330 GEOGRAPHIC WORK OF GENERAL GOVERNMENT 

 GEOLOGICAL SURVEY 



The Geological Survey is charged by law with the examina- 

 tion of the geological structure, the mineral resources, and with 

 the classification of the public lands of the United States. It 

 was organized in 1879, upon the discontinuance of the Hayden, 

 Wheeler, and Powell surveys of the Rocky Mountain region. 



As the successful prosecution of the work confided to it re- 

 quired the possession of accurate topographic maps, the prepa- 

 ration of such *naps was commenced in 1882, and a large pro- 

 portion of the appropriations for the Survey have been devoted 

 to this work. 



The work of the Survey, as at present organized, is as follows : 



The preparation of topographic maps. 



The preparation of geologic maps. 



The technical and statistical study of mineral resources. 



The study of the water resources of the arid region. 



The examination of the forests of the west. 



Chemistry and paleontology as accessories to the geologic work. 



The Geological Survey began, in 1882, the construction of a 

 topographic map of the country. The work has now been in 

 progress 16 3 T ears, and about 650,000 square miles have been 

 mapped. The areas shown on these maps are scattered widely 

 over the country, and represent a great variety of topographic 

 features, and the map sheets can be used to illustrate topographic 

 forms. These maps differ in scale. Some of them are on the scale 

 1 : 62.500, which is very nearly one mile to one inch. Another 

 scale is 1 : 125,000, which is very nearly two miles to one inch, 

 and a third scale is 1 : 250,000, or nearly four miles to one inch. 



Sheets. — For convenience this map is published in sheets of 

 nearly uniform size, the portion of the sheet covered by the map- 

 ping being usually 17? inches in height, with a breadth ranging, 

 according to latitude, from 12a to 15 inches. Each sheet on the 

 scale 1 : 250,000 includes what is commonly called a " square 

 degree," an area one degree in extent in each dimension (for in- 

 stance, latitude 40° to 41° and longitude 90° to 91°). A sheet 

 on the scale 1 : 125,000, which is of approximately the same size, 

 includes a tract of country 30' in latitude by 30' in longitude, or 

 one-fourth of a square degree, and a sheet on the largest scale, 

 1 : 62,500, includes an area 15' in latitude by 15' in longitude, or 

 one-sixteenth of a square degree. 



Contents. — This map shows features which, for convenience, 



