102 Association of American Geologists and Naturalists. 



which classes of I'ocks might be distinguished, although concealed at con- 

 siderable depths, the magnetical instruments in this respect answering 

 the general purpose of a mineral or divining rod. 



The method by which this became apparent was as follows : a hor- 

 izontal base line was laid off, and divided say into millimetres, and up- 

 on this as a line of latitude were set up, at distances corresponding to 

 the distances of the stations, perpendiculars, or ordinates, of such lengths 

 as to represent the dip at the given station. This formed a line of points 

 through which a curve was drawn, called a curve of dip. The scale 

 adopted was a millimetre to every geographical mile of latitude, and a 

 half millimetre to every minute of dip. As we desired only the differ- 

 ences of these ordinates to generate the curve, only the upper ends 

 were represented — the lower portion, including the zero, or point of no 

 dip, was supposed to be concealed, or cut off below the chart. A simi- 

 lar method was adopted to produce a curve of total intensity, indeed, 

 which was drawn on the same ordinates ; the total intensity at Cincin- 

 nati being reckoned a thousand, and a millimetre a unit. And here 

 a^ain the lower portion of the scale was omitted. The curves are ex- 

 hibited on these engravings, [here a diagram was exhibited,] and here 

 it appears that within the region of our horizontal limestone, destitute of 

 dykes, either of trap or any other igneous rock, for many hundreds of 

 miles, and destitute also of local disturbances, the curves ascend in pro- 

 ceeding to the northward with a gentle acclivity, as exhibited on the 

 line from St. Louis along the Mississippi to Prairie du Chien, a distance 

 of four and a half degrees ; and again on a line from Lexington, Ken- 

 tucky, north two degrees, to Piqua, in Ohio. Here it will be seen, that 

 the curve of dip, although it varies its rate of ascent in many places, 

 yet it never descends in progressing northwardly. But a curve similarly 

 generated along the line from Baltimore to New York, where the sub- 

 jacent rocks are igneous, exhibits ascending and descending undulations, 

 like the outlines of distant primitive mountains ; and on a line from 

 west to east, along the south shore of Lake Superior, not only undulate, 

 but so abruptly as to generate sharp needle-shaped points, so sharp in- 

 deed that it was impossible to preserve the scale and still delineate 

 them — the horizontal scale at one place being enlarged. The subjacent 

 rocks, in this curve, are various ; but they consist mostly of trap rocks, 

 and exhibit abundant signs of igneous action. 



I have lately examined the trappean rocks in the vicinity of New 

 York, and find the results diversified and interesting. Here the partic- 

 ulars were stated, and it was suggested as probably true that certain 

 rocky portions of the earth, if they be conductors of magnetism, as trap- 

 pean rocks, and especially if they be elevated, and of an oblong form, 

 become feebly magnetized by the induction of the earth itself, and ac- 



