iv • PREFACE. 



follow the steps which have led to present 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 uncer- 

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

 the purpose has been to take the student into the fraternity of geolo- 

 gists, and to reveal to him the true state of the development of the 

 science, giving an accurate and proportionate view of the positive knowl- 

 edge 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 hypo- 

 thetical elements as such, and to avoid confusing the interpretations 

 based on hypothesis, with the statements of fact and established doc- 

 trines. 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. 

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

 should also be regarded as giving so much the more emphasis to that 

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

 which is doubtful. Hypothetical and unsolved problems have been 

 treated, so far as practicable, 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 famihar 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 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 



