Whittlesey.] 186 [November 18, 



along their line of bearing, and of dip, the same disappearances. 

 Their identity often rests upon fossils alone, of which the specimens 

 are few. In such circumstances, in cases of doubt, especially where 

 the beds are not conformable, the most sure solution lies in the use of 

 the spirit level, and geometry. It often happens that the vertical 

 distance between coal seams is less than the unavoidable errors of 

 the barometer, and therefore in Ohio I have never used it. The local 

 undulations in large mines are often twenty and twenty-five feet, and 

 the extreme ones forty to fifty feet. 



Opposite the Ohio division of this coal field in West Virginia, the 

 central line of the basin lies fifteen to twenty miles beyond the Ohio 

 River, its general direction being about northeast, and, though not 

 always straight, is roughly parallel to the valley. 



The bottom of the basin here is probably boat-shaped (its thickness 

 unknown), the central line being vertically in the plane of the keel, 

 crossing the river near the great bend below Pittsburg, east of the 

 Ohio line, and pointing in the direction of the forks of the Alle- 

 ghany at Franklin, Pa. Here the keel rises into the prow. Easterly 

 of it, the dip changes from southerly to westerly, in a region which 

 H. D. Rogers has shown to be peculiarly irregular. The three tests 

 I propose to apply are as follows : — 



First. — To take several points on the reputed out-crop of No. 6, 

 as given in the Report, and draw lines from them converging to • a 

 point on or near the central line of the basin, in Washington County, 

 Pennsylvania. The points are six in number, covering a section 

 of the coal field of about 90° in arc from west to north; all of 

 which lines must cut the supposed undulations. All the elevations of 

 Seam No. 6, on and near these radial lines, will be given. If there 

 are swells and reverse inclinations in the strata, it must appear on 

 these lines. 



Secondly. — To give a net work of triangulations for dip covering 

 the same territory. If there are reverse planes it is impossible they 

 should escape observation among so many lines and levels. 



Thirdly. — Taking Seam No. 6 as a persistent stratum, as it is rep- 

 resented to be in the Report, to refer to it all beds of coal and lime- 

 stone below it. If it shall appear that no other bed is persistent, 

 the presumption is that this is not an exception. 



I begin with the outcrop of No. 6 in Coshocton County, O., which 

 is nearly west of the assumed central point, and number the radii 

 around to the north in succession. The figures for elevation, where 



