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  THE 
  J 
  

  

  AMERICAN 
  

  

  JOURNAL 
  OF 
  SCIENCE. 
  

  

  Established 
  by 
  BENJAMIN 
  SILLIMAN 
  in 
  1818. 
  

  

  EDITOES 
  

  

  JAMES 
  D. 
  and 
  EDWARD 
  S. 
  DANA. 
  

  

  ASSOCIATE 
  EDITOES 
  

  

  Professors 
  JOSIAII 
  P. 
  COOKE, 
  GEORGE 
  L. 
  GOODALE 
  

   and 
  JOHN 
  TROWBRIDGE, 
  of 
  Cambkidge. 
  

  

  Professors 
  H. 
  A. 
  NEWTON 
  and 
  A. 
  E. 
  YERRILL, 
  of 
  

   New 
  Haven, 
  

  

  Professor 
  GEORGE 
  F. 
  BARKER, 
  of 
  Philadelphia. 
  

  

  THIRD 
  SERIES. 
  

  

  YOL. 
  XLIL— 
  [WHOLE 
  NUMBER, 
  OXLIL] 
  

  

  Nos. 
  21H—252. 
  

  

  JULY 
  TO 
  DECEMBEK, 
  1891. 
  

  

  2 
  5" 
  G 
  I 
  G 
  3 
  

  

  NEW 
  HAYEN, 
  CONN. 
  : 
  J. 
  D. 
  & 
  E. 
  S. 
  DANA. 
  

   1891. 
  

  

  