﻿G. 
  H. 
  Wieland 
  — 
  On 
  Liassic 
  Floras. 
  261 
  

  

  differentiated 
  series 
  of 
  oscillatory 
  subsidences, 
  which 
  increased 
  

   to 
  a 
  maximum 
  at 
  the 
  time 
  of 
  the 
  deposition 
  of 
  the 
  quartzose 
  con- 
  

   glomerate 
  !No. 
  61. 
  Meantime, 
  the 
  subsidences, 
  followed 
  locally 
  

   by 
  conditions 
  of 
  moderately 
  quiet 
  waters 
  with 
  shore 
  distance 
  

   and 
  depth 
  suitable 
  to 
  the 
  deposition 
  of 
  plants 
  in 
  fine 
  muds, 
  

   were 
  very 
  frequent. 
  They 
  were 
  marked 
  by 
  the 
  laying 
  down 
  

   of 
  abundant 
  plant 
  remains 
  and 
  where 
  pronounced 
  of 
  coal.* 
  

   With 
  the 
  later 
  sharp 
  encroachment 
  of 
  the 
  oolitic 
  seas 
  following 
  

   the 
  deep 
  water 
  bed 
  No. 
  67 
  the 
  formation 
  of 
  shell 
  limestones 
  

   with 
  alternate 
  marine 
  or 
  semi-marine 
  sandstones 
  begins, 
  while 
  

   later 
  still, 
  the 
  deep 
  Jurassic 
  and 
  Cretaceous 
  oceans 
  entirely 
  

   transgressed 
  the 
  plant 
  bed 
  region. 
  

  

  That 
  the 
  history 
  of 
  the 
  plant 
  beds 
  consists 
  in 
  a 
  single 
  unit 
  is 
  

   not 
  believed 
  ; 
  even 
  if 
  relatively 
  short, 
  there 
  may 
  have 
  been 
  

   three, 
  if 
  not 
  four, 
  series 
  of 
  events 
  involved 
  in 
  their 
  deposition. 
  

   But 
  it 
  seems 
  much 
  the 
  more 
  scientific 
  method 
  to 
  avoid 
  hasty 
  

   and 
  perforce 
  arbitrary 
  methods 
  of 
  division, 
  whether 
  resting 
  

   the 
  case 
  on 
  either 
  the 
  plant 
  or 
  the 
  physical 
  record 
  as 
  now 
  

   known. 
  And 
  it 
  must 
  be 
  far 
  preferable 
  to 
  use, 
  for 
  the 
  time 
  

   being, 
  the 
  purely 
  tentative 
  demarcation 
  into 
  lower 
  and 
  upper 
  

   beds 
  and 
  await 
  the 
  accumulation 
  of 
  exact 
  evidence 
  as 
  to 
  the 
  

   actual 
  course 
  of 
  deposition 
  and 
  floral 
  change. 
  

  

  B. 
  Composition, 
  Age, 
  and 
  Source 
  of 
  the 
  Mixteca 
  Alta 
  Flora. 
  

  

  In 
  further 
  comparing 
  representative 
  Jurassic 
  floras 
  like 
  those 
  

   of 
  Italy, 
  Bornholm, 
  Yorkshire, 
  India, 
  and 
  California, 
  it 
  seems 
  

   that 
  the 
  age 
  usually 
  assigned 
  to 
  the 
  plants 
  of 
  these 
  several 
  

   regions 
  is 
  in 
  the 
  main 
  correct. 
  That 
  is 
  to 
  say, 
  however 
  uncer- 
  

   tain 
  in 
  their 
  application, 
  however 
  illy 
  defined 
  and 
  variant 
  may 
  

   be 
  the 
  methods 
  for 
  determining 
  the 
  succession 
  of 
  the 
  earlier 
  

   Mesozoic 
  florae, 
  the 
  general 
  mode 
  of 
  procedure 
  is 
  at 
  least 
  

   fairly 
  enough 
  understood 
  and 
  agreed 
  upon 
  to 
  yield 
  approxima- 
  

   tions 
  to 
  the 
  true 
  age. 
  

  

  But 
  aside 
  from 
  exigencies 
  of 
  fossilization, 
  climatic 
  varia- 
  

   tions, 
  and 
  rates 
  of 
  migration 
  or 
  dispersal, 
  all 
  comparisons 
  of 
  

  

  * 
  Neither 
  the 
  position 
  nor 
  local 
  trend 
  of 
  the 
  land 
  masses 
  which 
  furnished 
  

   the 
  fossil 
  plants 
  of 
  the 
  Consuelo 
  section 
  has 
  been 
  worked 
  out. 
  The 
  plant 
  

   beds 
  may 
  indeed 
  lie 
  near 
  to 
  a 
  long, 
  approximately 
  east-west 
  shore 
  line. 
  But, 
  

   in 
  any 
  event, 
  there 
  is 
  much 
  doubt 
  if 
  the 
  coal 
  seams, 
  always 
  or 
  ever, 
  indicate 
  

   swamp 
  bottoms. 
  The 
  comminuted 
  condition 
  of 
  much 
  of 
  the 
  plant 
  material 
  

   of 
  the 
  coal 
  seams 
  may 
  as 
  well 
  indicate 
  flotation 
  of 
  rafts 
  into 
  deeper 
  waters 
  

   as 
  swamp 
  bottom 
  conditions 
  ; 
  while 
  the 
  fine 
  clays, 
  which 
  are 
  so 
  generally 
  

   intermingled 
  and 
  alternate 
  so 
  often, 
  may 
  mostly 
  be 
  the 
  result 
  of 
  sedimenta- 
  

   tion 
  in 
  deeper 
  waters. 
  

  

  Fortunately, 
  we 
  may 
  presently 
  expect 
  some 
  direct 
  information 
  as 
  to 
  the 
  

   nature 
  of 
  these 
  coals, 
  the 
  Instituto 
  Geologico 
  having, 
  through, 
  the 
  courtesy 
  

   of 
  the 
  director, 
  Senor 
  Villerello, 
  arranged 
  to 
  send 
  a 
  complete 
  series 
  of 
  the 
  

   Mexican 
  coals 
  to 
  Professor 
  Jeffrey 
  of 
  Harvard 
  for 
  study 
  by 
  the 
  highly 
  

   effective 
  methods 
  of 
  thin 
  sectioning 
  he 
  has 
  developed. 
  

  

  