﻿162 
  EEPORT 
  UNITED 
  STATES 
  GEOLOGICAL 
  SURVEY. 
  

  

  trending 
  north 
  and 
  south. 
  From 
  there 
  eastward 
  the 
  country 
  is 
  more 
  

   broken, 
  showing, 
  however, 
  in 
  its 
  character, 
  that 
  the 
  contours 
  produced 
  

   are 
  but 
  the 
  result 
  of 
  the 
  cutting 
  of 
  an 
  extensive 
  plateau 
  that 
  formerly 
  

   existed 
  there. 
  Again 
  we 
  meet 
  the 
  basalt, 
  overlying 
  trachyte 
  on 
  a 
  small 
  

   plateau, 
  10,631 
  feet 
  above 
  sea-level, 
  upon 
  which 
  station 
  90 
  is 
  located. 
  

   As 
  at 
  station 
  88, 
  so 
  we 
  find 
  here 
  an 
  easterly 
  dip 
  of 
  the 
  volcanic 
  strata, 
  

   both 
  of 
  the 
  trachyte 
  and 
  of 
  the 
  basalt. 
  Looking 
  southward 
  from 
  that 
  

   station, 
  we 
  can 
  observe 
  the 
  well-defined 
  western 
  edge 
  of 
  the 
  plateau, 
  

   striking 
  about 
  north 
  to 
  south, 
  sloping 
  eastward 
  at 
  an 
  angle 
  of 
  6° 
  to 
  8°. 
  

   This 
  expresses 
  the 
  dip 
  of 
  the 
  volcanic 
  beds. 
  Upon 
  the 
  edge 
  of 
  the 
  pla- 
  

   teau, 
  south 
  of 
  Rio 
  San 
  Antonio, 
  station 
  96 
  was 
  made, 
  at 
  an 
  elevation 
  of 
  

   10,294 
  feet. 
  From 
  here 
  we 
  may 
  regard 
  the 
  basaltic 
  outcrop 
  as 
  contin- 
  

   uous. 
  It 
  is 
  cut 
  by 
  every 
  stream 
  leading 
  from 
  the 
  mountains 
  into 
  the 
  

   valley 
  beyond, 
  but 
  all 
  the 
  bluffs 
  remaining 
  are 
  covered 
  with 
  the 
  basalt. 
  

   The 
  entire 
  area 
  belongs 
  to 
  one 
  flow, 
  stratigraphically 
  as 
  well 
  as 
  lithologi- 
  

   cally. 
  Throughout 
  its 
  entire 
  extent 
  the 
  even 
  dip 
  eastward 
  is 
  preserved, 
  

   diminishing 
  gradually 
  as 
  we 
  approach 
  the 
  broad 
  expanse 
  of 
  San 
  Luis 
  

   Valley. 
  An 
  idea 
  of 
  the 
  arrangement 
  may 
  be 
  obtained 
  from 
  Section 
  IX. 
  

   Distribution 
  and 
  character 
  of 
  the 
  basalt, 
  after 
  it 
  has 
  entered 
  the 
  valley, 
  

   is 
  discussed 
  in 
  Chapter 
  II. 
  A 
  comparison 
  of 
  a 
  few 
  basalts 
  collected 
  

   from 
  various 
  stations 
  throughout 
  the 
  region 
  under 
  consideration, 
  will 
  

   show 
  the 
  general 
  uniformity 
  of 
  their 
  character, 
  as 
  well 
  as 
  the 
  constancy 
  

   of 
  the 
  variations 
  occurring. 
  

  

  1. 
  Station 
  91. 
  Basalt. 
  

  

  Paste, 
  microcrystalline, 
  color 
  middle 
  to 
  dark 
  grey; 
  weathers 
  dark- 
  

   brown. 
  Contains 
  small 
  spherical 
  cavities, 
  which 
  appear 
  glazed. 
  Brown 
  

   decomposed 
  inclosures 
  of 
  olivine 
  have 
  a 
  splendent 
  luster. 
  Magnetite 
  

   is 
  segregated 
  in 
  small, 
  octahedral 
  crystals. 
  Is 
  altogether 
  very 
  homogen- 
  

   eous, 
  heavy 
  and 
  hard. 
  

  

  2. 
  Station 
  96. 
  Basalt. 
  

  

  a. 
  Paste, 
  microcrystalline, 
  color 
  dark-grey, 
  weathering 
  brown. 
  Olivine 
  

   in 
  exceedingly 
  minute 
  particles. 
  Spherical 
  vesicles, 
  containing 
  ismall 
  

   crystals 
  of 
  zeolites. 
  Magnetite 
  not 
  visible. 
  

  

  h. 
  Paste, 
  crystalline, 
  color 
  reddish-brown, 
  weathering 
  lighter. 
  Crys- 
  

   tals 
  of 
  black 
  biotite 
  occur 
  sparingly. 
  Olivine, 
  decomposed, 
  dark, 
  splen- 
  

   dent 
  brown. 
  Irregular 
  vesicles 
  distributed 
  throughout 
  the 
  entire 
  mass, 
  

   some 
  of 
  them 
  containing 
  zeolites. 
  Decomposition 
  of 
  magnetite 
  produces 
  

   the 
  browu'color, 
  

  

  3. 
  Station 
  97. 
  Basalt. 
  

  

  a. 
  Paste 
  cryptocrystalline, 
  color 
  dark-gray 
  to 
  black. 
  Slightly 
  vesicu- 
  

   lar, 
  vesicles 
  either 
  spheroid 
  or 
  drawn 
  out. 
  In 
  some 
  of 
  them 
  deposits 
  of 
  

   zeolites. 
  Olivine 
  the 
  only 
  segregated 
  mineral 
  distinguishable. 
  

  

  1). 
  Essentially 
  the 
  same 
  as 
  above, 
  but 
  highly 
  vesicular. 
  Vesicles 
  fre- 
  

   quently 
  round, 
  while 
  on 
  the 
  same 
  bowlder 
  in 
  another 
  zone 
  they 
  are 
  

   drawn 
  out. 
  Ko 
  mineral 
  distinguishable 
  but 
  olivine, 
  which 
  is 
  partly 
  

   decomposed. 
  

  

  c. 
  Paste 
  microcrystalline, 
  color 
  pitch-black, 
  with 
  fatty 
  luster. 
  Very 
  

   compact. 
  Vesicles 
  too 
  minute 
  to 
  be 
  visible. 
  No 
  segregated 
  minerals. 
  

   Eesembles 
  the 
  typical 
  melaphyrs 
  of 
  Europe. 
  Large 
  percentage 
  of 
  mag- 
  

   netite. 
  This 
  variety 
  is 
  subject 
  to, 
  mainly, 
  three 
  modifications. 
  

  

  d. 
  Physical 
  characters 
  as 
  above, 
  excepting 
  the 
  presence 
  of 
  vesicles 
  ; 
  

   these 
  are 
  very 
  flat, 
  drawn 
  out 
  to 
  the 
  length 
  of 
  half 
  an 
  inch. 
  Between 
  the 
  

   larger 
  ones 
  are 
  very 
  minute 
  ones. 
  The 
  rock 
  breaks 
  into 
  shaly 
  frag- 
  

   ments, 
  owing 
  to 
  the 
  fact 
  that 
  the 
  vesicles 
  have 
  been 
  compressed 
  in 
  on.e 
  

   direction. 
  This 
  latter 
  feature 
  still 
  more 
  modified 
  in 
  — 
  

  

  