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to welcome cordially, its future advances. Where practicable, the text has 

 been so shaped that the student may follow the steps which have led to pres- 

 ent conclusions. To this end the working methods of the practical geologist 

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

 been frankness of statement relative to the limitations of knowledge and the 

 uncertainty of many tentative conclusions. In these and in other respects, 

 the purpose has been to take the student into the fraternity of geologists, and 

 to reveal to him the true state of the development of the science, giving an 

 accurate and proportionate view of the positive knowledge attained, of the 

 problems yet unsolved, or but partially solved, and of solutions still to be 

 attained. 



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 

 and complete conceptions, and because they stimulate thought. The aim has 

 been, however, to characterize hypothetical elements as such, and to avoid 

 confusing 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 over- 

 looked. If this shall seem to give prominence to the hypothetical element, it 

 should also be regarded as giving so much the more em.phasis to that which 

 is really trustworthy, in that it sets forth more frankly that which is doubt- 

 ful. Hvpothetical and unsolved problems have been treated, so far as prac- 

 ticable, on the multiple basis ; that is, alternative hypotheses and alternative 

 interpretations are frequently presented where knowledge does not warrant 

 positive conclusions. 



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 fulness ; and 

 in still others the emphasis is laid on points which have not commonly 

 been brought into prominence. Whether the authors have been wise in 

 departing to this extent from beaten paths, the users of the volumes must 

 decide. 



The work is intended primarily for mature students, and is designed to 

 furnish the basis for a year's work in the later part of the college course. 

 By judicious selection of material to be presented and omitted, the volumes 

 will be found useful for briefer courses, and by the use of the numerous refer- 

 ences to the fuller discussions of special treatises they may be made the basis 

 for more extended courses than are commonly given in undergraduate work. 

 The attempt has also been made to make the volumes readable, in the belief 

 that many persons not in colleges or universities will be interested in follow- 



