LETTEE TO THE SECEETAEY. 



Denver, Colorado Teeritoiiy, 



Octoher 15, 1869. 



Sir : In accordance witli your iustructions dated Washington, April 1, 

 1869, I have the honor to transmit my preliminary field report of the 

 United States geological survey of Colorado and iSTew Mexico, con- 

 ducted by me, under your direction, during the past season. A i3ortion 

 of your instructions is as follows : 



" You will proceed to the field of your labors as soon as the necessary 

 arrangements can be made and the season will permit, and your attentioii 

 will be especially directed to the geological, mineralogical and agricul- 

 tural resources of the Territories herein designated ) you will be required 

 to ascertain the age, order of succession, relative position, dip, and com- 

 parative thickness of the different strata and geological formations, 

 and examine with care all the beds, veins, and other deposits, of ores, 

 coals, clays, marls, peat, and other mineral substances, as well as the 

 fossil remains of the different formations ; and you will also make full 

 collections in geology, mineralogy, and i^aleontology, to illustrate your 

 notes taken in the field." 



In accordance with the above instructions I proceeded to Cheyenne, 

 Wyoming Territory, where my preparations and outfit were made. 



My assistants were selected as follows : 



1. James Stevenson, managing director and general assistant. 



2. Henry W. Elliott, artist. 



3. Rev. Cyrus Thomas, entomologist and botanist. 



4. Persifer Frazer, jr., mining engineer and metallurgist. 



5. E. C. Carrington, jr., zoologist. 



6. B. H. Cheever, jr., general assistant. 



Five men were also employed, three of them as teamsters, one as 

 laborer, and the other one as cook. 



As soon as my preparations were completed, my field labors com- 

 menced, June 29, at Cheyenne. Limited somewhat as to time and means, 

 I arranged my plans so as to cover as much ground as possible and secure 

 the greatest amount of geological information. On the plains the 

 geological structure is very simple, and frequently over large areas the 

 basis rocks are concealed by superficial deposits. It seemed best, there- 

 fore, to make my examinations southward along the eastern base of the 

 Eocky Mountains for the purpose of studying the upheaved ridges, or 

 " hog backs," as they are called in this country. These ridges afford 

 peculiar facilities for working out the geological structure of the country. 

 Indeed, they are like the pages of an open book upon which the geolo- 

 gist can read what the Creator has written upon each formation known 

 in the country from the granite mass that forms the nucleus of the 

 loftiest mountain range to the most recent tertiary formation inclusive. 

 Often in a little belt, from half a mile to four or five in width, one may 

 travel over the upturned edges of nearly all the formations in the geolo- 

 gical scale, and the opportunity was presented, in this way, for tracing 



