﻿Chas. 
  D. 
  Walcott, 
  

  

  U. 
  S. 
  Geological 
  Survey. 
  

  

  VOL. 
  XLII. 
  DECEMBER, 
  1891. 
  

  

  Established 
  by 
  BENJAMIN 
  SILLIMAN 
  in 
  1818. 
  

  

  THE 
  

  

  AMERICAN 
  

  

  JOURNAL 
  OF 
  SCIENCE. 
  

  

  EDITORS 
  

  

  JAMES 
  D. 
  and 
  EDWARD 
  S. 
  DANA. 
  

  

  ASSOCIATE 
  EDITORS 
  

  

  Professors 
  JOSIAH 
  P. 
  COOKE, 
  GEORGE 
  L. 
  GOODALE 
  

   and 
  JOHN 
  TROWBRIDGE, 
  of 
  Cambridge. 
  

  

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

   New 
  Hayen, 
  

  

  Professor 
  GEORGE 
  F. 
  BARKER, 
  of 
  Philadelphia. 
  

  

  THIRD 
  SERIES. 
  

   YOL. 
  XLII.— 
  [WHOLE 
  NUMBER, 
  CXLIL] 
  

  

  No. 
  252.— 
  DECEMBER, 
  1891. 
  

  

  WITH 
  PLATE 
  XYI. 
  

  

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

   1891. 
  

  

  TUTTLE, 
  MOREHOUSE 
  & 
  TAYLOR, 
  PRINTERS, 
  371 
  STATE 
  STREET. 
  

  

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