REFLEX INFLUENCE OF GRAPHIC METHODS. 187 



The geologist who is forging a continuous chain of evidence becomes ap- 

 prehensive of missing links, and the visible continuity of his record leads 

 him to close observation ; he skips nothing. And herein is a reflex action 

 of graphic methods upon the observer, which is one of their best recom- 

 mendations. The graphic record invites close attention to the accumulating 

 facts ; it is suggestive and directs observation to possible undiscovered facts. 

 Graphic methods make keen observers. 



Types of Field Notes. 



Verbal descriptive Notes. — Example : 



" Above these beds of Clinton ore, which lie in yellowish shales, is a white sand- 

 stone, forming the summit of Walden's ridge and its southern slope to the fault on 

 the south. This sandstone is precisely like that observed on Poor valley ridge, Cum- 

 berland mountain, but it here lies above Clinton ores and there it underlies them. At 

 the summit of Walden's ridge this sandstone and the Clinton shales with ore are folded 

 in a sharp broken anticlinal and gentle synclinal, which restores the southern dip ; 

 this structure is well exposed in the sandstone cliff." 



This form of record is often imperatively necessary to supply descriptions 

 of relations or of physical characteristics of rocks which cannot easily be 

 graphically expressed. The description written on the spot has an authority 

 and value no subsequent statement can have, and the verbal form permits 

 comparisons of facts, as the graphic form does not in the same degree. 



Verbal Notes for Stratigraphy. — Example : 



" Descending Cumberland mountain at White Rocks. Dip of strata 15° to 20° 

 northwestward ; aneroid 2,740 feet. The summit and escarpment of the mountain 

 are formed of jfine-grained sandstones, cross-stratified and containing layers of quartz 

 pebbles, 1 inch and less in diameter. 



715 feet (aneroid) below the summit the sandstone talus covers a light-greenish 

 shale ; anei'oid 2,025 feet. 



Outcrop of yellowish sandstone below shale ; aneroid 1,850 feet. 



Highest outcrop of compact gray limestone ; aneroid 1,780 feet. 



Highest outcrop of purple shales ; aneroid 1,720 feet." 



Such aneroid notes are of value for determinations of thickness of strata 

 where the beds lie flat or at a gentle dip and the observations can be made 

 on a steep slope ; that is where the vertical measurement is of principal value 

 and the relations in horizontal plan are not essential to the result desired. 

 Again, given an adequate contour map and a simple problem of stratigraphy 

 and structure, such notes may answer for the location of boundaries and 

 structural facts ; but their value is in proportion to the accuracy of the map 

 and the simplicity of the problem, so that they may at any point become 

 valueless through inaccuracies of the one or through unexpected complica- 

 tions of the other. 



