132 Prof. B. S. Lyman — Contour Lines on Geological Maps. 



A 



VI. — Contour-Lines on Geological Maps. 



By Prof. Benjamin Smith Lyman. 



S one of your reviewers now some years ago criticized unfavour- 

 ably the method of representing geological structure in maps 

 by contour lines upon a bed of coal or other rock, and as, like hira, 

 most geologists are not yet practically familiar with the method, 

 useful though it be, and nobody that might be thought wholly 

 unbiassed has replied in your pages to his attacks, it may not be 

 improper to call your attention to the unprejudiced opinion and 

 explanation of Mr. Charles A. Ashburner, the geologist in charge of 

 the survey of the anthracite region, under the Geological Survey of 

 Pennsylvania, who has up to this time published 26 large sheets 

 of most elaborate maps and sections with one volume of text. After 

 careful examination before adopting the method and after using it 

 several years he writes of it in the report (page 8) as follows : — 



"The first time the system has been used on an extensive scale in 

 America has been in the construction of the mining geological sheets 

 of the anthracite coal fields. The data which are available for con- 

 structing these maps are very extensive and very accurate. The 

 method has been found to furnish the best means for interpreting 

 the geological structure and the best way of representing it ; so that 

 in this the demands of the geological investigator are satisfied. At 

 the same time that this end is accomplished, the facts relating to the 

 structure of the coal bed in the mines and adjoining tracts are best 

 classified and placed in such a form as to be of practical use to the 

 mine superintendent and engineer. 



" The information which a geological mining map constructed on 

 this plan contains relative to the coal bed which is contoured may be 

 classified under the following heads : a, elevation above tide of the 

 coal outcrop ; h, dip of the bed ; e, strike of the bed ; d, depth of 

 the coal basins ; e, rate of fall or rise of the basins along their axes ; 

 /, position of the synclinals and ancticlinal crests in the coal bed ; 

 g, data from which a vertical cross-section may be made at any point 

 across the basins ; h, data from which the absolute area of the coal 

 bed may be obtained, and the amount of coal contained in any special 

 surface area estimated. 



" a. In order to obtain the elevation of the outcrop at every point, 

 it is necessary that the bed should be contoured on a topographical 

 map of the surface as a basis. 



" 6. The horizontal distance between the contour curves represents 

 the cosine of the angle of the dip . . . 



" c. The direction of the contour curves shows the strike of the bed 

 and the direction of the gangways which have already been driven, 

 or the most probable direction which approximately level gangways 

 will assume if driven beyond the present mined area. 



" d. The most probable depth of the coal basins can be better 

 estimated by this method of construction than by any other . . . 



" e. The rate of rise and fall of the basins along their axes is a 

 very important fact to determine, in order to locate mining works 



