﻿Titicaca 
  Island 
  and 
  Adjoining 
  Areas. 
  201 
  

  

  Structure. 
  

  

  The 
  meager 
  and 
  disconnected 
  geological 
  mapping 
  resulting 
  

   from 
  studies 
  of 
  Titicaca 
  is 
  an 
  insufficient 
  basis 
  for 
  the 
  discus- 
  

   sion 
  of 
  structural 
  detail, 
  and 
  has 
  allowed 
  the 
  perpetuation 
  of 
  

   such, 
  notions 
  as 
  that 
  the 
  basin 
  is 
  igneous 
  in 
  origin 
  and 
  that 
  

   " 
  smoldering 
  fires 
  still 
  keep 
  the 
  waters 
  warm 
  ;" 
  that 
  the 
  Straits 
  

   of 
  Tiquina 
  resulted 
  from 
  a 
  volcanic 
  eruption 
  ; 
  that 
  the 
  coal 
  is 
  

   Tertiary 
  ; 
  that 
  the 
  succession 
  of 
  strata 
  and 
  the 
  age 
  of 
  the 
  rocks 
  

   are 
  identical 
  with 
  those 
  represented 
  in 
  the 
  Alps, 
  etc. 
  The 
  inves- 
  

   tigations 
  of 
  Forbes 
  and 
  D'Orbigny 
  are, 
  however, 
  sufficient 
  

   to 
  indicate 
  that 
  the 
  strata, 
  Devonian 
  and 
  Carboniferous 
  in 
  age, 
  

   have 
  been 
  thrown 
  into 
  folds, 
  the 
  truncated 
  edges 
  of 
  whose 
  

   limbs 
  are 
  responsible 
  for 
  the 
  innumerable 
  rounded 
  hogbacks 
  

   which 
  form 
  such 
  conspicuous 
  features 
  of 
  the 
  landscape. 
  

  

  The 
  basin 
  of 
  Lago 
  Grande 
  has 
  the 
  appearance 
  of 
  a 
  

   "graben" 
  — 
  one 
  or 
  more 
  down-sunken 
  fault 
  blocks 
  bordered 
  

   in 
  places 
  by 
  escarpments, 
  elsewhere 
  by 
  areas 
  of 
  steep 
  faulting. 
  

  

  Faults 
  are 
  present 
  in 
  large 
  numbers, 
  but 
  their 
  length 
  and 
  

   amount 
  of 
  displacement 
  and 
  structural 
  control 
  are 
  as 
  yet 
  un- 
  

   known. 
  In 
  a 
  section 
  extending 
  from 
  Achacachi 
  to 
  La 
  Guardia, 
  

   entirely 
  across 
  the 
  lake 
  basin, 
  Forbes 
  encountered 
  volcanics 
  

   and 
  highly 
  contorted 
  Devonian 
  (D'Orbigny) 
  strata, 
  presumably 
  

   cut 
  off 
  from 
  overlying 
  strata 
  by 
  a 
  fault, 
  the 
  Carboniferous 
  

   dipping 
  westward 
  until 
  the 
  Straits 
  of 
  Tiquina 
  are 
  reached, 
  

   then 
  dipping 
  eastward 
  across 
  Copacabana. 
  The 
  straits 
  are 
  de- 
  

   scribed 
  as 
  the 
  locus 
  of 
  a 
  fault, 
  a 
  "broken 
  arch." 
  Agassiz 
  

   believed 
  the 
  Carboniferous 
  to 
  consist 
  of 
  a 
  series 
  of 
  rather 
  

   limited 
  elongated 
  basins 
  with 
  axes 
  (determined 
  by 
  Forbes) 
  

   running 
  northwest-southeast. 
  " 
  By 
  a 
  series 
  of 
  such 
  faults 
  [as 
  

   at 
  Tiquina] 
  , 
  more 
  or 
  less 
  prominent, 
  the 
  successive 
  basins 
  of 
  

   Carboniferous 
  .... 
  have 
  been 
  separated."* 
  

  

  Structurally, 
  Titicaca 
  Island 
  is 
  a 
  continuation 
  of 
  Copacabana 
  

   peninsula. 
  The 
  strata 
  are 
  alike 
  in 
  composition, 
  exhibit 
  the 
  

   same 
  folds 
  and 
  nearly 
  identical 
  strikes 
  and 
  dips. 
  This 
  con- 
  

   nection 
  is 
  well 
  shown 
  by 
  a 
  narrow 
  sandstone 
  hogback 
  which 
  

   projects 
  as 
  a 
  cape 
  from 
  Copacabana 
  and 
  terminates 
  in 
  a 
  line 
  of 
  

   islands, 
  progressively 
  decreasing 
  in 
  size. 
  On 
  Titicaca 
  Island, 
  

   the 
  same 
  bed 
  apparently 
  is 
  included 
  in 
  the 
  strata 
  carried 
  north- 
  

   west 
  along 
  the 
  line 
  of 
  strike. 
  With 
  the 
  exception 
  of 
  the 
  

   rounded 
  knobs 
  and 
  eastward-facing 
  cliffs 
  carved 
  by 
  waves 
  from 
  

   the 
  limestone, 
  the 
  whole 
  surface 
  of 
  the 
  island 
  is 
  formed 
  of 
  

   more 
  or 
  less 
  modified 
  hogbacks 
  facing 
  southwest 
  and 
  exhibit- 
  

   ing 
  the 
  influence 
  of 
  structure. 
  From 
  a 
  structural 
  standpoint 
  

   the 
  island 
  may 
  be 
  considered 
  as 
  a 
  group 
  of 
  eroded, 
  folded 
  

   strata, 
  drowned 
  by 
  the 
  waters 
  of 
  the 
  lake. 
  The 
  northwest- 
  

  

  * 
  Agassiz, 
  p. 
  283. 
  

  

  