MAGNETIC SURVEY OF JAPAN FOR THE EPOCH 1895.0. 177 



precaution, may lead ns to thoiiglits with regard to tectonic condi- 

 tions which might otherwise lie too hidden for our mental sight. 



The above cited English magneticians have inaugurated a 

 convenient nomenclature to designate various groups of disturb- 

 ing forces ; according to them, a place is called a magnetic ridge 

 or peak when disturbing forces converge towards it, and a valley 

 when they diverge from it. In land topography an eye estimate 

 is of great help : even from a few barometric determinations of 

 heights on prominent points, a fairly approximate set of contour 

 lines can be drawn in this way. In magnetic survey w^e are 

 utterly deprived of such means ; the circumstance is even worse 

 than that of sea sounding, where nothing but the depths of 

 observed points can be known ; for we do not get wdiat corres- 

 ponds to depths and heights directly, but only wdiat corresponds 

 to slopes or rales of gradiaiit. If a magnetic survey could be so 

 extended as to enable us to draw equipotentials of the disturb- 

 ing forces, the ridges and valleys Avould become more distinct. 

 In the absence of such we are much involved in ambiguity at 

 present. Thus if there are numbers of stations which give dis- 

 turbing forces pointing in the same direction, w^e are not certain 

 whether this is due to attraction to the one side or repulsion 

 from the other. 



In Kitalcami Plateau in the north east of Honsyfi, this is 

 exemplified. Almost all disturbing forces round this plateau 

 diverge from it, showing apparently the presence of a magnetic 

 valley along its length as indicated by the thick dotted line. 

 But looking on the west, the station No. 185, Kakunodate, has 

 a disturbing force pointing towards the east, w^e are thus called 

 upon to reflect whether the observed disturbances along the banks 

 of the Kitakami Eiver (Nos. 165, 166, 167, 168) are due to re- 



