iv PREFACE 



Throughout the work the central purpose has been not only to 

 set forth the present status of knowledge, but to present it so that 

 the student will be introduced to the methods and spirit of the 

 science. To this end the working methods of the geologist have 

 been implied as frequently as practicable. . To this end also there 

 has been frankness of statement relative to the limitation of 

 knowledge and the uncertainty of many tentative conclusions. 



The theoretical and interpretative elements which enter into 

 the general conceptions of geology have been freely used, because 

 they are regarded as an essential part of the evolution of the science, 

 because they often help to clear the complete conceptions, and 

 because they stimulate thought. The aim has been, however, 

 to characterize hypothetical elements as such, and to avoid con- 

 fusing the interpretations based on hypothesis, with the statements 

 of fact and established doctrines. Especial care has been taken 

 to recognize the uncertain nature of prevalent interpretations when 

 they are dependent on unverified hypotheses, especially if this 

 dependence is likely to be overlooked. 



In many cases the topics discussed will be found to be presented 

 in ways differing widely from those which have become familiar. 

 In some cases, fundamentally new conceptions of familiar subjects 

 are involved; in others, topics not usually discussed in text-books 

 are stated with some fullness; and in still others, the emphasis 

 is laid on points which have not commonly been brought into prom- 

 inence. Whether the authors have been wise in departing to this 

 extent from beaten paths, the users of the volume must decide. 



Especial attention is directed to the map work suggested at vari- 

 ous points in the text, as on pages 109, 194, 222, 288, 331, 366, 413, 

 475, 506, 659, 726, 771, and 845. The use of the topographic maps, 

 folios, and other publications of the United States Geological Sur- 

 vey, somewhat as suggested, will be of great service in making 

 the subject real. The reports of the several State Geological 

 Surveys of the states where this book is used, will also !><> service- 

 able. It is suggested that instructors who use the maps and folios 

 mentioned in the text will do well to plan for this work before 

 reaching the ends of the chapters where mention is ma<le of this 

 work. The map work should be interwoven with the class-room 



