236 



4. Setting- Bench Marks with Aneroid Barometer. 



5. Reconnaissance (Scouting) Using Aneroid Barometer and Hand 

 Level. 



Method No 1. 



For close detail work the plane table and stadia traverse is by far 

 the most accurate method and no doubt favored by all geologists. 



With this method the party consists of a geologist in charge and 

 an instrument man. The geologist carries the stadia and follows the 

 outcrop, giving stadia readings for location and elevation as fre- 

 quently as he deems necessary. Between stations the instrument man 

 sketches the drainage, roads and any and other featui'cs necessary to 

 make a complete geological map. 



At intervals of an hour or an hour and a half the geologist should 

 return to the plane table and sketch the various outcrops on the map 

 and record the vertical interval between the different strata he has 

 mapped. 



At night the day's work is inked in and that portion of the map 

 is complete with exception of the structural contours. 



This method is favored for open countiy and areas free from tim- 

 ber growth, and is fairly rapid. 



The small telescopic alidade used by the United States Geological 

 Survey is commonly used. The size of plane table depends upon hori- 

 zontal scale used, varying from 15 inches to 24 inches square. 



Method No. 2. 



The second method used is not as accurate as first but is much 

 more rapid for use in timber-covered areas. 



With this method an instrument man with plane table, stadia and 

 a rodman are sent into the particular area to be mapped. They run 

 a stadia traverse along the roads, establishing bench marks at the 

 corners and other conspicuous places, at least every one-half mile. If 

 the roads are few the bench marks should be established at the end of 

 spurs that extend toward the main stream between tributary valleys. A 

 key system being used to mark the bench marks, the rodman paints 

 the bench marks according to the key used. The elevation and number 

 or key is recorded on the map for use of the geologist. 



