﻿170 
  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  i 
  

  

  small 
  area 
  of 
  metamorphic 
  Trenton 
  and 
  Calciferous 
  limestones, 
  

   but 
  it 
  is 
  exceedingly 
  improbable 
  that 
  any 
  rocks 
  which 
  have 
  been 
  

   subjected 
  to 
  metamorphic 
  changes 
  are 
  in 
  any 
  degree 
  permeable. 
  

   This 
  catchment 
  must 
  therefore 
  be 
  considered 
  as 
  underlain 
  by 
  an 
  

   impermeable 
  formation. 
  All 
  of 
  the 
  water 
  falling 
  upon 
  it 
  except 
  

   that 
  absorbed 
  by 
  evaporation, 
  chemical 
  changes, 
  etc. 
  reappears 
  as 
  

   runoff 
  in 
  the 
  streams. 
  It 
  may 
  be 
  safely 
  assumed 
  that 
  there 
  are 
  

   no 
  other 
  losses. 
  Nevertheless, 
  the 
  evaporation 
  of 
  this 
  stream 
  is 
  

   that 
  tentatively 
  placed 
  upon 
  other 
  deforested 
  areas. 
  Moreover, 
  

   there 
  is 
  another 
  interesting 
  consideration 
  of 
  which 
  brief 
  note 
  may 
  

   be 
  taken 
  at 
  this 
  place. 
  In 
  deference 
  to 
  the 
  Water 
  Supply 
  Depart- 
  

   ment 
  of 
  the 
  City 
  of 
  New 
  York, 
  the 
  writer 
  has 
  used 
  in 
  computing 
  

   the 
  monthly 
  runoff 
  the 
  catchment 
  area 
  of 
  339 
  square 
  miles. 
  Mr 
  

   Vermeule, 
  however, 
  asserts 
  that 
  this 
  area 
  is 
  not 
  the 
  true 
  one. 
  He 
  

   says 
  the 
  true 
  area 
  above 
  old 
  Croton 
  dam 
  is 
  353 
  square 
  miles. 
  

   If 
  we 
  assume 
  this 
  to 
  be 
  true, 
  it 
  follows 
  that 
  the 
  average 
  runoff, 
  

   instead 
  of 
  being 
  22.8 
  inches, 
  is 
  over 
  4 
  per 
  cent 
  less, 
  or 
  is, 
  roundly, 
  

   21.8 
  inches. 
  This 
  raises 
  the 
  evaporation 
  from 
  26.6 
  inches 
  to 
  27.6 
  

   inches. 
  In 
  his 
  report 
  on 
  forests, 
  Mr 
  Vermeule 
  has 
  placed 
  the 
  

   evaporation 
  of 
  his 
  second 
  Croton 
  series, 
  which 
  the 
  Avriter 
  under- 
  

   stands 
  him 
  to 
  consider 
  more 
  reliable, 
  at 
  22.6 
  inches, 
  a 
  difference 
  

   of 
  5 
  inches 
  from 
  the 
  foregoing 
  figures, 
  which 
  it 
  may 
  be 
  remarked 
  

   is 
  based 
  upon 
  the 
  latest 
  revision 
  and 
  is 
  presumably 
  more 
  likely 
  

   to 
  be 
  correct. 
  

  

  On 
  the 
  upper 
  Hudson 
  river, 
  with 
  a 
  catchment 
  above 
  Mechanic- 
  

   ville 
  of 
  4500 
  square 
  miles, 
  the 
  average 
  rainfall 
  for 
  the 
  fourteen 
  

   years 
  from 
  1888 
  to 
  1901, 
  inclusive, 
  was 
  about 
  44.2 
  inches, 
  the 
  

   average 
  runoff 
  23.3 
  inches, 
  and 
  the 
  evaporation 
  20.9 
  inches. 
  

   Above 
  Glens 
  Falls 
  -this 
  stream 
  lies 
  almost 
  entirely 
  in 
  the 
  Pre- 
  

   cambrian 
  gneiss, 
  from 
  which 
  it 
  is 
  improbable 
  that 
  there 
  is 
  any 
  

   loss 
  of 
  water. 
  Its 
  main 
  tributary 
  to 
  the 
  west, 
  Sacandaga, 
  is, 
  by 
  

   observation, 
  an 
  exceedingly 
  prolific 
  water 
  yielder. 
  To 
  the 
  east, 
  

   the 
  Battenkill 
  and 
  Hoosic 
  rivers 
  have 
  a 
  different 
  geologic 
  history. 
  

   The 
  Battenkill 
  flows 
  across 
  the 
  Hudson 
  shales, 
  the 
  Georgia 
  lime- 
  

   stones 
  and 
  shales, 
  finally 
  rising 
  in 
  the 
  metamorphic 
  Hudson 
  and 
  

   Trenton 
  formations. 
  The 
  Hoosic 
  river 
  has 
  a 
  similar 
  geologic 
  his- 
  

  

  