VI PKEFACE. 



logical companion, and the trip was made largely for the purpose of collect- 

 ing fossils at localities where they had previously been discovered, but 

 to which sufficient time had not been given to make good collections. But 

 many places of interest on account of geological structure were also visited, 

 and I had the good fortune to have with me an experienced geologist on 

 this my final review of the region ; and to Professor White, whose paper on 

 paleontology appears in this volume, I am greatly indebted. Nor must I 

 fail to mention the valuable services of Mr. Steward; as he was with me in 

 one of the earlier years of the work, and only in a portion of the region, his 

 studies were but fragmentary, and the results have not been directly incor- 

 porated in my general account of the geology of the country. I feel that I 

 have not done him full justice in this matter, but the plan of publication 

 would not permit the incorporation of his notes bodily; nor would such a 

 course have done him justice, from the fact that a more extended study lias 

 greatly modified opinions entertained by both Mr. Steward and myself at 

 that time. 



For the map I am indebted primarily to the labor and skill of Prof. A. 

 II. Thompson, who has been my collaborator for many years, but in the 

 work he has had several able assistants; and in the year 1874, Prof. H. C. 

 De Motte, of the Illinois Wesleyan University, traveled with me for the 

 purpose of more thoroughly studying the details of Jhe geography more 

 distant from the river, and he somewhat extended the area of the survey 

 embraced on the map. 



To Mr. Gilbert, I am indebted for great assistance in the preparation of 

 the graphic representation employed in illustrating the Report. 



The diagram, Plate VII of the atlas, was prepared for me by Mr. 

 Archibald R. Marvine. In the earlier years of my travels in the Rocky 

 Mountain region, I studied to some extent the Park Mountains. Subse- 

 quently the region was more thoroughly studied by parties under the 

 direction of Dr. Hayden, and in his connection with that work Mr. Marvine 

 traveled over much of the same ground that I had seen. It thus happened 

 that we frequently discussed together the country which had been visited 

 by both of us, and when I came to the preparation of this volume Mr. 

 Marvine kindly proposed to construct this illustration for my use. This is 



