CLASSIFICATION OP MINERAL LANDS. 58 



a reduced cost of the work and a great degree of accuracy in location. 

 In work of this kind the geologist usually examines the territory in 

 advance of the topographer, prospecting the coal bed and carefully 

 marking by means of flags all prospects and coal outcrops which he 

 desires the topographer to locate. The topographer, in the course of 

 his mapping, determines both the horizontal and the vertical positions 

 of all points flagged by the geologist. This work is done instru- 

 mentally and with great accuracy, the probable limit of error in 

 the horizontal location of any point being less than 20 feet and 

 in the vertical location less than 5 feet. With many points on the 

 outcrop thus accurately located, underground structure can be de- 

 termined with a degree of accuracy that has seldom been attained in 

 geologic work. 



With this preliminary statement, it is proposed' to set forth briefly 

 some of the methods now in use under the various conditions that 

 confront the geologist who is assigned to the task of making a mineral 

 classification of a specific area. 



A. If he is required to make a detailed survey, he will find one of 

 the following conditions existing: 



(a) A topographic base is available. 

 (h) A base map is not available but must be made. 

 (c) A topographic base is not available and is not essential, the 

 land survey sufficing as a base map. 



B. If he is making a preliminary survey only he will use recon- 

 naissance methods. 



DETAII4ED SURVET. 

 METHODS T7SBB WHEN A TOPOOKAPHIC BASB 13 AVATLABIiE. 



It is desirable that the examination of the mineral deposits in 

 regions of especial economic importance be based on an accurate 

 topographic map prepared in advance of the geologic examination. 

 Such a map, on which are shown the relief by contours of appropri- 

 ate intervals, the position and character of the drainagewaySj and the 

 location of land corners, roads, houses, and the bench marks and tri- 

 angulation stations established by the topographers, enables the geolo- 

 gist to determine by surveyor's methods the location of the numerous 

 points at which he makes observations, with a minimum expenditure 

 of time and with a degree of precision adequate to the scale of the 

 map and the work in hand. The scales which are generally used by 

 the Geological Survey in work of this character are about IJ or 2 

 inches to the mile, with contour intervals of 10, 20, or 50 feet. The 

 degree of accuracy attained in work on such maps is represented by 

 the width of a carefully drawn pencil line, or less than 20 feet for lo- 

 cations where refined plane-table methods are employed — for example, 

 along the outcrop of the coal, oil sand, or phosphate bed — and by a 



