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  A. 
  M. 
  Edwards 
  — 
  Infusorial 
  Earths 
  of 
  Pacific 
  Coast. 
  

  

  From 
  what 
  lias 
  been 
  said 
  with 
  regard 
  to 
  distinguishing 
  the 
  

   origin 
  of 
  a 
  deposit 
  by 
  means 
  of 
  the 
  minute 
  remains 
  present 
  

   in 
  it, 
  it 
  will 
  be 
  readily 
  understood 
  that 
  we 
  can 
  thus 
  determine 
  

   to 
  a 
  certain 
  extent 
  its 
  age, 
  as 
  to 
  whether 
  the 
  overlying 
  water 
  

   has 
  been 
  fresh, 
  brackish 
  or 
  salt. 
  In 
  the 
  last 
  case 
  we 
  shall 
  find 
  

   present 
  such 
  oceanic 
  genera 
  as 
  Triceratium, 
  Coscinodiscus, 
  

   Aulacodiscus, 
  or 
  Actinocyclus. 
  If 
  the 
  source 
  of 
  the 
  deposit 
  

   has 
  been 
  the 
  shallow 
  water 
  along 
  shore 
  we 
  should 
  expect 
  to 
  

   find 
  littoral 
  species 
  among 
  which 
  would 
  be 
  some 
  of 
  the 
  Pleu- 
  

   rosigma 
  or 
  Amphiprora 
  ; 
  often, 
  of 
  course 
  mixed 
  with 
  deeper 
  

   water 
  forms 
  or 
  even 
  fresh 
  water 
  varieties 
  accidentally 
  mixed 
  by 
  

   being 
  washed 
  down 
  from 
  elevated 
  stations. 
  On 
  the 
  other 
  

   hand 
  if 
  we 
  find 
  the 
  genera 
  Tabellaria, 
  Cocconema 
  or 
  Himan- 
  

   tidium 
  to 
  be 
  present, 
  the 
  fresh-water 
  origin 
  of 
  the 
  gathering 
  is 
  

   established. 
  So 
  a 
  group 
  of 
  mixed 
  marine 
  and 
  fresh-water 
  

   species 
  would 
  indicate 
  the 
  formation 
  of 
  such 
  a 
  deposit 
  under 
  

   very 
  peculiar 
  circumstances, 
  but 
  such 
  mixtures 
  are 
  extremely 
  

   rare. 
  One 
  of 
  the 
  few 
  of 
  this 
  character 
  which 
  I 
  have 
  seen 
  

   being 
  a 
  gathering 
  of 
  living 
  specimens 
  from 
  the 
  St. 
  Johns 
  

   river 
  in 
  Florida, 
  which 
  on 
  account 
  of 
  its 
  course 
  being 
  nearly 
  

   North 
  and 
  South, 
  is 
  so 
  affected 
  by 
  the 
  tides 
  that 
  the 
  marine 
  

   species 
  of 
  Diatomacese 
  at 
  least 
  are 
  carried 
  up 
  almost 
  to 
  its 
  head. 
  

   At 
  some 
  future 
  time 
  when 
  the 
  life 
  history 
  of 
  these 
  minute 
  

   forms 
  is 
  better 
  understood 
  observers 
  will 
  doubtless 
  be 
  able 
  to 
  

   ascertain 
  from 
  the 
  examination 
  of 
  gatherings 
  of 
  the 
  siliceous 
  

   skeletons 
  whether 
  they 
  have 
  grown 
  and 
  been 
  deposited 
  in 
  a 
  

   lake, 
  river 
  or 
  brook, 
  near 
  the 
  level 
  of 
  the 
  sea 
  or 
  at 
  high 
  alti- 
  

   tudes 
  as 
  well 
  as 
  the 
  fact 
  of 
  the 
  fresh 
  or 
  salt 
  character 
  of 
  the 
  

   water. 
  In 
  fact 
  I 
  feel 
  convinced 
  that 
  a 
  time 
  will 
  come 
  when 
  

   this 
  mode 
  of 
  study 
  applied 
  to 
  deposits 
  generally 
  will 
  reveal 
  

   many 
  circumstances 
  connected 
  with 
  the 
  formation 
  of 
  most 
  of 
  

   the 
  strata 
  constituting 
  the 
  available 
  mass 
  of 
  the 
  earth. 
  At 
  the 
  

   present 
  time 
  so 
  little 
  is 
  known 
  of 
  certainty 
  with 
  regard 
  to 
  the 
  

   life-history 
  of 
  the 
  Diatomacese 
  ; 
  the 
  attention 
  of 
  observers 
  

   having 
  been 
  mainly 
  turned 
  towards 
  the 
  finding 
  of 
  new 
  forms 
  

   and 
  manufacturing 
  them, 
  when 
  found, 
  into 
  so-called 
  species, 
  

   that 
  little 
  can 
  be 
  stated 
  definitely 
  with 
  regard 
  to 
  their 
  distribu- 
  

   tion 
  or 
  habit. 
  For 
  years 
  I 
  have 
  been 
  engaged 
  in 
  gathering 
  

   material 
  to 
  illustrate 
  this 
  point 
  and 
  I 
  am 
  in 
  hopes 
  that, 
  as 
  

   facilities 
  for 
  collection 
  increase 
  valuable 
  information 
  will 
  be 
  

   accumulated. 
  With 
  regard 
  to 
  the 
  mixture 
  of 
  forms 
  considered 
  

   peculiar 
  to 
  fresh 
  or 
  salt 
  water 
  respectively, 
  a 
  case 
  of 
  supposed 
  

   mixing 
  of 
  species 
  in 
  a 
  lake 
  into 
  which 
  the 
  ocean 
  had 
  access 
  at 
  

   certain 
  periods 
  of 
  high 
  tides 
  is 
  recorded 
  by 
  Dr. 
  Gregory 
  in 
  the 
  - 
  

   celebrated 
  ' 
  Glenshira 
  sand,' 
  as 
  it 
  has 
  been 
  called, 
  and 
  such 
  may 
  

   have 
  been 
  the 
  circumstances 
  under 
  which 
  this 
  deposit 
  was 
  

   thrown 
  down 
  for 
  w 
  T 
  e 
  have 
  an 
  example 
  of 
  a 
  similar 
  phenomenon 
  

  

  