). Walcott, 



Geological Survey. 

 >L. XLL APRIL, 1891 



Established by BENJAMIN SILLIMAN in 1818. 



THE 



AMERICAN 



JOURNAL OF SCIENCE. 



EDITORS 



JAMES D. and EDWARD S. DANA. 



ASSOCIATE EDITORS 



•rofessors JOSIAH P. COOKE, GEORGE L. GOODALE 

 and JOHN TROWBRIDGE, of Cambridge. 



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

 New Hayen, 



r GEORGE F. BARKER, of Philadelphia. 



THIRD SERIES. 

 VOL. XLL— [WHOLE NUMBER, CXLL] 

 No. 244.— APRIL, 1891. 



WITH PLATES XY-XVII. 



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

 1891, 



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



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

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