﻿66 
  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  formation 
  was 
  the 
  absence 
  above 
  of 
  the 
  sandstone 
  beds 
  which 
  

   are 
  interstratified 
  with 
  the 
  limestones 
  in 
  the 
  lower 
  division. 
  

   Otherwise 
  the 
  formation 
  constitutes 
  an 
  apparent 
  lithologic 
  unit 
  

   and 
  appears 
  as 
  such 
  on 
  the 
  maps; 
  and 
  it 
  seems 
  better 
  to 
  leave 
  

   it 
  as 
  such 
  instead 
  of 
  attempting 
  to 
  subdivide 
  it 
  at 
  this 
  juncture. 
  

   If, 
  however, 
  it 
  does 
  consist 
  of 
  these 
  two 
  separate 
  formations 
  the 
  

   necessity 
  for 
  the 
  name 
  largely 
  disappears, 
  and 
  it 
  is 
  rather 
  a 
  pity 
  

   that 
  it 
  was 
  ever 
  suggested. 
  It 
  is 
  likely, 
  however, 
  to 
  prove 
  use- 
  

   ful 
  as 
  a 
  name 
  for 
  the 
  considerable 
  thickness 
  of 
  alternating 
  beds 
  

   of 
  sandstone 
  and 
  magnesian 
  limestone 
  which 
  everywhere 
  im- 
  

   mediately 
  overlie 
  the 
  Potsdam 
  sandstone 
  in 
  northern 
  New 
  York, 
  

   and 
  which 
  should 
  be 
  mapped 
  separately. 
  

  

  There 
  is 
  then 
  some 
  reason 
  to 
  believe 
  that 
  there 
  is 
  present 
  in 
  

   this 
  district 
  a 
  thickness 
  of 
  from 
  20 
  to 
  30 
  feet 
  of 
  limestone 
  of 
  

   lower 
  Beekmantown 
  age, 
  quite 
  like 
  a 
  similar 
  thickness 
  of 
  rock 
  

   at 
  the 
  summit 
  of 
  the 
  Little 
  Falls 
  dolomite 
  at 
  Little 
  Falls 
  (where 
  

   an 
  unquestioned 
  unconformity 
  exists 
  between 
  the 
  two), 
  and 
  hold- 
  

   ing 
  the 
  same 
  fauna. 
  This 
  is 
  to 
  be 
  separated 
  from 
  the 
  Little 
  

   P'alls 
  dolomite 
  under 
  the 
  name 
  of 
  the 
  Tribes 
  Hill 
  limestone, 
  and 
  

   the 
  same 
  separation 
  needs 
  to 
  be 
  made 
  in 
  this 
  district. 
  The 
  

   Theresa 
  formation 
  is 
  to 
  be 
  restricted 
  to 
  the 
  alternations 
  of 
  sand- 
  

   stone 
  and 
  magnesian 
  limestone 
  which 
  constitute 
  the 
  lower 
  half 
  

   of 
  the 
  formation 
  as 
  mapped. 
  

  

  Age 
  of 
  the 
  Potsdam 
  and 
  Theresa 
  formations. 
  These 
  two 
  

   formations, 
  with 
  a 
  maximum 
  thickness 
  in 
  this 
  district 
  of 
  from 
  125 
  

   to 
  150 
  feet 
  only, 
  represent 
  the 
  attenuated 
  western 
  edge 
  of 
  forma- 
  

   tions 
  which, 
  in 
  the 
  Champlain 
  valley, 
  have 
  tenfold 
  that 
  thickness. 
  

   Their 
  distribution 
  shows 
  that 
  they 
  were 
  deposited 
  in 
  a 
  subsid- 
  

   ing 
  trough 
  along 
  the 
  present 
  St 
  Lawrence 
  valley 
  line, 
  and 
  that 
  

   their 
  deposit 
  commenced 
  at 
  the 
  east 
  and 
  worked 
  westward. 
  

   Everywhere 
  along 
  this 
  line 
  we 
  find 
  a 
  sandstone 
  beneath, 
  grading 
  

   upward 
  into 
  an 
  overlying 
  dolomite, 
  and 
  everywhere 
  the 
  horizon 
  

   is 
  characterized 
  by 
  the 
  presence 
  of 
  the 
  fossil 
  L 
  i 
  n 
  g 
  u 
  1 
  e 
  p 
  i 
  s 
  

   acuminata. 
  Everywhere 
  along 
  this 
  line 
  too 
  there 
  seems 
  to 
  

   be 
  a 
  break 
  between 
  these 
  formations 
  and 
  the 
  next 
  formation 
  

   above. 
  The 
  two 
  formations 
  seem 
  then 
  to 
  be 
  indissolubly 
  bound 
  

   together, 
  to 
  rest 
  unconformably 
  on 
  the 
  Precambric, 
  and 
  to 
  be 
  

   separated 
  by 
  an 
  unconformity 
  from 
  the 
  overlying 
  formation. 
  

   Since 
  the 
  formations 
  are 
  thin 
  in 
  the 
  immediate 
  district, 
  and 
  are 
  

   thinning 
  to 
  the 
  west 
  and 
  south, 
  it 
  follows 
  that 
  we 
  are 
  here 
  in 
  the 
  

   vicinity 
  of 
  the 
  western 
  edge 
  of 
  the 
  subsiding 
  trough. 
  Just 
  how 
  

   far 
  west 
  its 
  deposits 
  extended 
  can 
  not 
  be 
  told. 
  According 
  to 
  

  

  