﻿H. 
  E. 
  Gregory 
  — 
  La 
  Paz 
  (Bolivia) 
  Gorge. 
  143 
  

  

  city 
  as 
  well 
  as 
  the 
  walls 
  of 
  the 
  valley, 
  and 
  extending 
  nearly 
  to 
  

   the 
  level 
  of 
  the 
  altiplano, 
  are 
  beds 
  of 
  gravels 
  and 
  sands 
  and 
  

   clays 
  eroded 
  into 
  a 
  bewildering 
  maze 
  of 
  forms. 
  Needles 
  in 
  

   groups 
  or 
  singly, 
  columns 
  unadorned 
  or 
  fluted 
  or 
  capped 
  by 
  

   tables, 
  rise 
  five 
  to 
  fifteen 
  feet 
  on 
  steep 
  slopes 
  and 
  five 
  to 
  fifty 
  

   feet 
  on 
  knife-edged, 
  dividing 
  ridges. 
  Innumerable 
  sharply- 
  

   cut, 
  miniature 
  canyons 
  with 
  sheer 
  walls 
  five 
  to 
  two 
  hundred 
  

   feet 
  in 
  height 
  together 
  with 
  tunnels 
  and 
  pits 
  in 
  great 
  variety 
  

   furnish 
  passage 
  for 
  water. 
  Landslides 
  with 
  slopes 
  as 
  great 
  as 
  

   50°, 
  frequently 
  accompanied 
  by 
  open 
  cracks, 
  are 
  numerous. 
  

   Here 
  and 
  there 
  benches 
  and 
  tables 
  composed 
  of 
  cemented 
  

   gravels 
  and 
  brown 
  concretions 
  project 
  from 
  vertical 
  surfaces 
  

   or 
  form 
  the 
  capping 
  of 
  columnar 
  masses. 
  The 
  whole 
  deposit 
  

   is 
  ash-gray 
  in 
  general 
  tone, 
  but 
  is 
  beautifully 
  striated 
  by 
  gray, 
  

   brown, 
  light 
  pink, 
  bright 
  yellow, 
  purple 
  and 
  white 
  bands 
  from 
  

   a 
  few 
  inches 
  to 
  one 
  hundred 
  feet 
  in 
  thickness. 
  Vegetation 
  is 
  

   absent 
  except 
  for 
  patches 
  of 
  wiry 
  grass 
  and 
  tough 
  shrubs 
  

   which 
  find 
  a 
  foothold 
  on 
  the 
  little 
  flat-topped 
  tables 
  and 
  gen- 
  

   tler 
  slopes. 
  The 
  beds 
  in 
  general 
  dip 
  slightly 
  to 
  the 
  south. 
  

  

  A 
  closer 
  examination 
  of 
  the 
  strata 
  exposed 
  reveals 
  the 
  

   presence 
  of 
  the 
  following 
  materials 
  : 
  (1) 
  Sand, 
  mostly 
  fine, 
  

   some 
  coarse, 
  composed 
  chiefly 
  of 
  quartz 
  grains, 
  and 
  arranged 
  

   in 
  beds 
  several 
  hundred 
  feet 
  in 
  extent, 
  or 
  in 
  short 
  lenses. 
  All 
  

   the 
  strata 
  are 
  more 
  or 
  less 
  cross-bedded, 
  with 
  laminae 
  dipping 
  

   0° 
  — 
  25°. 
  (2) 
  Gravel, 
  composed 
  of 
  rounded 
  pebbles 
  from 
  the 
  

   size 
  of 
  a 
  small 
  pea 
  to 
  three 
  inches 
  in 
  diameter, 
  arranged 
  as 
  

   lenses 
  which 
  exhibit 
  marked 
  and 
  sudden 
  variation 
  in 
  position 
  

   and 
  size 
  both 
  horizontally 
  and 
  vertically. 
  The 
  gravels 
  are 
  

   everywhere 
  cross-bedded 
  and 
  frequently 
  inclose 
  lenses 
  of 
  sand. 
  

   The 
  component 
  materials 
  in 
  the 
  upper 
  part 
  of 
  the 
  section 
  were 
  

   found 
  to 
  consist 
  approximately 
  of 
  sedimentary 
  fragments 
  75 
  

   per 
  cent, 
  igneous 
  15 
  per 
  cent, 
  metamorphic 
  10 
  per 
  cent. 
  The 
  

   following 
  types 
  of 
  rock 
  were 
  recognized 
  : 
  gray 
  sandstone, 
  brown 
  

   sandstone, 
  white 
  granite, 
  granite-gneiss, 
  diorite-gneiss, 
  garneti- 
  

   ferous 
  granite-gneiss, 
  black 
  slate, 
  mica 
  or 
  chlorite 
  slate, 
  gray 
  

   quartzite, 
  brown 
  quartzite. 
  Quartz 
  pebbles 
  are 
  rare 
  and 
  no 
  

   limestone 
  or 
  volcanic 
  material 
  was 
  observed. 
  All 
  the 
  pebbles 
  

   are 
  worn, 
  about 
  half 
  of 
  them 
  well-rounded, 
  and 
  a 
  few 
  are 
  

   faulted 
  and 
  veined. 
  The 
  gravel 
  increases 
  in 
  amount 
  and 
  

   becomes 
  coarser 
  toward 
  the 
  top, 
  and 
  along 
  the 
  electric 
  railway 
  

   from 
  La 
  Paz 
  to 
  Alto 
  contains 
  angular, 
  sub-angular 
  and 
  rounded 
  

   bowlders 
  four 
  inches 
  to 
  one 
  and 
  one-half 
  feet 
  in 
  diameter. 
  At 
  

   this 
  locality 
  the 
  gravel 
  forms 
  beds 
  of 
  considerable 
  thickness 
  or 
  

   occurs 
  as 
  lenses 
  embedded 
  in 
  finer 
  gravels, 
  sands 
  and 
  clays, 
  

   and 
  resembles 
  morainal 
  deposits 
  except 
  for 
  the 
  irregular 
  strati- 
  

   fication. 
  (3) 
  Clays, 
  rarely 
  pure, 
  usually 
  highly 
  arenaceous, 
  

   generally 
  distributed 
  as 
  lenses 
  within 
  the 
  finer 
  sands. 
  In 
  the 
  

  

  