NOTE. y 



Each full atlas sheet represents two degrees and forty-five minutes in 

 longitude and one degree and forty minutes in latitude (an area of from 

 17,000 to 18,000 square miles, or an average of 11,200,000 acres), and is 

 so constructed, upon a special projection, as to admit that the several sheets 

 may be joined to comprise entire political or other divisions. 



The plan for the systematic prosecution of a detailed topographical 

 survey of the territory of the United States west of the one hundredth 

 meridian, as the main object, was submitted to the Engineer Department 

 by the officer in charge shortly after the return of the Expedition of 1871, 

 was then approved by Brig. Gen. A A. Humphreys, Chief of Engineers, 

 and the Honorable the Secretary of War, and received the sanction of 

 Congress by a specific act approved June 10, 1872. 



In addition to the astronomic, geodetic, topographical, and meteor- 

 ological observations needed for the preparation and construction of the 

 map, such observations as are required, and are commensurate witli the 

 present condition of development of this region, are made in the branches 

 of mineralogy and mining, geology, paleontology, zoology, botany, archae- 

 ology, ethnology, and philology. 



The quarto reports embrace the results of the special branches of the 

 Survey that are completed at the date at which each is separately submitted, 

 while annual reports of operations of the work, accompanied by maps, 

 showing progress during the fiscal year, are regularly submitted to the 

 Chief of fmgineers, and appear as appendixes to his Annual Eeports 



From the accumulation of field data, the finished topographical maps 

 are completed as fast as the draughting force permits, and with the neces- 

 sary additions, special editions showing geological formations and land clas- 

 sification, are issued from time to time. Neither atlas will be complete until 

 the whole work is finished. 



