﻿64 
  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  Theresa 
  and 
  Tribes 
  Hiil 
  formations 
  

  

  These 
  fonnations, 
  as 
  mapped, 
  consist 
  chiefly 
  of 
  rather 
  thin 
  bedded 
  

   layers 
  of 
  bkie 
  gray, 
  sandy 
  magnesian 
  limestone 
  which 
  are 
  exceed- 
  

   ingly 
  tough 
  and 
  resistant 
  rocks 
  when 
  fresh, 
  but 
  weather 
  rapidly 
  

   to 
  an 
  ocherous, 
  rotten 
  stone 
  [pi. 
  15]. 
  The 
  basal 
  portion, 
  through 
  a 
  

   thickness 
  of 
  15 
  feet, 
  carries 
  frequent 
  beds 
  of 
  weak, 
  calcareous 
  

   sandstone 
  in 
  alternation 
  with 
  the 
  limestone, 
  the 
  sandstone 
  being 
  

   identical 
  in 
  character 
  and 
  appearance 
  with 
  that 
  forming 
  the 
  sum- 
  

   mit 
  of 
  the 
  Potsdam. 
  These 
  form 
  apparent 
  " 
  passage 
  beds 
  " 
  between 
  

   the 
  sandstone 
  and 
  the 
  limestone 
  above. 
  The 
  overlying 
  beds 
  consist 
  

   chiefly 
  of 
  magnesian 
  limestone 
  though 
  occasional 
  sand 
  streaks 
  con- 
  

   tinue 
  throughout, 
  and 
  there 
  is 
  a 
  varWng 
  and. 
  in 
  general, 
  consider- 
  

   able 
  amount 
  of 
  sand 
  in 
  most 
  of 
  the 
  beds. 
  While 
  this 
  tends 
  to 
  have 
  

   a 
  streaky 
  distribution, 
  it 
  seldom 
  wholly 
  gives 
  out. 
  The 
  sand 
  is 
  

   chiefly 
  of 
  quartz, 
  certainly 
  90'^ 
  of 
  it 
  consisting 
  of 
  that 
  mineral, 
  but 
  

   grains 
  of 
  feldspar, 
  mica, 
  magnetite, 
  pyrite, 
  titanite 
  and 
  zircon 
  are 
  

   also 
  present 
  and 
  all 
  in 
  quite 
  fresh 
  condition. 
  

  

  All 
  the 
  rock 
  ei¥ervesces 
  freely 
  with 
  acid, 
  and 
  the 
  thin 
  section 
  

   shows 
  this 
  to 
  be 
  chiefly 
  due 
  to 
  the 
  presence 
  of 
  calcite 
  cement, 
  most 
  

   prominent 
  in 
  the 
  more 
  sandy 
  portions 
  of 
  the 
  rock. 
  A 
  prevailing 
  

   and 
  highly 
  characteristic 
  feature 
  of 
  the 
  rock 
  is 
  the 
  appearance, 
  on 
  

   freshly 
  broken 
  surfaces, 
  of 
  lustrous 
  calcite 
  cleavages. 
  These 
  are 
  

   due 
  to 
  the 
  coarsely 
  crystalline 
  character 
  of 
  the 
  calcite 
  cement, 
  the 
  

   crystals 
  ranging 
  from 
  ^ 
  inch 
  to 
  i 
  inch 
  in 
  length, 
  and 
  inclosing 
  

   a 
  number 
  of 
  sand 
  grains, 
  so 
  that 
  they 
  are 
  veritable 
  sand 
  crystals. 
  

   This 
  lithologic 
  peculiarity 
  is 
  a 
  feature 
  of 
  the 
  rock 
  of 
  this 
  horizon 
  

   across 
  the 
  entire 
  northern 
  border 
  of 
  the 
  Adirondacks. 
  

  

  As 
  mapped 
  the 
  general 
  thickness 
  of 
  the 
  formation 
  over 
  the 
  

   district 
  is 
  from 
  60 
  to 
  70 
  feet, 
  but 
  the 
  thickness 
  is 
  variable. 
  The 
  

   thickness 
  steadily 
  diminishes 
  to 
  the 
  west 
  and 
  to 
  the 
  south 
  in 
  

   the 
  same 
  fashion 
  as 
  the 
  Potsdam's. 
  But 
  there 
  are 
  also 
  local 
  

   variations 
  in 
  thickness 
  which 
  are 
  to 
  be 
  ascribed 
  to 
  wear 
  of 
  its 
  

   summit 
  during 
  an 
  erosion 
  interval 
  which 
  separated 
  its 
  completed 
  

   deposition 
  from 
  the 
  beginning 
  accumulation 
  of 
  the 
  succeeding 
  

   formation. 
  For 
  instance 
  it 
  has 
  a 
  thickness 
  of 
  but 
  20 
  feet 
  near 
  

   the 
  north 
  end 
  of 
  Perch 
  lake 
  (Theresa 
  sheet) 
  though 
  recovering 
  

   its 
  normal 
  thickness 
  of 
  60 
  feet 
  both 
  to 
  the 
  east 
  and 
  to 
  the 
  west: 
  

   and 
  that 
  the 
  diminution 
  in 
  thickness 
  is 
  because 
  of 
  the 
  wearing 
  

   away 
  of 
  its 
  upper 
  beds 
  with 
  the 
  production 
  of 
  a 
  shallow 
  valley 
  

   is 
  shown 
  by 
  the 
  fact 
  that 
  the 
  overlying 
  formations 
  thicken 
  here 
  

   by 
  the 
  same 
  amount 
  that 
  the 
  Theresa 
  thins, 
  and 
  that 
  the 
  thicken- 
  

  

  