﻿196 
  E. 
  E. 
  Gregory— 
  Geologic 
  Sketch 
  of 
  

  

  In 
  this 
  connection 
  Agassiz's 
  statement, 
  quoted 
  above, 
  that 
  

   "the 
  terraces 
  of 
  its 
  [Titicaca] 
  former 
  shore 
  line 
  are 
  everywhere 
  

   most 
  distinctly 
  to 
  be 
  traced," 
  at 
  "an 
  elevation 
  of 
  300 
  or 
  400 
  feet 
  

   at 
  least 
  higher 
  than 
  its 
  present 
  level," 
  deserves 
  attention. 
  Such 
  

   high 
  level 
  terraces 
  were 
  not 
  observed 
  by 
  the 
  waiter 
  at 
  Puno, 
  

   Guaqui, 
  Tiquina, 
  Yampupata, 
  or 
  on 
  Titicaca 
  Island, 
  — 
  a 
  fact 
  

   which 
  surprised 
  me 
  not 
  a 
  little, 
  since 
  I 
  had 
  assumed 
  that 
  such 
  

   evidences 
  of 
  higher 
  level 
  were 
  to 
  be 
  found 
  on 
  all 
  sides. 
  The 
  

   shale, 
  sandstone, 
  and 
  limestone, 
  tilted 
  at 
  various 
  angles 
  and 
  of 
  

   different 
  degrees 
  of 
  firmness, 
  fretted 
  by 
  waves 
  of 
  considerable 
  

   power, 
  especially 
  during 
  the 
  southern 
  winter, 
  would 
  be 
  expected 
  

   to 
  produce 
  unmistakable 
  rock 
  benches, 
  and 
  the 
  low-lying 
  bor- 
  

   ders 
  of 
  parts 
  of 
  the 
  lake 
  offer 
  favorable 
  opportunities 
  for 
  beach- 
  

   making. 
  Moreover, 
  the 
  conditions 
  for 
  preserval 
  of 
  the 
  shore 
  

   forms 
  in 
  a 
  semi-arid 
  climate 
  and 
  where 
  freezing 
  is 
  unusual 
  are 
  

   favorable. 
  This 
  does 
  not 
  prove, 
  of 
  course, 
  that 
  no 
  such 
  evi- 
  

   dences 
  of 
  high-water 
  level 
  exist, 
  for 
  no 
  detailed 
  survey 
  has 
  as 
  

   yet 
  been 
  made 
  ; 
  but 
  raised 
  terraces 
  are 
  not 
  " 
  distinctly 
  to 
  be 
  

   traced." 
  In 
  fact, 
  no 
  rock 
  shelf 
  or 
  raised 
  beach 
  has 
  been 
  mapped 
  

   or 
  described, 
  and 
  there 
  is 
  no 
  direct 
  evidence 
  of 
  former 
  high 
  

   levels 
  except 
  for 
  the 
  relatively 
  slight 
  fluctuations 
  discussed 
  

   above. 
  It 
  is 
  significant 
  that 
  La 
  Puente, 
  who 
  stoutly 
  affirms 
  

   the 
  former 
  existence 
  of 
  a 
  vast 
  interior 
  sea, 
  made 
  a 
  traverse 
  of 
  

   the 
  lake 
  borders 
  and 
  visited 
  many 
  islands 
  without 
  recording 
  

   the 
  presence 
  of 
  ancient 
  shore 
  forms, 
  and 
  that 
  Bowman 
  in 
  1908 
  

   saw 
  no 
  signs 
  of 
  raised 
  terraces.* 
  

  

  It 
  will 
  be 
  noted 
  that 
  the 
  argument 
  against 
  the 
  presence 
  of 
  

   an 
  ancient 
  interior 
  sea 
  of 
  vast 
  dimensions 
  rests 
  chiefly 
  on 
  evi- 
  

   dence 
  of 
  a 
  negative 
  value, 
  and 
  in 
  the 
  absence 
  of 
  topographic 
  

   maps 
  and 
  of 
  detailed 
  physiographic 
  studies 
  must 
  remain 
  so. 
  

   The 
  problem 
  involves 
  the 
  unraveling 
  of 
  the 
  geologic 
  history 
  

   of 
  the 
  entire 
  plateau 
  region. 
  From 
  the 
  data 
  at 
  hand 
  it 
  appears 
  

   that 
  the 
  great 
  interior 
  depression, 
  itself 
  a 
  plateau, 
  owes 
  its 
  

   existence 
  to 
  faulting 
  as 
  implied 
  by 
  Bowman, 
  f 
  and 
  that 
  the 
  

   downfaulted 
  area 
  was 
  given 
  its 
  relative 
  position 
  after 
  uplift 
  

   and 
  peneplanation 
  of 
  both 
  the 
  eastern 
  and 
  the 
  Maritime 
  Cor- 
  

   dilleras 
  in 
  early 
  Tertiary 
  time. 
  It 
  is 
  also 
  probable 
  that 
  the 
  

   floor 
  of 
  the 
  sunken 
  area 
  was 
  further 
  modified 
  by 
  warping 
  and 
  

   selective 
  faulting 
  which 
  produced 
  a 
  number 
  of 
  secondary 
  

   depressions 
  at 
  considerable 
  depths 
  below 
  the 
  general 
  floor. 
  

   It 
  is 
  reasonable 
  to 
  suppose 
  that 
  such 
  a 
  downfaulted, 
  warped 
  

   surface 
  would 
  be 
  occupied 
  by 
  a 
  number 
  of 
  lakes, 
  whose 
  extent 
  

   and 
  permanency 
  and 
  degree 
  of 
  salinity 
  would 
  bear 
  direct 
  rela- 
  

   tions 
  to 
  the 
  original 
  topography, 
  abundance 
  of 
  waste 
  and 
  

   climatic 
  fluctuations. 
  

  

  -«• 
  Private 
  communication. 
  

  

  f 
  This 
  Journal, 
  vol. 
  xxviii, 
  1909. 
  

  

  