﻿252 
  G. 
  E. 
  Wieland—On 
  Liassic 
  Floras. 
  

  

  dilleran 
  system, 
  facing 
  the 
  Pacific 
  and 
  extending 
  through 
  

   central 
  and 
  western 
  Oaxaca 
  well 
  into 
  Guerrero 
  as 
  well 
  as 
  

   northerly 
  into 
  Puebla. 
  Nor 
  is 
  it 
  in 
  the 
  main 
  a 
  markedly 
  ele- 
  

   vated 
  portion 
  of 
  the 
  so-called 
  high 
  plateau 
  region 
  of 
  Mexico, 
  

   for 
  to 
  the 
  north 
  lie 
  the 
  lofty 
  peaks 
  about 
  Puebla, 
  and 
  to 
  the 
  

   southeast 
  the 
  great 
  Sierra 
  Juarez 
  mountain 
  knot, 
  the 
  latter 
  

   resting 
  almost 
  directly 
  on 
  the 
  Cordilleran 
  front. 
  In 
  fact 
  the 
  

   upper 
  Mixtecan 
  region 
  in 
  which 
  the 
  Jurassic 
  plant 
  beds 
  are 
  a 
  

   great 
  geologic 
  feature 
  is 
  in 
  part 
  only 
  a 
  moderately 
  elevated 
  

   but 
  much 
  folded 
  and 
  faulted 
  basin. 
  Or 
  at 
  least 
  there 
  are 
  

   border 
  or 
  transverse 
  ranges 
  which 
  have 
  partially 
  protected 
  

   both 
  the 
  marine 
  and 
  underlying 
  freshwater 
  deposits 
  from 
  the 
  

   full 
  effects 
  of 
  dissection 
  and 
  erosion 
  and 
  all 
  those 
  tremendous 
  

   tectonics 
  to 
  which 
  the 
  superbly 
  beautiful 
  and 
  picturesque 
  

   Mixteca 
  Alta 
  region 
  has 
  long 
  been 
  subjected. 
  

  

  The 
  ten 
  or 
  twelve 
  thousand 
  feet 
  of 
  Mesozoic 
  strata 
  which 
  

   give 
  character 
  to 
  the 
  Mixteca 
  Alta 
  are 
  about 
  equally 
  divided 
  

   between 
  the 
  Jura 
  and 
  Cretaceous, 
  any 
  Trias 
  that 
  may 
  occur 
  

   not 
  being 
  as 
  yet 
  distinctly 
  classified. 
  The 
  Mesozoic 
  mass 
  is 
  

   often 
  entirely 
  freed 
  from 
  Tertiary 
  eruptives, 
  ash 
  or 
  conglomer- 
  

   ates, 
  and 
  rests 
  on 
  older 
  sedimentaries 
  of 
  undetermined 
  age, 
  or 
  

   against 
  intrusives. 
  The 
  topography 
  is 
  exceedingly 
  rough. 
  

   The 
  streams 
  in 
  cutting 
  through 
  the 
  massive 
  Cretaceous 
  lime- 
  

   stones 
  capping 
  Jurassic 
  strata 
  of 
  lesser 
  induration 
  form 
  a 
  

   tremendous 
  system 
  of 
  deeply 
  cut 
  valleys, 
  gorges, 
  and 
  canons. 
  

   Yet 
  because 
  of 
  the 
  distinctly 
  varied 
  vegetation 
  and 
  rather 
  free 
  

   growth 
  of 
  pine 
  and 
  oak 
  with 
  relatively 
  few 
  barren 
  stretches, 
  

   scenic 
  aspects 
  are 
  far 
  softer 
  than 
  in 
  the 
  upland 
  mountain 
  coun- 
  

   try 
  of 
  central 
  Mexico. 
  

  

  The 
  general 
  physiography 
  is 
  especially 
  well 
  developed 
  along 
  

   the 
  valley 
  of 
  the 
  Tlaxiaco 
  river, 
  and 
  the 
  adjoining 
  photograph 
  

   of 
  this 
  valley 
  as 
  seen 
  from 
  a 
  point 
  about 
  eight 
  kilometers 
  

   westerly 
  from 
  the 
  town 
  of 
  Tlaxiaco 
  conveys 
  a 
  clear 
  idea 
  of 
  

   scenery 
  in 
  the 
  uplands 
  of 
  the 
  Mixteca 
  Alta. 
  The 
  view 
  is 
  from 
  

   the 
  right, 
  the 
  course 
  of 
  the 
  valley 
  northerly, 
  and 
  to 
  reach 
  the 
  

   river 
  bed 
  some 
  300 
  meters 
  lower, 
  the 
  " 
  trail 
  " 
  here 
  makes 
  a 
  

   long 
  detour. 
  Just 
  over 
  the 
  pack 
  train 
  a 
  long 
  steep 
  side 
  valley 
  

   is 
  plainly 
  seen 
  to 
  lead 
  down 
  into 
  the 
  main 
  valley 
  just 
  to 
  the 
  

   right 
  of 
  the 
  frontal 
  mountain 
  mass 
  of 
  Jura-Cretaceous 
  strata. 
  

   And 
  where 
  this 
  lateral 
  valley 
  debouches 
  a 
  thick 
  series 
  of 
  

   Liassic 
  sandstones 
  and 
  shales 
  bearing 
  thin 
  seams 
  of 
  coal 
  is 
  cut 
  

   into 
  for 
  some 
  distance 
  along 
  the 
  river 
  bank. 
  In 
  fact 
  here 
  are 
  

   a 
  number 
  of 
  coal 
  prospects 
  ; 
  but 
  unfortunately 
  the 
  dumps 
  were 
  

   located 
  directly 
  on 
  the 
  river 
  bank, 
  and 
  had 
  been 
  nearly 
  all 
  

   washed 
  away 
  during 
  the 
  rainy 
  season 
  preceding 
  my 
  visit. 
  So 
  

   that 
  in 
  the 
  absence 
  of 
  fresh 
  excavation 
  or 
  new 
  quarries, 
  there 
  

   were 
  recovered 
  at 
  this 
  particular 
  point 
  but 
  a 
  very 
  few 
  of 
  the 
  

   rich 
  store 
  of 
  fossil 
  cycads 
  unquestionably 
  present. 
  

  

  