﻿GEOLOGY 
  OF 
  THOUSAND 
  ISLANDS 
  REGION 
  65 
  

  

  ing 
  is 
  due 
  to 
  the 
  presence 
  of 
  basal 
  layers 
  which 
  disappear 
  to 
  the 
  

   east 
  and 
  west 
  as 
  the 
  Theresa 
  thickens. 
  

  

  The 
  field 
  work 
  in 
  the 
  district 
  was 
  completed, 
  and 
  this 
  report 
  

   written 
  under 
  the 
  impression 
  that 
  this 
  comparatively 
  thin 
  forma- 
  

   tion 
  was 
  a 
  unit 
  and 
  all 
  of 
  the 
  same 
  age. 
  In 
  its 
  lower 
  portion 
  L 
  i 
  n 
  - 
  

   gulepis 
  acuminata 
  is 
  abundant 
  ; 
  above, 
  specimens 
  of 
  a 
  

   rather 
  large, 
  fiat-coiled 
  gastropod 
  occur 
  abundantly 
  in 
  places 
  ; 
  

   occasional 
  cystid 
  plates 
  are 
  found, 
  and 
  unrecognizable 
  traces 
  of 
  

   other 
  forms. 
  The 
  horizon 
  seemed 
  the 
  same 
  as, 
  and 
  the 
  beds 
  

   identical 
  with 
  beds 
  which 
  directly 
  overlie 
  the 
  Potsdam 
  sandstone 
  

   all 
  across 
  northern 
  New 
  York, 
  a 
  length 
  of 
  outcrop 
  of 
  150 
  miles, 
  

   and 
  which 
  have 
  heretofore 
  been 
  called 
  " 
  passage 
  beds 
  " 
  between 
  

   the 
  Potsdam 
  and 
  the 
  Beekmantown, 
  the 
  Beekmantown 
  being 
  the 
  

   formation 
  which 
  overlies 
  the 
  Potsdam 
  for 
  much 
  of 
  this 
  distance. 
  

   In 
  the 
  belief 
  that 
  no 
  Beekmantown 
  was 
  present 
  here, 
  and 
  yet 
  

   that 
  there 
  was 
  here 
  a 
  formation 
  which 
  required 
  mapping 
  separate 
  

   from 
  the 
  Potsdam, 
  the 
  name 
  Theresa 
  was 
  proposed 
  for 
  this 
  

   magnesian 
  limestone 
  formation, 
  it 
  being 
  well 
  exposed 
  in 
  the 
  

   township 
  of 
  that 
  name.^ 
  Recent 
  work 
  by 
  Ulrich, 
  Ruedemann 
  

   and 
  myself 
  in 
  the 
  Mohawk 
  valley 
  has, 
  however, 
  tended 
  to 
  throw 
  

   much 
  doubt 
  upon 
  the 
  entire 
  correctness 
  of 
  this 
  position. 
  We 
  

   find 
  that 
  the 
  formation 
  in 
  the 
  Mohawk 
  valley 
  known 
  as 
  the 
  

   Little 
  Falls 
  dolomite, 
  and 
  heretofore 
  regarded 
  as 
  of 
  Beekman- 
  

   town 
  age, 
  is 
  made 
  up 
  of 
  two 
  unconformable 
  formations, 
  the 
  

   uppermost 
  of 
  which 
  is 
  of 
  lower 
  Beekmantown 
  age, 
  and 
  is 
  a 
  quite 
  

   fossiliferous 
  limestone 
  which 
  we 
  are 
  proposing 
  to 
  separate 
  and 
  

   call 
  the 
  Tribes 
  Hill 
  formation; 
  while 
  the 
  underlying 
  dolomite 
  

   formation 
  is 
  of 
  Upper 
  Cambric 
  (Ozarkic) 
  age. 
  Now 
  the 
  Tribes 
  

   Hill 
  formation 
  contains, 
  as 
  one 
  of 
  its 
  fossils, 
  a 
  gastropod 
  (named 
  

   Pleurotomaria 
  hunterensis 
  by 
  Cleland) 
  which 
  

   Ulrich 
  regards 
  as 
  identical 
  with 
  the 
  gastropod 
  from 
  the 
  Theresa 
  

   formation; 
  it 
  also 
  contains 
  numerous 
  cystid 
  plates, 
  and 
  these 
  

   he 
  also 
  regards 
  as 
  identical 
  with 
  those 
  from 
  the 
  Theresa. 
  The 
  

   Lingula, 
  however, 
  occurs 
  in 
  the 
  Little 
  Falls 
  dolomite, 
  instead 
  of 
  

   the 
  Tribes 
  Hill 
  formation, 
  and 
  is 
  in 
  fact 
  a 
  characteristic 
  Ozarkic 
  

   fossil 
  all 
  around 
  the 
  Adirondack 
  region. 
  Ulrich's 
  present 
  view 
  

   is 
  therefore 
  that 
  the 
  Theresa 
  formation, 
  as 
  here 
  mapped, 
  is 
  in 
  

   part 
  of 
  Ozarkic, 
  and 
  in 
  part 
  of 
  Tribes 
  Hill 
  (lower 
  Beekmantown) 
  

   age. 
  If 
  this 
  be 
  true 
  there 
  must 
  be 
  an 
  undetected 
  unconformity 
  

   between 
  the 
  two 
  portions. 
  In 
  the 
  field 
  the 
  only 
  lithologic 
  

   difference 
  noted 
  between 
  the 
  upper 
  and 
  lower 
  portions 
  of 
  the 
  

  

  ^Geol. 
  Soc. 
  Am. 
  Bui. 
  19:155-76. 
  

  

  