﻿COHOES 
  MASTODON. 
  

  

  129 
  

  

  Notes 
  of 
  investigations 
  at 
  Cohoes 
  with 
  reference 
  to 
  the 
  cir- 
  

   cumstances 
  OF 
  THE 
  DEPOSITION 
  OF 
  THE 
  SKELETON 
  OF 
  MASTODON; 
  

  

  by 
  G. 
  K. 
  Gilbert, 
  under 
  the 
  direction 
  of 
  James 
  Hall. 
  

  

  Pot-Holes. 
  — 
  The 
  pot-holes 
  of 
  the 
  river 
  bed 
  exhibit 
  all 
  stages 
  

   of 
  formation 
  and 
  obliteration. 
  The 
  shales 
  of 
  the 
  Hudson 
  Biver 
  

   Group 
  present 
  everywhere 
  a 
  surface 
  quite 
  rough 
  in 
  detail, 
  though 
  

  

  even 
  and 
  regular 
  in 
  its 
  general 
  features. 
  

  

  S3 
  O 
  

  

  An 
  insensible 
  gradation 
  

  

  Fig. 
  l. 
  

  

  Fij?- 
  2. 
  

  

  Fig. 
  3. 
  

  

  

  H 
  

  

  « 
  y 
  

  

  may 
  be 
  seen 
  above 
  the 
  crest 
  of 
  the 
  fall, 
  from 
  irregular 
  hollows 
  

   bounded 
  by 
  sharp 
  fractures 
  of 
  slate, 
  to 
  deep, 
  rounded, 
  smoothed 
  

   pot-holes. 
  Upon 
  the 
  plateau 
  below 
  the 
  fall 
  the 
  few 
  remaining 
  

   pot-holes 
  are 
  filled 
  nearly 
  to 
  the 
  brim 
  by 
  gravel 
  and 
  mud, 
  and 
  are 
  

   gradually 
  disappearing 
  as 
  the 
  surface 
  of 
  the 
  surrounding 
  rock 
  is 
  

   lowered 
  by 
  wearing 
  without 
  any 
  corresponding 
  wear 
  in 
  the 
  bottom 
  

   of 
  the 
  cavity. 
  The 
  typical 
  form 
  of 
  these 
  pot-holes 
  is 
  a 
  cylinder 
  

   with 
  rounded 
  bottom 
  and 
  rounded 
  brim 
  like 
  a 
  chemist's 
  test-tube 
  

   (fig. 
  1.) 
  Where 
  the 
  river 
  bed 
  is 
  undergoing 
  erosion 
  the 
  brim 
  

  

  often 
  becomes 
  angular 
  (fig. 
  2) 
  ; 
  and 
  

   upon 
  the 
  fall 
  one 
  margin 
  is 
  frequently 
  

   much 
  depressed 
  (fig. 
  3). 
  The 
  type 
  

   is 
  modified 
  in 
  some 
  shallow 
  pot-holes 
  

   by 
  an 
  enlargement 
  of 
  the 
  mouth 
  (fig. 
  

   4), 
  and 
  in 
  some 
  deep 
  ones 
  by 
  expan- 
  

   sions 
  and 
  constrictions 
  in 
  the 
  shaft 
  

   (fig. 
  5), 
  the 
  greatest 
  expansion 
  being 
  

   generally 
  at 
  the 
  bottom. 
  The 
  union 
  

   of 
  several 
  holes 
  gives 
  rise 
  to 
  many 
  

   irregular 
  forms, 
  and 
  any 
  very 
  decided 
  

   departure 
  from 
  a 
  circular 
  cross-sec- 
  

   tion 
  may 
  be 
  attributed 
  to 
  this 
  cause. 
  

   In 
  a 
  few 
  instances 
  the 
  mouth 
  of 
  a 
  

   slender 
  pot-hole 
  was 
  noticed 
  in 
  the 
  bottom 
  of 
  a 
  broader 
  one 
  (fig. 
  6). 
  

   The 
  interior 
  surfaces 
  are 
  rounded, 
  smoothed 
  and 
  even 
  polished, 
  

   especially 
  toward 
  the 
  bottom. 
  The 
  axis 
  is 
  always 
  vertical 
  or 
  nearly 
  

   so, 
  and 
  independent 
  of 
  the 
  clip 
  of 
  the 
  strata. 
  In 
  my 
  examination 
  I 
  

   saw 
  nothing 
  to 
  controvert 
  the 
  theory 
  that 
  they 
  were 
  formed 
  by 
  the 
  

   grinding 
  action 
  of 
  stones 
  moved 
  by 
  water. 
  The 
  force 
  of 
  the 
  cur- 
  

   rents 
  within 
  the 
  pot-holes 
  is 
  attested 
  by 
  the 
  fact 
  that 
  some 
  of 
  them 
  

   of 
  considerable 
  depth 
  were 
  found 
  empty 
  (of 
  aught 
  else 
  than 
  water), 
  

   while 
  in 
  many 
  a 
  few 
  stones 
  were 
  found. 
  Several 
  of 
  them 
  held 
  so 
  

   much 
  gravel 
  as 
  to 
  preclude 
  the 
  idea 
  that 
  they 
  are 
  now 
  deepening, 
  and 
  

  

  Fis. 
  6. 
  

  

  