viii PREFACE. 



treatises, they may be made the basis for much more extended courses 

 than are commonly given in undergraduate work. They will, it is 

 hoped, be of service as reference books where short courses only are 

 given. 



No attempt has been made to make these volumes a manual, and 

 the details of the geology of individual regions will not be found in 

 them, except as these details have been thought necessary to the inter- 

 pretation of the general phases of the subject. For local details, State 

 and National Survey reports, and other special monographs, must be 

 consulted. 



In the account of the geology of each period, the text is so arranged 

 that the physical history of the continent may be followed through 

 consecutively, without especial reference to the history of the life 

 and conversely, so that the history of life may be followed through 

 as an independent, unified subject. This is an adaptation to the con- 

 venience of those institutions whose division of instructional labor 

 permits a separation of the treatment of the physical from the bio- 

 logical phases of the subject. 



Acknowledgments, in addition to those made in Volume I, are 

 due to many colleagues who have rendered valuable aid in the prepa- 

 ration of these volumes. In the portions relating to invertebrate 

 paleontology, the assistance of Dr. Stuart Weller has been so gen- 

 erous and unreserved that he has been scarcely less than a collaborator. 

 Almost as much may be said of the assistance of Drs. S. W. Williston 

 and E. C. Case in the portions relating to vertebrate paleontology, 

 and of Drs. J. M. Coulter and H. C. Cowles in paleobotany. The selec- 

 tion of the invertebrate illustrations and the preparation of the anno- 

 tated legends have been largely the work of Professor Weller, and 

 many of the figures for these illustrations have been redrawn for this 

 work under his supervision by Miss Mildred Marvin. He has also 

 read both the manuscript and the proof. Dr. Williston has read the 

 manuscript and proof of the portions relating to vertebrate life. In 

 the cosmological portions, the collaboration of Dr. Forest R. Moulton 

 has been of indispensable service. In certain mathematical prob- 

 lems, Dr. A. C. Lunn has given valuable advice, and Dr. Julius A. 

 Stieglitz has done the same in connection with chemical subjects. 

 The colored map of the American formations has been prepared by 

 Professor Bailey Willis. The series of analytical maps of the several 



