﻿200 
  

  

  II. 
  E. 
  Gregory 
  — 
  Geologic 
  Sketch 
  of 
  

  

  which 
  occupy 
  sheltered 
  coves 
  slope 
  gently 
  lake 
  ward 
  for 
  a 
  few 
  

   feet 
  only 
  to 
  drop 
  abruptly 
  into 
  the 
  depths. 
  The 
  one 
  hundred- 
  

   foot 
  bathy 
  metric 
  contour 
  lies 
  very 
  close 
  in-shore, 
  except 
  to 
  the 
  

   northwest, 
  where 
  the 
  under-water 
  platform 
  forms 
  a 
  foundation 
  

   for 
  six 
  islands, 
  the 
  largest 
  only 
  a 
  little 
  over 
  a 
  square 
  mile 
  in 
  

   area, 
  and 
  all 
  within 
  two 
  miles 
  of 
  Titicaca 
  itself. 
  

  

  The 
  agencies 
  concerned 
  in 
  molding 
  Titicaca 
  are 
  none 
  of 
  

   them 
  vigorous 
  except 
  the 
  waves, 
  which 
  are 
  very 
  efficient 
  tools, 
  

  

  Fig. 
  

  

  Fig. 
  7. 
  Ahyjadero 
  Bay, 
  Titicaca 
  Island 
  , 
  showing 
  beach, 
  lake 
  cliff, 
  and 
  

   terraced 
  fields 
  (andenes). 
  The 
  strata 
  exposed 
  are 
  limestones 
  of 
  Pennsyl- 
  

   vanian 
  age 
  and 
  mark 
  the 
  northwest 
  side 
  of 
  the 
  Ahyjadero 
  fault. 
  K. 
  C. 
  

   Heald, 
  photo. 
  

  

  particularly 
  during 
  the 
  southern 
  summer. 
  Chemical 
  decom- 
  

   position 
  is 
  favored 
  by 
  continuously 
  moist 
  atmosphere 
  ; 
  stream 
  

   erosion 
  is 
  checked 
  by 
  inf 
  requency 
  of 
  rain 
  and 
  by 
  fiat 
  gradi- 
  

   ents, 
  which 
  is 
  offset 
  only 
  in 
  part 
  by 
  the 
  severity 
  of 
  sudden 
  

   showers. 
  The 
  peeling 
  of 
  rocks 
  occasioned 
  by 
  diurnal 
  changes 
  of 
  

   temperature 
  is 
  little 
  in 
  evidence, 
  owing 
  to 
  a 
  nearly 
  constant 
  

   temperature 
  which, 
  influenced 
  by 
  the 
  lake, 
  maintains 
  a 
  mean 
  

   of 
  about 
  55° 
  and 
  very 
  rarely 
  drops 
  below 
  freezing. 
  The 
  

   waves, 
  however, 
  are 
  vigorously 
  attacking 
  the 
  shores 
  and 
  by 
  

   selective 
  erosion 
  have 
  developed 
  dikes 
  of 
  sandstone 
  by 
  remov- 
  

   ing 
  the 
  less 
  resistant 
  shales, 
  and 
  here 
  and 
  there 
  have 
  sur- 
  

   rounded 
  masses 
  which 
  stand 
  as 
  stacks 
  and 
  skerries. 
  

  

  