PREFACE BY THE EDITOR. 



At. the time of Mr. Newton's death his manuscript was incomplete. It 

 was chiefly in the form of a first draft, with much erasure and interlineation, 

 but by no means perfected. Two important sections were unwritten, and 

 he was engaged in making a re-examination of some parts of his field, for 

 the purpose of settling certain geological questions that had arisen in his 

 mind while he was at work upon his report. He regarded no part of his 

 manuscript as a finished composition, but intended to revise the whole and 

 recast some portions on his return. The editor has therefore felt called 

 upon to put himself, in a certain sense, in the place of the author, and make 

 such emendation of form as seemed necessary to harmonize the whole. 

 But while he has freely modified the language in such ways as he conceived 

 the author might have modified it had he been able to revise it, he has, 

 with a single exception, carefully preserved the substance. 



At the time of Newton's first examination of the Black Hills, gold had 

 been discovered only in the modern gravels and in a few Archaean quartz 

 ledges. It was natural to assume that all the placer gold originated in the 

 Archaean ledges, and Mr. Newton and Mr. Jenney made that assumption. 

 The logic of events has shown their error, and Newton was aware of it 

 before his death, although he had no opportunity to record his later under- 

 standing. In this single case the editor has modified the substance of the 

 manuscript so far as was necessary to eliminate the error. 



In Newton's scheme of the third chapter, there were two sections indi- 

 cated by title but unwritten ; the first on "The Quatenary,"the second on the 

 " Structure and Age of the Black Hills." It was impossible to supply data 



