xx PREFACE. 



horizons throughoul the whole Rocky Mountain region wherever they bad 

 hoi yel been developed by actual mine workings. 



It was therefore judged wise to extend the scope of the work to an 

 accurate and detailed study of the geologj and economic resources of the 

 Denver Basin area, which would thus serve as a typeof many similar basins 

 along the eastern front of the Rocky Mountains. For such work an ade- 

 quate topographical map was a uecessar) preliminary. It was at lirst 

 supposed thai the enlarged Bayden map, corrected by data from hand 

 Office and railroad surveys, would be sufficiently accurate for the purpose, 

 but as the work progressed and the complexity of the geological structure 



became more and more apparent frequent revisions had to he made, lirst 



l>v topographers as they could be spared from other field work, and finally 

 l>\ the geologisl himself. Mr. Eldridge commenced his study of the 

 Mesozoic rocks of the region in November, L885, and remained in the field 

 almosl continuously, except when driven ill by stress of weather, until 

 March, 1887. Iii L888 papers were published in scientific journals bj 

 Messrs. Cross and Eldridge discussing the principal orographic movements 

 deduced from the stratigraphic study of the field, and their bearing upon 

 the age of the Denver beds, for the purpose of calling the attention of 

 paleontologists and of geologists working in other fields to the conclusions 

 which it seemed legitimate to draw from these studies, that they might be 



led to Contribute tacts which would confirm or oppose these conclusions. 



Mr. Eldridge's extremely detailed field work necessitated a correspond- 

 ingly extended study in the office and laboratory, and owing to the interim 

 nature of the work he was interrupted before its completion to compile 

 coal and artesian maps for the Director and to undertake an economic 

 survey of Florida, which could be done only in winter, in the months 

 Usually devoted to office work. The delays in the publication of the work 

 which have been thus occasioned have not, however, been entirely disad- 

 vantageous, for they have resulted in the discovery of formations similar to 

 the Denver in other regions, notably the Livingston beds of Montana, and 

 in the revision of the old ami the addition of much new faunal and floral 

 evidence on the hasis of the corrected stratigraphy. Although upon the 



question of the absolute age of the Denver beds paleontologists are not yet 



