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PREFACE. IX 



periods were compiled and drawn by Mr. C. E. Siebenthal under the 

 supervision of the authors. Many of the figures in the text were drawn 

 by Mr. G. A. Johnson. In other ways, much valued aid has been 

 rendered by Professors E. B. Frost, A. A. Michelson, C. R. Barnes, 

 J. P. Goode, H. G. Gale, and Messrs. Edson S. Bastin and Eugene W. 

 Shaw. Various individual investigators, publishers, and surveys, 

 preeminently the U. S. Geological Survey, have furnished material 

 for illustrations. In general these contributions are acknowledged 

 in the text. 



The references to Volume I are to the second edition, in which 

 the pagination numbers are higher than those of the first edition by 

 a little more than one in twenty, as a rule. 



University of Chicago, December, 1905. 



