178 BAILEY WILLIS — GRAPHIC FIELD NOTES. 



antly narrow; in the underbrush of the plateau they wound about in such 

 manner as to exaggerate the impression of distance. It was shrewdly cal- 

 culated that any geologist by these means topographically misled might be 

 geologically confused and led to count a single coal bed seen at different 

 openings as several beds. And this calculation was justified by the result. 

 An expert of high standing, whose experience and reputation fairly com- 

 manded confidence, reported the coal at nearly three times its actual thick- 

 ness, and $750,000 was paid on his mistake. The error in stratigraphy fol- 

 lowed from ignorance of the local geologic structure, both avoidable had the 

 geologist determined relations of distance and direction among observed 

 sections. 



The point of this story is the point of this article : A knowledge of rela- 

 tions in space among geologic facts is essential to the solution of problems 

 of stratigraphy and structure, and it follows that the geologist must locate 

 his observations on a map either prepared in advance or surveyed simul- 

 taneously with his work. The possession of an adequate map constitutes the 

 ideal initial condition for geologic work. 



Definition of an adequate Map for Geologic Purposes. 



General Definition. — An "adequate map" is one which accurately de- 

 scribes the character of the features delineated : it is so characteristically 

 true to the facts of topography aud culture that it oflfers many tie-points, 

 i. e., many points which can be definitely recognized as the representatives 

 of specific locations on the ground. Such points are essential to the location 

 of a geologist's observations of outcrops, strikes and dips, or formation 

 boundaries, which may be of very limited extent but which must be placed 

 on the map with such accuracy that the error, reduced to the scale of the 

 map, is insignificant. Such tie-points are bends of roads, cross-roads, cross- 

 ings of roads and streams, sharp turns in streams, stream junctions, springs, 

 mountain peaks, ridges, gaps, spurs, abrupt changes of slope ; in a word all 

 characteristic features. 



Maps are sketches fitted to a geometric control. If we compare them 

 with works of higher art, we may liken the painfully exact military maps of 

 Europe to miniature portraits, while some American maps, produced under 

 demand for quantity rather than quality, suggest paintings executed with 

 a palette knife. The diflTerence lies in the minuteness of control, in the 

 number of points accurately determined per square inch of map. 



Methods of Control. — The measurements which constitute control are 

 obtained by two methods, triangulation and meander, each of which has its 

 advocates, each of which requires certain natural conditions for economic 

 working, but which in most regions can advantageously be combined. 



