ACTIVITIES 41 



A "geologic folio" is thus described by the Survey : 2 



A standard geologic folio comprises a descriptive text, in 

 which the geography, topography, and geology of the area 

 are described; a topographic map; a geologic map, printed 

 in colors, showing the areal distribution of the geologic for- 

 mations; a geologic map showing the deposits of economic 

 value in stronger or more brilliant colors; a geologic map on 

 which the underground structure of the rocks is exhibited by 

 vertical sections representing what would be seen in deep 

 trenches cut across the area; and a columnar section, in which 

 the rocks are represented in a vertical column in their nor- 

 mal relation one to another, accompanied by a condensed 

 description of their composition, thickness, and relations. 



For special areas other maps or illustrations are included. 

 Often it is desirable to present with the description a page of 

 illustrations reproduced from photographs or sketches. For 

 artesian-water areas the distribution of the water-bearing 

 strata, the areas where flowing water may be obtained, and 

 the depths to the water horizon are shown on a special sheet. 

 For active mining regions a map on a scale sufficiently large 

 to show important details of the geology, mines, and ore 

 deposits is inserted. For coal regions of economic importance 

 special methods are used to show the structure or lay of the 

 coal beds — of so much importance to the mine worker. This 

 is done by means of contour lines which show the elevation 

 above sea of the coal bed throughout its occurrence in the 

 area, and then furnish the data for determining the depth 

 of the coal below the surface at any point. Sections of coal 

 beds which have been opened or worked are shown on a large 

 scale and their relation to one another is indicated. 



Generally speaking, each folio relates to a single quadrangle, 

 that term having the same meaning as when applied to a topo- 

 graphic map; but some folios cover two or more quadrangles. 



Occasionally, the advance of geologic knowledge of a par- 

 ticular area, or the demand for more detailed information, 

 necessitates a revision of maps already published. 



The geologic survey now carried on discloses the distribu- 



2 The U. S. Geological Survey : Its Origin, Development, Organiza- 

 tion and Operations, pi 99-100, 1904. 



