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  COHOES 
  MASTODON. 
  

   Fig. 
  11. 
  

  

  139 
  

  

  At 
  a 
  and 
  b 
  are 
  striated 
  surfaces 
  dipping 
  from 
  each 
  other, 
  and 
  

   shown 
  at 
  c 
  to 
  be 
  parts 
  of 
  a 
  curved 
  surface. 
  The 
  straight 
  slip 
  d 
  dips 
  

  

  toward 
  the 
  curved 
  one. 
  

   A 
  section 
  (fig. 
  12) 
  makes 
  

   this 
  plainer. 
  Theory: 
  the 
  

   direction 
  of 
  motion 
  of 
  the 
  

  

  upper 
  mass 
  is 
  shown 
  by 
  the 
  arrow 
  (tig. 
  13). 
  A 
  large 
  concretion 
  

   at 
  g 
  meets 
  another 
  in 
  the 
  bed, 
  and 
  loosens 
  the 
  mass 
  M, 
  so 
  that 
  it 
  

   assumes 
  the 
  position 
  in 
  fig. 
  14, 
  until 
  the 
  projections 
  pass 
  each 
  other. 
  

   We 
  might 
  object 
  that 
  so 
  slight 
  a 
  motion 
  seems 
  insufficient 
  to 
  pro- 
  

   duce 
  the 
  groves 
  of 
  the 
  curved 
  surface. 
  

  

  Above 
  Cohoes 
  town 
  the 
  bed-rock 
  of 
  the 
  west 
  shore 
  is, 
  to 
  a 
  large 
  

   extent, 
  hidden 
  by 
  hills 
  of 
  drift 
  and 
  a 
  terrace 
  of 
  clay. 
  On 
  the 
  

   east 
  side, 
  an 
  area 
  nearly 
  a 
  mile 
  broad 
  and 
  two 
  miles 
  long, 
  stretch- 
  

   ing 
  along 
  the 
  river 
  from 
  the 
  falls 
  toward 
  Middletown. 
  shows 
  the 
  

   bed-rock, 
  covered 
  by 
  a 
  thin 
  clay 
  soil 
  derived 
  from 
  the 
  decomposi- 
  

   tion 
  of 
  the 
  rock 
  itself. 
  This 
  area 
  is 
  elevated 
  about 
  175 
  feet 
  above 
  

   the 
  water 
  of 
  the 
  Hudson. 
  Its 
  character 
  is 
  very 
  uniform. 
  It 
  is 
  

   undulating, 
  and 
  the 
  axis 
  of 
  each 
  hill 
  is 
  due 
  N. 
  and 
  S. 
  Some 
  of 
  

   the 
  declivities 
  facing 
  E. 
  or 
  W. 
  are 
  steep, 
  but 
  none 
  of 
  those 
  facing 
  

   N. 
  or 
  S. 
  Its 
  eastern 
  boundary 
  is 
  a 
  line 
  of 
  drift 
  hills 
  and 
  terrace. 
  

   The 
  latter 
  is 
  of 
  clay 
  covered 
  by 
  sand, 
  and 
  extends 
  eastward 
  to 
  

  

  