Chas. D. Walcott, 



U. S. Geological Survey. 



VOL. XLI. 





MAY, 1891. 



Established by BENJAMIN SILLIMAN in 1818. 



T H E 



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 Haven, 



Professor GEORGE F. BARKER, of Philadelphia. 



THIRD SERIES. 



YOL. XLI.— [WHOLE NUMBER, CXLL] 



No. 245.— MAY, 1891. 



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

 1891. 



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



Published monthly. Six dollars per year (postage prepaid). $6.40 to foreign sub- 

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