ACTIVITIES 39 



est foot ; second, by lists of bench marks in the annual reports 

 or bulletins of the Survey, with a full description of each 

 bench mark and its exact elevation above sea level to the 

 thousandth of a foot, as adjusted and referred to the various 

 central datum points, these lists being corrected in publica- 

 tions from time to time as better connections are made with 

 sea level. These publications are issued separately for each 

 state. 



Geological Survey. As perhaps its basic activity, the Survey 

 is carrying on a survey of the areal geology and geological 

 structure of the United States. By means of this survey it 

 seeks to ascertain the distribution, structural relations, miner- 

 alogic character, economic value, and geologic history of the 

 formations of each area surveyed. The results of this survey 

 it publishes in the form of maps, illustrations, and textual de- 

 scriptions. 



The selection of areas for geologic survey is determined by 

 a variety of considerations, corresponding closely to those 

 mentioned in connection with the distribution of the topo- 

 graphic surveys. Still another element is the desirability of 

 giving preference to the public land areas because of the value 

 of geologic mapping in furnishing information needed by the 

 General Land Office in the administration of the land laws. 



The degree of refinement of the geologic survey varies with 

 the character of the area and the results desired. The scale 

 of the map on which the results are to be published is deter- 

 mined by the same considerations as in the case of topographic 

 mapping. The three scales used in the topographic mapping — 

 1 1250,000, 1 :i25,ooo and 1 :62,50c) — are used also in the geo- 

 logic mapping and according as one or the other map scale is 

 contemplated the survey is said to be exploratory, reconnais- 

 sance, or detailed. Larger scales also are used. 



The geologic survey of an area is usually begun after the 

 topographic map has been completed, and that map is used as 

 a base. Occasionally, however, the geologist works with the 



