MAGNETIC OBSERVATIONS. 343 



of the dip curves is reaLy the subject sought, and since these, in the pres- 

 ence of considerable disturbances, are sensibh' independent of small differ- 

 ences in the index error, it is not indispensable that the needles should be 

 exactly together. 



These instruments are simple, and, of course, do not give precise results. 

 But the observations are rapidl}^ and cheaply made, and to a sufficient 

 degree of accuracy for the end in view. It may be stated again that the 

 object is to detect and compare relative magnetic disturbances, and to find 

 out the bearing of these disturbances on the distribution and attitude of the 

 rocks which produce them. For this piirpose the instruments are exceed- 

 ingly well adapted. 



The field work was carried out by parties of two men each, one of 

 whom, a skilled woodsman, carried along the line and observed the hori- 

 zontal needle, while the other read the dip needle, kept the notes, and 

 attended to tlie geology. According to the general plan of the field work, 

 . a series of parallel lines was run either north and south or east and west 

 across each section. The direction of the lines of travel was chosen so as 

 to cut the strike of the rocks at the largest angle. The probable direction 

 of strike for each day's work could be inferred in advance from what had 

 gone before. If it were more nearly north and south than east and west, 

 the traverse lines were run east and west, and vice versa. These directions 

 were in many cases not the most desirable for the magnetic work alone, but 

 the choice was practically limited by the lines of the United States Land 

 Survey, which give for each square mile eight points of departure (at the 

 four corners and four quarter posts of each section)^ which are generally 

 identifiable on the ground. On these lines of travel the instruments were 

 read at various intervals, from 5 to 10 or 100 paces, depending upon the 

 local complications. The intervals between the lines varied from one- 

 sixteenth to one-fourth of a mile, and were determined not only by the 

 mag-netic complications, but by the character of the surface, it being- 

 especially desirable that the ground should be so closely covered that no 

 outcrop could escape detection. The distances along and ofP the lines of 

 travel were measured by pacing. The general accuracy of the pacing is 

 remarkable, and is essentially within the platting error of the scale of the 

 maps. The average closing error for August, 1892, during which about 

 100 miles of traverse lines were run, was 20 paces per mile, or 1 per cent. 



