212 KNOTT AND TANAKADATE 



probal)le error becomos ± 5' .9 — n value hy no means eonsiderahje in 

 the circumstances. 



This seems the natural place to refer to Dr. Xaumann's discussion 

 of the iso-map-netic lines of Japan in relation to its geology. Basing 

 on the broad features of Sekino's chart, IS^aumann finds in the form 

 of the isoo-onic line of 5° W. a close relation to the so-called Fossa Mckj- 

 na. Just where this grent break in the geological continuity of the 

 country occurs, there a large sinuosity seemed to show itself on the 

 isoo-onic line. This Fossa Magna almost stretches right j'.cross the 

 central part of Japan in a nearly north and south direction. Fuji- 

 yama is mcluded in it and so, it is generally supposed, is the line of 

 volcanic islands stretching south-easterly. The Fossa Magna hardly 

 reaches the Northern coast of Japan ; but, if continued northwards, 

 it wonld be found to run between the Peninsula of Notc^ on the west 

 and the Island of Sndo on the east. Now it is just at this region 

 that Sekino's 5° Isogonic line makes a great bend to the north, 

 doublinp' back just over the islnnd of Sado and then, after an easterly 

 sweep, continuing north-easterly across the country. It is extremely 

 doubtful whether the observations warrant such a delineati(^n of the 

 line of 5° Declination. A careful scrutiny of Sekino's numbers brings 

 out certain discrepancies which should not nltogerher be neglected. 

 Further, there is a complete lack of observations nlong the coast to the 

 south and south-west of Sado — just where observations seem most 

 called for. The stations chosen are all inland, and show striking 

 irreo-ularities in the values of the declinations. True, the declinations 

 at the tlu-ee Stations on Sado are all considerably less than the values 

 at maiidand stations immediately to the east ; whereas we should 

 expect to find them greater. P)Ut that seems hardly a sufficient 

 reason for making the isogone of the form represented. For it is well 

 known that the isogonif lines at and near islands often present 



