﻿32 
  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  hibited 
  in 
  contact 
  with 
  each 
  other. 
  Generally 
  the 
  older 
  rocks 
  have 
  

   been 
  worn 
  down 
  to 
  an 
  'undulating- 
  plain 
  (or 
  peneplain), 
  and 
  the 
  

   succeeding 
  beds 
  made 
  from 
  the 
  fragments 
  which 
  were 
  worn 
  from 
  

   the 
  old 
  land. 
  

  

  In 
  the 
  area 
  under 
  consideration, 
  the 
  ancient 
  erosion 
  surface 
  of 
  

   pre-Cambric 
  rocks 
  was 
  overspread 
  by 
  a 
  deposit 
  of 
  sand 
  and 
  occas- 
  

   ionally 
  gravel, 
  which 
  commonly 
  possesses 
  characteristics 
  pointing- 
  

   to 
  a 
  very 
  local 
  origin. 
  Thus 
  the 
  pebbles 
  found 
  in 
  the 
  lowest 
  layers 
  

   of 
  the 
  covering 
  sands, 
  i. 
  e. 
  the 
  Potsdam 
  sandstone, 
  are 
  sometimes 
  

   of 
  the 
  same 
  lithic 
  character 
  as 
  the 
  crystalline 
  rocks 
  near 
  by. 
  

   The 
  Potsdam 
  sandstone 
  is 
  a 
  shallow-water 
  rock, 
  and 
  during 
  its 
  

   accumulation 
  a 
  progressive 
  subsidence 
  of 
  the 
  sea 
  floor 
  took 
  place, 
  

   thus 
  allowing 
  the 
  deposition 
  of 
  beds 
  of 
  considerable 
  thickness. 
  

   This 
  subsidence 
  brought 
  with 
  it 
  a 
  northward 
  migration 
  of 
  the 
  shore 
  

   line 
  of 
  the 
  sea, 
  so 
  that 
  the 
  region 
  of 
  the 
  former 
  coast 
  line 
  gradually 
  

   became 
  more 
  remote 
  from 
  the 
  shore. 
  As 
  a 
  consequence, 
  land- 
  

   derived 
  material 
  became 
  less 
  abundant 
  in 
  this 
  off-shore 
  district, 
  

   being 
  deposited 
  mainly 
  along 
  the 
  new 
  coast 
  line, 
  while 
  farther 
  out 
  

   to 
  sea 
  calcareous 
  deposits, 
  resulting 
  in 
  part 
  from 
  the 
  shells 
  of 
  organ- 
  

   isms, 
  became 
  relatively 
  more 
  abundant. 
  A 
  profile 
  through 
  the 
  

   strata 
  of 
  this 
  region, 
  such 
  as 
  would 
  be 
  obtained 
  in 
  a 
  well 
  or 
  shaft 
  

   sunk 
  to 
  the 
  crystalline 
  floor, 
  would 
  show 
  a 
  progressive 
  decrease 
  in 
  

   the 
  land-derived, 
  or 
  terrigenous 
  material 
  from 
  the 
  Potsdam 
  sand- 
  

   stone 
  upward 
  to 
  the 
  top 
  of 
  the 
  Trenton 
  limestone, 
  and 
  a 
  correspond- 
  

   ingly 
  progressive 
  increase 
  in 
  the 
  amount 
  of 
  calcareous 
  matter. 
  

   This 
  indicates 
  a 
  sustained 
  subsidence 
  of 
  the 
  sea 
  floor, 
  and 
  hence 
  a 
  

   migration 
  of 
  the 
  shore 
  with 
  its 
  attendant 
  terrigenous 
  deposit. 
  It 
  

   will 
  also 
  be 
  seen 
  that 
  the 
  lithic 
  character 
  of 
  any 
  particular 
  formation 
  

   is 
  not 
  the 
  same 
  throughout 
  its 
  extent, 
  but 
  that 
  the 
  local 
  characteris- 
  

   tics, 
  or 
  fades, 
  show 
  considerable 
  variation. 
  Close 
  to 
  the 
  shore 
  each 
  

   formation 
  would 
  present 
  a 
  terrigenous 
  character, 
  i. 
  e. 
  would 
  show 
  

   gravelly, 
  sandy 
  or 
  clayey 
  facies, 
  while 
  away 
  from 
  shore 
  each 
  forma- 
  

   tion 
  would 
  pass 
  into 
  its 
  calcareous 
  facies, 
  which 
  would 
  increase 
  in 
  

   purity 
  with 
  the 
  increase 
  in 
  distance 
  from 
  the 
  source 
  of 
  supply 
  of 
  

   terrigenous 
  sediment. 
  Thus 
  the 
  Potsdam 
  formation 
  has 
  calcareous 
  

   as 
  well 
  as 
  sandy 
  facies, 
  with 
  facies 
  of 
  intermediate 
  type 
  connecting 
  

   them. 
  

  

  