ACKNOWLEDGMENTS. 



To the many among my colleagues and students zvho have ren- 

 dered assistance in the production of this ivork I zuish to tender 

 heartiest acknoivledgments. Of those whose helpful attitude aided 

 the early stages of my labors I zvant especially to mention Alpheus 

 Hyatt and Robert Tracy Jackson, leaders in American Palceon- 

 tology, and further, William Otis Crosby, Nathaniel Southgate 

 Shaler, William Morris Davis and Jay Backus Woodzvorth, masters 

 all of inorganic geology, and knozvn as such in both hemispheres. 

 More recently my colleagues at Columbia University, Professors 

 James F. Kemp, Douglas Wilson Johnson, and especially Charles 

 Peter Berkey have put me under obligations by helpful suggestions 

 and criticisms. My graduate students, too, during the last ten years 

 have aided me more than many of them perhaps realize, for only a 

 teacher can knozv the help and inspiration that comes from daily 

 contact zvith eager and, above all, inquiring minds, such as our 

 splendid body of American graduate students furnishes. May these 

 pages recall to them the many spirited discussions, in zvhich they 

 usually bore their part so zvell. 



To one of them. Miss Marjorie O'Connell, A.M., instructor in 

 Geology in Adelphi College, Brooklyn, my special thanks are due 

 for the careful and critical attention given for a period of a full 

 year or over to both nmnuscript and proof, and to the verification of 

 the literary references, and the endeavor, by patient library re- 

 search, to make the bibliograpJiies as serviceable as possible. To 

 her prolonged search of the literature for available material, I also 

 ozve manv important references zvhich I zvould otherzvise have 

 missed. And, finally, she has to her credit the very complete index 

 of this volume. One other I may mention by name, Mr. George L. 

 Cannon, of Denver, Colorado, zvhose broad conceptions of the prin- 

 ciples of classification have rendered our discussions highly prof- 

 itable. 



Finally, J cannot forbear to mention my many foreign friends 

 who, by word of mouth or by their zvritings, have led me into paths 

 replete zvith interest and profitable adventure. To their zvillingness 

 to guide me in my studies zvithin their home-lands, and to discuss 

 the problems zvhich interested us in common, I ozve many stimulat- 

 ing hours. Some, like Kittl of Vienna, Koken of Tubingen, and 

 Holsapfel of Strassburg, have since passed azvay, and I may think 

 of them here in sorrozv. To the many, however, zvho still hold 

 high the torch of learning, these pages bring a greeting from the 

 land zvhich has yet much to learn from the culture, and the devo- 

 tion to Pure Science, so characteristic of the European leaders in 

 the Earth Sciences. And he, to zvhom these pages are inscribed, 

 zvill knozv that the days spent in his inspiriting companionship in 

 field and study are among the potent influences which helped to 

 shape the character of this book. 



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