
  there 
  is 
  

   a 
  large 
  accnmulation 
  of 
  fine 
  sand 
  in 
  the 
  shape 
  of 
  "dunes."* 
  Upon 
  

   examination 
  it 
  was 
  found 
  that 
  this 
  sand 
  owed 
  its 
  existence 
  to 
  the 
  

   western 
  range, 
  fully 
  fifty 
  miles 
  distant. 
  It 
  is 
  accounted 
  for, 
  however, 
  

   by 
  the 
  fact 
  of 
  the 
  sand 
  having 
  been 
  blown 
  there, 
  thus 
  covering 
  the 
  

   drift 
  that 
  would 
  naturally 
  be 
  expected 
  to 
  present 
  itself, 
  in 
  that 
  case 
  drift 
  

   from 
  the 
  iSaugre 
  de 
  Cristo. 
  These 
  '• 
  dunes" 
  seem 
  to 
  be 
  of 
  com})arative]y 
  

   recent 
  date, 
  geologically 
  speaking, 
  and 
  belong 
  to 
  the 
  Post-GIacial 
  age. 
  

   North 
  of 
  the 
  Eio 
  Grande 
  no 
  evidence 
  was 
  found, 
  indicating 
  positively 
  

   the 
  former 
  presence 
  of 
  glaciers, 
  though 
  it 
  seems 
  to 
  me 
  possible 
  that 
  

   such 
  evidence 
  may 
  still 
  be 
  discovered. 
  Taking 
  sand 
  from 
  the 
  central 
  

   portion 
  of 
  the 
  valley, 
  it 
  was 
  seen 
  that 
  it 
  is 
  composed 
  in 
  part 
  by 
  the 
  feld- 
  

   spars 
  (the 
  only 
  mineral 
  there 
  which 
  can 
  lead 
  to 
  a 
  decision) 
  of 
  the 
  vol- 
  

   canic 
  region 
  of 
  the 
  west, 
  as 
  well 
  as 
  from 
  the 
  metamorphic 
  rocks 
  on 
  the 
  

   east 
  side. 
  Sanidite, 
  orthoclase, 
  and 
  oligoclase 
  were 
  found 
  in 
  such 
  

   quantities 
  that 
  it 
  would 
  be 
  impossible 
  to 
  determine 
  which 
  of 
  the 
  two 
  

   groups 
  furnished 
  the 
  greater 
  portion 
  of 
  the 
  material. 
  One 
  orographic 
  

   feature 
  observed 
  very 
  frequently 
  north 
  of 
  Del 
  Norte 
  deserves 
  mention. 
  

   The 
  western 
  mountain 
  country 
  has 
  broken 
  into 
  "blulf 
  country," 
  and 
  con- 
  

   tinues 
  as 
  such 
  eastward 
  to 
  the 
  edge 
  of 
  San 
  Luis 
  Valley. 
  The 
  last 
  out- 
  

   posts 
  of 
  the 
  foot-hills 
  appear, 
  viewed 
  from 
  above, 
  very 
  much 
  like 
  islands 
  

   in 
  a 
  sea 
  of 
  sand 
  and 
  pebbles. 
  I 
  state 
  this 
  to 
  illustrate 
  the 
  evidently 
  

   very 
  even 
  distribution 
  of 
  these 
  latter 
  materials, 
  as 
  regards 
  relative 
  ele- 
  

   vation. 
  Pinous 
  covering 
  the 
  knolls, 
  but 
  wanting 
  below 
  them, 
  tend 
  to 
  

   increase 
  the 
  similarity 
  of 
  the 
  picture. 
  

  

  Far 
  more 
  varied 
  and 
  interesting 
  do 
  we 
  find 
  the 
  character 
  of 
  the 
  valley 
  

   south 
  and 
  west 
  of 
  the 
  Eio 
  Grande. 
  For 
  about 
  fifteen 
  miles 
  south 
  of 
  

   del 
  Norte 
  the 
  features 
  of 
  the 
  valley-border 
  change 
  but 
  little. 
  The 
  same 
  

   bluffs 
  are 
  there, 
  showing 
  the 
  same 
  island 
  like 
  appearance. 
  Station 
  14, 
  

   there 
  located, 
  shows 
  a 
  capping 
  of 
  basalt. 
  Though 
  not 
  more 
  than 
  

   40 
  feet 
  in 
  thickness 
  at 
  some 
  places, 
  it 
  was 
  found 
  to 
  be 
  of 
  the 
  same 
  char- 
  

   acter, 
  lithologically, 
  as 
  that 
  farther 
  south, 
  which 
  occurs 
  in 
  such 
  large 
  

   quantities. 
  A 
  short 
  distance 
  north 
  of 
  Eio 
  Alamosa 
  the 
  continuous 
  

   area 
  of 
  basalt 
  sets 
  in, 
  and 
  is 
  found 
  southward 
  on 
  the 
  west 
  side 
  

   of 
  the 
  valley 
  as 
  far 
  as 
  our 
  explorations 
  were 
  extended 
  (north 
  latitude 
  

   30° 
  45'). 
  From 
  that 
  first 
  point 
  jnst 
  mentioned 
  basalt 
  caps 
  the 
  trach- 
  

   orheites 
  of 
  the 
  main 
  range, 
  and 
  sloping 
  eastward 
  gently, 
  forms 
  the 
  

   bottom 
  of 
  that 
  portion 
  of 
  San 
  Luis 
  Valley. 
  On 
  the 
  higher 
  portions 
  

   of 
  the 
  foot-hills 
  it 
  is 
  found 
  merely 
  as 
  a 
  capping, 
  while 
  the 
  streams 
  

   flowing 
  eastward 
  have 
  their 
  beds 
  in 
  trachyte 
  or 
  tracliytic 
  conglom- 
  

   erate. 
  Isolated 
  points 
  beyond 
  the 
  foot-hills 
  testily 
  to 
  subsequent 
  

   eruptions 
  of 
  the 
  same 
  material, 
  and 
  afford 
  good 
  locations 
  for 
  topo- 
  

   graphical 
  stations. 
  It 
  may 
  be 
  observed 
  that 
  at 
  these 
  points 
  the 
  regular 
  

   stratification, 
  so 
  noticeable 
  elsewhere, 
  can 
  no 
  longer 
  be 
  traced. 
  Al- 
  

   though 
  it 
  seems 
  that 
  the 
  flows 
  from 
  such 
  eruptions 
  have 
  extended 
  but 
  

   a 
  short 
  distance, 
  their 
  existence 
  can 
  easily 
  be 
  recognized, 
  not 
  only 
  

   from 
  the 
  lithological 
  constitution 
  of 
  their 
  material, 
  but 
  also 
  from 
  the 
  

   evidently 
  secondary 
  positions 
  they 
  occupy 
  with 
  reference 
  to 
  the 
  under- 
  

   lying 
  basalt. 
  Crossing 
  the 
  Eios 
  La 
  Jara, 
  Conejos, 
  and 
  San 
  Antonio 
  we 
  

   find 
  that 
  but 
  very 
  little 
  soil 
  covers 
  the 
  volcanic 
  rock. 
  Although 
  

   this 
  feature 
  is 
  frequently 
  regarded 
  as 
  indicative 
  of 
  late 
  geological 
  

   age, 
  I 
  cannot 
  entirely 
  accept 
  this 
  view. 
  We 
  know 
  of 
  no 
  more 
  powerful 
  

  

  ' 
  Report 
  United 
  States 
  Geological 
  and 
  Geographical 
  Survey, 
  1875, 
  page 
  33o. 
  

  

  