﻿A. 
  M. 
  Edwards 
  — 
  Infusorial 
  Earths 
  of 
  Pacific 
  Coast 
  375 
  

  

  in 
  the 
  case 
  of 
  the 
  Mystic 
  Pond, 
  near 
  Boston, 
  Mass. 
  Here 
  the 
  

   bed 
  of 
  the 
  pond 
  is 
  much 
  below 
  that 
  of 
  the 
  river 
  which 
  serves 
  

   as 
  its 
  outlet, 
  so 
  that 
  at 
  the 
  time 
  of 
  high 
  tides 
  the 
  salt 
  water, 
  

   which 
  on 
  account 
  of 
  its 
  superior 
  density 
  creeps 
  up 
  beneath 
  the 
  

   fresh 
  water, 
  runs 
  over 
  the 
  bar 
  at 
  the 
  entrance 
  and 
  flows 
  down 
  

   into 
  the 
  pond, 
  thus 
  mixing 
  the 
  forms 
  of 
  life 
  found 
  therein. 
  

   An 
  account 
  of 
  this 
  locality 
  with 
  a 
  list 
  of 
  the 
  forms 
  of 
  Diato- 
  

   macese 
  observed 
  in 
  the 
  mud 
  brought 
  up 
  from 
  the 
  bottom 
  of 
  

   the 
  pond 
  by 
  Messrs. 
  Greenleaf 
  and 
  Stodder 
  will 
  be 
  found 
  in 
  

   the 
  Proceedings 
  of 
  the 
  Boston 
  Society 
  of 
  Natural 
  History, 
  

   vol. 
  viii, 
  page 
  119, 
  So 
  also 
  I 
  have 
  examined 
  a 
  locality 
  of 
  a 
  

   like 
  kind 
  upon 
  Phillips' 
  Beach 
  between 
  Swampscott 
  and 
  

   Marblehead, 
  Mass. 
  Here 
  a 
  small 
  mass 
  of 
  fresh 
  water 
  fed 
  at 
  

   uncertain 
  periods 
  by 
  intermitting 
  streams, 
  by 
  drainage 
  or 
  by 
  

   iniiltration 
  of 
  water 
  through 
  the 
  beach 
  sand, 
  by 
  which 
  the 
  salt 
  

   is 
  removed, 
  lies 
  a 
  short 
  distance 
  within 
  and 
  at 
  a 
  lower 
  level 
  

   than 
  the 
  shore 
  and 
  in 
  it 
  grow 
  many 
  fresh 
  water 
  plants 
  and 
  

   are 
  found 
  several 
  fresh 
  water 
  animals. 
  Yet 
  at 
  times 
  of 
  high 
  

   tide 
  or 
  during 
  storms 
  the 
  salt 
  water 
  must 
  find 
  egress, 
  for 
  in 
  it 
  

   I 
  observed 
  marine 
  species 
  of 
  Diatomacese 
  in 
  the 
  mud 
  taken 
  

   from 
  the 
  bottom 
  and, 
  in 
  fact, 
  some 
  few 
  were 
  noticed 
  living 
  in 
  

   the 
  water 
  of 
  the 
  pond. 
  

  

  The 
  microscope 
  thus 
  applied 
  to 
  geology, 
  in 
  the 
  hands 
  of 
  

   experienced 
  and 
  competent 
  observers, 
  besides 
  the 
  above, 
  re- 
  

   veals 
  the 
  fact 
  as 
  to 
  whether 
  a 
  gathering 
  under 
  examination 
  be 
  

   of 
  recent 
  origin, 
  deposited 
  in 
  a 
  pond, 
  lake, 
  river, 
  marsh, 
  bay, 
  

   or 
  ocean 
  in 
  existence 
  at 
  the 
  time 
  ; 
  or 
  contain 
  mostly 
  extinct 
  

   forms 
  or 
  be 
  situated 
  in 
  time 
  below 
  the 
  alluvial, 
  and 
  hence 
  to 
  

   be 
  classed 
  among 
  the 
  truly 
  fossil 
  strata. 
  So 
  that 
  by 
  means 
  of 
  

   such 
  an 
  examination 
  we 
  come 
  to 
  classify 
  specimens 
  containing 
  

   Diatomacese 
  according 
  to 
  the 
  age 
  or 
  mode 
  of 
  occurrence 
  of 
  

   these 
  forms, 
  and 
  I 
  have 
  provisionally 
  grouped 
  my 
  gatherings 
  

   into, 
  first 
  : 
  Recent, 
  both 
  marine 
  and 
  fresh 
  water 
  ; 
  second, 
  

   Lacustrine 
  sedimentary, 
  now 
  forming, 
  although 
  in 
  many 
  cases 
  

   dating 
  their 
  period 
  or 
  origin 
  as 
  far 
  back 
  as 
  the 
  Post-Glacial. 
  

   The 
  recorded 
  occurrences 
  of 
  similar 
  deposits 
  of 
  fresh 
  water 
  

   forms 
  in 
  the 
  Tertiary 
  I 
  consider 
  extremely 
  doubtful 
  ; 
  third, 
  

   we 
  have 
  then 
  the 
  deposits 
  to 
  which 
  I 
  have 
  given 
  the 
  distinc- 
  

   tive 
  title 
  of 
  sub-Plutonic 
  and 
  the 
  mode 
  of 
  occurrence 
  of 
  which 
  

   I 
  have 
  alluded 
  to 
  above 
  ; 
  fourth, 
  thereafter 
  and 
  lastly 
  we 
  have 
  

   the 
  true 
  Marine 
  Fossiliferous 
  strata 
  which, 
  as 
  far 
  as 
  recorded, 
  

   have 
  been 
  found 
  only 
  in 
  the 
  Lower 
  Miocene 
  Tertiary. 
  A 
  

   subdivision 
  of 
  some 
  of 
  these 
  groups 
  is 
  convenient 
  ; 
  as, 
  for 
  

   instance, 
  the 
  recent 
  gatherings 
  may 
  be 
  so 
  arranged 
  as 
  to 
  indi- 
  

   cate 
  the 
  peculiar 
  habitat 
  of 
  the 
  species 
  contained 
  in 
  it 
  ; 
  the 
  

   so-called 
  "natural 
  leathers" 
  and 
  "paper;" 
  the 
  soundings 
  from 
  

  

  Am. 
  Jour. 
  Sci. 
  — 
  Third 
  Series, 
  Vol. 
  XLII, 
  No. 
  251. 
  — 
  November, 
  1891. 
  

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