PREFACE. 



To account for what might otherwise appear as an unnecessary and 

 unwarrantable delay in the publication of the materia] comprised in the 

 accompanying volume, it is needful to give a somewhat detailed history <>t' 

 the work since its inception. 



While the office of tli<' Rocky Mountain division \v;is still located at 

 Denver, and after the completion of ili<' t i « ■ 1 < I work for the monograph 

 ujniii Leadville, it seemed wise to have some work in progress which 

 could be carried <>n during seasons when that in the high mountains was 

 rendered impracticable by snow, and which might serve to occupy such 

 moments <>t' leisure as members of the corps might have in intervals of 

 more important economic work. With this view authority was obtained 

 from the I >irector to have Mr. Whitman < Iross make a study of the basaltic 

 mesas at Golden, Colo., and of the zeolitic minerals known to occur there 

 This work was carried <>n ;it convenient intervals in other work from the 

 autumn of L881 to that of l ss -'i. when the mineralogical results were 

 published in the American Journal of Science' Incidental to this work it 

 was discovered that a series of beds occurring there which had hitherto 

 been considered pari of the coal-bearing Laramie formation were distinctly 

 unconformable with the latter and separated from it by ;i long geological 

 time interval. To these was given the name Denver beds. This discovery 

 was of far-reaching importance both from a strictly scientific and from an 

 economic point of view. Upon the correct and final determination of the 

 age of these beds depended not only the accurate fixing of the age of the 



Rocky Mountain uplift and of the line between Cretac is and Tertiary 



formations in general, but also the means of recognizing the coal-bearing 



'Am. Jour. Bci., 3d series, Vol. Will, L882, p. 152; ibid., Vol. XXIV. 1882, p. 129. 



xix 



